[PATCH 0/2] Fix built-in rebase perf regression

[PATCH 0/2] Fix built-in rebase perf regression

[Johannes Schindelin via GitGitGadget]

In our tests with large repositories, we noticed a serious regression of the
performance of git rebase when using the built-in vs the shell script
version. It boils down to an incorrect conversion of a git checkout -q:
instead of using a twoway_merge as git checkout does, we used a oneway_merge 
as git reset does. The latter, however, calls lstat() on all files listed in
the index, while the former essentially looks only at the files that are
different between the given two revisions.

Let's reinstate the original behavior by introducing a flag to the 
reset_head() function to indicate whether we want to emulate reset --hard 
(in which case we use the oneway_merge, otherwise we use twoway_merge).

Johannes Schindelin (2):
  rebase: consolidate clean-up code before leaving reset_head()
  built-in rebase: reinstate `checkout -q` behavior where appropriate

 builtin/rebase.c | 60 ++++++++++++++++++++++++++----------------------
 1 file changed, 33 insertions(+), 27 deletions(-)


base-commit: 8858448bb49332d353febc078ce4a3abcc962efe
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-72%2Fdscho%2Fbuiltin-rebase-perf-regression-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-72/dscho/builtin-rebase-perf-regression-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/72
-- 
gitgitgadget

[PATCH 1/2] rebase: consolidate clean-up code before leaving reset_head()

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The same clean-up code is repeated quite a few times; Let's DRY up the
code some.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 0ee06aa363..6f6d7de156 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -569,16 +569,13 @@ static int reset_head(struct object_id *oid, const char *action,
 	}
 
 	if (!fill_tree_descriptor(&desc, oid)) {
-		error(_("failed to find tree of %s"), oid_to_hex(oid));
-		rollback_lock_file(&lock);
-		free((void *)desc.buffer);
-		return -1;
+		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
+		goto leave_reset_head;
 	}
 
 	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
-		rollback_lock_file(&lock);
-		free((void *)desc.buffer);
-		return -1;
+		ret = -1;
+		goto leave_reset_head;
 	}
 
 	tree = parse_tree_indirect(oid);
@@ -586,10 +583,9 @@ static int reset_head(struct object_id *oid, const char *action,
 
 	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
 		ret = error(_("could not write index"));
-	free((void *)desc.buffer);
 
 	if (ret)
-		return ret;
+		goto leave_reset_head;
 
 	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
 	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
@@ -622,7 +618,10 @@ static int reset_head(struct object_id *oid, const char *action,
 					 UPDATE_REFS_MSG_ON_ERR);
 	}
 
+leave_reset_head:
 	strbuf_release(&msg);
+	rollback_lock_file(&lock);
+	free((void *)desc.buffer);
 	return ret;
 }
 
-- 
gitgitgadget

Re: [PATCH 1/2] rebase: consolidate clean-up code before leaving reset_head()

[Jeff King]

On Fri, Nov 09, 2018 at 01:34:17AM -0800, Johannes Schindelin via GitGitGadget wrote:

> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 0ee06aa363..6f6d7de156 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -569,16 +569,13 @@ static int reset_head(struct object_id *oid, const char *action,
>  	}
>  
>  	if (!fill_tree_descriptor(&desc, oid)) {
> -		error(_("failed to find tree of %s"), oid_to_hex(oid));
> -		rollback_lock_file(&lock);
> -		free((void *)desc.buffer);
> -		return -1;
> +		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
> +		goto leave_reset_head;
>  	}

If fill_tree_descriptor() fails, what is left in desc.buffer? Looking at
the implementation, I think it's always NULL or a valid buffer. But I
think all code paths actually die() unless we pass a NULL oid (and in
that case desc.buffer would be NULL, too).

So I think the original here that calls free() doesn't ever do anything
but it did not hurt. After your patch, the leave_reset_head code would
continue to call free(), and that's OK.

There are a few earlier conditionals in reset_head() that do only
rollback_lock_file() that could similarly be converted to use the goto.
But they would need desc.buffer to be initialized to NULL. I could go
either way on converting them or not.

> @@ -586,10 +583,9 @@ static int reset_head(struct object_id *oid, const char *action,
>  
>  	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
>  		ret = error(_("could not write index"));
> -	free((void *)desc.buffer);
>  
>  	if (ret)
> -		return ret;
> +		goto leave_reset_head;
>  
>  	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
>  	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
> @@ -622,7 +618,10 @@ static int reset_head(struct object_id *oid, const char *action,
>  					 UPDATE_REFS_MSG_ON_ERR);
>  	}
>  
> +leave_reset_head:
>  	strbuf_release(&msg);
> +	rollback_lock_file(&lock);
> +	free((void *)desc.buffer);
>  	return ret;

We get here on success, too. So we may call rollback_lock_file() on an
already-committed lock. This is explicitly documented as a no-op by the
lock code, so that's OK.

So overall looks good to me.

-Peff

Re: [PATCH 1/2] rebase: consolidate clean-up code before leaving reset_head()

[Johannes Schindelin]

Hi Peff,

On Fri, 9 Nov 2018, Jeff King wrote:

> On Fri, Nov 09, 2018 at 01:34:17AM -0800, Johannes Schindelin via GitGitGadget wrote:
> 
> > diff --git a/builtin/rebase.c b/builtin/rebase.c
> > index 0ee06aa363..6f6d7de156 100644
> > --- a/builtin/rebase.c
> > +++ b/builtin/rebase.c
> > @@ -569,16 +569,13 @@ static int reset_head(struct object_id *oid, const char *action,
> >  	}
> >  
> >  	if (!fill_tree_descriptor(&desc, oid)) {
> > -		error(_("failed to find tree of %s"), oid_to_hex(oid));
> > -		rollback_lock_file(&lock);
> > -		free((void *)desc.buffer);
> > -		return -1;
> > +		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
> > +		goto leave_reset_head;
> >  	}
> 
> If fill_tree_descriptor() fails, what is left in desc.buffer? Looking at
> the implementation, I think it's always NULL or a valid buffer. But I
> think all code paths actually die() unless we pass a NULL oid (and in
> that case desc.buffer would be NULL, too).
> 
> So I think the original here that calls free() doesn't ever do anything
> but it did not hurt. After your patch, the leave_reset_head code would
> continue to call free(), and that's OK.

Right, that was my thinking, too.

> There are a few earlier conditionals in reset_head() that do only
> rollback_lock_file() that could similarly be converted to use the goto.
> But they would need desc.buffer to be initialized to NULL. I could go
> either way on converting them or not.

Whoops. I should have checked more carefully?

> > @@ -586,10 +583,9 @@ static int reset_head(struct object_id *oid, const char *action,
> >  
> >  	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
> >  		ret = error(_("could not write index"));
> > -	free((void *)desc.buffer);
> >  
> >  	if (ret)
> > -		return ret;
> > +		goto leave_reset_head;
> >  
> >  	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
> >  	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
> > @@ -622,7 +618,10 @@ static int reset_head(struct object_id *oid, const char *action,
> >  					 UPDATE_REFS_MSG_ON_ERR);
> >  	}
> >  
> > +leave_reset_head:
> >  	strbuf_release(&msg);
> > +	rollback_lock_file(&lock);
> > +	free((void *)desc.buffer);
> >  	return ret;
> 
> We get here on success, too. So we may call rollback_lock_file() on an
> already-committed lock. This is explicitly documented as a no-op by the
> lock code, so that's OK.

Indeed. I did not check the documentation, but the code, and came to the
same conclusion.

> So overall looks good to me.

Thanks!
Dscho

[PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When we converted a `git checkout -q $onto^0` call to use
`reset_head()`, we inadvertently incurred a change from a twoway_merge
to a oneway_merge, as if we wanted a `git reset --hard` instead.

This has performance ramifications under certain, though, as the
oneway_merge needs to lstat() every single index entry whereas
twoway_merge does not.

So let's go back to the old behavior.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 45 ++++++++++++++++++++++++++-------------------
 1 file changed, 26 insertions(+), 19 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 6f6d7de156..c1cc50f3f8 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -523,11 +523,12 @@ finished_rebase:
 #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
 
 static int reset_head(struct object_id *oid, const char *action,
-		      const char *switch_to_branch, int detach_head,
+		      const char *switch_to_branch,
+		      int detach_head, int reset_hard,
 		      const char *reflog_orig_head, const char *reflog_head)
 {
 	struct object_id head_oid;
-	struct tree_desc desc;
+	struct tree_desc desc[2];
 	struct lock_file lock = LOCK_INIT;
 	struct unpack_trees_options unpack_tree_opts;
 	struct tree *tree;
@@ -536,7 +537,7 @@ static int reset_head(struct object_id *oid, const char *action,
 	size_t prefix_len;
 	struct object_id *orig = NULL, oid_orig,
 		*old_orig = NULL, oid_old_orig;
-	int ret = 0;
+	int ret = 0, nr = 0;
 
 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
@@ -544,20 +545,20 @@ static int reset_head(struct object_id *oid, const char *action,
 	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
 		return -1;
 
-	if (!oid) {
-		if (get_oid("HEAD", &head_oid)) {
-			rollback_lock_file(&lock);
-			return error(_("could not determine HEAD revision"));
-		}
-		oid = &head_oid;
+	if (get_oid("HEAD", &head_oid)) {
+		rollback_lock_file(&lock);
+		return error(_("could not determine HEAD revision"));
 	}
 
+	if (!oid)
+		oid = &head_oid;
+
 	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
 	setup_unpack_trees_porcelain(&unpack_tree_opts, action);
 	unpack_tree_opts.head_idx = 1;
 	unpack_tree_opts.src_index = the_repository->index;
 	unpack_tree_opts.dst_index = the_repository->index;
-	unpack_tree_opts.fn = oneway_merge;
+	unpack_tree_opts.fn = reset_hard ? oneway_merge : twoway_merge;
 	unpack_tree_opts.update = 1;
 	unpack_tree_opts.merge = 1;
 	if (!detach_head)
@@ -568,12 +569,17 @@ static int reset_head(struct object_id *oid, const char *action,
 		return error(_("could not read index"));
 	}
 
-	if (!fill_tree_descriptor(&desc, oid)) {
+	if (!reset_hard && !fill_tree_descriptor(&desc[nr++], &head_oid)) {
+		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
+		goto leave_reset_head;
+	}
+
+	if (!fill_tree_descriptor(&desc[nr++], oid)) {
 		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
 		goto leave_reset_head;
 	}
 
-	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
+	if (unpack_trees(nr, desc, &unpack_tree_opts)) {
 		ret = -1;
 		goto leave_reset_head;
 	}
@@ -621,7 +627,8 @@ static int reset_head(struct object_id *oid, const char *action,
 leave_reset_head:
 	strbuf_release(&msg);
 	rollback_lock_file(&lock);
-	free((void *)desc.buffer);
+	while (nr)
+		free((void *)desc[--nr].buffer);
 	return ret;
 }
 
@@ -999,7 +1006,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		rerere_clear(&merge_rr);
 		string_list_clear(&merge_rr, 1);
 
-		if (reset_head(NULL, "reset", NULL, 0, NULL, NULL) < 0)
+		if (reset_head(NULL, "reset", NULL, 0, 1, NULL, NULL) < 0)
 			die(_("could not discard worktree changes"));
 		if (read_basic_state(&options))
 			exit(1);
@@ -1015,7 +1022,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (read_basic_state(&options))
 			exit(1);
 		if (reset_head(&options.orig_head, "reset",
-			       options.head_name, 0, NULL, NULL) < 0)
+			       options.head_name, 0, 1, NULL, NULL) < 0)
 			die(_("could not move back to %s"),
 			    oid_to_hex(&options.orig_head));
 		ret = finish_rebase(&options);
@@ -1379,7 +1386,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			write_file(autostash, "%s", oid_to_hex(&oid));
 			printf(_("Created autostash: %s\n"), buf.buf);
 			if (reset_head(&head->object.oid, "reset --hard",
-				       NULL, 0, NULL, NULL) < 0)
+				       NULL, 0, 1, NULL, NULL) < 0)
 				die(_("could not reset --hard"));
 			printf(_("HEAD is now at %s"),
 			       find_unique_abbrev(&head->object.oid,
@@ -1433,7 +1440,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				strbuf_addf(&buf, "rebase: checkout %s",
 					    options.switch_to);
 				if (reset_head(&oid, "checkout",
-					       options.head_name, 0,
+					       options.head_name, 0, 0,
 					       NULL, NULL) < 0) {
 					ret = !!error(_("could not switch to "
 							"%s"),
@@ -1499,7 +1506,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			 "it...\n"));
 
 	strbuf_addf(&msg, "rebase: checkout %s", options.onto_name);
-	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
+	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
 	    NULL, msg.buf))
 		die(_("Could not detach HEAD"));
 	strbuf_release(&msg);
@@ -1515,7 +1522,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		strbuf_addf(&msg, "rebase finished: %s onto %s",
 			options.head_name ? options.head_name : "detached HEAD",
 			oid_to_hex(&options.onto->object.oid));
-		reset_head(NULL, "Fast-forwarded", options.head_name, 0,
+		reset_head(NULL, "Fast-forwarded", options.head_name, 0, 0,
 			   "HEAD", msg.buf);
 		strbuf_release(&msg);
 		ret = !!finish_rebase(&options);
-- 
gitgitgadget

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Jeff King]

On Fri, Nov 09, 2018 at 01:34:19AM -0800, Johannes Schindelin via GitGitGadget wrote:

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> 
> When we converted a `git checkout -q $onto^0` call to use
> `reset_head()`, we inadvertently incurred a change from a twoway_merge
> to a oneway_merge, as if we wanted a `git reset --hard` instead.
> 
> This has performance ramifications under certain, though, as the
> oneway_merge needs to lstat() every single index entry whereas
> twoway_merge does not.
> 
> So let's go back to the old behavior.

Makes sense. I didn't think too hard about any possible gotchas with the
twoway/oneway switch, but if that's what git-checkout was doing before,
it seems obviously safe.

> diff --git a/builtin/rebase.c b/builtin/rebase.c
> index 6f6d7de156..c1cc50f3f8 100644
> --- a/builtin/rebase.c
> +++ b/builtin/rebase.c
> @@ -523,11 +523,12 @@ finished_rebase:
>  #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
>  
>  static int reset_head(struct object_id *oid, const char *action,
> -		      const char *switch_to_branch, int detach_head,
> +		      const char *switch_to_branch,
> +		      int detach_head, int reset_hard,

It might be worth switching to a single flag variable here. It would
make calls like this:

> -	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
> +	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
>  	    NULL, msg.buf))

a little more self-documenting (if a little more verbose).

> -	if (!oid) {
> -		if (get_oid("HEAD", &head_oid)) {
> -			rollback_lock_file(&lock);
> -			return error(_("could not determine HEAD revision"));
> -		}
> -		oid = &head_oid;
> +	if (get_oid("HEAD", &head_oid)) {
> +		rollback_lock_file(&lock);
> +		return error(_("could not determine HEAD revision"));
>  	}

This one could actually turn into:

  ret = error(...);
  goto leave_reset_head;

now. We don't have to worry about an uninitialized desc.buffer anymore
(as I mentioned in the previous email), because "nr" would be 0.

It doesn't save any lines, though (but maybe having a single
cleanup/exit point would make things easier to read; I dunno).

Take all my comments as observations, not objections. This looks OK to
me either way.

-Peff

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Johannes Schindelin]

Hi Peff,

On Fri, 9 Nov 2018, Jeff King wrote:

> On Fri, Nov 09, 2018 at 01:34:19AM -0800, Johannes Schindelin via GitGitGadget wrote:
> 
> > From: Johannes Schindelin <johannes.schindelin@gmx.de>
> > 
> > When we converted a `git checkout -q $onto^0` call to use
> > `reset_head()`, we inadvertently incurred a change from a twoway_merge
> > to a oneway_merge, as if we wanted a `git reset --hard` instead.
> > 
> > This has performance ramifications under certain, though, as the
> > oneway_merge needs to lstat() every single index entry whereas
> > twoway_merge does not.
> > 
> > So let's go back to the old behavior.
> 
> Makes sense. I didn't think too hard about any possible gotchas with the
> twoway/oneway switch, but if that's what git-checkout was doing before,
> it seems obviously safe.

Right.

> > diff --git a/builtin/rebase.c b/builtin/rebase.c
> > index 6f6d7de156..c1cc50f3f8 100644
> > --- a/builtin/rebase.c
> > +++ b/builtin/rebase.c
> > @@ -523,11 +523,12 @@ finished_rebase:
> >  #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
> >  
> >  static int reset_head(struct object_id *oid, const char *action,
> > -		      const char *switch_to_branch, int detach_head,
> > +		      const char *switch_to_branch,
> > +		      int detach_head, int reset_hard,
> 
> It might be worth switching to a single flag variable here. It would
> make calls like this:
> 
> > -	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
> > +	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
> >  	    NULL, msg.buf))
> 
> a little more self-documenting (if a little more verbose).

I thought about that, but for just two flags? Well, let me try it and see
whether I like the result ;-)

> > -	if (!oid) {
> > -		if (get_oid("HEAD", &head_oid)) {
> > -			rollback_lock_file(&lock);
> > -			return error(_("could not determine HEAD revision"));
> > -		}
> > -		oid = &head_oid;
> > +	if (get_oid("HEAD", &head_oid)) {
> > +		rollback_lock_file(&lock);
> > +		return error(_("could not determine HEAD revision"));
> >  	}
> 
> This one could actually turn into:
> 
>   ret = error(...);
>   goto leave_reset_head;
> 
> now. We don't have to worry about an uninitialized desc.buffer anymore
> (as I mentioned in the previous email), because "nr" would be 0.
> 
> It doesn't save any lines, though (but maybe having a single
> cleanup/exit point would make things easier to read; I dunno).

But you're right, of course. Consistency makes for easier-to-read code.

> Take all my comments as observations, not objections. This looks OK to
> me either way.

Actually, you got me thinking about the desc.buffer. And I think there is
one corner case where it could cause a problem: `struct tree_desc desc[2]`
does not initialize the buffers to NULL. And what if
fill_tree_descriptor() function returns NULL? Then the buffer is still
uninitialized.

In practice, our *current* version of fill_tree_descriptor() never returns
NULL if the oid parameter is non-NULL. It would die() in the error case
instead (bad!). So to prepare for a less bad version, I'd rather
initialize the `desc` array and be on the safe (and easier to reason
about) side.

Thank you for your helpful review,
Dscho

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Jeff King]

On Fri, Nov 09, 2018 at 06:21:41PM +0100, Johannes Schindelin wrote:

> Actually, you got me thinking about the desc.buffer. And I think there is
> one corner case where it could cause a problem: `struct tree_desc desc[2]`
> does not initialize the buffers to NULL. And what if
> fill_tree_descriptor() function returns NULL? Then the buffer is still
> uninitialized.
> 
> In practice, our *current* version of fill_tree_descriptor() never returns
> NULL if the oid parameter is non-NULL. It would die() in the error case
> instead (bad!). So to prepare for a less bad version, I'd rather
> initialize the `desc` array and be on the safe (and easier to reason
> about) side.

Yeah, I agree with all of that.

One solution would just be to increment only after success:

  if (fill_tree_descriptor(&desc[nr], ..) < 0)
	goto error;
  nr++; /* now we know it's valid! */

But there are lots of alternatives.  :)

-Peff

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Johannes Schindelin]

Hi Peff,

On Fri, 9 Nov 2018, Jeff King wrote:

> On Fri, Nov 09, 2018 at 06:21:41PM +0100, Johannes Schindelin wrote:
> 
> > Actually, you got me thinking about the desc.buffer. And I think there is
> > one corner case where it could cause a problem: `struct tree_desc desc[2]`
> > does not initialize the buffers to NULL. And what if
> > fill_tree_descriptor() function returns NULL? Then the buffer is still
> > uninitialized.
> > 
> > In practice, our *current* version of fill_tree_descriptor() never returns
> > NULL if the oid parameter is non-NULL. It would die() in the error case
> > instead (bad!). So to prepare for a less bad version, I'd rather
> > initialize the `desc` array and be on the safe (and easier to reason
> > about) side.
> 
> Yeah, I agree with all of that.
> 
> One solution would just be to increment only after success:
> 
>   if (fill_tree_descriptor(&desc[nr], ..) < 0)
> 	goto error;
>   nr++; /* now we know it's valid! */
> 
> But there are lots of alternatives.  :)

True. I simply prefer to initialize it and be done with it. ;-)

Ciao,
Dscho

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Junio C Hamano]

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

>> >  static int reset_head(struct object_id *oid, const char *action,
>> > -		      const char *switch_to_branch, int detach_head,
>> > +		      const char *switch_to_branch,
>> > +		      int detach_head, int reset_hard,
>> 
>> It might be worth switching to a single flag variable here. It would
>> make calls like this:
>> 
>> > -	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
>> > +	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
>> >  	    NULL, msg.buf))
>> 
>> a little more self-documenting (if a little more verbose).
>
> I thought about that, but for just two flags? Well, let me try it and see
> whether I like the result ;-)

My rule of thumb is that repeating three times is definitely when we
should consolidate separate ones into a single flag word, and twice
is a borderline.

For two-time repetition, it is often worth fixing when somebody
notices it during the review, as that is a sign that repetition,
even a minor one, disturbed a reader enough to point out.  On the
other hand, for a file-scope static that is likely to stay as a
non-API function but a local helper, it may not be worth it.

So I am OK either way, slightly in favor of fixing it while we
remember.


>> This one could actually turn into:
>> 
>>   ret = error(...);
>>   goto leave_reset_head;
>> 
>> now. We don't have to worry about an uninitialized desc.buffer anymore
>> (as I mentioned in the previous email), because "nr" would be 0.
>> 
>> It doesn't save any lines, though (but maybe having a single
>> cleanup/exit point would make things easier to read; I dunno).
>
> But you're right, of course. Consistency makes for easier-to-read code.

Yup, that does sound good.

Thanks.

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Johannes Schindelin]

Hi Junio,

On Mon, 12 Nov 2018, Junio C Hamano wrote:

> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
> 
> >> >  static int reset_head(struct object_id *oid, const char *action,
> >> > -		      const char *switch_to_branch, int detach_head,
> >> > +		      const char *switch_to_branch,
> >> > +		      int detach_head, int reset_hard,
> >> 
> >> It might be worth switching to a single flag variable here. It would
> >> make calls like this:
> >> 
> >> > -	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
> >> > +	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
> >> >  	    NULL, msg.buf))
> >> 
> >> a little more self-documenting (if a little more verbose).
> >
> > I thought about that, but for just two flags? Well, let me try it and see
> > whether I like the result ;-)
> 
> My rule of thumb is that repeating three times is definitely when we
> should consolidate separate ones into a single flag word, and twice
> is a borderline.
> 
> For two-time repetition, it is often worth fixing when somebody
> notices it during the review, as that is a sign that repetition,
> even a minor one, disturbed a reader enough to point out.

That's my thought exactly, hence I looked into it. The end result is
actually easier to read, in particular the commit that re-introduces the
`reset --hard` behavior: it no longer touches *all* callsites of
`reset_head()` but only the relevant ones.

> On the other hand, for a file-scope static that is likely to stay as a
> non-API function but a local helper, it may not be worth it.

Oh, do you think that `reset_head()` is likely to stay as non-API
function? I found myself in the need of repeating this tedious
unpack_trees() dance quite a few times over the past two years, and it is
*always* daunting because the API is *that* unintuitive.

So I *do* hope that `reset_head()` will actually be moved to reset.[ch]
eventually, and callsites e.g. in `sequencer.c` will be converted from
calling `unpack_trees()` to calling `reset_head()` instead.

v2 on the way,
Dscho

> So I am OK either way, slightly in favor of fixing it while we
> remember.
> 
> 
> >> This one could actually turn into:
> >> 
> >>   ret = error(...);
> >>   goto leave_reset_head;
> >> 
> >> now. We don't have to worry about an uninitialized desc.buffer anymore
> >> (as I mentioned in the previous email), because "nr" would be 0.
> >> 
> >> It doesn't save any lines, though (but maybe having a single
> >> cleanup/exit point would make things easier to read; I dunno).
> >
> > But you're right, of course. Consistency makes for easier-to-read code.
> 
> Yup, that does sound good.
> 
> Thanks.
> 

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Junio C Hamano]

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

>> On the other hand, for a file-scope static that is likely to stay as a
>> non-API function but a local helper, it may not be worth it.
>
> Oh, do you think that `reset_head()` is likely to stay as non-API
> function? I found myself in the need of repeating this tedious
> unpack_trees() dance quite a few times over the past two years, and it is
> *always* daunting because the API is *that* unintuitive.
>
> So I *do* hope that `reset_head()` will actually be moved to reset.[ch]
> eventually, and callsites e.g. in `sequencer.c` will be converted from
> calling `unpack_trees()` to calling `reset_head()` instead.

As long as the public API function is well thought out, of course it
is OK.  Ideally, builtin/reset.c can detect when it is making a hard
reset to HEAD and call that same function.  Which may or may not
mean you would also want to tell it to record ORIG_HEAD and remove
MERGE_HEAD and others), perhaps with another bit in that flag word.

Re: [PATCH 2/2] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Junio C Hamano]

Junio C Hamano <gitster@pobox.com> writes:

> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
>
>>> On the other hand, for a file-scope static that is likely to stay as a
>>> non-API function but a local helper, it may not be worth it.
>>
>> Oh, do you think that `reset_head()` is likely to stay as non-API
>> function? I found myself in the need of repeating this tedious
>> unpack_trees() dance quite a few times over the past two years, and it is
>> *always* daunting because the API is *that* unintuitive.
>>
>> So I *do* hope that `reset_head()` will actually be moved to reset.[ch]
>> eventually, and callsites e.g. in `sequencer.c` will be converted from
>> calling `unpack_trees()` to calling `reset_head()` instead.
>
> As long as the public API function is well thought out, of course it
> is OK.  Ideally, builtin/reset.c can detect when it is making a hard
> reset to HEAD and call that same function.  Which may or may not
> mean you would also want to tell it to record ORIG_HEAD and remove
> MERGE_HEAD and others), perhaps with another bit in that flag word.

Nah, forget about that one.  It sometimes does branch switching and
sometimes does hard reset, which looks like a strange chimera.  We
shouldn't burden it further by adding "while at it remove cruft, as
'reset --hard' needs to do that" to it.

[PATCH v2 0/3] Fix built-in rebase perf regression

[Johannes Schindelin via GitGitGadget]

In our tests with large repositories, we noticed a serious regression of the
performance of git rebase when using the built-in vs the shell script
version. It boils down to an incorrect conversion of a git checkout -q:
instead of using a twoway_merge as git checkout does, we used a oneway_merge 
as git reset does. The latter, however, calls lstat() on all files listed in
the index, while the former essentially looks only at the files that are
different between the given two revisions.

Let's reinstate the original behavior by introducing a flag to the 
reset_head() function to indicate whether we want to emulate reset --hard 
(in which case we use the oneway_merge, otherwise we use twoway_merge).

Johannes Schindelin (3):
  rebase: consolidate clean-up code before leaving reset_head()
  rebase: prepare reset_head() for more flags
  built-in rebase: reinstate `checkout -q` behavior where appropriate

 builtin/rebase.c | 79 ++++++++++++++++++++++++++++--------------------
 1 file changed, 46 insertions(+), 33 deletions(-)


base-commit: 8858448bb49332d353febc078ce4a3abcc962efe
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-72%2Fdscho%2Fbuiltin-rebase-perf-regression-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-72/dscho/builtin-rebase-perf-regression-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/72

Range-diff vs v1:

 1:  64597fe827 ! 1:  28e24d98ab rebase: consolidate clean-up code before leaving reset_head()
     @@ -11,6 +11,33 @@
      --- a/builtin/rebase.c
      +++ b/builtin/rebase.c
      @@
     + 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
     + 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
     + 
     +-	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
     +-		return -1;
     ++	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0) {
     ++		ret = -1;
     ++		goto leave_reset_head;
     ++	}
     + 
     + 	if (!oid) {
     + 		if (get_oid("HEAD", &head_oid)) {
     +-			rollback_lock_file(&lock);
     +-			return error(_("could not determine HEAD revision"));
     ++			ret = error(_("could not determine HEAD revision"));
     ++			goto leave_reset_head;
     + 		}
     + 		oid = &head_oid;
     + 	}
     +@@
     + 		unpack_tree_opts.reset = 1;
     + 
     + 	if (read_index_unmerged(the_repository->index) < 0) {
     +-		rollback_lock_file(&lock);
     +-		return error(_("could not read index"));
     ++		ret = error(_("could not read index"));
     ++		goto leave_reset_head;
       	}
       
       	if (!fill_tree_descriptor(&desc, oid)) {
     @@ -31,15 +58,17 @@
       	}
       
       	tree = parse_tree_indirect(oid);
     -@@
     + 	prime_cache_tree(the_repository->index, tree);
       
     - 	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
     +-	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
     ++	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0) {
       		ret = error(_("could not write index"));
      -	free((void *)desc.buffer);
     - 
     - 	if (ret)
     +-
     +-	if (ret)
      -		return ret;
      +		goto leave_reset_head;
     ++	}
       
       	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
       	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
 -:  ---------- > 2:  db963b2094 rebase: prepare reset_head() for more flags
 2:  070092b430 ! 3:  a7360b856f built-in rebase: reinstate `checkout -q` behavior where appropriate
     @@ -20,15 +20,18 @@
      @@
       #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
       
     + #define RESET_HEAD_DETACH (1<<0)
     ++#define RESET_HEAD_HARD (1<<1)
     + 
       static int reset_head(struct object_id *oid, const char *action,
     --		      const char *switch_to_branch, int detach_head,
     -+		      const char *switch_to_branch,
     -+		      int detach_head, int reset_hard,
     + 		      const char *switch_to_branch, unsigned flags,
       		      const char *reflog_orig_head, const char *reflog_head)
       {
     + 	unsigned detach_head = flags & RESET_HEAD_DETACH;
     ++	unsigned reset_hard = flags & RESET_HEAD_HARD;
       	struct object_id head_oid;
      -	struct tree_desc desc;
     -+	struct tree_desc desc[2];
     ++	struct tree_desc desc[2] = { { NULL }, { NULL } };
       	struct lock_file lock = LOCK_INIT;
       	struct unpack_trees_options unpack_tree_opts;
       	struct tree *tree;
     @@ -42,18 +45,18 @@
       	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
       		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
      @@
     - 	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
     - 		return -1;
     + 		goto leave_reset_head;
     + 	}
       
      -	if (!oid) {
      -		if (get_oid("HEAD", &head_oid)) {
     --			rollback_lock_file(&lock);
     --			return error(_("could not determine HEAD revision"));
     +-			ret = error(_("could not determine HEAD revision"));
     +-			goto leave_reset_head;
      -		}
      -		oid = &head_oid;
     -+	if (get_oid("HEAD", &head_oid)) {
     -+		rollback_lock_file(&lock);
     -+		return error(_("could not determine HEAD revision"));
     ++	if ((!oid || !reset_hard) && get_oid("HEAD", &head_oid)) {
     ++		ret = error(_("could not determine HEAD revision"));
     ++		goto leave_reset_head;
       	}
       
      +	if (!oid)
     @@ -70,7 +73,7 @@
       	unpack_tree_opts.merge = 1;
       	if (!detach_head)
      @@
     - 		return error(_("could not read index"));
     + 		goto leave_reset_head;
       	}
       
      -	if (!fill_tree_descriptor(&desc, oid)) {
     @@ -104,7 +107,8 @@
       		string_list_clear(&merge_rr, 1);
       
      -		if (reset_head(NULL, "reset", NULL, 0, NULL, NULL) < 0)
     -+		if (reset_head(NULL, "reset", NULL, 0, 1, NULL, NULL) < 0)
     ++		if (reset_head(NULL, "reset", NULL, RESET_HEAD_HARD,
     ++			       NULL, NULL) < 0)
       			die(_("could not discard worktree changes"));
       		if (read_basic_state(&options))
       			exit(1);
     @@ -113,7 +117,8 @@
       			exit(1);
       		if (reset_head(&options.orig_head, "reset",
      -			       options.head_name, 0, NULL, NULL) < 0)
     -+			       options.head_name, 0, 1, NULL, NULL) < 0)
     ++			       options.head_name, RESET_HEAD_HARD,
     ++			       NULL, NULL) < 0)
       			die(_("could not move back to %s"),
       			    oid_to_hex(&options.orig_head));
       		ret = finish_rebase(&options);
     @@ -122,34 +127,7 @@
       			printf(_("Created autostash: %s\n"), buf.buf);
       			if (reset_head(&head->object.oid, "reset --hard",
      -				       NULL, 0, NULL, NULL) < 0)
     -+				       NULL, 0, 1, NULL, NULL) < 0)
     ++				       NULL, RESET_HEAD_HARD, NULL, NULL) < 0)
       				die(_("could not reset --hard"));
       			printf(_("HEAD is now at %s"),
       			       find_unique_abbrev(&head->object.oid,
     -@@
     - 				strbuf_addf(&buf, "rebase: checkout %s",
     - 					    options.switch_to);
     - 				if (reset_head(&oid, "checkout",
     --					       options.head_name, 0,
     -+					       options.head_name, 0, 0,
     - 					       NULL, NULL) < 0) {
     - 					ret = !!error(_("could not switch to "
     - 							"%s"),
     -@@
     - 			 "it...\n"));
     - 
     - 	strbuf_addf(&msg, "rebase: checkout %s", options.onto_name);
     --	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
     -+	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
     - 	    NULL, msg.buf))
     - 		die(_("Could not detach HEAD"));
     - 	strbuf_release(&msg);
     -@@
     - 		strbuf_addf(&msg, "rebase finished: %s onto %s",
     - 			options.head_name ? options.head_name : "detached HEAD",
     - 			oid_to_hex(&options.onto->object.oid));
     --		reset_head(NULL, "Fast-forwarded", options.head_name, 0,
     -+		reset_head(NULL, "Fast-forwarded", options.head_name, 0, 0,
     - 			   "HEAD", msg.buf);
     - 		strbuf_release(&msg);
     - 		ret = !!finish_rebase(&options);

-- 
gitgitgadget

[PATCH v2 1/3] rebase: consolidate clean-up code before leaving reset_head()

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The same clean-up code is repeated quite a few times; Let's DRY up the
code some.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 0ee06aa363..e173654d56 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -541,13 +541,15 @@ static int reset_head(struct object_id *oid, const char *action,
 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
 
-	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
-		return -1;
+	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0) {
+		ret = -1;
+		goto leave_reset_head;
+	}
 
 	if (!oid) {
 		if (get_oid("HEAD", &head_oid)) {
-			rollback_lock_file(&lock);
-			return error(_("could not determine HEAD revision"));
+			ret = error(_("could not determine HEAD revision"));
+			goto leave_reset_head;
 		}
 		oid = &head_oid;
 	}
@@ -564,32 +566,27 @@ static int reset_head(struct object_id *oid, const char *action,
 		unpack_tree_opts.reset = 1;
 
 	if (read_index_unmerged(the_repository->index) < 0) {
-		rollback_lock_file(&lock);
-		return error(_("could not read index"));
+		ret = error(_("could not read index"));
+		goto leave_reset_head;
 	}
 
 	if (!fill_tree_descriptor(&desc, oid)) {
-		error(_("failed to find tree of %s"), oid_to_hex(oid));
-		rollback_lock_file(&lock);
-		free((void *)desc.buffer);
-		return -1;
+		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
+		goto leave_reset_head;
 	}
 
 	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
-		rollback_lock_file(&lock);
-		free((void *)desc.buffer);
-		return -1;
+		ret = -1;
+		goto leave_reset_head;
 	}
 
 	tree = parse_tree_indirect(oid);
 	prime_cache_tree(the_repository->index, tree);
 
-	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
+	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0) {
 		ret = error(_("could not write index"));
-	free((void *)desc.buffer);
-
-	if (ret)
-		return ret;
+		goto leave_reset_head;
+	}
 
 	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
 	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
@@ -622,7 +619,10 @@ static int reset_head(struct object_id *oid, const char *action,
 					 UPDATE_REFS_MSG_ON_ERR);
 	}
 
+leave_reset_head:
 	strbuf_release(&msg);
+	rollback_lock_file(&lock);
+	free((void *)desc.buffer);
 	return ret;
 }
 
-- 
gitgitgadget

[PATCH v2 2/3] rebase: prepare reset_head() for more flags

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

Currently, we only accept the flag indicating whether the HEAD should be
detached not. In the next commit, we want to introduce another flag: to
toggle between emulating `reset --hard` vs `checkout -q`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index e173654d56..074594cf10 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -522,10 +522,13 @@ finished_rebase:
 
 #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
 
+#define RESET_HEAD_DETACH (1<<0)
+
 static int reset_head(struct object_id *oid, const char *action,
-		      const char *switch_to_branch, int detach_head,
+		      const char *switch_to_branch, unsigned flags,
 		      const char *reflog_orig_head, const char *reflog_head)
 {
+	unsigned detach_head = flags & RESET_HEAD_DETACH;
 	struct object_id head_oid;
 	struct tree_desc desc;
 	struct lock_file lock = LOCK_INIT;
@@ -1500,8 +1503,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			 "it...\n"));
 
 	strbuf_addf(&msg, "rebase: checkout %s", options.onto_name);
-	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
-	    NULL, msg.buf))
+	if (reset_head(&options.onto->object.oid, "checkout", NULL,
+		       RESET_HEAD_DETACH, NULL, msg.buf))
 		die(_("Could not detach HEAD"));
 	strbuf_release(&msg);
 
-- 
gitgitgadget

[PATCH v2 3/3] built-in rebase: reinstate `checkout -q` behavior where appropriate

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When we converted a `git checkout -q $onto^0` call to use
`reset_head()`, we inadvertently incurred a change from a twoway_merge
to a oneway_merge, as if we wanted a `git reset --hard` instead.

This has performance ramifications under certain, though, as the
oneway_merge needs to lstat() every single index entry whereas
twoway_merge does not.

So let's go back to the old behavior.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 074594cf10..dc78c1497d 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -523,14 +523,16 @@ finished_rebase:
 #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
 
 #define RESET_HEAD_DETACH (1<<0)
+#define RESET_HEAD_HARD (1<<1)
 
 static int reset_head(struct object_id *oid, const char *action,
 		      const char *switch_to_branch, unsigned flags,
 		      const char *reflog_orig_head, const char *reflog_head)
 {
 	unsigned detach_head = flags & RESET_HEAD_DETACH;
+	unsigned reset_hard = flags & RESET_HEAD_HARD;
 	struct object_id head_oid;
-	struct tree_desc desc;
+	struct tree_desc desc[2] = { { NULL }, { NULL } };
 	struct lock_file lock = LOCK_INIT;
 	struct unpack_trees_options unpack_tree_opts;
 	struct tree *tree;
@@ -539,7 +541,7 @@ static int reset_head(struct object_id *oid, const char *action,
 	size_t prefix_len;
 	struct object_id *orig = NULL, oid_orig,
 		*old_orig = NULL, oid_old_orig;
-	int ret = 0;
+	int ret = 0, nr = 0;
 
 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
@@ -549,20 +551,20 @@ static int reset_head(struct object_id *oid, const char *action,
 		goto leave_reset_head;
 	}
 
-	if (!oid) {
-		if (get_oid("HEAD", &head_oid)) {
-			ret = error(_("could not determine HEAD revision"));
-			goto leave_reset_head;
-		}
-		oid = &head_oid;
+	if ((!oid || !reset_hard) && get_oid("HEAD", &head_oid)) {
+		ret = error(_("could not determine HEAD revision"));
+		goto leave_reset_head;
 	}
 
+	if (!oid)
+		oid = &head_oid;
+
 	memset(&unpack_tree_opts, 0, sizeof(unpack_tree_opts));
 	setup_unpack_trees_porcelain(&unpack_tree_opts, action);
 	unpack_tree_opts.head_idx = 1;
 	unpack_tree_opts.src_index = the_repository->index;
 	unpack_tree_opts.dst_index = the_repository->index;
-	unpack_tree_opts.fn = oneway_merge;
+	unpack_tree_opts.fn = reset_hard ? oneway_merge : twoway_merge;
 	unpack_tree_opts.update = 1;
 	unpack_tree_opts.merge = 1;
 	if (!detach_head)
@@ -573,12 +575,17 @@ static int reset_head(struct object_id *oid, const char *action,
 		goto leave_reset_head;
 	}
 
-	if (!fill_tree_descriptor(&desc, oid)) {
+	if (!reset_hard && !fill_tree_descriptor(&desc[nr++], &head_oid)) {
+		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
+		goto leave_reset_head;
+	}
+
+	if (!fill_tree_descriptor(&desc[nr++], oid)) {
 		ret = error(_("failed to find tree of %s"), oid_to_hex(oid));
 		goto leave_reset_head;
 	}
 
-	if (unpack_trees(1, &desc, &unpack_tree_opts)) {
+	if (unpack_trees(nr, desc, &unpack_tree_opts)) {
 		ret = -1;
 		goto leave_reset_head;
 	}
@@ -625,7 +632,8 @@ static int reset_head(struct object_id *oid, const char *action,
 leave_reset_head:
 	strbuf_release(&msg);
 	rollback_lock_file(&lock);
-	free((void *)desc.buffer);
+	while (nr)
+		free((void *)desc[--nr].buffer);
 	return ret;
 }
 
@@ -1003,7 +1011,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		rerere_clear(&merge_rr);
 		string_list_clear(&merge_rr, 1);
 
-		if (reset_head(NULL, "reset", NULL, 0, NULL, NULL) < 0)
+		if (reset_head(NULL, "reset", NULL, RESET_HEAD_HARD,
+			       NULL, NULL) < 0)
 			die(_("could not discard worktree changes"));
 		if (read_basic_state(&options))
 			exit(1);
@@ -1019,7 +1028,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (read_basic_state(&options))
 			exit(1);
 		if (reset_head(&options.orig_head, "reset",
-			       options.head_name, 0, NULL, NULL) < 0)
+			       options.head_name, RESET_HEAD_HARD,
+			       NULL, NULL) < 0)
 			die(_("could not move back to %s"),
 			    oid_to_hex(&options.orig_head));
 		ret = finish_rebase(&options);
@@ -1383,7 +1393,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			write_file(autostash, "%s", oid_to_hex(&oid));
 			printf(_("Created autostash: %s\n"), buf.buf);
 			if (reset_head(&head->object.oid, "reset --hard",
-				       NULL, 0, NULL, NULL) < 0)
+				       NULL, RESET_HEAD_HARD, NULL, NULL) < 0)
 				die(_("could not reset --hard"));
 			printf(_("HEAD is now at %s"),
 			       find_unique_abbrev(&head->object.oid,
-- 
gitgitgadget

Re: [PATCH v2 0/3] Fix built-in rebase perf regression

[Johannes Schindelin]

Hi,

On Mon, 12 Nov 2018, Johannes Schindelin via GitGitGadget wrote:

> In our tests with large repositories, we noticed a serious regression of the
> performance of git rebase when using the built-in vs the shell script
> version. It boils down to an incorrect conversion of a git checkout -q:
> instead of using a twoway_merge as git checkout does, we used a oneway_merge 
> as git reset does. The latter, however, calls lstat() on all files listed in
> the index, while the former essentially looks only at the files that are
> different between the given two revisions.
> 
> Let's reinstate the original behavior by introducing a flag to the 
> reset_head() function to indicate whether we want to emulate reset --hard 
> (in which case we use the oneway_merge, otherwise we use twoway_merge).
> 
> Johannes Schindelin (3):
>   rebase: consolidate clean-up code before leaving reset_head()
>   rebase: prepare reset_head() for more flags
>   built-in rebase: reinstate `checkout -q` behavior where appropriate
> 
>  builtin/rebase.c | 79 ++++++++++++++++++++++++++++--------------------
>  1 file changed, 46 insertions(+), 33 deletions(-)

I forgot to specify the changes vs v1:

- More error paths are not consolidated via `goto leave_reset_head`.
- The `desc` array is not initialized to all-zero, to avoid bogus
  addresses being passed to `free()`.
- The `detach_head` and `reset_hard` parameters have been consolidated
  into a `flags` parameter.
- The `reset_head()` function once again only initializes `head_oid`
  when needed.

Sorry for the omission,
Johannes

> 
> 
> base-commit: 8858448bb49332d353febc078ce4a3abcc962efe
> Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-72%2Fdscho%2Fbuiltin-rebase-perf-regression-v2
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-72/dscho/builtin-rebase-perf-regression-v2
> Pull-Request: https://github.com/gitgitgadget/git/pull/72
> 
> Range-diff vs v1:
> 
>  1:  64597fe827 ! 1:  28e24d98ab rebase: consolidate clean-up code before leaving reset_head()
>      @@ -11,6 +11,33 @@
>       --- a/builtin/rebase.c
>       +++ b/builtin/rebase.c
>       @@
>      + 	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
>      + 		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
>      + 
>      +-	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
>      +-		return -1;
>      ++	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0) {
>      ++		ret = -1;
>      ++		goto leave_reset_head;
>      ++	}
>      + 
>      + 	if (!oid) {
>      + 		if (get_oid("HEAD", &head_oid)) {
>      +-			rollback_lock_file(&lock);
>      +-			return error(_("could not determine HEAD revision"));
>      ++			ret = error(_("could not determine HEAD revision"));
>      ++			goto leave_reset_head;
>      + 		}
>      + 		oid = &head_oid;
>      + 	}
>      +@@
>      + 		unpack_tree_opts.reset = 1;
>      + 
>      + 	if (read_index_unmerged(the_repository->index) < 0) {
>      +-		rollback_lock_file(&lock);
>      +-		return error(_("could not read index"));
>      ++		ret = error(_("could not read index"));
>      ++		goto leave_reset_head;
>        	}
>        
>        	if (!fill_tree_descriptor(&desc, oid)) {
>      @@ -31,15 +58,17 @@
>        	}
>        
>        	tree = parse_tree_indirect(oid);
>      -@@
>      + 	prime_cache_tree(the_repository->index, tree);
>        
>      - 	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
>      +-	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0)
>      ++	if (write_locked_index(the_repository->index, &lock, COMMIT_LOCK) < 0) {
>        		ret = error(_("could not write index"));
>       -	free((void *)desc.buffer);
>      - 
>      - 	if (ret)
>      +-
>      +-	if (ret)
>       -		return ret;
>       +		goto leave_reset_head;
>      ++	}
>        
>        	reflog_action = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
>        	strbuf_addf(&msg, "%s: ", reflog_action ? reflog_action : "rebase");
>  -:  ---------- > 2:  db963b2094 rebase: prepare reset_head() for more flags
>  2:  070092b430 ! 3:  a7360b856f built-in rebase: reinstate `checkout -q` behavior where appropriate
>      @@ -20,15 +20,18 @@
>       @@
>        #define GIT_REFLOG_ACTION_ENVIRONMENT "GIT_REFLOG_ACTION"
>        
>      + #define RESET_HEAD_DETACH (1<<0)
>      ++#define RESET_HEAD_HARD (1<<1)
>      + 
>        static int reset_head(struct object_id *oid, const char *action,
>      --		      const char *switch_to_branch, int detach_head,
>      -+		      const char *switch_to_branch,
>      -+		      int detach_head, int reset_hard,
>      + 		      const char *switch_to_branch, unsigned flags,
>        		      const char *reflog_orig_head, const char *reflog_head)
>        {
>      + 	unsigned detach_head = flags & RESET_HEAD_DETACH;
>      ++	unsigned reset_hard = flags & RESET_HEAD_HARD;
>        	struct object_id head_oid;
>       -	struct tree_desc desc;
>      -+	struct tree_desc desc[2];
>      ++	struct tree_desc desc[2] = { { NULL }, { NULL } };
>        	struct lock_file lock = LOCK_INIT;
>        	struct unpack_trees_options unpack_tree_opts;
>        	struct tree *tree;
>      @@ -42,18 +45,18 @@
>        	if (switch_to_branch && !starts_with(switch_to_branch, "refs/"))
>        		BUG("Not a fully qualified branch: '%s'", switch_to_branch);
>       @@
>      - 	if (hold_locked_index(&lock, LOCK_REPORT_ON_ERROR) < 0)
>      - 		return -1;
>      + 		goto leave_reset_head;
>      + 	}
>        
>       -	if (!oid) {
>       -		if (get_oid("HEAD", &head_oid)) {
>      --			rollback_lock_file(&lock);
>      --			return error(_("could not determine HEAD revision"));
>      +-			ret = error(_("could not determine HEAD revision"));
>      +-			goto leave_reset_head;
>       -		}
>       -		oid = &head_oid;
>      -+	if (get_oid("HEAD", &head_oid)) {
>      -+		rollback_lock_file(&lock);
>      -+		return error(_("could not determine HEAD revision"));
>      ++	if ((!oid || !reset_hard) && get_oid("HEAD", &head_oid)) {
>      ++		ret = error(_("could not determine HEAD revision"));
>      ++		goto leave_reset_head;
>        	}
>        
>       +	if (!oid)
>      @@ -70,7 +73,7 @@
>        	unpack_tree_opts.merge = 1;
>        	if (!detach_head)
>       @@
>      - 		return error(_("could not read index"));
>      + 		goto leave_reset_head;
>        	}
>        
>       -	if (!fill_tree_descriptor(&desc, oid)) {
>      @@ -104,7 +107,8 @@
>        		string_list_clear(&merge_rr, 1);
>        
>       -		if (reset_head(NULL, "reset", NULL, 0, NULL, NULL) < 0)
>      -+		if (reset_head(NULL, "reset", NULL, 0, 1, NULL, NULL) < 0)
>      ++		if (reset_head(NULL, "reset", NULL, RESET_HEAD_HARD,
>      ++			       NULL, NULL) < 0)
>        			die(_("could not discard worktree changes"));
>        		if (read_basic_state(&options))
>        			exit(1);
>      @@ -113,7 +117,8 @@
>        			exit(1);
>        		if (reset_head(&options.orig_head, "reset",
>       -			       options.head_name, 0, NULL, NULL) < 0)
>      -+			       options.head_name, 0, 1, NULL, NULL) < 0)
>      ++			       options.head_name, RESET_HEAD_HARD,
>      ++			       NULL, NULL) < 0)
>        			die(_("could not move back to %s"),
>        			    oid_to_hex(&options.orig_head));
>        		ret = finish_rebase(&options);
>      @@ -122,34 +127,7 @@
>        			printf(_("Created autostash: %s\n"), buf.buf);
>        			if (reset_head(&head->object.oid, "reset --hard",
>       -				       NULL, 0, NULL, NULL) < 0)
>      -+				       NULL, 0, 1, NULL, NULL) < 0)
>      ++				       NULL, RESET_HEAD_HARD, NULL, NULL) < 0)
>        				die(_("could not reset --hard"));
>        			printf(_("HEAD is now at %s"),
>        			       find_unique_abbrev(&head->object.oid,
>      -@@
>      - 				strbuf_addf(&buf, "rebase: checkout %s",
>      - 					    options.switch_to);
>      - 				if (reset_head(&oid, "checkout",
>      --					       options.head_name, 0,
>      -+					       options.head_name, 0, 0,
>      - 					       NULL, NULL) < 0) {
>      - 					ret = !!error(_("could not switch to "
>      - 							"%s"),
>      -@@
>      - 			 "it...\n"));
>      - 
>      - 	strbuf_addf(&msg, "rebase: checkout %s", options.onto_name);
>      --	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1,
>      -+	if (reset_head(&options.onto->object.oid, "checkout", NULL, 1, 0,
>      - 	    NULL, msg.buf))
>      - 		die(_("Could not detach HEAD"));
>      - 	strbuf_release(&msg);
>      -@@
>      - 		strbuf_addf(&msg, "rebase finished: %s onto %s",
>      - 			options.head_name ? options.head_name : "detached HEAD",
>      - 			oid_to_hex(&options.onto->object.oid));
>      --		reset_head(NULL, "Fast-forwarded", options.head_name, 0,
>      -+		reset_head(NULL, "Fast-forwarded", options.head_name, 0, 0,
>      - 			   "HEAD", msg.buf);
>      - 		strbuf_release(&msg);
>      - 		ret = !!finish_rebase(&options);
> 
> -- 
> gitgitgadget
>