BUG: 2.11-era rebase regression when @{upstream} is implicitly used

BUG: 2.11-era rebase regression when @{upstream} is implicitly used

[Ævar Arnfjörð Bjarmason]

This is not a 2.21 release issue, and pre-dates the built-in rebase.

When you clone any repository, e.g. git.git, and add one commit on top
of the cloned branch, then run "git rebase" you'll get e.g.:

    $ git rebase
    First, rewinding head to replay your work on top of it...
    Applying: foo

Before 4f21454b55 ("merge-base: handle --fork-point without reflog",
2016-10-12) you'd get:

    $ git rebase
    Current branch master is up to date.

The results are not the same for "git rebase @{u}" or "git rebase $(git
rev-parse @{u})":

    $ git rev-parse HEAD; ~/g/git/git --exec-path=/home/avar/g/git rebase; git rev-parse HEAD; ~/g/git/git --exec-path=/home/avar/g/git rebase @{u}; git rev-parse HEAD; ~/g/git/git --exec-path=/home/avar/g/git rebase $(git rev-parse @{u}); git rev-parse HEAD
    d0a1e49341cac6db3226eb0f76ec4a5912f18af8
    First, rewinding head to replay your work on top of it...
    Applying: foo
    3a9261d6e34d9f6d00c8e8411d7ddd8cffa02d97
    Current branch master is up to date.
    3a9261d6e34d9f6d00c8e8411d7ddd8cffa02d97
    Current branch master is up to date.
    3a9261d6e34d9f6d00c8e8411d7ddd8cffa02d97

With 4f21454b55^ checked-out the SHA-1 always stays the same, i.e. no
work is done for the same command:

    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b
    Current branch master is up to date.
    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b
    Current branch master is up to date.
    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b
    Current branch master is up to date.
    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b

Re: BUG: 2.11-era rebase regression when @{upstream} is implicitly used

[Jeff King]

On Thu, Feb 14, 2019 at 02:23:04PM +0100, Ævar Arnfjörð Bjarmason wrote:

> This is not a 2.21 release issue, and pre-dates the built-in rebase.
> 
> When you clone any repository, e.g. git.git, and add one commit on top
> of the cloned branch, then run "git rebase" you'll get e.g.:
> 
>     $ git rebase
>     First, rewinding head to replay your work on top of it...
>     Applying: foo
> 
> Before 4f21454b55 ("merge-base: handle --fork-point without reflog",
> 2016-10-12) you'd get:
> 
>     $ git rebase
>     Current branch master is up to date.

I'm not entirely sure this is a regression, and not the patch bringing
the behavior into line with what would happen when you _do_ have a
reflog.

> The results are not the same for "git rebase @{u}" or "git rebase $(git
> rev-parse @{u})":

Those aren't using "--fork-point", so they're going to behave
differently. The default with no arguments is basically "--fork-point
@{u}".

-Peff

Re: BUG: 2.11-era rebase regression when @{upstream} is implicitly used

[Ævar Arnfjörð Bjarmason]

On Thu, Feb 21 2019, Jeff King wrote:

> On Thu, Feb 14, 2019 at 02:23:04PM +0100, Ævar Arnfjörð Bjarmason wrote:
>
>> This is not a 2.21 release issue, and pre-dates the built-in rebase.
>>
>> When you clone any repository, e.g. git.git, and add one commit on top
>> of the cloned branch, then run "git rebase" you'll get e.g.:
>>
>>     $ git rebase
>>     First, rewinding head to replay your work on top of it...
>>     Applying: foo
>>
>> Before 4f21454b55 ("merge-base: handle --fork-point without reflog",
>> 2016-10-12) you'd get:
>>
>>     $ git rebase
>>     Current branch master is up to date.
>
> I'm not entirely sure this is a regression, and not the patch bringing
> the behavior into line with what would happen when you _do_ have a
> reflog.
>
>> The results are not the same for "git rebase @{u}" or "git rebase $(git
>> rev-parse @{u})":
>
> Those aren't using "--fork-point", so they're going to behave
> differently. The default with no arguments is basically "--fork-point
> @{u}".

Yeah, that's what it *should* do, but it's not equivalent to using
--fork-point manually:

    # my series on top of origin/master
    $ git rev-parse HEAD
    2a67977d3f70fa7fc4bce89db24a1218dc9ab2aa
    
    # Junio's origin/master upstream
    $ git rev-parse @{u}
    35ee755a8c43bcb3c2786522d423f006c23d32df
    
    # Where my fork point is
    $ git merge-base --fork-point @{u}
    35ee755a8c43bcb3c2786522d423f006c23d32df
    
    # OK
    $ git rebase 35ee755a8c43bcb3c2786522d423f006c23d32df
    Current branch master is up to date.
    
    # OK
    $ git rebase $(git merge-base --fork-point @{u})
    Current branch master is up to date.
    
    # ???
    $ git rebase
    First, rewinding head to replay your work on top of it...
    [...]

Re: BUG: 2.11-era rebase regression when @{upstream} is implicitly used

[Jeff King]

On Thu, Feb 21, 2019 at 03:50:38PM +0100, Ævar Arnfjörð Bjarmason wrote:

> > Those aren't using "--fork-point", so they're going to behave
> > differently. The default with no arguments is basically "--fork-point
> > @{u}".
> 
> Yeah, that's what it *should* do, but it's not equivalent to using
> --fork-point manually:
> 
>     # my series on top of origin/master
>     $ git rev-parse HEAD
>     2a67977d3f70fa7fc4bce89db24a1218dc9ab2aa
>     
>     # Junio's origin/master upstream
>     $ git rev-parse @{u}
>     35ee755a8c43bcb3c2786522d423f006c23d32df
>     
>     # Where my fork point is
>     $ git merge-base --fork-point @{u}
>     35ee755a8c43bcb3c2786522d423f006c23d32df
>     
>     # OK
>     $ git rebase 35ee755a8c43bcb3c2786522d423f006c23d32df
>     Current branch master is up to date.
>     
>     # OK
>     $ git rebase $(git merge-base --fork-point @{u})
>     Current branch master is up to date.
>     
>     # ???
>     $ git rebase
>     First, rewinding head to replay your work on top of it...
>     [...]

Have you tried with "git rebase --fork-point"? It does more than just
pass --fork-point to merge-base. It seems to also skip some of the "is
up to date", I think due to 1e0dacdbdb (rebase: omit patch-identical
commits with --fork-point, 2014-07-16).

I'm still not clear on whether my 4f21454b55 actually changed something
menaingful here, or if it's simply that you're getting the fork-point
behavior more consistently.

-Peff

[PATCH 0/2] rebase: fix 2.11.0-era --fork-point regression

[Ævar Arnfjörð Bjarmason]

On Thu, Feb 21 2019, Jeff King wrote:

> On Thu, Feb 21, 2019 at 03:50:38PM +0100, Ævar Arnfjörð Bjarmason wrote:
>
>> > Those aren't using "--fork-point", so they're going to behave
>> > differently. The default with no arguments is basically "--fork-point
>> > @{u}".
>> 
>> Yeah, that's what it *should* do, but it's not equivalent to using
>> --fork-point manually:
>> 
>>     # my series on top of origin/master
>>     $ git rev-parse HEAD
>>     2a67977d3f70fa7fc4bce89db24a1218dc9ab2aa
>>     
>>     # Junio's origin/master upstream
>>     $ git rev-parse @{u}
>>     35ee755a8c43bcb3c2786522d423f006c23d32df
>>     
>>     # Where my fork point is
>>     $ git merge-base --fork-point @{u}
>>     35ee755a8c43bcb3c2786522d423f006c23d32df
>>     
>>     # OK
>>     $ git rebase 35ee755a8c43bcb3c2786522d423f006c23d32df
>>     Current branch master is up to date.
>>     
>>     # OK
>>     $ git rebase $(git merge-base --fork-point @{u})
>>     Current branch master is up to date.
>>     
>>     # ???
>>     $ git rebase
>>     First, rewinding head to replay your work on top of it...
>>     [...]
>
> Have you tried with "git rebase --fork-point"? It does more than just
> pass --fork-point to merge-base. It seems to also skip some of the "is
> up to date", I think due to 1e0dacdbdb (rebase: omit patch-identical
> commits with --fork-point, 2014-07-16).
>
> I'm still not clear on whether my 4f21454b55 actually changed something
> menaingful here, or if it's simply that you're getting the fork-point
> behavior more consistently.

Yes it's a regression. Here's a fix for it. As seen in 2/2 how we got
to the point of regressing was an interesting combination of factors.

Ævar Arnfjörð Bjarmason (2):
  rebase tests: test linear branch topology
  rebase: don't rebase linear topology with --fork-point

 builtin/rebase.c                  |  6 +++--
 t/t3421-rebase-topology-linear.sh | 44 +++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+), 2 deletions(-)

-- 
2.21.0.rc0.258.g878e2cd30e

[PATCH 1/2] rebase tests: test linear branch topology

[Ævar Arnfjörð Bjarmason]

Add tests rebasing a linear branch topology to linear rebase tests
added in 2aad7cace2 ("add simple tests of consistency across rebase
types", 2013-06-06).

These tests are duplicates of two surrounding tests that do the same
with tags pointing to the same objects. Right now there's no change in
behavior being introduced, but as we'll see in a subsequent change
rebase can have different behaviors when working implicitly with
remote tracking branches.

While I'm at it add a --fork-point test, strictly speaking this is
redundant to the existing '' test, as no argument to rebase implies
--fork-point. But now it's easier to grep for tests that explicitly
stress --fork-point.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/t3421-rebase-topology-linear.sh | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
index 7274dca40b..b847064f91 100755
--- a/t/t3421-rebase-topology-linear.sh
+++ b/t/t3421-rebase-topology-linear.sh
@@ -31,6 +31,16 @@ test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
 
+test_expect_success 'setup branches and remote tracking' '
+	git tag -l >tags &&
+	for tag in $(cat tags)
+	do
+		git branch branch-$tag $tag || return 1
+	done &&
+	git remote add origin "file://$PWD" &&
+	git fetch origin
+'
+
 test_run_rebase () {
 	result=$1
 	shift
@@ -57,10 +67,28 @@ test_run_rebase () {
 	"
 }
 test_run_rebase success ''
+test_run_rebase success --fork-point
 test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase failure -p
 
+test_run_rebase () {
+	result=$1
+	shift
+	test_expect_$result "rebase $* -f rewrites even if remote upstream is an ancestor" "
+		reset_rebase &&
+		git rebase $* -f branch-b branch-e &&
+		! test_cmp_rev branch-e origin/branch-e &&
+		test_cmp_rev branch-b HEAD~2 &&
+		test_linear_range 'd e' branch-b..
+	"
+}
+test_run_rebase success ''
+test_run_rebase success --fork-point
+test_run_rebase success -m
+test_run_rebase success -i
+test_have_prereq !REBASE_P || test_run_rebase success -p
+
 test_run_rebase () {
 	result=$1
 	shift
@@ -71,6 +99,7 @@ test_run_rebase () {
 	"
 }
 test_run_rebase success ''
+test_run_rebase success --fork-point
 test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
-- 
2.21.0.rc0.258.g878e2cd30e

Re: [PATCH 1/2] rebase tests: test linear branch topology

[Jeff King]

On Thu, Feb 21, 2019 at 10:40:58PM +0100, Ævar Arnfjörð Bjarmason wrote:

> Add tests rebasing a linear branch topology to linear rebase tests
> added in 2aad7cace2 ("add simple tests of consistency across rebase
> types", 2013-06-06).

I had trouble parsing this. Did you mean s/topology to/topology, similar
to/?

> These tests are duplicates of two surrounding tests that do the same
> with tags pointing to the same objects. Right now there's no change in
> behavior being introduced, but as we'll see in a subsequent change
> rebase can have different behaviors when working implicitly with
> remote tracking branches.

It took me a while to figure out what was new in these tests. Maybe:

  These tests are duplicates of two surrounding tests, but with one
  change: the existing tests refer to objects by their tag names, but
  here we'll use branches (pointing at the same objects).

But then I'm left wondering why that's important.

> While I'm at it add a --fork-point test, strictly speaking this is
> redundant to the existing '' test, as no argument to rebase implies
> --fork-point. But now it's easier to grep for tests that explicitly
> stress --fork-point.

That makes sense.

> +test_expect_success 'setup branches and remote tracking' '
> +	git tag -l >tags &&
> +	for tag in $(cat tags)
> +	do
> +		git branch branch-$tag $tag || return 1
> +	done &&

I don't think we need this extra tmpfile and cat, do we? I.e.,

  for tag in $(git tag -l)

would work. We should probably avoid depending on the exact output of
the porcelain "tag", though. Maybe:

  git for-each-ref \
    --format='create refs/heads/branch-%(refname:strip=2) %(objectname)' \
    refs/tags |
  git update-ref --stdin

which has the added bonus of using a constant number of processes.

-Peff

Re: [PATCH 1/2] rebase tests: test linear branch topology

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

>> While I'm at it add a --fork-point test, strictly speaking this is
>> redundant to the existing '' test, as no argument to rebase implies
>> --fork-point. But now it's easier to grep for tests that explicitly
>> stress --fork-point.
>
> That makes sense.
>
>> +test_expect_success 'setup branches and remote tracking' '
>> +	git tag -l >tags &&
>> +	for tag in $(cat tags)
>> +	do
>> +		git branch branch-$tag $tag || return 1
>> +	done &&
>
> I don't think we need this extra tmpfile and cat, do we? I.e.,
>
>   for tag in $(git tag -l)
>
> would work.

I think it is being (overly) defensive not to lose the exit status
of "git tag".

> We should probably avoid depending on the exact output of
> the porcelain "tag", though. Maybe:
>
>   git for-each-ref \
>     --format='create refs/heads/branch-%(refname:strip=2) %(objectname)' \
>     refs/tags |
>   git update-ref --stdin
>
> which has the added bonus of using a constant number of processes.

Much better ;-)

[PATCH 2/2] rebase: don't rebase linear topology with --fork-point

[Ævar Arnfjörð Bjarmason]

Fix a regression introduced in 4f21454b55 ("merge-base: handle
--fork-point without reflog", 2016-10-12).

Before that change having a linear history on top of an upstream
master would with --fork-point (aka argument-less rebase) tell us
there was nothing to be done:

    $ git rebase
    Current branch master is up to date.

After that change "rebase" will always redundantly find that it has
work to do (it doesn't):

    $ git rebase
    First, rewinding head to replay your work on top of it...
    Applying: [...]

Whereas equivalently running:

    $ git rebase @{upstream}
    $ git rebase $(git merge-base --fork-point @{u})

Gives us the old behavior of doing nothing.

Now, why did we have this regression? Fully digging into it yields an
interesting combination of causes:

Way back in 1308c17b3e ("Allow rebase to run if upstream is completely
merged", 2007-07-04) "rebase" learned to not do this redundant work
when asked to rebase on a commit that was already an ancestor of the
current commit.

Then in 1e0dacdbdb ("rebase: omit patch-identical commits with
--fork-point", 2014-07-16) a rebase bug was fixed for a case where the
history to be rebased was divergent by entirely skipping the 2007-era
logic if --fork-point was provided.

But here's the critical thing, *only* if the --fork-point was
divergent. At that time "git merge-base --fork-point A B" would return
nothing if the two commits weren't divergent.

Then in 4f21454b55 ("merge-base: handle --fork-point without reflog",
2016-10-12) which introduced the regression being fixed here, a bug
fix for "git merge-base --fork-point" being run stand-alone by proxy
broke this use-case git-rebase.sh was relying on, since it was still
assuming that if we didn't have divergent history we'd have no output.

Finally, when "rebase" was rewritten in C a combination of
9a48a615b4 ("builtin rebase: try to fast forward when possible",
2018-09-04), 103148aad8 ("merge-base --fork-point: extract libified
function", 2018-09-04) and 92d0d74e8d ("builtin rebase: support
`fork-point` option", 2018-09-04) faithfully re-implemented the
then-buggy behavior.

So let's fix this. It's easy enough, we just stop explicitly excluding
--fork-point from the can_fast_forward(...) test we're doing, which as
discussed above is faithfully ported over from buggy shellscript-era
logic.

I'm not bothering to fix this in the legacy rebase mode. As discussed
in 9aea5e9286 ("rebase: fix regression in rebase.useBuiltin=false test
mode", 2019-02-13) it'll be going away shortly after 2.21.0 lands.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/rebase.c                  |  6 ++++--
 t/t3421-rebase-topology-linear.sh | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 7c7bc13e91..7a16b8051c 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1664,9 +1664,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	 * and if this is not an interactive rebase.
 	 */
 	if (can_fast_forward(options.onto, &options.orig_head, &merge_base) &&
-	    !is_interactive(&options) && !options.restrict_revision &&
+	    !is_interactive(&options) &&
 	    options.upstream &&
-	    !oidcmp(&options.upstream->object.oid, &options.onto->object.oid)) {
+	    (options.restrict_revision
+	     ? !oidcmp(&options.upstream->object.oid, &options.restrict_revision->object.oid)
+	     : !oidcmp(&options.upstream->object.oid, &options.onto->object.oid))) {
 		int flag;
 
 		if (!(options.flags & REBASE_FORCE)) {
diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
index b847064f91..1754537789 100755
--- a/t/t3421-rebase-topology-linear.sh
+++ b/t/t3421-rebase-topology-linear.sh
@@ -55,6 +55,21 @@ test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
 
+test_run_rebase () {
+	result=$1
+	shift
+	test_expect_$result "rebase $* is no-op if remote upstream is an ancestor" "
+		reset_rebase &&
+		GIT_TEST_REBASE_USE_BUILTIN=true git rebase $* branch-b branch-e &&
+		test_cmp_rev e HEAD
+	"
+}
+test_run_rebase success ''
+test_run_rebase success --fork-point
+test_run_rebase success -m
+test_run_rebase success -i
+test_have_prereq !REBASE_P || test_run_rebase success -p
+
 test_run_rebase () {
 	result=$1
 	shift
-- 
2.21.0.rc0.258.g878e2cd30e

Re: [PATCH 2/2] rebase: don't rebase linear topology with --fork-point

[Jeff King]

On Thu, Feb 21, 2019 at 10:40:59PM +0100, Ævar Arnfjörð Bjarmason wrote:

> Fix a regression introduced in 4f21454b55 ("merge-base: handle
> --fork-point without reflog", 2016-10-12).
> [...]

OK, your explanation mostly makes sense to me, except for one thing.

> Then in 4f21454b55 ("merge-base: handle --fork-point without reflog",
> 2016-10-12) which introduced the regression being fixed here, a bug
> fix for "git merge-base --fork-point" being run stand-alone by proxy
> broke this use-case git-rebase.sh was relying on, since it was still
> assuming that if we didn't have divergent history we'd have no output.

I still don't quite see how 4f21454b55 is involved here, except by
returning a fork-point value when there is no reflog, and thus
triggering the bug in more cases.

In particular, imagine this case:

  git init
  for i in $(seq 1 3); do echo $i >$i; git add $i; git commit -m $i; done
  git checkout -t -b other
  for i in $(seq 4 6); do echo $i >$i; git add $i; git commit -m $i; done
  git rebase

With the current code, that will rewind and replay 4-6, and I understand
that to be a bug from your description. And it happens at 4f21454b55,
too. But it _also_ happens at 4f21454b55^.

I.e., I still think that the only thing that commit changed is that we
found a fork-point in more cases. But the bug was still demonstrably
there when you actually have a reflog entry.

With the fix you have here, that case now produces "Current branch other
is up to date".

This is splitting hairs a little (and of course I'm trying to exonerate
the commit I'm responsible for ;) ), but I just want to make sure we
understand fully what's going on. Your fix looks plausibly correct to
me, but I admit I don't quite grok all the details of that conditional.

-Peff

Re: [PATCH 2/2] rebase: don't rebase linear topology with --fork-point

[Ævar Arnfjörð Bjarmason]

On Fri, Feb 22 2019, Jeff King wrote:

> On Thu, Feb 21, 2019 at 10:40:59PM +0100, Ævar Arnfjörð Bjarmason wrote:
>
>> Fix a regression introduced in 4f21454b55 ("merge-base: handle
>> --fork-point without reflog", 2016-10-12).
>> [...]
>
> OK, your explanation mostly makes sense to me, except for one thing.
>
>> Then in 4f21454b55 ("merge-base: handle --fork-point without reflog",
>> 2016-10-12) which introduced the regression being fixed here, a bug
>> fix for "git merge-base --fork-point" being run stand-alone by proxy
>> broke this use-case git-rebase.sh was relying on, since it was still
>> assuming that if we didn't have divergent history we'd have no output.
>
> I still don't quite see how 4f21454b55 is involved here, except by
> returning a fork-point value when there is no reflog, and thus
> triggering the bug in more cases.
>
> In particular, imagine this case:
>
>   git init
>   for i in $(seq 1 3); do echo $i >$i; git add $i; git commit -m $i; done
>   git checkout -t -b other
>   for i in $(seq 4 6); do echo $i >$i; git add $i; git commit -m $i; done
>   git rebase
>
> With the current code, that will rewind and replay 4-6, and I understand
> that to be a bug from your description. And it happens at 4f21454b55,
> too. But it _also_ happens at 4f21454b55^.
>
> I.e., I still think that the only thing that commit changed is that we
> found a fork-point in more cases. But the bug was still demonstrably
> there when you actually have a reflog entry.
>
> With the fix you have here, that case now produces "Current branch other
> is up to date".
>
> This is splitting hairs a little (and of course I'm trying to exonerate
> the commit I'm responsible for ;) ), but I just want to make sure we
> understand fully what's going on.

Yes. I didn't dig far enough into this and will re-word & re-submit,
also with the feedback you had on 1/2.

So here's my current understanding of this.

It's b6266dc88b ("rebase--am: use --cherry-pick instead of
--ignore-if-in-upstream", 2014-07-15) that broke this in the general
case.

I.e. if you set a tracking branch within the same repo (which I'd
betnobody does) but *also* if you have an established clone you have a
reflog for the upstream. Then we'll find the fork point, and we'll
always redundantly rebase.

But this hung on by a thread until your 4f21454b55 ("merge-base: handle
--fork-point without reflog", 2016-10-12). In particular when you:

 1. Clone some *new* repo
 2. commit on top
 3. git pull --rebase

You'll redundantly rebase on top, even though you have nothing to
do. Since there's no reflog.

This is why I ran into this most of the time, because my "patch some
random thing" is that, and I have pull.rebase=true in my config.

What had me confused about this being the primary cause was that when
trying to test this I was re-cloning, so I'd always get this empty
reflog case.

> Your fix looks plausibly correct to me, but I admit I don't quite grok
> all the details of that conditional.

We just consider whether we can fast-forward now, and then don't need to
rebase (unless "git rebase -i" etc.). I.e. that --fork-point was
considered for "do we need to do stuff" was a bug introduced in
b6266dc88b.

Re: [PATCH 2/2] rebase: don't rebase linear topology with --fork-point

[Jeff King]

On Fri, Feb 22, 2019 at 05:49:57PM +0100, Ævar Arnfjörð Bjarmason wrote:

> Yes. I didn't dig far enough into this and will re-word & re-submit,
> also with the feedback you had on 1/2.
> 
> So here's my current understanding of this.
> 
> It's b6266dc88b ("rebase--am: use --cherry-pick instead of
> --ignore-if-in-upstream", 2014-07-15) that broke this in the general
> case.
> 
> I.e. if you set a tracking branch within the same repo (which I'd
> betnobody does) but *also* if you have an established clone you have a
> reflog for the upstream. Then we'll find the fork point, and we'll
> always redundantly rebase.
> 
> But this hung on by a thread until your 4f21454b55 ("merge-base: handle
> --fork-point without reflog", 2016-10-12). In particular when you:
> 
>  1. Clone some *new* repo
>  2. commit on top
>  3. git pull --rebase
> 
> You'll redundantly rebase on top, even though you have nothing to
> do. Since there's no reflog.
> 
> This is why I ran into this most of the time, because my "patch some
> random thing" is that, and I have pull.rebase=true in my config.
> 
> What had me confused about this being the primary cause was that when
> trying to test this I was re-cloning, so I'd always get this empty
> reflog case.

OK, thanks, that all makes sense to me and matches what I'm seeing. I
think we're on the same page now.

> > Your fix looks plausibly correct to me, but I admit I don't quite grok
> > all the details of that conditional.
> 
> We just consider whether we can fast-forward now, and then don't need to
> rebase (unless "git rebase -i" etc.). I.e. that --fork-point was
> considered for "do we need to do stuff" was a bug introduced in
> b6266dc88b.

Right, that makes sense. But I'm just not clear on the new oidcmp() rule
that's put in place.

When we're not using fork point, in the old code we'd check whether
upstream and onto are the same. Which makes sense; if we're rebuilding
onto the same spot, then it's a noop.

And in the fork-point case, we'd instead want to do something similar
with restrict_revision. But you compare upstream and restrict_revision,
and my understanding is that with --fork-point the latter basically
replaces the former.  Shouldn't we be comparing restrict_revision and
onto?

E.g., if I do:

  git checkout -b foo origin
  echo content >file && git add file && git commit -m foo
  git rebase --onto origin^ --fork-point origin

we should do an actual rebase, but with your patch we get "Current
branch foo is up to date". Whereas dropping --fork-point, it works.

-Peff