Re: [PATCH] diff: batch fetching of missing blobs

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Jonathan,

On Tue, 26 Mar 2019, Jonathan Tan wrote:

> When running a command like "git show" or "git diff" in a partial clone,
> batch all missing blobs to be fetched as one request.
>
> This is similar to c0c578b33c ("unpack-trees: batch fetching of missing
> blobs", 2017-12-08), but for another command.
>
> Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
> ---
> Here's an improvement for those having partial clones.
>
> I couldn't find a good place to place the test (a place that checks how
> diff interfaces with the object store would be ideal), so I created a
> new one. Let me know if there's a better place to put it.

My 2c: that's fine.

> diff --git a/diff.c b/diff.c
> index ec5c095199..0e08d05b14 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -25,6 +25,7 @@
>  #include "packfile.h"
>  #include "parse-options.h"
>  #include "help.h"
> +#include "fetch-object.h"
>
>  #ifdef NO_FAST_WORKING_DIRECTORY
>  #define FAST_WORKING_DIRECTORY 0
> @@ -6067,6 +6068,32 @@ static void diff_flush_patch_all_file_pairs(struct diff_options *o)
>  	if (o->color_moved)
>  		o->emitted_symbols = &esm;
>
> +	if (repository_format_partial_clone) {
> +		/*
> +		 * Prefetch the diff pairs that are about to be flushed.
> +		 */
> +		struct oid_array to_fetch = OID_ARRAY_INIT;
> +		int fetch_if_missing_store = fetch_if_missing;
> +
> +		fetch_if_missing = 0;
> +		for (i = 0; i < q->nr; i++) {
> +			struct diff_filepair *p = q->queue[i];
> +			if (!check_pair_status(p))
> +				continue;
> +			if (p->one && p->one->oid_valid &&
> +			    !has_object_file(&p->one->oid))
> +				oid_array_append(&to_fetch, &p->one->oid);
> +			if (p->two && p->two->oid_valid &&
> +			    !has_object_file(&p->two->oid))
> +				oid_array_append(&to_fetch, &p->two->oid);
> +		}
> +		if (to_fetch.nr)
> +			fetch_objects(repository_format_partial_clone,
> +				      to_fetch.oid, to_fetch.nr);
> +		fetch_if_missing = fetch_if_missing_store;

I am slightly uneasy about the fact that this is totally *not*
multi-thread safe. If only we had a way to pass the `fetch_if_missing`
information as a parameter to `has_object_file()`...

Do you think that would be easy to do? If so, I think now would be as fine
an opportunity as ever to introduce that.

Otherwise the patch looks just fine to me.

Thanks,
Dscho

> +		oid_array_clear(&to_fetch);
> +	}
> +
>  	for (i = 0; i < q->nr; i++) {
>  		struct diff_filepair *p = q->queue[i];
>  		if (check_pair_status(p))
> diff --git a/t/t4067-diff-partial-clone.sh b/t/t4067-diff-partial-clone.sh
> new file mode 100755
> index 0000000000..38f03be114
> --- /dev/null
> +++ b/t/t4067-diff-partial-clone.sh
> @@ -0,0 +1,76 @@
> +#!/bin/sh
> +
> +test_description='behavior of diff when reading objects in a partial clone'
> +
> +. ./test-lib.sh
> +
> +test_expect_success 'git show batches blobs' '
> +	test_create_repo server &&
> +	echo a >server/a &&
> +	echo b >server/b &&
> +	git -C server add a b &&
> +	git -C server commit -m x &&
> +
> +	test_config -C server uploadpack.allowfilter 1 &&
> +	test_config -C server uploadpack.allowanysha1inwant 1 &&
> +	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
> +
> +	# Ensure that there is exactly 1 negotiation by checking that there is
> +	# only 1 "done" line sent. ("done" marks the end of negotiation.)
> +	GIT_TRACE_PACKET="$(pwd)/trace" git -C client show HEAD &&
> +	grep "git> done" trace >done_lines &&
> +	test_line_count = 1 done_lines
> +'
> +
> +test_expect_success 'diff batches blobs' '
> +	rm -rf server client trace &&
> +
> +	test_create_repo server &&
> +	echo a >server/a &&
> +	echo b >server/b &&
> +	git -C server add a b &&
> +	git -C server commit -m x &&
> +	echo c >server/c &&
> +	echo d >server/d &&
> +	git -C server add c d &&
> +	git -C server commit -m x &&
> +
> +	test_config -C server uploadpack.allowfilter 1 &&
> +	test_config -C server uploadpack.allowanysha1inwant 1 &&
> +	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
> +
> +	# Ensure that there is exactly 1 negotiation by checking that there is
> +	# only 1 "done" line sent. ("done" marks the end of negotiation.)
> +	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff HEAD^ HEAD &&
> +	grep "git> done" trace >done_lines &&
> +	test_line_count = 1 done_lines
> +'
> +
> +test_expect_success 'diff skips same-OID blobs' '
> +	rm -rf server client trace &&
> +
> +	test_create_repo server &&
> +	echo a >server/a &&
> +	echo b >server/b &&
> +	git -C server add a b &&
> +	git -C server commit -m x &&
> +	echo another-a >server/a &&
> +	git -C server add a &&
> +	git -C server commit -m x &&
> +
> +	test_config -C server uploadpack.allowfilter 1 &&
> +	test_config -C server uploadpack.allowanysha1inwant 1 &&
> +	git clone --bare --filter=blob:limit=0 "file://$(pwd)/server" client &&
> +
> +	echo a | git hash-object --stdin >hash-old-a &&
> +	echo another-a | git hash-object --stdin >hash-new-a &&
> +	echo b | git hash-object --stdin >hash-b &&
> +
> +	# Ensure that only a and another-a are fetched.
> +	GIT_TRACE_PACKET="$(pwd)/trace" git -C client diff HEAD^ HEAD &&
> +	grep "want $(cat hash-old-a)" trace &&
> +	grep "want $(cat hash-new-a)" trace &&
> +	! grep "want $(cat hash-b)" trace
> +'
> +
> +test_done
> --
> 2.21.0.155.ge902e9bcae.dirty
>
>