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

[Junio C Hamano <gitster@pobox.com>]

Duy Nguyen <pclouds@gmail.com> writes:

>> Avoid a usage of the_repository by propagating the configured repository
>> to add_if_missing(). Also, prefetch only if the repository being diffed
>> is the_repository (because we do not support lazy fetching for any other
>> repository anyway).

If we are willing to stay limited to the default repository anyway,
allowing add_if_missing() to take an arbitrary repository does not
really matter, but before the caller of add_if_missing() befcomes
ready to work on an arbitrary repository, this change has to happen.

To update the caller, it seems to me that fetch_objects() must learn
to take an arbitrary repository, but is that the only thing needed?
After that, the function that the caller resides in and callchain
upwards can learn to take a repository instance if we want to be
able to diff inside an arbitrary repository.

But.  Such a change still would not allow us to compare a tree in
one repository against a tree in another repository.  It is likely
that a caller with such a need would simply make sure that objects
in both repositories are available by using the in-core alternate
object store mechanism, making it a more-or-less moot point to be
able to pass a repository instance through the callchain X-<.  We
probably should make it, and spell it out somewhere in a long term
vision shared among the developers, an explicit goal to get rid of
the internal (ab)use of the alternate object store mechanism.

With squashing the fix-up commit in, the 2/2 patch has become like
so.

Thanks, both.

-- >8 --
From: Jonathan Tan <jonathantanmy@google.com>
Date: Fri, 5 Apr 2019 10:09:34 -0700
Subject: [PATCH] diff: batch fetching of missing blobs

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 diff.c                        |  33 +++++++++++
 t/t4067-diff-partial-clone.sh | 103 ++++++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+)
 create mode 100755 t/t4067-diff-partial-clone.sh

diff --git a/diff.c b/diff.c
index ec5c095199..811afbdfb1 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
@@ -6366,8 +6367,40 @@ void diffcore_fix_diff_index(void)
 	QSORT(q->queue, q->nr, diffnamecmp);
 }
 
+static void add_if_missing(struct oid_array *to_fetch, struct repository *r,
+			   const struct diff_filespec *filespec)
+{
+	if (filespec && filespec->oid_valid &&
+	    oid_object_info_extended(r, &filespec->oid, NULL,
+				     OBJECT_INFO_FOR_PREFETCH))
+		oid_array_append(to_fetch, &filespec->oid);
+}
+
 void diffcore_std(struct diff_options *options)
 {
+	if (options->repo == the_repository &&
+	    repository_format_partial_clone) {
+		/*
+		 * Prefetch the diff pairs that are about to be flushed.
+		 */
+		int i;
+		struct diff_queue_struct *q = &diff_queued_diff;
+		struct oid_array to_fetch = OID_ARRAY_INIT;
+
+		for (i = 0; i < q->nr; i++) {
+			struct diff_filepair *p = q->queue[i];
+			add_if_missing(&to_fetch, options->repo, p->one);
+			add_if_missing(&to_fetch, options->repo, p->two);
+		}
+		if (to_fetch.nr)
+			/*
+			 * NEEDSWORK: Consider deduplicating the OIDs sent.
+			 */
+			fetch_objects(repository_format_partial_clone,
+				      to_fetch.oid, to_fetch.nr);
+		oid_array_clear(&to_fetch);
+	}
+
 	/* NOTE please keep the following in sync with diff_tree_combined() */
 	if (options->skip_stat_unmatch)
 		diffcore_skip_stat_unmatch(options);
diff --git a/t/t4067-diff-partial-clone.sh b/t/t4067-diff-partial-clone.sh
new file mode 100755
index 0000000000..90c8fb2901
--- /dev/null
+++ b/t/t4067-diff-partial-clone.sh
@@ -0,0 +1,103 @@
+#!/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_when_finished "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 &&
+
+	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' '
+	test_when_finished "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' '
+	test_when_finished "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 $(cat hash-b)" trace
+'
+
+test_expect_success 'diff with rename detection batches blobs' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	echo a >server/a &&
+	printf "b\nb\nb\nb\nb\n" >server/b &&
+	git -C server add a b &&
+	git -C server commit -m x &&
+	rm server/b &&
+	printf "b\nb\nb\nb\nbX\n" >server/c &&
+	git -C server add c &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --bare --filter=blob:limit=0 "file://$(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 -M HEAD^ HEAD >out &&
+	grep "similarity index" out &&
+	grep "git> done" trace >done_lines &&
+	test_line_count = 1 done_lines
+'
+
+test_done
-- 
2.21.0-196-g041f5ea1cf