[PATCH 8/8] pack-revindex: ensure that on-disk reverse indexes are given precedence

[Taylor Blau <me@ttaylorr.com>]

When an on-disk reverse index exists, there is no need to generate one
in memory. In fact, doing so can be slow, and require large amounts of
the heap.

Let's make sure that we treat the on-disk reverse index with precedence
(i.e., that when it exists, we don't bother trying to generate an
equivalent one in memory) by teaching Git how to conditionally die()
when generating a reverse index in memory.

Then, add a test to ensure that when (a) an on-disk reverse index
exists, and (b) when setting GIT_TEST_REV_INDEX_DIE_IN_MEMORY, that we
do not die, implying that we read from the on-disk one.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pack-revindex.c          | 4 ++++
 pack-revindex.h          | 1 +
 t/t5325-reverse-index.sh | 8 ++++++++
 3 files changed, 13 insertions(+)

diff --git a/pack-revindex.c b/pack-revindex.c
index 1baaf2c42a..683d22e898 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -2,6 +2,7 @@
 #include "pack-revindex.h"
 #include "object-store.h"
 #include "packfile.h"
+#include "config.h"
 
 struct revindex_entry {
 	off_t offset;
@@ -166,6 +167,9 @@ static void create_pack_revindex(struct packed_git *p)
 
 static int load_pack_revindex_from_memory(struct packed_git *p)
 {
+	if (git_env_bool(GIT_TEST_REV_INDEX_DIE_IN_MEMORY, 0))
+		die("dying as requested by '%s'",
+		    GIT_TEST_REV_INDEX_DIE_IN_MEMORY);
 	if (open_pack_index(p))
 		return -1;
 	create_pack_revindex(p);
diff --git a/pack-revindex.h b/pack-revindex.h
index d2d466e298..e271da871a 100644
--- a/pack-revindex.h
+++ b/pack-revindex.h
@@ -2,6 +2,7 @@
 #define PACK_REVINDEX_H
 
 #define GIT_TEST_WRITE_REV_INDEX "GIT_TEST_WRITE_REV_INDEX"
+#define GIT_TEST_REV_INDEX_DIE_IN_MEMORY "GIT_TEST_REV_INDEX_DIE_IN_MEMORY"
 
 struct packed_git;
 
diff --git a/t/t5325-reverse-index.sh b/t/t5325-reverse-index.sh
index 5a59cc71e4..9d4eecccc9 100755
--- a/t/t5325-reverse-index.sh
+++ b/t/t5325-reverse-index.sh
@@ -83,4 +83,12 @@ test_expect_success 'pack-objects respects pack.writeReverseIndex' '
 	test_path_is_file pack-1-*.rev
 '
 
+test_expect_success 'reverse index is not generated when available on disk' '
+	git index-pack --rev-index $packdir/pack-$pack.pack &&
+
+	git rev-parse HEAD >tip &&
+	GIT_TEST_REV_INDEX_DIE_IN_MEMORY=1 git cat-file \
+		--batch-check="%(objectsize:disk)" <tip
+'
+
 test_done
-- 
2.30.0.138.g6d7191ea01