pack-revindex: ensure that on-disk reverse indexes are given precedence

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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 create_pack_revindex_in_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);

pack-revindex.h

@@ -21,6 +21,7 @@
 #define RIDX_VERSION 1
 
 #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;
 

t/t5325-reverse-index.sh

@@ -85,4 +85,13 @@ 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' '
+	test_index_pack true &&
+	test_path_is_file $rev &&
+
+	git rev-parse HEAD >tip &&
+	GIT_TEST_REV_INDEX_DIE_IN_MEMORY=1 git cat-file \
+		--batch-check="%(objectsize:disk)" <tip
+'
+
 test_done