[v4,8/9] midx: read `RIDX` chunk when present

Commit Message

Taylor Blau Jan. 25, 2022, 10:41 p.m. UTC

When a MIDX contains the new `RIDX` chunk, ensure that the reverse index
is read from it instead of the on-disk .rev file. Since we need to
encode the object order in the MIDX itself for correctness reasons,
there is no point in storing the same data again outside of the MIDX.

So, this patch stops writing separate .rev files, and reads it out of
the MIDX itself. This is possible to do with relatively little new code,
since the format of the RIDX chunk is identical to the data in the .rev
file. In other words, we can implement this by pointing the
`revindex_data` field at the reverse index chunk of the MIDX instead of
the .rev file without any other changes.

Note that we have two knobs that are adjusted for the new tests:
GIT_TEST_MIDX_WRITE_REV and GIT_TEST_MIDX_READ_RIDX. The former controls
whether the MIDX .rev is written at all, and the latter controls whether
we read the MIDX's RIDX chunk.

Both are necessary to ensure that the test added at the beginning of
this series continues to work. This is because we always need to write
the RIDX chunk in the MIDX in order to change its checksum, but we want
to make sure reading the existing .rev file still works (since the RIDX
chunk takes precedence by default).

Arguably this isn't a very interesting mode to test, because the
precedence rules mean that we'll always read the RIDX chunk over the
.rev file. But it makes it impossible for a user to induce corruption in
their repository by adjusting the test knobs (since if we had an
either/or knob they could stop writing the RIDX chunk, allowing them to
tweak the MIDX's object order without changing its checksum).

Signed-off-by: Taylor Blau <[e-mail]>
---
 midx.c                            |  6 +++++-
 midx.h                            |  1 +
 pack-revindex.c                   | 17 +++++++++++++++++
 t/lib-bitmap.sh                   |  4 ++--
 t/t5326-multi-pack-bitmaps.sh     |  6 ++++++
 t/t5327-multi-pack-bitmaps-rev.sh | 23 +++++++++++++++++++++++
 t/t7700-repack.sh                 |  4 ----
 7 files changed, 54 insertions(+), 7 deletions(-)
 create mode 100755 t/t5327-multi-pack-bitmaps-rev.sh

Comments

Ævar Arnfjörð Bjarmason Jan. 26, 2022, 3:10 p.m. UTC | #1

On Tue, Jan 25 2022, Taylor Blau wrote:

> @@ -298,9 +298,26 @@ int load_midx_revindex(struct multi_pack_index *m)
>  {
>  	struct strbuf revindex_name = STRBUF_INIT;
>  	int ret;
> +

nit: good addition for style in general, but a stay whitespace change in
an otherwise narrowly focused patch.

>  	if (m->revindex_data)
>  		return 0;
>  
> +	if (m->chunk_revindex) {
> +		/*
> +		 * If the MIDX `m` has a `RIDX` chunk, then use its contents for
> +		 * the reverse index instead of trying to load a separate `.rev`
> +		 * file.
> +		 *
> +		 * Note that we do *not* set `m->revindex_map` here, since we do
> +		 * not want to accidentally call munmap() in the middle of the
> +		 * MIDX.
> +		 */
> +		trace2_data_string("load_midx_revindex", the_repository,
> +				   "source", "midx");
> +		m->revindex_data = (const uint32_t *)m->chunk_revindex;
> +		return 0;
> +	}
> +
>  	trace2_data_string("load_midx_revindex", the_repository,
>  			   "source", "rev");

This trace2_data_string() is repeated with just "midx" v.s. "rev"
parameters.

I was going to suggest that maybe we should add a "goto" here, since
you're trying to juggle the early return and logging this.

But wouldn't this be both simpler and give you better logging if it was
the usual region start/end pattern, with just the "data string"
in-between?

Then you'd get both performance data on the index loading, and the
source.

Or maybe it's overkill in this case, I don't know...

Taylor Blau Jan. 26, 2022, 8:23 p.m. UTC | #2

On Wed, Jan 26, 2022 at 04:10:08PM +0100, Ævar Arnfjörð Bjarmason wrote:
>
> On Tue, Jan 25 2022, Taylor Blau wrote:
>
> > @@ -298,9 +298,26 @@ int load_midx_revindex(struct multi_pack_index *m)
> >  {
> >  	struct strbuf revindex_name = STRBUF_INIT;
> >  	int ret;
> > +
>
> nit: good addition for style in general, but a stay whitespace change in
> an otherwise narrowly focused patch.
>
> >  	if (m->revindex_data)
> >  		return 0;
> >
> > +	if (m->chunk_revindex) {
> > +		/*
> > +		 * If the MIDX `m` has a `RIDX` chunk, then use its contents for
> > +		 * the reverse index instead of trying to load a separate `.rev`
> > +		 * file.
> > +		 *
> > +		 * Note that we do *not* set `m->revindex_map` here, since we do
> > +		 * not want to accidentally call munmap() in the middle of the
> > +		 * MIDX.
> > +		 */
> > +		trace2_data_string("load_midx_revindex", the_repository,
> > +				   "source", "midx");
> > +		m->revindex_data = (const uint32_t *)m->chunk_revindex;
> > +		return 0;
> > +	}
> > +
> >  	trace2_data_string("load_midx_revindex", the_repository,
> >  			   "source", "rev");
>
> This trace2_data_string() is repeated with just "midx" v.s. "rev"
> parameters.
>
> I was going to suggest that maybe we should add a "goto" here, since
> you're trying to juggle the early return and logging this.

Right; that matches how we handle the two cases separately here. I'm not
sure exactly what you're suggesting with by adding a "goto" label; I
think it would make things more awkward rather than more readable, but
it's certainly possible that I'm missing something.

> But wouldn't this be both simpler and give you better logging if it was
> the usual region start/end pattern, with just the "data string"
> in-between?
>
> Then you'd get both performance data on the index loading, and the
> source.
>
> Or maybe it's overkill in this case, I don't know...

I think it's overkill; in the "embedded" case (which we expect to be
pretty common in the future) we're not really measuring anything except
for the pointer assignment.

We *could* instrument pack-revindex.c::load_revindex_from_disk(), but I
think that's a separate issue from this one.

Thanks,
Taylor

Patch

diff --git a/midx.c b/midx.c
index 0248c4577c..6e6cb0eb04 100644
--- a/midx.c
+++ b/midx.c
@@ -162,6 +162,9 @@  struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 
 	pair_chunk(cf, MIDX_CHUNKID_LARGEOFFSETS, &m->chunk_large_offsets);
 
+	if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
+		pair_chunk(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex);
+
 	m->num_objects = ntohl(m->chunk_oid_fanout[255]);
 
 	CALLOC_ARRAY(m->pack_names, m->num_packs);
@@ -1429,7 +1432,8 @@  static int write_midx_internal(const char *object_dir,
 	finalize_hashfile(f, midx_hash, CSUM_FSYNC | CSUM_HASH_IN_STREAM);
 	free_chunkfile(cf);
 
-	if (flags & MIDX_WRITE_REV_INDEX)
+	if (flags & MIDX_WRITE_REV_INDEX &&
+	    git_env_bool("GIT_TEST_MIDX_WRITE_REV", 0))
 		write_midx_reverse_index(midx_name.buf, midx_hash, &ctx);
 	if (flags & MIDX_WRITE_BITMAP) {
 		if (write_midx_bitmap(midx_name.buf, midx_hash, &ctx,
diff --git a/midx.h b/midx.h
index b7d79a515c..22e8e53288 100644
--- a/midx.h
+++ b/midx.h
@@ -36,6 +36,7 @@  struct multi_pack_index {
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_object_offsets;
 	const unsigned char *chunk_large_offsets;
+	const unsigned char *chunk_revindex;
 
 	const char **pack_names;
 	struct packed_git **packs;
diff --git a/pack-revindex.c b/pack-revindex.c
index bd15ebad03..08dc160167 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -298,9 +298,26 @@  int load_midx_revindex(struct multi_pack_index *m)
 {
 	struct strbuf revindex_name = STRBUF_INIT;
 	int ret;
+
 	if (m->revindex_data)
 		return 0;
 
+	if (m->chunk_revindex) {
+		/*
+		 * If the MIDX `m` has a `RIDX` chunk, then use its contents for
+		 * the reverse index instead of trying to load a separate `.rev`
+		 * file.
+		 *
+		 * Note that we do *not* set `m->revindex_map` here, since we do
+		 * not want to accidentally call munmap() in the middle of the
+		 * MIDX.
+		 */
+		trace2_data_string("load_midx_revindex", the_repository,
+				   "source", "midx");
+		m->revindex_data = (const uint32_t *)m->chunk_revindex;
+		return 0;
+	}
+
 	trace2_data_string("load_midx_revindex", the_repository,
 			   "source", "rev");
 
diff --git a/t/lib-bitmap.sh b/t/lib-bitmap.sh
index 253895c04e..a95537e759 100644
--- a/t/lib-bitmap.sh
+++ b/t/lib-bitmap.sh
@@ -290,7 +290,7 @@  test_rev_exists () {
 }
 
 midx_bitmap_core () {
-	rev_kind="${1:-rev}"
+	rev_kind="${1:-midx}"
 
 	setup_bitmap_history
 
@@ -434,7 +434,7 @@  midx_bitmap_core () {
 }
 
 midx_bitmap_partial_tests () {
-	rev_kind="${1:-rev}"
+	rev_kind="${1:-midx}"
 
 	test_expect_success 'setup partial bitmaps' '
 		test_commit packed &&
diff --git a/t/t5326-multi-pack-bitmaps.sh b/t/t5326-multi-pack-bitmaps.sh
index 100ac90d15..c0924074c4 100755
--- a/t/t5326-multi-pack-bitmaps.sh
+++ b/t/t5326-multi-pack-bitmaps.sh
@@ -9,6 +9,12 @@  test_description='exercise basic multi-pack bitmap functionality'
 GIT_TEST_MULTI_PACK_INDEX=0
 GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
 
+# This test exercise multi-pack bitmap functionality where the object order is
+# stored and read from a special chunk within the MIDX, so use the default
+# behavior here.
+sane_unset GIT_TEST_MIDX_WRITE_REV
+sane_unset GIT_TEST_MIDX_READ_RIDX
+
 midx_bitmap_core
 
 bitmap_reuse_tests() {
diff --git a/t/t5327-multi-pack-bitmaps-rev.sh b/t/t5327-multi-pack-bitmaps-rev.sh
new file mode 100755
index 0000000000..d30ba632c8
--- /dev/null
+++ b/t/t5327-multi-pack-bitmaps-rev.sh
@@ -0,0 +1,23 @@ 
+#!/bin/sh
+
+test_description='exercise basic multi-pack bitmap functionality (.rev files)'
+
+. ./test-lib.sh
+. "${TEST_DIRECTORY}/lib-bitmap.sh"
+
+# We'll be writing our own midx and bitmaps, so avoid getting confused by the
+# automatic ones.
+GIT_TEST_MULTI_PACK_INDEX=0
+GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
+
+# Unlike t5326, this test exercise multi-pack bitmap functionality where the
+# object order is stored in a separate .rev file.
+GIT_TEST_MIDX_WRITE_REV=1
+GIT_TEST_MIDX_READ_RIDX=0
+export GIT_TEST_MIDX_WRITE_REV
+export GIT_TEST_MIDX_READ_RIDX
+
+midx_bitmap_core rev
+midx_bitmap_partial_tests rev
+
+test_done
diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
index e489869dd9..5922fb5bdd 100755
--- a/t/t7700-repack.sh
+++ b/t/t7700-repack.sh
@@ -312,16 +312,13 @@  test_expect_success 'cleans up MIDX when appropriate' '
 		checksum=$(midx_checksum $objdir) &&
 		test_path_is_file $midx &&
 		test_path_is_file $midx-$checksum.bitmap &&
-		test_path_is_file $midx-$checksum.rev &&
 
 		test_commit repack-3 &&
 		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adb --write-midx &&
 
 		test_path_is_file $midx &&
 		test_path_is_missing $midx-$checksum.bitmap &&
-		test_path_is_missing $midx-$checksum.rev &&
 		test_path_is_file $midx-$(midx_checksum $objdir).bitmap &&
-		test_path_is_file $midx-$(midx_checksum $objdir).rev &&
 
 		test_commit repack-4 &&
 		GIT_TEST_MULTI_PACK_INDEX=0 git repack -Adb &&
@@ -354,7 +351,6 @@  test_expect_success '--write-midx with preferred bitmap tips' '
 		test_line_count = 1 before &&
 
 		rm -fr $midx-$(midx_checksum $objdir).bitmap &&
-		rm -fr $midx-$(midx_checksum $objdir).rev &&
 		rm -fr $midx &&
 
 		# instead of constructing the snapshot ourselves (c.f., the test