commit-graph: fix parsing the Chunk Lookup table

The commit-graph file format specifies that the chunks may be in any
order.  However, if the OID Lookup chunk happens to be the last one in
the file, then any command attempting to access the commit-graph data
will fail with:

  fatal: invalid commit position. commit-graph is likely corrupt

In this case the error is wrong, the commit-graph file does conform to
the specification, but the parsing of the Chunk Lookup table is a bit
buggy, and leaves the field holding the number of commits in the
commit-graph zero-initialized.

The number of commits in the commit-graph is determined while parsing
the Chunk Lookup table, by dividing the size of the OID Lookup chunk
with the hash size.  However, the Chunk Lookup table doesn't actually
store the size of the chunks, but it stores their starting offset.
Consequently, the size of a chunk can only be calculated by
subtracting the starting offsets of that chunk from the offset of the
subsequent chunk, or in case of the last chunk from the offset
recorded in the terminating label.  This is currenly implemented in a
bit complicated way: as we iterate over the entries of the Chunk
Lookup table, we check the ID of each chunk and store its starting
offset, then we check the ID of the last seen chunk and calculate its
size using its previously saved offset if necessary (at the moment
it's only necessary for the OID Lookup chunk).  Alas, while parsing
the Chunk Lookup table we only interate through the "real" chunks, but
never look at the terminating label, thus don't even check whether
it's necessary to calulate the size of the last chunk.  Consequently,
if the OID Lookup chunk is the last one, then we don't calculate its
size and turn don't run the piece of code determining the number of
commits in the commit graph, leaving the field holding that number
unchanged (i.e. zero-initialized), eventually triggering the sanity
check in load_oid_from_graph().

Fix this by iterating through all entries in the Chunk Lookup table,
including the terminating label.

Note that this is the minimal fix, suitable for the maintenance track.
A better fix would be to simplify how the chunk sizes are calculated,
but that is a more invasive change, less suitable for 'maint', so that
will be done in later patches.

This additional flexibility of scanning more chunks breaks a test for
"git commit-graph verify" so alter that test to mutate the commit-graph
to have an even lower chunk count.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -277,7 +277,7 @@ struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 	last_chunk_id = 0;
 	last_chunk_offset = 8;
 	chunk_lookup = data + 8;
-	for (i = 0; i < graph->num_chunks; i++) {
+	for (i = 0; i <= graph->num_chunks; i++) {
 		uint32_t chunk_id;
 		uint64_t chunk_offset;
 		int chunk_repeated = 0;

t/t5318-commit-graph.sh

@@ -488,7 +488,7 @@ test_expect_success 'detect bad hash version' '
 '
 
 test_expect_success 'detect low chunk count' '
-	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\02" \
+	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\01" \
 		"missing the .* chunk"
 '