Merge branch 'ds/commit-graph-gen-v2-fixes'

Fixes to the way generation number v2 in the commit-graph files are
(not) handled.

* ds/commit-graph-gen-v2-fixes:
  commit-graph: declare bankruptcy on GDAT chunks
  commit-graph: fix generation number v2 overflow values
  commit-graph: start parsing generation v2 (again)
  commit-graph: fix ordering bug in generation numbers
  t5318: extract helpers to lib-commit-graph.sh
  test-read-graph: include extra post-parse info

1 files changed, 10 insertions, 2 deletions

diff --git a/Documentation/technical/commit-graph-format.txt b/Documentation/technical/commit-graph-format.txt
index 87971c27dd..484b185ba9 100644
--- a/Documentation/technical/commit-graph-format.txt
+++ b/Documentation/technical/commit-graph-format.txt
@@ -93,7 +93,7 @@ CHUNK DATA:
       2 bits of the lowest byte, storing the 33rd and 34th bit of the
       commit time.
 
-  Generation Data (ID: {'G', 'D', 'A', 'T' }) (N * 4 bytes) [Optional]
+  Generation Data (ID: {'G', 'D', 'A', '2' }) (N * 4 bytes) [Optional]
     * This list of 4-byte values store corrected commit date offsets for the
       commits, arranged in the same order as commit data chunk.
     * If the corrected commit date offset cannot be stored within 31 bits,
@@ -104,7 +104,7 @@ CHUNK DATA:
       by compatible versions of Git and in case of split commit-graph chains,
       the topmost layer also has Generation Data chunk.
 
-  Generation Data Overflow (ID: {'G', 'D', 'O', 'V' }) [Optional]
+  Generation Data Overflow (ID: {'G', 'D', 'O', '2' }) [Optional]
     * This list of 8-byte values stores the corrected commit date offsets
       for commits with corrected commit date offsets that cannot be
       stored within 31 bits.
@@ -156,3 +156,11 @@ CHUNK DATA:
 TRAILER:
 
 	H-byte HASH-checksum of all of the above.
+
+== Historical Notes:
+
+The Generation Data (GDA2) and Generation Data Overflow (GDO2) chunks have
+the number '2' in their chunk IDs because a previous version of Git wrote
+possibly erroneous data in these chunks with the IDs "GDAT" and "GDOV". By
+changing the IDs, newer versions of Git will silently ignore those older
+chunks and write the new information without trusting the incorrect data.