"Derrick Stolee via GitGitGadget" <gitgitgadget@xxxxxxxxx> writes:
> From: Derrick Stolee <derrickstolee@xxxxxxxxxx>
>
> The commit-graph file format v2 alters how it stores the corrected
> commit date offsets within the Commit Data chunk instead of a separate
> chunk. The idea is to significantly reduce the amount of data loaded
> from disk while parsing the commit-graph.
>
> We need to alter the error message when we see a file format version
> outside of our range now that multiple are possible. This has a
> non-functional side-effect of altering a use of GRAPH_VERSION within
> write_commit_graph().
>
> By storing the file format version in 'struct commit_graph', we can
> alter the parsing code to depend on that version value. This involves
> changing where we look for the corrected commit date offset, but also
> which constants we use for jumping into the Generation Data Overflow
> chunk. The Commit Data chunk only has 30 bits available for the offset
> while the Generation Data chunk has 32 bits. This only makes a
> meaningful difference in very malformed repositories.
>
> Also, we need to be careful about how we enable using corrected commit
> dates and generation numbers to rely upon the read_generation_data value
> instead of a non-zero value in the Commit Date chunk. In
> generation_numbers_enabled(), the first_generation variable is
> attemptint to look for the first topological level stored to see that it
> is nonzero. However, for a v2 commit-graph, this value is actually
> likely to be zero because the corrected commit date offset is probably
> zero.
I see references to OVERFLOW_V3 that comes after OVERFLOW, but there
is no OVERFLOW_V2. Intended, or should it be V2 to match the "file
format v2" of "generation number v2"? It is getting awkward to have
two different version scheme ("gen v2" means corrected committer
timestamp, whose on-disk representation is different between "file
v1" and "file v2", and this OVERFLOW vs OVERFLOW_V3 is about the
difference between "file v1" and "file v2" if I am following the
series correctly).
