commit-graph.c: gracefully handle file descriptor exhaustion

commit-graph.c: gracefully handle file descriptor exhaustion

When writing a layered commit-graph, the commit-graph machinery uses
'commit_graph_filenames_after' and 'commit_graph_hash_after' to keep
track of the layers in the chain that we are in the process of writing.

When the number of commit-graph layers shrinks, we initialize all
entries in the aforementioned arrays, because we know the structure of
the new commit-graph chain immediately (since there are no new layers,
there are no unknown hash values).

But when the number of commit-graph layers grows (i.e., that
'num_commit_graphs_after > num_commit_graphs_before'), then we leave
some entries in the filenames and hashes arrays as uninitialized,
because we will fill them in later as those values become available.

For instance, we rely on 'write_commit_graph_file's to store the
filename and hash of the last layer in the new chain, which is the one
that it is responsible for writing. But, it's possible that
'write_commit_graph_file' may fail, e.g., from file descriptor
exhaustion. In this case it is possible that 'git_mkstemp_mode' will
fail, and that function will return early *before* setting the values
for the last commit-graph layer's filename and hash.

This causes a number of upleasant side-effects. For instance, trying to
'free()' each entry in 'ctx->commit_graph_filenames_after' (and
similarly for the hashes array) causes us to 'free()' uninitialized
memory, since the area is allocated with 'malloc()' and is therefore
subject to contain garbage (which is left alone when
'write_commit_graph_file' returns early).

This can manifest in other issues, like a general protection fault,
and/or leaving a stray 'commit-graph-chain.lock' around after the
process dies. (The reasoning for this is still a mystery to me, since
we'd otherwise usually expect the kernel to run tempfile.c's 'atexit()'
handlers in the case of a normal death...)

To resolve this, initialize the memory with 'CALLOC_ARRAY' so that
uninitialized entries are filled with zeros, and can thus be 'free()'d
as a noop instead of causing a fault.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>