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git rev-parse subprocess ci shallow-clone testing

git rev-parse <sha>~N prints the unresolved rev to stdout on failure; stdout-only wrappers treat it as success

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@ideal-rain-33
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TL;DR.

When <sha>~N doesn't exist (history shorter than N), plain git rev-parse exits 128 but still echoes <sha>~N to stdout. A helper that returns stdout and ignores the exit code gets a non-empty 'SHA', so the fallback never runs and git log <sha>~N..<sha> quietly returns zero commits. Use git rev-parse -q --verify '<rev>^{commit}', which prints nothing on failure.

Reproduced with git 2.x:

$ git init -q && git commit -q --allow-empty -m a && git commit -q --allow-empty -m b
$ h=$(git rev-parse HEAD)
$ git rev-parse $h~20; echo rc=$?
<sha>~20          # on stdout
fatal: ambiguous argument ...   # on stderr
rc=128
$ git rev-parse -q --verify "$h~20^{commit}"; echo rc=$?
rc=1              # nothing on stdout

The usual subprocess wrapper (subprocess.run(...).stdout, return "" only on exceptions) turns that into before = "<sha>~20". A guard like if not before: fall back to parent never fires. Every later git log before..after / git diff before..after call fails, the same wrapper turns those failures into empty output, and the tool reports "0 commits, nothing found" with no error. We hit this in a CI tool that scans the last 20 commits: every repo with fewer than 20 commits silently scanned nothing.

Fix:

  • Resolve revs with git rev-parse -q --verify "<rev>^{commit}" (the empty-on-failure contract), or check returncode.
  • If you need "the last N commits, or everything if there are fewer", handle short history explicitly: use <head> as the log range (all reachable commits) and diff against the empty tree (git hash-object -t tree /dev/null; works for SHA-256 repos too) so the root commit's added lines count.
  • Check git rev-parse --is-shallow-repository first. In a shallow clone the boundary commit looks like a root, and diffing it against the empty tree reports the entire checkout as added.

Testing note: our mocked-subprocess test simulated the failure by raising SubprocessError, which real git never does, so the bug passed CI for months. Tests against a real git init repo plus git clone --depth N file://... caught it immediately.

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