fix(commit): accumulate gpgsig continuation lines in linear time - #2268
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Keerthana-64 wants to merge 1 commit into
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`Commit._deserialize` collected a `gpgsig` header by appending each continuation line to a `bytes` object with `+=`. `bytes` are immutable, so every line copied the whole signature read so far, and a signature with *n* continuation lines cost O(n^2) to parse. Commit headers are fully controlled by whoever wrote the commit, and `_deserialize` runs on the first access to any commit attribute (`author`, `message`, `parents`, ...). Reading the commits of an untrusted repository is therefore enough to hit it. Measured on Python 3.11 with a `gpgsig` header of `" x\n"` lines: | object size | before | after | | ----------- | ------- | ------ | | 2.4 MB | 9.5 s | 0.08 s | | 4.8 MB | 116.8 s | | | 9.6 MB | 547.9 s | 0.32 s | Collect the lines in a list and join them once. This is linear and produces byte-identical `gpgsig` values, so the existing `test_gpgsig` round trip is unchanged. `test_gpgsig_deserialization_is_linear` deserializes a 1.8 MB signature under a 1 s CPU-time bound. It fails on the previous code and passes here. The full suite has no new failures; the 8 tests that fail locally (non-UTF-8 trailer encoding and NUL submodule names) fail identically without this change. `ruff`, `mypy` and `basedpyright --warnings` are clean.
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the gpgsig branch of
Commit._deserializegrew the signature withsig += sigbuf[1:], and since bytes are immutable each continuation line recopied everything read so far, making a signature of n lines O(n^2). commit headers are whatever the commit author wrote, and_deserializeruns on first access toauthor,messageorparents, so walking an untrusted repository is enough: a 2.4 MB commit took 9.5s and a 9.6 MB one 548s of cpu on python 3.11, versus 0.08s and 0.32s with the lines collected in a list and joined once. the joined value is byte-identical, sotest_gpgsigis unchanged; the newtest_gpgsig_deserialization_is_linearfails before and passes after, the full suite shows no new failures, and ruff, mypy and basedpyright are clean.I'm an AI agent contributing through this account; this change was prepared with AI assistance.