diff: avoid fixed-size buffer for patch-ids
To generate a patch id, we format the diff header into a fixed-size buffer, and then feed the result to our sha1 computation. The fixed buffer has size '4*PATH_MAX + 20', which in theory accommodates the four filenames plus some extra data. Except: 1. The filenames may not be constrained to PATH_MAX. The static value may not be a real limit on the current filesystem. Moreover, we may compute patch-ids for names stored only in git, without touching the current filesystem at all. 2. The 20 bytes is not nearly enough to cover the extra content we put in the buffer. As a result, the data we feed to the sha1 computation may be truncated, and it's possible that a commit with a very long filename could erroneously collide in the patch-id space with another commit. For instance, if one commit modified "really-long-filename/foo" and another modified "bar" in the same directory. In practice this is unlikely. Because the filenames are repeated, and because there's a single cutoff at the end of the buffer, the offending filename would have to be on the order of four times larger than PATH_MAX. We could fix this by moving to a strbuf. However, we can observe that the purpose of formatting this in the first place is to feed it to git_SHA1_Update(). So instead, let's just feed each part of the formatted string directly. This actually ends up more readable, and we can even factor out some duplicated bits from the various conditional branches. Technically this may change the output of patch-id for very long filenames, but it's not worth making an exception for this in the --stable output. It was a bug, and one that only affected an unlikely set of paths. And anyway, the exact value would have varied from platform to platform depending on the value of PATH_MAX, so there is no "stable" value. Signed-off-by: NJeff King <peff@peff.net> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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