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update comment based on further research

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This commit is contained in:
Ralf Jung 2018-08-22 20:55:14 +02:00
parent 08843a0457
commit b1fbafd066
1 changed files with 17 additions and 4 deletions
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21
src/cargo/core/compiler/fingerprint.rs
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@ -744,10 +744,23 @@ where
return true;
}
};
// Equal mtimes could mean that the input was changed in that same second,
// but *after* the output was generated. So this means they are stale.
// In theory, cargo is using nanosecond precision throughout so this
// should not make a difference -- but it was necessary to fix #5918.
// Note that equal mtimes are considered "stale". For filesystems with
// not much timestamp precision like 1s this is a conservative approximation
// to handle the case where a file is modified within the same second after
// a build finishes. We want to make sure that incremental rebuilds pick that up!
//
// For filesystems with nanosecond precision it's been seen in the wild that
// its "nanosecond precision" isn't really nanosecond-accurate. It turns out that
// kernels may cache the current time so files created at different times actually
// list the same nanosecond precision. Some digging on #5919 picked up that the
// kernel caches the current time between timer ticks, which could mean that if
// a file is updated at most 10ms after a build finishes then Cargo may not
// pick up the build changes.
//
// All in all, the equality check here is a conservative assumption that,
// if equal, files were changed just after a previous build finished.
// It's hoped this doesn't cause too many issues in practice!
if mtime2 >= mtime {
info!("stale: {} -- {} vs {}", path.display(), mtime2, mtime);
true