Fix path::deep_dependencies_trigger_rebuild often failing in CI

[dwijnand]

A shallow fix for path::deep_dependencies_trigger_rebuild in particular as it's failed my PRs often.

See #5935 (comment), and #5940 for the bigger picture.

[rust-highfive]

r? @matklad

(rust_highfive has picked a reviewer for you, use r? to override)

[ehuss]

👍 This should fix this particular test.

[dwijnand]

r=ehuss?

[alexcrichton]

Hm is this the right fix? The test here is that if we change a file it'll cause a rebuild, but the sleep has to be before the file is edited to simulate time passing between the build and the edit?

[ehuss]

I'm not sure I understand. Any change that happens between builds should trigger a rebuild, regardless of when it is made. The problem is in this test:

1. Build foo/bar/baz
2. Modify baz
3. Build foo/bar/baz
4. Modify bar
5. Build foo/bar

If step 2 is done immediately before 3, the mtime of baz.rs is N and its output is also N. Then, in step 5, it checks baz and because N==N it recompiles baz when it shouldn't.

If you sleep after the files are modified, then the output mtime for baz will be N+1 in step 3, so it won't recompile baz in step 5 because N < N+1.

[alexcrichton]

Ah ok I think I misunderstood! I wrote up some more thoughts in #5940 about a longer-term solution, but otherwise this looks good!

@bors: r+

[bors]

📌 Commit 274c162 has been approved by alexcrichton

[bors]

⌛ Testing commit 274c162 with merge e954520...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing e954520 to master...