Fix freshness when linking is interrupted.

[ehuss]

Cargo was incorrectly determining a build is fresh in the following scenario.

1. cargo build
2. Modify source file.
3. cargo build.
4. Hit ctrl-c while the linker is running.
5. cargo run — Cargo will think it is "fresh" because the output file exists, but will attempt to run a corrupt file.

The solution here is to delete the fingerprint when something is determined to be dirty. There are a variety of other possible approaches, but this seemed the simplest.

I was only able to reproduce this with gcc on Linux.

I couldn't think of a good way to write a test for this. I did test this a bit locally, by using the repro steps above. I also did some tests with a custom linker that wrote out an empty file and then went to sleep so I could kill Cargo. I was reluctant to include a test using that technique because it needs to parse the linker command-line flags to find the output filename, and I'm not sure how stable/reliable that would be with different platforms and linker flavors. It is also a bit cumbersome to signal when linking has started.

Fixes #7767

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

Nice find! This is definitely a good fix to have.

I do think it'd be good if we could get a test for this though since it seems very easy to accidentally leave out in a refactoring. For signaling when the linker started I think a TCP server should work (the test waits for a connection and the linker hangs after making a connection). For identifying the output filename I think we could get something semi-robust by basically looking at all arguments that look like they're paths starting with the target directory, and make sure that all of those exist as files or directories?

[ehuss]

I'm glad you said something. The technique I was using to repro this wasn't valid (I was deleting the outputs instead of changing the mtime of the inputs). It'll need some more investigation to figure out the original issue.