Go: trimpath doesn't work if working in a symlinked directory

[TyBrown]

Please does not seem to trimpath correctly when building within a directory that is symlinked. This is really easy to reproduce on MacOS because /tmp is really a symlink to /private/tmp.

Given the following:

Environment Info:

$ plz --version
Please version 15.11.0
$ go version
go version go1.15.5 darwin/amd64

$PLZREPO/tybrown_net/testing/BUILD

go_binary(
    name = "testing",
    srcs = glob(
        ["*.go"],
        exclude = ["*_test.go"],
    ),
)

$PLZREPO/tybrown_net/testing/main.go

package main

func testing() {
	panic("foo")
}

func main() {
	testing()
}

Results

Working in a standard directory

$ cd /private/tmp/plzrepo-copy
$ plz clean -f
$ plz build //tybrown_net/testing:testing
Build finished; total time 1.67s, incrementality 0.0%. Outputs:
//tybrown_net/testing:testing:
  plz-out/bin/tybrown_net/testing/testing
$ ./plz-out/bin/tybrown_net/testing/testing
panic: foo

goroutine 1 [running]:
main.testing(...)
	tybrown_net/testing/main.go:4
main.main()
	tybrown_net/testing/main.go:8 +0x39

Working in a symlinked directory

$ cd /tmp/plzrepo-copy
$ plz clean -f
$ plz build //tybrown_net/testing:testing
Build finished; total time 1.59s, incrementality 0.0%. Outputs:
//tybrown_net/testing:testing:
  plz-out/bin/tybrown_net/testing/testing
$ ./plz-out/bin/tybrown_net/testing/testing
panic: foo

goroutine 1 [running]:
main.testing(...)
	/private/tmp/plzrepo-copy/plz-out/tmp/tybrown_net/testing/_testing#lib._build/tybrown_net/testing/main.go:4
main.main()
	/private/tmp/plzrepo-copy/plz-out/tmp/tybrown_net/testing/_testing#lib._build/tybrown_net/testing/main.go:8 +0x39

[Tatskaari]

So everything on the Please side is actually correct. We're falling afoul of this issue: https://github.com/golang/go/issues/17198

Essentially we're setting the command directory to be $(os.Getwd())/plz-out/tmp/... where os.Getwd() is returning the unexpanded fake path from bash. os/exec doesn't have the same smarts.

One way to fix this would be to make Please follow the symlinks for the repo root using something like this: https://github.com/yookoala/realpath/blob/master/realpath.go

Seems quite awkward to fix... Is this causing you serious problems?

[TyBrown]

@Tatskaari Thanks for the reply... Not serious/critical for me by any means, it's a nice to have. Was confused as to why the stacktraces were so long/hard to read, and that the packages built locally on my Mac were not matching what was building in CI... this explained it. :)

[stale[bot]]

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[sagikazarmark]

Is it solved?

[Tatskaari]

No, but I suppose it could be with a little effort. Will revisit when I have v16 out the door. It's going to be quite slow to build in /tmp though isn't it? Please will have to do full copies because it can't hardlink files across filesystems. Is there a reason you want to build out of a symlinked dir?

[TyBrown]

The biggest reason I use a symlink is because I have a case-sensitive filesystem on my Mac for dealing with repos that require case-sensitivity, but prefer to access it at something like ~/repos instead of /Volumes/repos, which is where macOS really, really wants it to be mounted... so I just put a symlink at ~/repos -> /Volumes/repos and it seems to work most of the time. This was one of those times where it tripped me up a bit.

Like I said, not a huge issue... just breaks some of the hermetic build guarantees that I was expecting. There are workarounds, so probably low priority, just figured I'd put it on y'alls radar. :)