cmd/dist: "moved GOROOT" test failing on plan9/386 builder

[0intro]

CL 48550 added "moved GOROOT" test, which is failing on the plan9/386 builder. The plan9/386 builder is running as a buildlet on GCE, while the other builders are old-style builders.

##### moved GOROOT
go: cannot find GOROOT directory: /tmp/workdir/go

See https://build.golang.org/log/6fc6155f74eeff35e8a05ce78ab0531a4f9018bf

[0intro]

The test is passing on the old-style builders because the GO_BUILDER_NAME environment variable hasn't been set on these builders, so the "moved GOROOT" test is skipped.

[ianlancetaylor]

I will disable the test, but ideally someone with a plan9 environment should figure out why it is failing. The error suggests that on Plan 9 the default GOROOT is overriding the one computed on the basis of where the cmd/go executable is found.

[gopherbot]

CL https://golang.org/cl/48650 mentions this issue.

[0intro]

Thanks. I'm currently investigating the issue on my side.

[0intro]

I think I figured out the issue.

In findGOROOT, the os.Executable function returns /bin/go. This is not the command being executed by exec.Command (/tmp/workdir_moved/bin/go), but since I did bind -a /tmp/workdir/bin /bin, this is also a valid path for the go tool (returned by fd2path on /proc/<pid>/text). However, it's not possible to recover the GOROOT from this path.

Since the go tool is called directly from the tests, it should be accessible from the $path environment variable. On Plan 9, $path usually only contains "/bin" and "." and the directories containing binaries are bound to the /bin directory.