cmd/go: module loader fails to resolve imports within symlinked source file

[zx2c4]

This succeeds:

mkdir source-files && cd source-files
cp /path/to/somewhere/file1.go file1.go
cp /path/to/somewhere/file2.go file2.go
cp /path/to/somewhere/file3.go file3.go
cp /path/to/somewhere/go.mod go.mod
cp /path/to/somewhere/go.sum go.sum
go build

However, this fails:

mkdir source-files && cd source-files
ln -s /path/to/somewhere/file1.go file1.go
ln -s /path/to/somewhere/file2.go file2.go
ln -s /path/to/somewhere/file3.go file3.go
ln -s /path/to/somewhere/go.mod go.mod
ln -s /path/to/somewhere/go.sum go.sum
go build
[...FAILURES...]

It seems like go's module system might be trying to resolve things relative to the symlink target and not the actual symlink, which makes all sorts of nice overlay arrangements impossible unless you either hardlink (ew) or copy (yuck).

Here's a full sequence to reproduce

zx2c4@thinkpad ~ $ cd /tmp
zx2c4@thinkpad /tmp $ mkdir blah
zx2c4@thinkpad /tmp $ cd blah/
zx2c4@thinkpad /tmp/blah $ export GOPATH=$PWD/gopath
zx2c4@thinkpad /tmp/blah $ printf 'package main\nimport "golang.org/x/sys/unix"\nfunc main() { unix.Exit(0) }\n' > blah.go
zx2c4@thinkpad /tmp/blah $ go mod init blah
go: creating new go.mod: module blah
zx2c4@thinkpad /tmp/blah $ go build
go: finding golang.org/x/sys/unix latest
go: finding golang.org/x/sys latest
go: downloading golang.org/x/sys v0.0.0-20181005133103-4497e2df6f9e
zx2c4@thinkpad /tmp/blah $ ./blah
zx2c4@thinkpad /tmp/blah $ chmod -R +w gopath && rm -rf gopath
zx2c4@thinkpad /tmp/blah $ mkdir symlink
zx2c4@thinkpad /tmp/blah $ cd symlink
zx2c4@thinkpad /tmp/blah/symlink $ export GOPATH=$PWD/gopath
zx2c4@thinkpad /tmp/blah/symlink $ ln -s ../go.mod ../go.sum ../blah.go .
zx2c4@thinkpad /tmp/blah/symlink $ ls -l *
lrwxrwxrwx 1 zx2c4 zx2c4 10 Oct 10 06:27 blah.go -> ../blah.go
lrwxrwxrwx 1 zx2c4 zx2c4  9 Oct 10 06:27 go.mod -> ../go.mod
lrwxrwxrwx 1 zx2c4 zx2c4  9 Oct 10 06:27 go.sum -> ../go.sum
zx2c4@thinkpad /tmp/blah/symlink $ go build
go: finding golang.org/x/sys v0.0.0-20181005133103-4497e2df6f9e
blah.go:2:8: unknown import path "golang.org/x/sys/unix": cannot find package

[agnivade]

@bcmills @myitcv

[bcmills]

Thanks for the straightforward reproducer. The problem seems to be the .go source file.

~/go/src/cmd/go$ pushd $(mktemp -d)
/tmp/tmp.0t9zEAmI9v ~/go/src/cmd/go

$ export GOPATH=$PWD/gopath

$ printf 'package main\nimport "golang.org/x/sys/unix"\nfunc main() { unix.Exit(0) }\n' > blah.go

$ go mod init blah
go: creating new go.mod: module blah

$ go build
go: finding golang.org/x/sys/unix latest
go: finding golang.org/x/sys latest
go: downloading golang.org/x/sys v0.0.0-20181005133103-4497e2df6f9e

$ ./blah

$ mkdir symlink

$ cd symlink

symlink$ export GOPATH=$PWD/gopath

symlink$ ln -s ../go.mod ../go.sum ../blah.go .

symlink$ go build
go: finding golang.org/x/sys v0.0.0-20181005133103-4497e2df6f9e
blah.go:2:8: unknown import path "golang.org/x/sys/unix": cannot find package

symlink$ rm blah.go

symlink$ cp ../blah.go ./

symlink$ go build
go: downloading golang.org/x/sys v0.0.0-20181005133103-4497e2df6f9e

symlink$

[gopherbot]

Change https://golang.org/cl/141097 mentions this issue: cmd/go/internal/imports: resolve symlinks in ScanDir

[zx2c4]

@bcmills I'm not too certain on the cause of this, but broadly speaking, shouldn't the fix involve resolving fewer symlinks, not more?

[bcmills]

Nope. The bug turned out to be in how we detect whether an entry within the package directory is a file or a directory: in order to do that for a symlink, we have to first resolve it.

[zx2c4]

👍

[gopherbot]

Change https://golang.org/cl/163517 mentions this issue: cmd/go/internal/imports: use the full path to resolve symlinks

[katiehockman]

@gopherbot please open a backport to 1.12. This is a low risk patch that has enough benefits to warrant a cherrypick.

[gopherbot]

Backport issue(s) opened: #31763 (for 1.12).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[gopherbot]

Change https://golang.org/cl/175441 mentions this issue: [release-branch.go1.12] cmd/go/internal/imports: use the full path to resolve symlinks