Suggested change, here, cribbing from the open code. Add:

_, _, err := fs.dirlookup(dp, elem)
if err == nil {
        return EEXIST
}

before the call to fs.dirlink() at line 639

@tenortim, do you want to send a change? (and include a new test)

@bradfitz Will do. Any pointers on how to test this (since it only lives in the NACL code)?

@tenortim, just add tests in package "net", or "os", or "syscall", wherever's convenient. You can protect it with runtime.GOOS == "nacl" if needed.

See the src/nacltest.bash test file for how to run the tests.

Change https://golang.org/cl/76110 mentions this issue: fix NaCl Link() syscall error handling (EEXIST)

Fix submitted.
Test added first and verified that it fails.
Fix added to fs_nacl.go. It's not tied to GOOS since it's a valid test regardless (though unlikely to fail on any other platform). The test is a modified copy of part of the existing rename tests.
Verified that the test passes both for the NaCl build and for linux x64.