cmd/go: infinite loop when an import path starts with /

[heschi]

@leitzler reported an infinite loop in the go command when an import path starts with /. The stack trace implicates the loop that calls path.Dir in cmd/go/internal/modload.keepSums, which terminates on .. That assumes that the import path is "relative". If the path happens to start with a slash, the path is "absolute" and the fixed point for path.Dir is /, not ., and the loop is infinite.

cc @jayconrod, who added this code in https://golang.org/cl/262781. FWIW, when I wrote a version of this somewhere I did it such that it terminated when path.Dir(s) == s.

[gopherbot]

Change https://golang.org/cl/274172 mentions this issue: cmd/go: fix infinite loop in modload.keepSums