Only add explicit -mod=vendor if needed (#2332)

* Only add explicit -mod=vendor if needed

-mod=vendor is not needed if it's already set in GOFLAGS. So lets not
set it unless it's needed.

This is an issue for commands such as `go test` and `go vet` (
see: golang/go#32471 ). This avois such an
issue.

* Add comments for -mod=vendor flag addition

This takes into us the suggestions from @estroz and adds the relevant comments for the addition of the -mod=vendor GOFLAG

Co-authored-by: Eric Stroczynski <[email protected]>

Author: JAORMX

internal/util/projutil/exec.go

@@ -124,7 +124,9 @@ func (opts GoCmdOptions) getGeneralArgsWithCmd(cmd string) ([]string, error) {
 		}
 		// Does the first "go" subcommand accept -mod=vendor?
 		_, ok := validVendorCmds[bargs[0]]
-		if err == nil && info.IsDir() && ok {
+		// TODO: remove needsModVendor when
+		// https://github.com/golang/go/issues/32471 is resolved.
+		if err == nil && info.IsDir() && ok && needsModVendor() {
 			bargs = append(bargs, "-mod=vendor")
 		}
 	}
@@ -135,6 +137,14 @@ func (opts GoCmdOptions) getGeneralArgsWithCmd(cmd string) ([]string, error) {
 	return bargs, nil
 }
 
+// needsModVendor resolves https://github.com/golang/go/issues/32471,
+// where any flags in GOFLAGS that are also set in the CLI are
+// duplicated, causing 'go' invocation errors.
+// TODO: remove once the issue is resolved.
+func needsModVendor() bool {
+	return !strings.Contains(os.Getenv("GOFLAGS"), "-mod=vendor")
+}
+
 func (opts GoCmdOptions) setCmdFields(c *exec.Cmd) {
 	c.Env = append(c.Env, os.Environ()...)
 	if len(opts.Env) != 0 {