cmd/go: more informative test failures when GOROOT is stale

If GOROOT is stale, test fail when commands unexpectedly write to GOROOT.
Include an message in the test failure indicating that this is a possible
and expected reason for the failure, and how to fix it.

For #48698.

Change-Id: I057c20260bab09aebf684e8f20794ab8fc0ede1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/448895
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>

diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
index 4f451e614b707d2b91e1a51b262bb3b56a25eb9c..eebc3266c89cc816077fc13e3c344bba0d92b775 100644 (file)
--- a/src/cmd/go/go_test.go
+++ b/src/cmd/go/go_test.go
@@ -129,7 +129,8 @@ func TestMain(m *testing.M) {
                                        }
                                        callerPos = fmt.Sprintf("%s:%d: ", file, line)
                                }
-                               return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s)", callerPos, filepath.Join("GOROOT", rel))
+                               notice := "This error error can occur if GOROOT is stale, in which case rerunning make.bash will fix it."
+                               return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s) (%v)", callerPos, filepath.Join("GOROOT", rel), notice)
                        }
                }