testing: synchronize writes to the root's Writer

Prior to this change it was possible to see interleaved messages:
<<<
=== RUN   Test/LongLongLongLongName48
=== RUN   Test/LongLon=== RUN   Test/LongLongLongLongName50
gLongLongName49
=== RUN   Test/LongLongLongLongName51
>>>

This change fixes it such that you see:
<<<
=== RUN   Test/LongLongLongLongName48
=== RUN   Test/LongLongLongLongName49
=== RUN   Test/LongLongLongLongName50
=== RUN   Test/LongLongLongLongName51
>>>

Fixes #18741

Change-Id: I2529d724065dc65b3e9eb3d7cbeeda82a2d0cfd4
Reviewed-on: https://go-review.googlesource.com/35556
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/testing/testing.go b/src/testing/testing.go
index bd19a31c27..5efbc244fe 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -259,7 +259,7 @@ var (
 // common holds the elements common between T and B and
 // captures common methods such as Errorf.
 type common struct {
-	mu         sync.RWMutex // guards output, failed, and done.
+	mu         sync.RWMutex // guards output, w, failed, and done.
 	output     []byte       // Output generated by test or benchmark.
 	w          io.Writer    // For flushToParent.
 	chatty     bool         // A copy of the chatty flag.
@@ -687,7 +687,9 @@ func (t *T) Run(name string, f func(t *T)) bool {
 		root := t.parent
 		for ; root.parent != nil; root = root.parent {
 		}
+		root.mu.Lock()
 		fmt.Fprintf(root.w, "=== RUN   %s\n", t.name)
+		root.mu.Unlock()
 	}
 	// Instead of reducing the running count of this test before calling the
 	// tRunner and increasing it afterwards, we rely on tRunner keeping the