cmd/go: deflake TestScript/test2json_interrupt

- Start handling signals in 'go test' just before starting the test
  subprocess instead of just after. (It is unlikely that starting the
  process will cause cmd/go to hang in a way that requires signals to
  debug, and it is possible that something the test does — such as
  sending os.Interrupt to its parent processes — will immediately
  send a signal that needs to be handled.)

- In the test-test, don't try to re-parse the parent PIDs after
  sending signals, and sleep for a much shorter time interval.
  (Overrunning the sleep caused the next call to strconv.Atoi — which
  shouldn't even happen! — to fail with a parse error, leading to the
  failure mode observed in
  https://build.golang.org/log/f0982dcfc6a362f9c737eec3e7072dcc7ef29e32.)

Fixes #56083.
Updates #53563.

Change-Id: I346a95bdda5619632659ea854f98a9e17a6aede7
Reviewed-on: https://go-review.googlesource.com/c/go/+/456115
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/test/test.go

@@ -1306,6 +1306,7 @@ func (r *runTestActor) Act(b *work.Builder, ctx context.Context, a *work.Action)
 		cmd.Env = env
 	}
 
+	base.StartSigHandlers()
 	t0 := time.Now()
 	err = cmd.Start()
 
@@ -1314,7 +1315,6 @@ func (r *runTestActor) Act(b *work.Builder, ctx context.Context, a *work.Action)
 	// running.
 	if err == nil {
 		tick := time.NewTimer(testKillTimeout)
-		base.StartSigHandlers()
 		done := make(chan error)
 		go func() {
 			done <- cmd.Wait()

src/cmd/go/testdata/script/test2json_interrupt.txt

@@ -7,8 +7,8 @@ stdout -count=1 '"Action":"pass","Package":"example","Test":"FuzzInterrupt"'
 stdout -count=1 '"Action":"pass","Package":"example","Elapsed":'
 
 mkdir $WORK/fuzzcache
-go test -c . -fuzz=. -o test2json_interrupt_obj
-? go tool test2json -p example -t ./test2json_interrupt_obj -test.v -test.paniconexit0 -test.fuzzcachedir $WORK/fuzzcache -test.fuzz FuzzInterrupt -test.run '^$' -test.parallel 1
+go test -c . -fuzz=. -o example_test.exe
+? go tool test2json -p example -t ./example_test.exe -test.v -test.paniconexit0 -test.fuzzcachedir $WORK/fuzzcache -test.fuzz FuzzInterrupt -test.run '^$' -test.parallel 1
 stdout -count=1 '"Action":"pass","Package":"example","Test":"FuzzInterrupt"'
 stdout -count=1 '"Action":"pass","Package":"example","Elapsed":'
 
@@ -37,7 +37,9 @@ func FuzzInterrupt(f *testing.F) {
 		os.Setenv("GO_TEST_INTERRUPT_PIDS", fmt.Sprintf("%d,%d", ppid, pid))
 	}
 
+	sentInterrupt := false
 	f.Fuzz(func(t *testing.T, orig string) {
+		if !sentInterrupt {
 			// Simulate a ctrl-C on the keyboard by sending SIGINT
 			// to the main test process and its parent.
 			for _, pid := range strings.Split(pids, ",") {
@@ -47,9 +49,10 @@ func FuzzInterrupt(f *testing.F) {
 				}
 				if p, err := os.FindProcess(i); err == nil {
 					p.Signal(os.Interrupt)
-				time.Sleep(10 * time.Millisecond)
-				pids = ""  // Only interrupt once.
+					sentInterrupt = true // Only send interrupts once.
 				}
 			}
+		}
+		time.Sleep(1 * time.Millisecond)  // Delay the fuzzer a bit to avoid wasting CPU.
 	})
 }