runtime: when disabling SIGPROF handler, ignore SIGPROF

If the runtime disables the SIGPROF handler, because this is Go code
that is linked into a non-Go program, then don't go back to the
default handling of SIGPROF; just start ignoring SIGPROF.
Otherwise the program can get killed by a stray SIGPROF that is
delivered, presumably to a different thread, after profiling is disabled.

Fixes #19320

Change-Id: Ifebae477d726699c8c82c867604b73110c1cf262
Reviewed-on: https://go-review.googlesource.com/c/go/+/200740
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/runtime/signal_unix.go

@@ -242,10 +242,26 @@ func setProcessCPUProfiler(hz int32) {
 		}
 	} else {
 		// If the Go signal handler should be disabled by default,
-		// disable it if it is enabled.
+		// switch back to the signal handler that was installed
+		// when we enabled profiling. We don't try to handle the case
+		// of a program that changes the SIGPROF handler while Go
+		// profiling is enabled.
+		//
+		// If no signal handler was installed before, then start
+		// ignoring SIGPROF signals. We do this, rather than change
+		// to SIG_DFL, because there may be a pending SIGPROF
+		// signal that has not yet been delivered to some other thread.
+		// If we change to SIG_DFL here, the program will crash
+		// when that SIGPROF is delivered. We assume that programs
+		// that use profiling don't want to crash on a stray SIGPROF.
+		// See issue 19320.
 		if !sigInstallGoHandler(_SIGPROF) {
 			if atomic.Cas(&handlingSig[_SIGPROF], 1, 0) {
-				setsig(_SIGPROF, atomic.Loaduintptr(&fwdSig[_SIGPROF]))
+				h := atomic.Loaduintptr(&fwdSig[_SIGPROF])
+				if h == _SIG_DFL {
+					h = _SIG_IGN
+				}
+				setsig(_SIGPROF, h)
 			}
 		}
 	}