os: don't try to signal PID -1 on Unix

This restores behavior that we lost in CL 588675.

Fixes #68496

Change-Id: I1740986bed647835986d54109071b7a6b37413d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/599015
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>

src/os/exec_unix.go

@@ -103,6 +103,9 @@ func (p *Process) signal(sig Signal) error {
 }
 
 func (p *Process) pidSignal(s syscall.Signal) error {
+	if p.Pid == pidReleased {
+		return errors.New("os: process already released")
+	}
 	if p.Pid == pidUnset {
 		return errors.New("os: process not initialized")
 	}

src/os/exec_unix_test.go

@@ -40,7 +40,7 @@ func TestProcessAlreadyDone(t *testing.T) {
 		// EINVAL (see waitid in usr/src/uts/common/os/exit.c in
 		// illumos). This is configurable via sysconf(_SC_MAXPID), but
 		// we'll just take the default.
-		pid = 30000-1
+		pid = 30000 - 1
 	}
 
 	p, err := FindProcess(pid)
@@ -76,3 +76,14 @@ func TestUNIXProcessAlive(t *testing.T) {
 		t.Errorf("OS reported error for running process: %v", err)
 	}
 }
+
+func TestProcessBadPID(t *testing.T) {
+	p, err := FindProcess(-1)
+	if err != nil {
+		t.Fatalf("unexpected FindProcess error: %v", err)
+	}
+	err = p.Signal(syscall.Signal(0))
+	if err == nil {
+		t.Error("p.Signal succeeded unexpectedly")
+	}
+}