os/exec: document method to check if a process is alive

Fixes golang#34396

src/os/exec.go

@@ -86,7 +86,9 @@ func Getppid() int { return syscall.Getppid() }
 // about the underlying operating system process.
 //
 // On Unix systems, FindProcess always succeeds and returns a Process
-// for the given pid, regardless of whether the process exists.
+// for the given pid, regardless of whether the process exists. To test whether
+// the process actually exists, see whether p.Signal(syscall.Signal(0)) reports
+// an error.
 func FindProcess(pid int) (*Process, error) {
 	return findProcess(pid)
 }

src/os/exec_unix_test.go

@@ -9,6 +9,7 @@ package os_test
 import (
 	"internal/testenv"
 	. "os"
+	"syscall"
 	"testing"
 )
 
@@ -25,3 +26,20 @@ func TestErrProcessDone(t *testing.T) {
 		t.Errorf("got %v want %v", got, ErrProcessDone)
 	}
 }
+
+func TestUNIXProcessAlive(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	t.Parallel()
+
+	p, err := StartProcess(testenv.GoToolPath(t), []string{"sleep", "1"}, &ProcAttr{})
+	if err != nil {
+		t.Skipf("starting test process: %v", err)
+	}
+	defer p.Kill()
+
+	proc, _ := FindProcess(p.Pid)
+	err = proc.Signal(syscall.Signal(0))
+	if err != nil {
+		t.Fail()
+	}
+}