os: export errFinished as ErrProcessDone

(*Process).Signal returns an error sentinel, previously errFinished,
when (*Process).done or syscall.ESRCH. Callers would like the ability to
test for this state, so the value has been exported as ErrProcessDone.

Fixes #39444

Change-Id: I510e7647cc032af290180de5149f35ab7b09a526
Reviewed-on: https://go-review.googlesource.com/c/go/+/242998
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>

src/os/exec_unix.go

@@ -59,7 +59,8 @@ func (p *Process) wait() (ps *ProcessState, err error) {
 	return ps, nil
 }
 
-var errFinished = errors.New("os: process already finished")
+// ErrProcessDone indicates a Process has finished.
+var ErrProcessDone = errors.New("os: process already finished")
 
 func (p *Process) signal(sig Signal) error {
 	if p.Pid == -1 {
@@ -71,7 +72,7 @@ func (p *Process) signal(sig Signal) error {
 	p.sigMu.RLock()
 	defer p.sigMu.RUnlock()
 	if p.done() {
-		return errFinished
+		return ErrProcessDone
 	}
 	s, ok := sig.(syscall.Signal)
 	if !ok {
@@ -79,7 +80,7 @@ func (p *Process) signal(sig Signal) error {
 	}
 	if e := syscall.Kill(p.Pid, s); e != nil {
 		if e == syscall.ESRCH {
-			return errFinished
+			return ErrProcessDone
 		}
 		return e
 	}

src/os/exec_unix_test.go

@@ -0,0 +1,29 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package os_test
+
+import (
+	"internal/testenv"
+	. "os"
+	"testing"
+)
+
+func TestErrProcessDone(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	path, err := testenv.GoTool()
+	if err != nil {
+		t.Errorf("finding go tool: %v", err)
+	}
+	p, err := StartProcess(path, []string{"go"}, &ProcAttr{})
+	if err != nil {
+		t.Errorf("starting test process: %v", err)
+	}
+	p.Wait()
+	if got := p.Signal(Kill); got != ErrProcessDone {
+		t.Errorf("got %v want %v", got, ErrProcessDone)
+	}
+}