os: add and use errProcessReleased

This error is already used in three places, so let's define it.

Change-Id: I73565d94aebcf3d5a278201d96839d82db85a2d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/702436
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/os/exec.go

@@ -14,8 +14,12 @@ import (
 	"time"
 )
 
-// ErrProcessDone indicates a [Process] has finished.
-var ErrProcessDone = errors.New("os: process already finished")
+var (
+	// ErrProcessDone indicates a [Process] has finished.
+	ErrProcessDone = errors.New("os: process already finished")
+	// errProcessReleased indicates a [Process] has been released.
+	errProcessReleased = errors.New("os: process already released")
+)
 
 // processStatus describes the status of a [Process].
 type processStatus uint32

src/os/exec_unix.go

@@ -92,7 +92,7 @@ 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")
+		return errProcessReleased
 	}
 	if p.Pid == pidUnset {
 		return errors.New("os: process not initialized")
@@ -105,7 +105,7 @@ func (p *Process) pidSignal(s syscall.Signal) error {
 	case statusDone:
 		return ErrProcessDone
 	case statusReleased:
-		return errors.New("os: process already released")
+		return errProcessReleased
 	}
 
 	return convertESRCH(syscall.Kill(p.Pid, s))

src/os/pidfd_linux.go

@@ -16,7 +16,6 @@
 package os
 
 import (
-	"errors"
 	"internal/syscall/unix"
 	"runtime"
 	"sync"
@@ -130,7 +129,7 @@ func (p *Process) pidfdSendSignal(s syscall.Signal) error {
 	case statusDone:
 		return ErrProcessDone
 	case statusReleased:
-		return errors.New("os: process already released")
+		return errProcessReleased
 	}
 	defer p.handleTransientRelease()