os: release process handle at the end of windows (*Process).Wait

Fixes #3154.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5707052

src/pkg/net/http/triv.go

diff --git a/src/pkg/net/http/triv.go b/src/pkg/net/http/triv.go
index c88a0fb..269af0c 100644
--- a/src/pkg/net/http/triv.go
+++ b/src/pkg/net/http/triv.go
@@ -108,7 +108,6 @@
 		fmt.Fprintf(rw, "fork/exec: %s\n", err)
 		return
 	}
-	defer p.Release()
 	io.Copy(rw, r)
 	wait, err := p.Wait(0)
 	if err != nil {

src/pkg/os/doc.go

diff --git a/src/pkg/os/doc.go b/src/pkg/os/doc.go
index ef857c0..546f864 100644
--- a/src/pkg/os/doc.go
+++ b/src/pkg/os/doc.go
@@ -11,6 +11,13 @@
 	return findProcess(pid)
 }
 
+// Release releases any resources associated with the Process p,
+// rendering it unusable in the future.
+// Release only needs to be called if Wait is not.
+func (p *Process) Release() error {
+	return p.release()
+}
+
 // Hostname returns the host name reported by the kernel.
 func Hostname() (name string, err error) {
 	return hostname()

src/pkg/os/exec_plan9.go

diff --git a/src/pkg/os/exec_plan9.go b/src/pkg/os/exec_plan9.go
index 1c9e2b9..a941d12 100644
--- a/src/pkg/os/exec_plan9.go
+++ b/src/pkg/os/exec_plan9.go
@@ -94,8 +94,7 @@
 	return ps, nil
 }
 
-// Release releases any resources associated with the Process.
-func (p *Process) Release() error {
+func (p *Process) release() error {
 	// NOOP for Plan 9.
 	p.Pid = -1
 	// no need for a finalizer anymore

src/pkg/os/exec_unix.go

diff --git a/src/pkg/os/exec_unix.go b/src/pkg/os/exec_unix.go
index 8d000e9..3f89fe8 100644
--- a/src/pkg/os/exec_unix.go
+++ b/src/pkg/os/exec_unix.go
@@ -51,8 +51,7 @@
 	return nil
 }
 
-// Release releases any resources associated with the Process.
-func (p *Process) Release() error {
+func (p *Process) release() error {
 	// NOOP for unix.
 	p.Pid = -1
 	// no need for a finalizer anymore

src/pkg/os/exec_windows.go

diff --git a/src/pkg/os/exec_windows.go b/src/pkg/os/exec_windows.go
index dab0dc9..3d07ab7 100644
--- a/src/pkg/os/exec_windows.go
+++ b/src/pkg/os/exec_windows.go
@@ -14,6 +14,7 @@
 
 // Wait waits for the Process to exit or stop, and then returns a
 // ProcessState describing its status and an error, if any.
+// Wait releases any resources associated with the Process.
 func (p *Process) Wait() (ps *ProcessState, err error) {
 	s, e := syscall.WaitForSingleObject(syscall.Handle(p.handle), syscall.INFINITE)
 	switch s {
@@ -30,6 +31,7 @@
 		return nil, NewSyscallError("GetExitCodeProcess", e)
 	}
 	p.done = true
+	defer p.Release()
 	return &ProcessState{p.Pid, syscall.WaitStatus{Status: s, ExitCode: ec}, new(syscall.Rusage)}, nil
 }
 
@@ -46,8 +48,7 @@
 	return syscall.Errno(syscall.EWINDOWS)
 }
 
-// Release releases any resources associated with the Process.
-func (p *Process) Release() error {
+func (p *Process) release() error {
 	if p.handle == uintptr(syscall.InvalidHandle) {
 		return syscall.EINVAL
 	}

src/pkg/os/os_test.go

diff --git a/src/pkg/os/os_test.go b/src/pkg/os/os_test.go
index 02f75b2..d1e241f 100644
--- a/src/pkg/os/os_test.go
+++ b/src/pkg/os/os_test.go
@@ -530,7 +530,6 @@
 	if err != nil {
 		t.Fatalf("StartProcess: %v", err)
 	}
-	defer p.Release()
 	w.Close()
 
 	var b bytes.Buffer
@@ -848,7 +847,6 @@
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer p.Release()
 	w.Close()
 
 	var b bytes.Buffer