syscall: implement Pipe2 on Linux and use it in ForkExec

Fixes #2656.

R=golang-dev, bradfitz, iant, minux.ma
CC=golang-dev
https://golang.org/cl/7062057

src/pkg/syscall/exec_bsd.go

@@ -221,3 +221,17 @@ childerror:
 	// and this shuts up the compiler.
 	panic("unreached")
 }
+
+// Try to open a pipe with O_CLOEXEC set on both file descriptors.
+func forkExecPipe(p []int) error {
+	err := Pipe(p)
+	if err != nil {
+		return err
+	}
+	_, err = fcntl(p[0], F_SETFD, FD_CLOEXEC)
+	if err != nil {
+		return err
+	}
+	_, err = fcntl(p[1], F_SETFD, FD_CLOEXEC)
+	return err
+}

src/pkg/syscall/exec_linux.go

@@ -233,3 +233,20 @@ childerror:
 	// and this shuts up the compiler.
 	panic("unreached")
 }
+
+// Try to open a pipe with O_CLOEXEC set on both file descriptors.
+func forkExecPipe(p []int) (err error) {
+	err = Pipe2(p, O_CLOEXEC)
+	// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
+	// might not be implemented.
+	if err == ENOSYS {
+		if err = Pipe(p); err != nil {
+			return
+		}
+		if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != nil {
+			return
+		}
+		_, err = fcntl(p[1], F_SETFD, FD_CLOEXEC)
+	}
+	return
+}

src/pkg/syscall/exec_unix.go

@@ -183,13 +183,7 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 	ForkLock.Lock()
 
 	// Allocate child status pipe close on exec.
-	if err = Pipe(p[0:]); err != nil {
-		goto error
-	}
-	if _, err = fcntl(p[0], F_SETFD, FD_CLOEXEC); err != nil {
-		goto error
-	}
-	if _, err = fcntl(p[1], F_SETFD, FD_CLOEXEC); err != nil {
+	if err = forkExecPipe(p[:]); err != nil {
 		goto error
 	}
 

src/pkg/syscall/syscall_linux.go

@@ -39,6 +39,18 @@ func Pipe(p []int) (err error) {
 	return
 }
 
+//sysnb pipe2(p *[2]_C_int, flags int) (err error)
+func Pipe2(p []int, flags int) (err error) {
+	if len(p) != 2 {
+		return EINVAL
+	}
+	var pp [2]_C_int
+	err = pipe2(&pp, flags)
+	p[0] = int(pp[0])
+	p[1] = int(pp[1])
+	return
+}
+
 //sys	utimes(path string, times *[2]Timeval) (err error)
 func Utimes(path string, tv []Timeval) (err error) {
 	if len(tv) != 2 {

src/pkg/syscall/zsyscall_linux_386.go

@@ -49,6 +49,16 @@ func pipe(p *[2]_C_int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func pipe2(p *[2]_C_int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_PIPE2, uintptr(unsafe.Pointer(p)), uintptr(flags), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func utimes(path string, times *[2]Timeval) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)

src/pkg/syscall/zsyscall_linux_amd64.go

@@ -49,6 +49,16 @@ func pipe(p *[2]_C_int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func pipe2(p *[2]_C_int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_PIPE2, uintptr(unsafe.Pointer(p)), uintptr(flags), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func utimes(path string, times *[2]Timeval) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)

src/pkg/syscall/zsyscall_linux_arm.go

@@ -49,6 +49,16 @@ func pipe(p *[2]_C_int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func pipe2(p *[2]_C_int, flags int) (err error) {
+	_, _, e1 := RawSyscall(SYS_PIPE2, uintptr(unsafe.Pointer(p)), uintptr(flags), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func utimes(path string, times *[2]Timeval) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)