os: start processes with posix_spawn

Use posix_spawn on hosted Linux and Darwin. Map process files, apply the
working directory and process group, and clear the child signal mask.
Use wait4 for process status and support Kill and Signal.

Mark Darwin pipes close-on-exec under ForkLock. Darwin also needs the
fcntl wrapper in PR #5612 and the libSystem symbols in PR #5636.
Concurrent spawn and pipe creation need the RWMutex fix in PR #5630.

Keep the process stubs on other targets and add process regression tests.

Author: yohimik

src/os/exec.go

@@ -5,6 +5,9 @@ import (
 	"syscall"
 )
 
+// Errors StartProcess returns for a ProcAttr that it cannot honour. On a
+// hosted OS only ErrNotImplementedSys is reachable. The other two stay because
+// they are part of the exported API of this package.
 var (
 	ErrNotImplementedDir   = errors.New("directory setting not implemented")
 	ErrNotImplementedSys   = errors.New("sys setting not implemented")
@@ -36,35 +39,12 @@ type ProcAttr struct {
 // ErrProcessDone indicates a Process has finished.
 var ErrProcessDone = errors.New("os: process already finished")
 
-type ProcessState struct {
-}
-
-func (p *ProcessState) String() string {
-	return "" // TODO
-}
-func (p *ProcessState) Success() bool {
-	return false // TODO
-}
-
-// Sys returns system-dependent exit information about
-// the process. Convert it to the appropriate underlying
-// type, such as syscall.WaitStatus on Unix, to access its contents.
-func (p *ProcessState) Sys() interface{} {
-	return nil // TODO
-}
-
-func (p *ProcessState) Exited() bool {
-	return false // TODO
-}
-
-// ExitCode returns the exit code of the exited process, or -1
-// if the process hasn't exited or was terminated by a signal.
-func (p *ProcessState) ExitCode() int {
-	return -1 // TODO
-}
-
 type Process struct {
 	Pid int
+
+	// done reports whether Wait reaped this process. A signal to a reaped pid
+	// is unsafe, because the number can belong to an unrelated process.
+	done int32
 }
 
 // StartProcess starts a new process with the program, arguments and attributes specified by name, argv and attr.
@@ -73,21 +53,6 @@ func StartProcess(name string, argv []string, attr *ProcAttr) (*Process, error)
 	return startProcess(name, argv, attr)
 }
 
-func (p *Process) Wait() (*ProcessState, error) {
-	if p.Pid == -1 {
-		return nil, syscall.EINVAL
-	}
-	return nil, ErrNotImplemented
-}
-
-func (p *Process) Kill() error {
-	return ErrNotImplemented
-}
-
-func (p *Process) Signal(sig Signal) error {
-	return ErrNotImplemented
-}
-
 func Ignore(sig ...Signal) {
 	// leave all the signals unaltered
 	return

src/os/exec_linux.go

@@ -1,4 +1,4 @@
-//go:build (!aix && !android && !freebsd && !linux && !netbsd && !openbsd && !plan9 && !solaris) || baremetal || tinygo.wasm || nintendoswitch
+//go:build (!aix && !android && !darwin && !freebsd && !linux && !netbsd && !openbsd && !plan9 && !solaris) || baremetal || tinygo.wasm || nintendoswitch
 
 package os
 
@@ -18,6 +18,49 @@ func (p *Process) release() error {
 	return nil
 }
 
+// ProcessState is a placeholder on targets that have no process model.
+type ProcessState struct {
+}
+
+func (p *ProcessState) String() string {
+	return "" // TODO
+}
+func (p *ProcessState) Success() bool {
+	return false // TODO
+}
+
+// Sys returns system-dependent exit information about
+// the process. Convert it to the appropriate underlying
+// type, such as syscall.WaitStatus on Unix, to access its contents.
+func (p *ProcessState) Sys() interface{} {
+	return nil // TODO
+}
+
+func (p *ProcessState) Exited() bool {
+	return false // TODO
+}
+
+// ExitCode returns the exit code of the exited process, or -1
+// if the process hasn't exited or was terminated by a signal.
+func (p *ProcessState) ExitCode() int {
+	return -1 // TODO
+}
+
+func (p *Process) Wait() (*ProcessState, error) {
+	if p.Pid == -1 {
+		return nil, syscall.EINVAL
+	}
+	return nil, ErrNotImplemented
+}
+
+func (p *Process) Kill() error {
+	return ErrNotImplemented
+}
+
+func (p *Process) Signal(sig Signal) error {
+	return ErrNotImplemented
+}
+
 func forkExec(_ string, _ []string, _ *ProcAttr) (pid int, err error) {
 	return 0, ErrNotImplemented
 }