os: start processes with posix_spawn on linux

src/os/exec_linux.go

@@ -35,15 +35,7 @@ func (p *Process) release() error {
 	return nil
 }
 
-// This function is a wrapper around the forkExec function, which is a wrapper around the fork and execve system calls.
-// The StartProcess function creates a new process by forking the current process and then calling execve to replace the current process with the new process.
-// It thereby replaces the newly created process with the specified command and arguments.
-// Differences to upstream golang implementation (https://cs.opensource.google/go/go/+/master:src/syscall/exec_unix.go;l=143):
-// * No setting of Process Attributes
-// * Ignoring Ctty
-// * No ForkLocking (might be introduced by #4273)
-// * No parent-child communication via pipes (TODO)
-// * No waiting for crashes child processes to prohibit zombie process accumulation / Wait status checking (TODO)
+// forkExec starts argv0 with posix_spawn.
 func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
 	if argv == nil {
 		return 0, errors.New("exec: no argv")
@@ -53,45 +45,25 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 		return 0, errors.New("exec: no argv")
 	}
 
+	return spawn(argv0, argv, attr.Env)
+}
+
+// startProcess does not support ProcAttr.Dir, Files or Sys yet.
+func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err error) {
 	if attr == nil {
-		attr = new(ProcAttr)
+		return nil, errors.New("exec: no ProcAttr")
 	}
 
-	p, err := fork()
-	pid = int(p)
-
-	if err != nil {
-		return 0, err
+	if attr.Dir != "" {
+		return nil, ErrNotImplementedDir
 	}
 
-	// else code runs in child, which then should exec the new process
-	err = execve(argv0, argv, attr.Env)
-	if err != nil {
-		// exec failed
-		return 0, err
+	if attr.Sys != nil {
+		return nil, ErrNotImplementedSys
 	}
-	// 3. TODO: use pipes to communicate back child status
-	return pid, nil
-}
-
-// In Golang, the idiomatic way to create a new process is to use the StartProcess function.
-// Since the Model of operating system processes in tinygo differs from the one in Golang, we need to implement the StartProcess function differently.
-// The startProcess function is a wrapper around the forkExec function, which is a wrapper around the fork and execve system calls.
-// The StartProcess function creates a new process by forking the current process and then calling execve to replace the current process with the new process.
-// It thereby replaces the newly created process with the specified command and arguments.
-func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err error) {
-	if attr != nil {
-		if attr.Dir != "" {
-			return nil, ErrNotImplementedDir
-		}
-
-		if attr.Sys != nil {
-			return nil, ErrNotImplementedSys
-		}
 
-		if len(attr.Files) != 0 {
-			return nil, ErrNotImplementedFiles
-		}
+	if len(attr.Files) != 0 {
+		return nil, ErrNotImplementedFiles
 	}
 
 	pid, err := forkExec(name, argv, attr)

src/os/exec_linux_test.go

@@ -5,6 +5,7 @@ package os_test
 import (
 	"errors"
 	. "os"
+	"path/filepath"
 	"runtime"
 	"syscall"
 	"testing"
@@ -67,7 +68,7 @@ func TestForkExecProcSys(t *testing.T) {
 }
 
 func TestForkExecProcFiles(t *testing.T) {
-	proc, err := StartProcess("/bin/echo", []string{"hello", "world"}, &ProcAttr{Files: []*File{}})
+	proc, err := StartProcess("/bin/echo", []string{"hello", "world"}, &ProcAttr{Files: []*File{Stdin, Stdout, Stderr}})
 	if !errors.Is(err, ErrNotImplementedFiles) {
 		t.Fatalf("wanted ErrNotImplementedFiles, got %v\n", err)
 	}
@@ -76,3 +77,37 @@ func TestForkExecProcFiles(t *testing.T) {
 		t.Fatalf("wanted nil, got %v\n", proc)
 	}
 }
+
+func TestForkExecNilAttr(t *testing.T) {
+	proc, err := StartProcess("/bin/echo", []string{"hello", "world"}, nil)
+	if err == nil {
+		t.Fatal("wanted an error for a nil ProcAttr")
+	}
+
+	if proc != nil {
+		t.Fatalf("wanted nil, got %v\n", proc)
+	}
+}
+
+// The caller keeps running and the command runs once.
+func TestForkExecParentContinues(t *testing.T) {
+	out := filepath.Join(t.TempDir(), "out")
+	proc, err := StartProcess("/bin/sh", []string{"sh", "-c", "echo child >> " + out}, &ProcAttr{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var ws syscall.WaitStatus
+	if _, err := syscall.Wait4(proc.Pid, &ws, 0, nil); err != nil {
+		t.Fatal(err)
+	}
+	if !ws.Exited() || ws.ExitStatus() != 0 {
+		t.Fatalf("child did not exit cleanly: %v", ws)
+	}
+	data, err := ReadFile(out)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(data) != "child\n" {
+		t.Fatalf("child output = %q, want %q", data, "child\n")
+	}
+}

src/os/osexec.go

@@ -7,39 +7,16 @@ import (
 	"unsafe"
 )
 
-func fork() (pid int32, err error) {
-	pid = libc_fork()
-	if pid != 0 {
-		if errno := *libc_errno(); errno != 0 {
-			err = syscall.Errno(*libc_errno())
-		}
+// spawn starts path with argv and envv and returns the pid of the new process.
+func spawn(path string, argv, envv []string) (int, error) {
+	pathp := cstring(path)
+	argvp := cstrings(argv)
+	envp := cstrings(envv)
+	var pid int32
+	if errno := libc_posix_spawn(&pid, &pathp[0], nil, nil, &argvp[0], &envp[0]); errno != 0 {
+		return 0, syscall.Errno(errno)
 	}
-	return
-}
-
-// the golang standard library does not expose interfaces for execve and fork, so we define them here the same way via the libc wrapper
-func execve(pathname string, argv []string, envv []string) error {
-	argv0 := cstring(pathname)
-
-	// transform argv and envv into the format expected by execve
-	argv1 := make([]*byte, len(argv)+1)
-	for i, arg := range argv {
-		argv1[i] = &cstring(arg)[0]
-	}
-	argv1[len(argv)] = nil
-
-	env1 := make([]*byte, len(envv)+1)
-	for i, env := range envv {
-		env1[i] = &cstring(env)[0]
-	}
-	env1[len(envv)] = nil
-
-	ret, _, err := syscall.Syscall(syscall.SYS_EXECVE, uintptr(unsafe.Pointer(unsafe.SliceData(argv0))), uintptr(unsafe.Pointer(unsafe.SliceData(argv1))), uintptr(unsafe.Pointer(unsafe.SliceData(env1))))
-	if int(ret) != 0 {
-		return err
-	}
-
-	return nil
+	return int(pid), nil
 }
 
 func cstring(s string) []byte {
@@ -49,10 +26,17 @@ func cstring(s string) []byte {
 	return data
 }
 
-//export fork
-func libc_fork() int32
+// cstrings returns ss as a nil terminated array of C strings.
+func cstrings(ss []string) []*byte {
+	p := make([]*byte, len(ss)+1)
+	for i, s := range ss {
+		p[i] = &cstring(s)[0]
+	}
+	return p
+}
 
-// Internal musl function to get the C errno pointer.
+// posix_spawn returns an error number instead of setting errno, see
+// https://pubs.opengroup.org/onlinepubs/9799919799/functions/posix_spawn.html
 //
-//export __errno_location
-func libc_errno() *int32
+//export posix_spawn
+func libc_posix_spawn(pid *int32, path *byte, fileActions, attrp unsafe.Pointer, argv, envp **byte) int32