os: TempDir(): obey TMPDIR on unix, TMP on win, etc

Uses upstream go code, lightly adjusted.

Adds smoke test.

src/os/file.go

@@ -1,3 +1,7 @@
+// Portions copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 // Package os implements a subset of the Go "os" package. See
 // https://godoc.org/os for details.
 //
@@ -260,7 +264,14 @@ func Readlink(name string) (string, error) {
 	return name, nil
 }
 
-// TempDir is a stub (for now), always returning the string "/tmp"
+// TempDir returns the default directory to use for temporary files.
+//
+// On Unix systems, it returns $TMPDIR if non-empty, else /tmp.
+// On Windows, it uses GetTempPath, returning the first non-empty
+// value from %TMP%, %TEMP%, %USERPROFILE%, or the Windows directory.
+//
+// The directory is neither guaranteed to exist nor have accessible
+// permissions.
 func TempDir() string {
-	return "/tmp"
+	return tempDir()
 }

src/os/file_anyos_test.go

@@ -0,0 +1,25 @@
+package os_test
+
+import (
+	. "os"
+	"testing"
+)
+
+// TestTempDir assumes that the test environment has a filesystem with a working temp dir.
+func TestTempDir(t *testing.T) {
+	name := TempDir() + "/_os_test_TestTempDir"
+	Remove(name)
+	f, err := OpenFile(name, O_RDWR|O_CREATE, 0644)
+	if err != nil {
+		t.Errorf("OpenFile %s: %s", name, err)
+		return
+	}
+	err = f.Close()
+	if err != nil {
+		t.Errorf("Close %s: %s", name, err)
+	}
+	err = Remove(name)
+	if err != nil {
+		t.Errorf("Remove %s: %s", name, err)
+	}
+}

src/os/file_other.go

@@ -56,3 +56,7 @@ func putchar(c byte)
 func Pipe() (r *File, w *File, err error) {
 	return nil, nil, ErrNotImplemented
 }
+
+func tempDir() string {
+	return "/tmp"
+}

src/os/file_unix.go

@@ -1,5 +1,9 @@
 // +build darwin linux,!baremetal
 
+// Portions copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package os
 
 import (
@@ -26,6 +30,14 @@ func Pipe() (r *File, w *File, err error) {
 	return
 }
 
+func tempDir() string {
+	dir := Getenv("TMPDIR")
+	if dir == "" {
+		dir = "/tmp"
+	}
+	return dir
+}
+
 // ReadAt reads up to len(b) bytes from the File starting at the given absolute offset.
 // It returns the number of bytes read and any error encountered, possibly io.EOF.
 // At end of file, Pread returns 0, io.EOF.

src/os/file_windows.go

@@ -1,8 +1,16 @@
+//go:build windows
 // +build windows
 
+// Portions copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package os
 
-import "syscall"
+import (
+	"syscall"
+	"unicode/utf16"
+)
 
 type syscallFd = syscall.Handle
 
@@ -23,6 +31,24 @@ func Pipe() (r *File, w *File, err error) {
 	return
 }
 
+func tempDir() string {
+	n := uint32(syscall.MAX_PATH)
+	for {
+		b := make([]uint16, n)
+		n, _ = syscall.GetTempPath(uint32(len(b)), &b[0])
+		if n > uint32(len(b)) {
+			continue
+		}
+		if n == 3 && b[1] == ':' && b[2] == '\\' {
+			// Do nothing for path, like C:\.
+		} else if n > 0 && b[n-1] == '\\' {
+			// Otherwise remove terminating \.
+			n--
+		}
+		return string(utf16.Decode(b[:n]))
+	}
+}
+
 // ReadAt reads up to len(b) bytes from the File starting at the given absolute offset.
 // It returns the number of bytes read and any error encountered, possibly io.EOF.
 // At end of file, Pread returns 0, io.EOF.