os, syscall: implement ReadAt for unix

Windows will take more work, so test is skipped there.

src/os/file.go

@@ -107,8 +107,15 @@ func (f *File) Read(b []byte) (n int, err error) {
 	return
 }
 
-func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
-	return 0, ErrNotImplemented
+// ReadAt reads up to len(b) bytes from the File at the given absolute offset.
+// It returns the number of bytes read and any error encountered, possible io.EOF.
+// At end of file, Read returns 0, io.EOF.
+func (f *File) ReadAt(b []byte, offset int64) (n int, err error) {
+	n, err = f.handle.ReadAt(b, offset)
+	if err != nil && err != io.EOF {
+		err = &PathError{"readat", f.name, err}
+	}
+	return
 }
 
 // Write writes len(b) bytes to the File. It returns the number of bytes written

src/os/file_other.go

@@ -27,6 +27,10 @@ func (f stdioFileHandle) Read(b []byte) (n int, err error) {
 	return 0, ErrUnsupported
 }
 
+func (f stdioFileHandle) ReadAt(b []byte, off int64) (n int, err error) {
+	return 0, ErrNotImplemented
+}
+
 // Write writes len(b) bytes to the output. It returns the number of bytes
 // written or an error if this file is not stdout or stderr.
 func (f stdioFileHandle) Write(b []byte) (n int, err error) {

src/os/file_unix.go

@@ -2,7 +2,10 @@
 
 package os
 
-import "syscall"
+import (
+	"io"
+	"syscall"
+)
 
 type syscallFd = int
 
@@ -22,3 +25,16 @@ func Pipe() (r *File, w *File, err error) {
 	}
 	return
 }
+
+// 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.
+// TODO: move to file_anyos once ReadAt is implemented for windows
+func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
+	n, err = syscall.Pread(syscallFd(f), b, offset)
+	err = handleSyscallError(err)
+	if n == 0 && err == nil {
+		err = io.EOF
+	}
+	return
+}

src/os/file_windows.go

@@ -22,3 +22,11 @@ func Pipe() (r *File, w *File, err error) {
 	}
 	return
 }
+
+// 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.
+// TODO: move to file_anyos once ReadAt is implemented for windows
+func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
+	return -1, ErrNotImplemented
+}

src/os/filesystem.go

@@ -46,6 +46,9 @@ type FileHandle interface {
 	// Read reads up to len(b) bytes from the file.
 	Read(b []byte) (n int, err error)
 
+	// ReadAt reads up to len(b) bytes from the file starting at the given absolute offset
+	ReadAt(b []byte, offset int64) (n int, err error)
+
 	// Write writes up to len(b) bytes to the file.
 	Write(b []byte) (n int, err error)
 

src/os/os_test.go

@@ -0,0 +1,48 @@
+// 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_test
+
+import (
+	"io"
+	. "os"
+	"runtime"
+	"testing"
+)
+
+// localTmp returns a local temporary directory not on NFS.
+func localTmp() string {
+	return TempDir()
+}
+
+func newFile(testName string, t *testing.T) (f *File) {
+	// TODO: use CreateTemp when it lands
+	f, err := OpenFile(TempDir()+"/_Go_"+testName, O_RDWR|O_CREATE, 0644)
+	if err != nil {
+		t.Fatalf("TempFile %s: %s", testName, err)
+	}
+	return
+}
+
+func TestReadAt(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("TODO: implement Pread for Windows")
+		return
+	}
+	f := newFile("TestReadAt", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	b := make([]byte, 5)
+	n, err := f.ReadAt(b, 7)
+	if err != nil || n != len(b) {
+		t.Fatalf("ReadAt 7: %d, %v", n, err)
+	}
+	if string(b) != "world" {
+		t.Fatalf("ReadAt 7: have %q want %q", string(b), "world")
+	}
+}