Add a standard Seeker interface.

R=rsc
APPROVED=rsc
DELTA=35  (30 added, 4 deleted, 1 changed)
OCL=33491
CL=33498

src/pkg/archive/tar/reader.go

@@ -91,16 +91,12 @@ func (ignoreWriter) Write(b []byte) (n int, err os.Error) {
 	return len(b), nil
 }
 
-type seeker interface {
-	Seek(offset int64, whence int) (ret int64, err os.Error);
-}
-
 // Skip any unread bytes in the existing file entry, as well as any alignment padding.
 func (tr *Reader) skipUnread() {
 	nr := tr.nb + tr.pad;	// number of bytes to skip
 
 	var n int64;
-	if sr, ok := tr.r.(seeker); ok {
+	if sr, ok := tr.r.(io.Seeker); ok {
 		n, tr.err = sr.Seek(nr, 1);
 	} else {
 		n, tr.err = io.Copyn(tr.r, ignoreWriter{}, nr);

src/pkg/io/io.go

@@ -59,6 +59,17 @@ type Closer interface {
 	Close() os.Error;
 }
 
+// Seeker is the interface that wraps the basic Seek method.
+//
+// Seek sets the offset for the next Read or Write to offset,
+// interpreted according to whence: 0 means relative to the origin of
+// the file, 1 means relative to the current offset, and 2 means
+// relative to the end.  Seek returns the new offset and an Error, if
+// any.
+type Seeker interface {
+	Seek(offset int64, whence int) (ret int64, err os.Error);
+}
+
 // ReadWrite is the interface that groups the basic Read and Write methods.
 type ReadWriter interface {
 	Reader;
@@ -84,6 +95,25 @@ type ReadWriteCloser interface {
 	Closer;
 }
 
+// ReadSeeker is the interface that groups the basic Read and Seek methods.
+type ReadSeeker interface {
+	Reader;
+	Seeker;
+}
+
+// WriteSeeker is the interface that groups the basic Write and Seek methods.
+type WriteSeeker interface {
+	Writer;
+	Seeker;
+}
+
+// ReadWriteSeeker is the interface that groups the basic Read, Write and Seek methods.
+type ReadWriteSeeker interface {
+	Reader;
+	Writer;
+	Seeker;
+}
+
 // WriteString writes the contents of the string s to w, which accepts an array of bytes.
 func WriteString(w Writer, s string) (n int, err os.Error) {
 	return w.Write(strings.Bytes(s))