make NewBufRead etc. idempotent

R=rsc
DELTA=63  (59 added, 0 deleted, 4 changed)
OCL=27143
CL=27143

src/lib/bufio.go

@@ -54,12 +54,18 @@ type BufRead struct {
 }
 
 // NewBufReadSize creates a new BufRead whose buffer has the specified size,
-// which must be greater than zero.
+// which must be greater than zero.  If the argument io.Read is already a
+// BufRead with large enough size, it returns the underlying BufRead.
 // It returns the BufRead and any error.
-func NewBufReadSize(rd io.Read, size int) (b *BufRead, err *os.Error) {
+func NewBufReadSize(rd io.Read, size int) (*BufRead, *os.Error) {
 	if size <= 0 {
 		return nil, BadBufSize
 	}
+	// Is it already a BufRead?
+	b, ok := rd.(*BufRead);
+	if ok && len(b.buf) >= size {
+		return b, nil
+	}
 	b = new(BufRead);
 	b.buf = make([]byte, size);
 	b.rd = rd;
@@ -381,12 +387,18 @@ type BufWrite struct {
 }
 
 // NewBufWriteSize creates a new BufWrite whose buffer has the specified size,
-// which must be greater than zero.
+// which must be greater than zero. If the argument io.Write is already a
+// BufWrite with large enough size, it returns the underlying BufWrite.
 // It returns the BufWrite and any error.
-func NewBufWriteSize(wr io.Write, size int) (b *BufWrite, err *os.Error) {
+func NewBufWriteSize(wr io.Write, size int) (*BufWrite, *os.Error) {
 	if size <= 0 {
 		return nil, BadBufSize
 	}
+	// Is it already a BufWrite?
+	b, ok := wr.(*BufWrite);
+	if ok && len(b.buf) >= size {
+		return b, nil
+	}
 	b = new(BufWrite);
 	b.buf = make([]byte, size);
 	b.wr = wr;

src/lib/bufio_test.go

@@ -330,3 +330,50 @@ func TestBufWrite(t *testing.T) {
 	}
 }
 
+func TestNewBufReadSizeIdempotent(t *testing.T) {
+	const BufSize = 1000;
+	b, err := NewBufReadSize(newByteReader(io.StringBytes("hello world")), BufSize);
+	if err != nil {
+		t.Error("NewBufReadSize create fail", err);
+	}
+	// Does it recognize itself?
+	b1, err2 := NewBufReadSize(b, BufSize);
+	if err2 != nil {
+		t.Error("NewBufReadSize #2 create fail", err2);
+	}
+	if b1 != b {
+		t.Error("NewBufReadSize did not detect underlying BufRead");
+	}
+	// Does it wrap if existing buffer is too small?
+	b2, err3 := NewBufReadSize(b, 2*BufSize);
+	if err3 != nil {
+		t.Error("NewBufReadSize #3 create fail", err3);
+	}
+	if b2 == b {
+		t.Error("NewBufReadSize did not enlarge buffer");
+	}
+}
+
+func TestNewBufWriteSizeIdempotent(t *testing.T) {
+	const BufSize = 1000;
+	b, err := NewBufWriteSize(newByteWriter(), BufSize);
+	if err != nil {
+		t.Error("NewBufWriteSize create fail", err);
+	}
+	// Does it recognize itself?
+	b1, err2 := NewBufWriteSize(b, BufSize);
+	if err2 != nil {
+		t.Error("NewBufWriteSize #2 create fail", err2);
+	}
+	if b1 != b {
+		t.Error("NewBufWriteSize did not detect underlying BufWrite");
+	}
+	// Does it wrap if existing buffer is too small?
+	b2, err3 := NewBufWriteSize(b, 2*BufSize);
+	if err3 != nil {
+		t.Error("NewBufWriteSize #3 create fail", err3);
+	}
+	if b2 == b {
+		t.Error("NewBufWriteSize did not enlarge buffer");
+	}
+}