zstd: Shorten checksum code (#795)

frameDec.consumeCRC is now inlineable. Its callers already check
for ignoreChecksum. frameDec.checkCRC's callers already check for
HasCheckSum and ignoreChecksum.

xxhash.Digest.Write is documented as always returning len(b), nil,
so no need to check its return values.

zstd/decoder.go

@@ -455,12 +455,7 @@ func (d *Decoder) nextBlock(blocking bool) (ok bool) {
 	}
 
 	if len(next.b) > 0 {
-		n, err := d.current.crc.Write(next.b)
-		if err == nil {
-			if n != len(next.b) {
-				d.current.err = io.ErrShortWrite
-			}
-		}
+		d.current.crc.Write(next.b)
 	}
 	if next.err == nil && next.d != nil && next.d.hasCRC {
 		got := uint32(d.current.crc.Sum64())

zstd/framedec.go

@@ -293,24 +293,16 @@ func (d *frameDec) next(block *blockDec) error {
 	return nil
 }
 
-// checkCRC will check the checksum if the frame has one.
+// checkCRC will check the checksum, assuming the frame has one.
 // Will return ErrCRCMismatch if crc check failed, otherwise nil.
 func (d *frameDec) checkCRC() error {
-	if !d.HasCheckSum {
-		return nil
-	}
-
 	// We can overwrite upper tmp now
 	buf, err := d.rawInput.readSmall(4)
 	if err != nil {
 		println("CRC missing?", err)
 		return err
 	}
 
-	if d.o.ignoreChecksum {
-		return nil
-	}
-
 	want := binary.LittleEndian.Uint32(buf[:4])
 	got := uint32(d.crc.Sum64())
 
@@ -326,17 +318,13 @@ func (d *frameDec) checkCRC() error {
 	return nil
 }
 
-// consumeCRC reads the checksum data if the frame has one.
+// consumeCRC skips over the checksum, assuming the frame has one.
 func (d *frameDec) consumeCRC() error {
-	if d.HasCheckSum {
-		_, err := d.rawInput.readSmall(4)
-		if err != nil {
-			println("CRC missing?", err)
-			return err
-		}
+	_, err := d.rawInput.readSmall(4)
+	if err != nil {
+		println("CRC missing?", err)
 	}
-
-	return nil
+	return err
 }
 
 // runDecoder will run the decoder for the remainder of the frame.
@@ -415,15 +403,8 @@ func (d *frameDec) runDecoder(dst []byte, dec *blockDec) ([]byte, error) {
 			if d.o.ignoreChecksum {
 				err = d.consumeCRC()
 			} else {
-				var n int
-				n, err = d.crc.Write(dst[crcStart:])
-				if err == nil {
-					if n != len(dst)-crcStart {
-						err = io.ErrShortWrite
-					} else {
-						err = d.checkCRC()
-					}
-				}
+				d.crc.Write(dst[crcStart:])
+				err = d.checkCRC()
 			}
 		}
 	}