protocol: Add simple helper class for parsing a pktline stream.

This has some overlap with the pktline reading code in Protocol. The
implementation seemed simple enough not to worry about that, but I'd
love to hear ideas on how to share more code between them.

dulwich/protocol.py

@@ -406,3 +406,36 @@ class BufferedPktLineWriter(object):
             self._write(data)
         self._len = 0
         self._wbuf = StringIO()
+
+
+class PktLineParser(object):
+    """Packet line parser that hands completed packets off to a callback.
+    """
+
+    def __init__(self, handle_pkt):
+        self.handle_pkt = handle_pkt
+        self._readahead = StringIO()
+
+    def parse(self, data):
+        """Parse a fragment of data and call back for any completed packets.
+        """
+        self._readahead.write(data)
+        buf = self._readahead.getvalue()
+        if len(buf) < 4:
+            return
+        while len(buf) >= 4:
+            size = int(buf[:4], 16)
+            if size == 0:
+                self.handle_pkt(None)
+                buf = buf[4:]
+            elif size <= len(buf):
+                self.handle_pkt(buf[4:size])
+                buf = buf[size:]
+            else:
+                break
+        self._readahead = StringIO()
+        self._readahead.write(buf)
+
+    def get_tail(self):
+        """Read back any unused data."""
+        return self._readahead.getvalue()

dulwich/tests/test_protocol.py

@@ -25,6 +25,7 @@ from dulwich.errors import (
     HangupException,
     )
 from dulwich.protocol import (
+    PktLineParser,
     Protocol,
     ReceivableProtocol,
     extract_capabilities,
@@ -280,3 +281,29 @@ class BufferedPktLineWriterTests(TestCase):
         self._writer.write('z')
         self._writer.flush()
         self.assertOutputEquals('0005z')
+
+
+class PktLineParserTests(TestCase):
+
+    def test_none(self):
+        pktlines = []
+        parser = PktLineParser(pktlines.append)
+        parser.parse("0000")
+        self.assertEquals(pktlines, [None])
+        self.assertEquals("", parser.get_tail())
+
+    def test_small_fragments(self):
+        pktlines = []
+        parser = PktLineParser(pktlines.append)
+        parser.parse("00")
+        parser.parse("05")
+        parser.parse("z0000")
+        self.assertEquals(pktlines, ["z", None])
+        self.assertEquals("", parser.get_tail())
+
+    def test_multiple_packets(self):
+        pktlines = []
+        parser = PktLineParser(pktlines.append)
+        parser.parse("0005z0006aba")
+        self.assertEquals(pktlines, ["z", "ab"])
+        self.assertEquals("a", parser.get_tail())