bundle2: extract logic for seeking bundle2 part into own class

bundle2: extract logic for seeking bundle2 part into own class

Currently, unbundlepart classes support bi-directional seeking.
Most consumers of unbundlepart only ever seek forward - typically
as part of moving to the end of the bundle part so they can move
on to the next one. But regardless of the actual usage of the
part, instances maintain an index mapping offsets within the
underlying raw payload to offsets within the decoded payload.

Maintaining the mapping of offset data can be expensive in terms of
memory use. Furthermore, many bundle2 consumers don't have access
to an underlying seekable stream. This includes all compressed
bundles. So maintaining offset data when the underlying stream
can't be seeked anyway is wasteful. And since many bundle2 streams
can't be seeked, it seems like a bad idea to expose a seek API
in bundle2 parts by default. If you provide them, people will
attempt to use them.

Seekable bundle2 parts should be the exception, not the rule. This
commit starts the process dividing unbundlepart into 2 classes: a
base class that supports linear, one-time reads and a child class
that supports bi-directional seeking. In this first commit, we
split various methods and attributes out into a new
"seekableunbundlepart" class. Previous instantiators of "unbundlepart"
now instantiate "seekableunbundlepart." This preserves backwards
compatibility. The coupling between the classes is still tight:
"unbundlepart" cannot be used on its own. This will be addressed
in subsequent commits.

Differential Revision: https://phab.mercurial-scm.org/D1386

--- a/mercurial/bundle2.py	Mon Nov 13 19:20:34 2017 -0800
+++ b/mercurial/bundle2.py	Mon Nov 13 19:22:11 2017 -0800
@@ -854,7 +854,7 @@
         indebug(self.ui, 'start extraction of bundle2 parts')
         headerblock = self._readpartheader()
         while headerblock is not None:
-            part = unbundlepart(self.ui, headerblock, self._fp)
+            part = seekableunbundlepart(self.ui, headerblock, self._fp)
             yield part
             # Seek to the end of the part to force it's consumption so the next
             # part can be read. But then seek back to the beginning so the
@@ -1155,7 +1155,7 @@
         if headerblock is None:
             indebug(self.ui, 'no part found during interruption.')
             return
-        part = unbundlepart(self.ui, headerblock, self._fp)
+        part = seekableunbundlepart(self.ui, headerblock, self._fp)
         op = interruptoperation(self.ui)
         hardabort = False
         try:
@@ -1207,10 +1207,8 @@
         self.advisoryparams = None
         self.params = None
         self.mandatorykeys = ()
-        self._payloadstream = None
         self._readheader()
         self._mandatory = None
-        self._chunkindex = [] #(payload, file) position tuples for chunk starts
         self._pos = 0
 
     def _fromheader(self, size):
@@ -1237,46 +1235,6 @@
         self.params.update(self.advisoryparams)
         self.mandatorykeys = frozenset(p[0] for p in mandatoryparams)
 
-    def _payloadchunks(self, chunknum=0):
-        '''seek to specified chunk and start yielding data'''
-        if len(self._chunkindex) == 0:
-            assert chunknum == 0, 'Must start with chunk 0'
-            self._chunkindex.append((0, self._tellfp()))
-        else:
-            assert chunknum < len(self._chunkindex), \
-                   'Unknown chunk %d' % chunknum
-            self._seekfp(self._chunkindex[chunknum][1])
-
-        pos = self._chunkindex[chunknum][0]
-        payloadsize = self._unpack(_fpayloadsize)[0]
-        indebug(self.ui, 'payload chunk size: %i' % payloadsize)
-        while payloadsize:
-            if payloadsize == flaginterrupt:
-                # interruption detection, the handler will now read a
-                # single part and process it.
-                interrupthandler(self.ui, self._fp)()
-            elif payloadsize < 0:
-                msg = 'negative payload chunk size: %i' %  payloadsize
-                raise error.BundleValueError(msg)
-            else:
-                result = self._readexact(payloadsize)
-                chunknum += 1
-                pos += payloadsize
-                if chunknum == len(self._chunkindex):
-                    self._chunkindex.append((pos, self._tellfp()))
-                yield result
-            payloadsize = self._unpack(_fpayloadsize)[0]
-            indebug(self.ui, 'payload chunk size: %i' % payloadsize)
-
-    def _findchunk(self, pos):
-        '''for a given payload position, return a chunk number and offset'''
-        for chunk, (ppos, fpos) in enumerate(self._chunkindex):
-            if ppos == pos:
-                return chunk, 0
-            elif ppos > pos:
-                return chunk - 1, pos - self._chunkindex[chunk - 1][0]
-        raise ValueError('Unknown chunk')
-
     def _readheader(self):
         """read the header and setup the object"""
         typesize = self._unpackheader(_fparttypesize)[0]
@@ -1328,6 +1286,69 @@
             self.consumed = True
         return data
 
+class seekableunbundlepart(unbundlepart):
+    """A bundle2 part in a bundle that is seekable.
+
+    Regular ``unbundlepart`` instances can only be read once. This class
+    extends ``unbundlepart`` to enable bi-directional seeking within the
+    part.
+
+    Bundle2 part data consists of framed chunks. Offsets when seeking
+    refer to the decoded data, not the offsets in the underlying bundle2
+    stream.
+
+    To facilitate quickly seeking within the decoded data, instances of this
+    class maintain a mapping between offsets in the underlying stream and
+    the decoded payload. This mapping will consume memory in proportion
+    to the number of chunks within the payload (which almost certainly
+    increases in proportion with the size of the part).
+    """
+    def __init__(self, ui, header, fp):
+        # (payload, file) offsets for chunk starts.
+        self._chunkindex = []
+
+        super(seekableunbundlepart, self).__init__(ui, header, fp)
+
+    def _payloadchunks(self, chunknum=0):
+        '''seek to specified chunk and start yielding data'''
+        if len(self._chunkindex) == 0:
+            assert chunknum == 0, 'Must start with chunk 0'
+            self._chunkindex.append((0, self._tellfp()))
+        else:
+            assert chunknum < len(self._chunkindex), \
+                   'Unknown chunk %d' % chunknum
+            self._seekfp(self._chunkindex[chunknum][1])
+
+        pos = self._chunkindex[chunknum][0]
+        payloadsize = self._unpack(_fpayloadsize)[0]
+        indebug(self.ui, 'payload chunk size: %i' % payloadsize)
+        while payloadsize:
+            if payloadsize == flaginterrupt:
+                # interruption detection, the handler will now read a
+                # single part and process it.
+                interrupthandler(self.ui, self._fp)()
+            elif payloadsize < 0:
+                msg = 'negative payload chunk size: %i' %  payloadsize
+                raise error.BundleValueError(msg)
+            else:
+                result = self._readexact(payloadsize)
+                chunknum += 1
+                pos += payloadsize
+                if chunknum == len(self._chunkindex):
+                    self._chunkindex.append((pos, self._tellfp()))
+                yield result
+            payloadsize = self._unpack(_fpayloadsize)[0]
+            indebug(self.ui, 'payload chunk size: %i' % payloadsize)
+
+    def _findchunk(self, pos):
+        '''for a given payload position, return a chunk number and offset'''
+        for chunk, (ppos, fpos) in enumerate(self._chunkindex):
+            if ppos == pos:
+                return chunk, 0
+            elif ppos > pos:
+                return chunk - 1, pos - self._chunkindex[chunk - 1][0]
+        raise ValueError('Unknown chunk')
+
     def tell(self):
         return self._pos