ssh: avoid reading beyond the end of stream when using compression

Compressed streams can be used as part of getbundle. The normal read()
operation of bufferedinputpipe will try to fulfill the request exactly
and can deadlock if the server sends less as it is done. At the same
time, the bundle2 logic will stop reading when it believes it has gotten
all parts of the bundle, which can leave behind end of stream markers as
used by bzip2 and zstd.

To solve this, introduce a new optional unbufferedread interface and
provided it in bufferedinputpipe and doublepipe. If there is buffered
data left, it will be returned, otherwise it will issue a single read
request and return whatever it obtains.

Reorganize the decompression handlers to try harder to read until the
end of stream, especially if the requested read can already be
fulfilled. Check for end of stream is messy with Python 2, none of the
standard compression modules properly exposes it. At least with zstd and
bzip2, decompressing will remember EOS and fail for empty input after
the EOS has been seen. For zlib, the only way to detect it with Python 2
is to duplicate the decompressobj and force some additional data into
it. The common handler can be further optimized, but works as PoC.

Differential Revision: https://phab.mercurial-scm.org/D3937

mercurial/sshpeer.py

@@ -99,6 +99,17 b' class doublepipe(object):'
             _forwardoutput(self._ui, self._side)
         return r
 
+    def unbufferedread(self, size):
+        r = self._call('unbufferedread', size)
+        if size != 0 and not r:
+            # We've observed a condition that indicates the
+            # stdout closed unexpectedly. Check stderr one
+            # more time and snag anything that's there before
+            # letting anyone know the main part of the pipe
+            # closed prematurely.
+            _forwardoutput(self._ui, self._side)
+        return r
+
     def readline(self):
         return self._call('readline')
 

mercurial/util.py

@@ -322,6 +322,11 b' class bufferedinputpipe(object):'
             self._fillbuffer()
         return self._frombuffer(size)
 
+    def unbufferedread(self, size):
+        if not self._eof and self._lenbuf == 0:
+            self._fillbuffer(max(size, _chunksize))
+        return self._frombuffer(min(self._lenbuf, size))
+
     def readline(self, *args, **kwargs):
         if 1 < len(self._buffer):
             # this should not happen because both read and readline end with a
@@ -363,9 +368,9 b' class bufferedinputpipe(object):'
             self._lenbuf = 0
         return data
 
-    def _fillbuffer(self):
+    def _fillbuffer(self, size=_chunksize):
         """read data to the buffer"""
-        data = os.read(self._input.fileno(), _chunksize)
+        data = os.read(self._input.fileno(), size)
         if not data:
             self._eof = True
         else:
@@ -3302,6 +3307,104 b' class compressionengine(object):'
         """
         raise NotImplementedError()
 
+class _CompressedStreamReader(object):
+    def __init__(self, fh):
+        if safehasattr(fh, 'unbufferedread'):
+            self._reader = fh.unbufferedread
+        else:
+            self._reader = fh.read
+        self._pending = []
+        self._pos = 0
+        self._eof = False
+
+    def _decompress(self, chunk):
+        raise NotImplementedError()
+
+    def read(self, l):
+        buf = []
+        while True:
+            while self._pending:
+                if len(self._pending[0]) > l + self._pos:
+                    newbuf = self._pending[0]
+                    buf.append(newbuf[self._pos:self._pos + l])
+                    self._pos += l
+                    return ''.join(buf)
+
+                newbuf = self._pending.pop(0)
+                if self._pos:
+                    buf.append(newbuf[self._pos:])
+                    l -= len(newbuf) - self._pos
+                else:
+                    buf.append(newbuf)
+                    l -= len(newbuf)
+                self._pos = 0
+
+            if self._eof:
+                return ''.join(buf)
+            chunk = self._reader(65536)
+            self._decompress(chunk)
+
+class _GzipCompressedStreamReader(_CompressedStreamReader):
+    def __init__(self, fh):
+        super(_GzipCompressedStreamReader, self).__init__(fh)
+        self._decompobj = zlib.decompressobj()
+    def _decompress(self, chunk):
+        newbuf = self._decompobj.decompress(chunk)
+        if newbuf:
+            self._pending.append(newbuf)
+        d = self._decompobj.copy()
+        try:
+            d.decompress('x')
+            d.flush()
+            if d.unused_data == 'x':
+                self._eof = True
+        except zlib.error:
+            pass
+
+class _BZ2CompressedStreamReader(_CompressedStreamReader):
+    def __init__(self, fh):
+        super(_BZ2CompressedStreamReader, self).__init__(fh)
+        self._decompobj = bz2.BZ2Decompressor()
+    def _decompress(self, chunk):
+        newbuf = self._decompobj.decompress(chunk)
+        if newbuf:
+            self._pending.append(newbuf)
+        try:
+            while True:
+                newbuf = self._decompobj.decompress('')
+                if newbuf:
+                    self._pending.append(newbuf)
+                else:
+                    break
+        except EOFError:
+            self._eof = True
+
+class _TruncatedBZ2CompressedStreamReader(_BZ2CompressedStreamReader):
+    def __init__(self, fh):
+        super(_TruncatedBZ2CompressedStreamReader, self).__init__(fh)
+        newbuf = self._decompobj.decompress('BZ')
+        if newbuf:
+            self._pending.append(newbuf)
+
+class _ZstdCompressedStreamReader(_CompressedStreamReader):
+    def __init__(self, fh, zstd):
+        super(_ZstdCompressedStreamReader, self).__init__(fh)
+        self._zstd = zstd
+        self._decompobj = zstd.ZstdDecompressor().decompressobj()
+    def _decompress(self, chunk):
+        newbuf = self._decompobj.decompress(chunk)
+        if newbuf:
+            self._pending.append(newbuf)
+        try:
+            while True:
+                newbuf = self._decompobj.decompress('')
+                if newbuf:
+                    self._pending.append(newbuf)
+                else:
+                    break
+        except self._zstd.ZstdError:
+            self._eof = True
+
 class _zlibengine(compressionengine):
     def name(self):
         return 'zlib'
@@ -3335,15 +3438,7 b' class _zlibengine(compressionengine):'
         yield z.flush()
 
     def decompressorreader(self, fh):
-        def gen():
-            d = zlib.decompressobj()
-            for chunk in filechunkiter(fh):
-                while chunk:
-                    # Limit output size to limit memory.
-                    yield d.decompress(chunk, 2 ** 18)
-                    chunk = d.unconsumed_tail
-
-        return chunkbuffer(gen())
+        return _GzipCompressedStreamReader(fh)
 
     class zlibrevlogcompressor(object):
         def compress(self, data):
@@ -3423,12 +3518,7 b' class _bz2engine(compressionengine):'
         yield z.flush()
 
     def decompressorreader(self, fh):
-        def gen():
-            d = bz2.BZ2Decompressor()
-            for chunk in filechunkiter(fh):
-                yield d.decompress(chunk)
-
-        return chunkbuffer(gen())
+        return _BZ2CompressedStreamReader(fh)
 
 compengines.register(_bz2engine())
 
@@ -3442,14 +3532,7 b' class _truncatedbz2engine(compressioneng'
     # We don't implement compressstream because it is hackily handled elsewhere.
 
     def decompressorreader(self, fh):
-        def gen():
-            # The input stream doesn't have the 'BZ' header. So add it back.
-            d = bz2.BZ2Decompressor()
-            d.decompress('BZ')
-            for chunk in filechunkiter(fh):
-                yield d.decompress(chunk)
-
-        return chunkbuffer(gen())
+        return _TruncatedBZ2CompressedStreamReader(fh)
 
 compengines.register(_truncatedbz2engine())
 
@@ -3544,9 +3627,7 b' class _zstdengine(compressionengine):'
         yield z.flush()
 
     def decompressorreader(self, fh):
-        zstd = self._module
-        dctx = zstd.ZstdDecompressor()
-        return chunkbuffer(dctx.read_from(fh))
+        return _ZstdCompressedStreamReader(fh, self._module)
 
     class zstdrevlogcompressor(object):
         def __init__(self, zstd, level=3):