From e27f97182e859fc6048ff13b028961da578dc340 Mon Sep 17 00:00:00 2001
From: Jan Pipek <jan.pipek@gmail.com>
Date: Tue, 10 Sep 2019 17:27:55 +0200
Subject: Implement _CachedStreamWrapper

---
 pyasn1/codec/ber/decoder.py     | 71 +++++++++++++++++++++++++++++++++++++----
 tests/codec/ber/test_decoder.py |  8 ++---
 2 files changed, 66 insertions(+), 13 deletions(-)

diff --git a/pyasn1/codec/ber/decoder.py b/pyasn1/codec/ber/decoder.py
index 0bd804c..b3a6c45 100644
--- a/pyasn1/codec/ber/decoder.py
+++ b/pyasn1/codec/ber/decoder.py
@@ -6,7 +6,7 @@
 #
 import os
 import sys
-from io import BytesIO, BufferedReader
+from io import BytesIO, BufferedReader, IOBase
 
 from pyasn1 import debug
 from pyasn1 import error
@@ -29,10 +29,68 @@ LOG = debug.registerLoggee(__name__, flags=debug.DEBUG_DECODER)
 noValue = base.noValue
 
 
-_BUFFER_SIZE = 1024
+_MAX_BUFFER_SIZE = 1024
 _PY2 = sys.version_info < (3,)
 
 
+class _CachedStreamWrapper(IOBase):
+    """Wrapper around non-seekable streams."""
+    def __init__(self, raw):
+        self._raw = raw
+        self._cache = BytesIO()
+        self._marked_position_ = 0
+
+    def peek(self, n):
+        pos = self._cache.tell()
+        result = self.read(n)
+        self._cache.seek(pos, os.SEEK_SET)
+        return result
+
+    def seekable(self):
+        return True
+
+    def seek(self, n=-1, whence=os.SEEK_SET):
+        return self._cache.seek(n, whence)
+
+    def read(self, n=-1):
+        read_from_cache = self._cache.read(n)
+        if n != -1:
+            n -= len(read_from_cache)
+        read_from_raw = self._raw.read(n)
+        self._cache.write(read_from_raw)
+        return read_from_cache + read_from_raw
+
+    @property
+    def _marked_position(self):
+        # This closely corresponds with how _marked_position attribute
+        # is manipulated with in Decoder.__call__ and (indefLen)ValueDecoder's
+        return self._marked_position_
+
+    @_marked_position.setter
+    def _marked_position(self, value):
+        self._marked_position_ = value
+        self.seek(value)
+        self.reset()
+
+    def tell(self):
+        return self._cache.tell()
+
+    def reset(self):
+        """Keep the buffered data reasonably large.
+
+        Whenever we se _marked_position, we know for sure
+        that we will not return back, and thus it is
+        safe to drop all cached data.
+        """
+        if self._cache.tell() > _MAX_BUFFER_SIZE:
+            current = self._cache.read()
+            self._cache.seek(0, os.SEEK_SET)
+            self._cache.truncate()
+            self._cache.write(current)
+            self._cache.seek(0, os.SEEK_SET)
+            self._marked_position_ = 0
+
+
 def asSeekableStream(substrate):
     """Convert object to seekable byte-stream.
 
@@ -54,13 +112,12 @@ def asSeekableStream(substrate):
     elif isinstance(substrate, univ.OctetString):
         return BytesIO(substrate.asOctets())
     try:
-        if _PY2 and isinstance(substrate, file):
-            return BytesIO(substrate.read())  # Not optimal for really large files
-        elif substrate.seekable():
+        if _PY2 and isinstance(substrate, file):  # Special case (it is not possible to set attributes)
+            return BufferedReader(substrate, _MAX_BUFFER_SIZE)
+        elif substrate.seekable():  # Will fail for most invalid types
             return substrate
         else:
-            # TODO: Implement for non-seekable streams
-            raise UnsupportedSubstrateError("Cannot use non-seekable bit stream: " + substrate.__class__.__name__)
+            return _CachedStreamWrapper(substrate)
     except AttributeError:
         raise UnsupportedSubstrateError("Cannot convert " + substrate.__class__.__name__ + " to a seekable bit stream.")
 
diff --git a/tests/codec/ber/test_decoder.py b/tests/codec/ber/test_decoder.py
index 0686c6d..141f7c7 100644
--- a/tests/codec/ber/test_decoder.py
+++ b/tests/codec/ber/test_decoder.py
@@ -1715,12 +1715,8 @@ class CompressedFilesTestCase(BaseTestCase):
 
             with zipfile.ZipFile(path, "r") as myzip:
                 with myzip.open("data", "r") as source:
-                    if sys.version_info < (3,):
-                        with self.assertRaises(UnsupportedSubstrateError):
-                            _ = list(decoder.decodeStream(source))
-                    else:
-                        values = list(decoder.decodeStream(source))
-                        assert values == [12, (1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 0, 1)]
+                    values = list(decoder.decodeStream(source))
+                    assert values == [12, (1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 0, 1)]
         finally:
             os.remove(path)
 
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