From 4d7d55330522f43472e8637c5f9a01778dea0f3a Mon Sep 17 00:00:00 2001
From: Jan Pipek <jan.pipek@gmail.com>
Date: Fri, 13 Sep 2019 14:26:37 +0200
Subject: CachingStreamWrapperTestCase

---
 pyasn1/codec/ber/decoder.py     |  2 +-
 tests/codec/ber/test_decoder.py | 51 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/pyasn1/codec/ber/decoder.py b/pyasn1/codec/ber/decoder.py
index cfdea7a..caf9c09 100644
--- a/pyasn1/codec/ber/decoder.py
+++ b/pyasn1/codec/ber/decoder.py
@@ -63,7 +63,7 @@ class _CachingStreamWrapper(IOBase):
         read_from_cache = self._cache.read(n)
         if n != -1:
             n -= len(read_from_cache)
-            if n <= 0:
+            if not n:  # 0 bytes left to read
                 return read_from_cache
 
         read_from_raw = self._raw.read(n)
diff --git a/tests/codec/ber/test_decoder.py b/tests/codec/ber/test_decoder.py
index 7b233b8..e72e025 100644
--- a/tests/codec/ber/test_decoder.py
+++ b/tests/codec/ber/test_decoder.py
@@ -1735,6 +1735,57 @@ class CompressedFilesTestCase(BaseTestCase):
             os.remove(path)
 
 
+class CachingStreamWrapperTestCase(BaseTestCase):
+    def setUp(self):
+        self.shortText = b"abcdefghij"
+        self.longText = self.shortText * (io.DEFAULT_BUFFER_SIZE * 5)
+        self.shortStream = io.BytesIO(self.shortText)
+        self.longStream = io.BytesIO(self.longText)
+
+    def testReadJustFromCache(self):
+        wrapper = decoder._CachingStreamWrapper(self.shortStream)
+        wrapper.read(6)
+        wrapper.seek(3)
+        assert wrapper.read(1) == b"d"
+        assert wrapper.read(1) == b"e"
+        assert wrapper.tell() == 5
+
+    def testReadFromCacheAndStream(self):
+        wrapper = decoder._CachingStreamWrapper(self.shortStream)
+        wrapper.read(6)
+        wrapper.seek(3)
+        assert wrapper.read(4) == b"defg"
+        assert wrapper.tell() == 7
+
+    def testReadJustFromStream(self):
+        wrapper = decoder._CachingStreamWrapper(self.shortStream)
+        assert wrapper.read(6) == b"abcdef"
+        assert wrapper.tell() == 6
+
+    def testPeek(self):
+        wrapper = decoder._CachingStreamWrapper(self.longStream)
+        read_bytes = wrapper.peek(io.DEFAULT_BUFFER_SIZE + 73)
+        assert len(read_bytes) == io.DEFAULT_BUFFER_SIZE + 73
+        assert read_bytes.startswith(b"abcdefg")
+        assert wrapper.tell() == 0
+        assert wrapper.read(4) == b"abcd"
+
+    def testMarkedPositionResets(self):
+        wrapper = decoder._CachingStreamWrapper(self.longStream)
+        wrapper.read(10)
+        wrapper._markedPosition = wrapper.tell()
+        assert wrapper._markedPosition == 10
+
+        # Reach the maximum capacity of cache
+        wrapper.read(io.DEFAULT_BUFFER_SIZE)
+        assert wrapper.tell() == 10 + io.DEFAULT_BUFFER_SIZE
+
+        # The following should clear the cache
+        wrapper._markedPosition = wrapper.tell()
+        assert wrapper._markedPosition == 0
+        assert len(wrapper._cache.getvalue()) == 0
+
+
 suite = unittest.TestLoader().loadTestsFromModule(sys.modules[__name__])
 
 if __name__ == '__main__':
-- 
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