Fix Unpacker.tell() (#427)

Fixes #426.

Co-authored-by: folz <joachim.folz@dfki.de>

2 files changed, 16 insertions, 9 deletions

diff --git a/msgpack/_unpacker.pyx b/msgpack/_unpacker.pyx
index 43c93a2..4340e04 100644
--- a/msgpack/_unpacker.pyx
+++ b/msgpack/_unpacker.pyx
@@ -484,8 +484,10 @@ cdef class Unpacker(object):
         nread = min(self.buf_tail - self.buf_head, nbytes)
         ret = PyBytes_FromStringAndSize(self.buf + self.buf_head, nread)
         self.buf_head += nread
-        if len(ret) < nbytes and self.file_like is not None:
-            ret += self.file_like.read(nbytes - len(ret))
+        if nread < nbytes and self.file_like is not None:
+            ret += self.file_like.read(nbytes - nread)
+            nread = len(ret)
+        self.stream_offset += nread
         return ret
 
     def unpack(self):
@@ -519,6 +521,10 @@ cdef class Unpacker(object):
         return self._unpack(read_map_header)
 
     def tell(self):
+        """Returns the current position of the Unpacker in bytes, i.e., the
+        number of bytes that were read from the input, also the starting
+        position of the next object.
+        """
         return self.stream_offset
 
     def __iter__(self):
diff --git a/msgpack/fallback.py b/msgpack/fallback.py
index 9f6665b..1e0bbe9 100644
--- a/msgpack/fallback.py
+++ b/msgpack/fallback.py
@@ -365,18 +365,19 @@ class Unpacker(object):
         return self._buffer[self._buff_i :]
 
     def read_bytes(self, n):
-        ret = self._read(n)
+        ret = self._read(n, raise_outofdata=False)
         self._consume()
         return ret
 
-    def _read(self, n):
+    def _read(self, n, raise_outofdata=True):
         # (int) -> bytearray
-        self._reserve(n)
+        self._reserve(n, raise_outofdata=raise_outofdata)
         i = self._buff_i
-        self._buff_i = i + n
-        return self._buffer[i : i + n]
+        ret = self._buffer[i : i + n]
+        self._buff_i = i + len(ret)
+        return ret
 
-    def _reserve(self, n):
+    def _reserve(self, n, raise_outofdata=True):
         remain_bytes = len(self._buffer) - self._buff_i - n
 
         # Fast path: buffer has n bytes already
@@ -404,7 +405,7 @@ class Unpacker(object):
             self._buffer += read_data
             remain_bytes -= len(read_data)
 
-        if len(self._buffer) < n + self._buff_i:
+        if len(self._buffer) < n + self._buff_i and raise_outofdata:
             self._buff_i = 0  # rollback
             raise OutOfData