Improve S3 read performance. Fix #152 (#157)

* Resolve #152: perform a .get only when necessary

The previous implementation performed a .get every time when reading to
the buffer.  This was wasteful and unnecessary.  A single .get is
sufficient to read from the current position until the end of the file.
Another .get is only necessary when seeking.

* decrease default buffer size to 128kB

This matches the behavior of 1.5.3 more closely, as observed during live
tests against S3.

smart_open/s3.py

@@ -30,7 +30,7 @@
 """Allowed I/O modes for working with S3."""
 
 BINARY_NEWLINE = b'\n'
-DEFAULT_BUFFER_SIZE = 256 * 1024
+DEFAULT_BUFFER_SIZE = 128 * 1024
 
 
 def _range_string(start, stop=None):
@@ -87,26 +87,39 @@ def read(self, size=-1):
 
 
 class SeekableRawReader(object):
-    """Read an S3 object.
+    """Read an S3 object."""
 
-    Support seeking around, but is slower than RawReader."""
     def __init__(self, s3_object):
-        self.position = 0
         self._object = s3_object
         self._content_length = self._object.content_length
+        self.seek(0)
+
+    def seek(self, position):
+        """Seek to the specified position (byte offset) in the S3 key.
+
+        :param int position: The byte offset from the beginning of the key.
+        """
+        self._position = position
+        range_string = _range_string(self._position)
+        logger.debug('content_length: %r range_string: %r', self._content_length, range_string)
+        if position == self._content_length == 0 or position == self._content_length:
+            #
+            # When reading, we can't seek to the first byte of an empty file.
+            # Similarly, we can't seek past the last byte.  Do nothing here.
+            #
+            self._body = io.BytesIO()
+        else:
+            self._body = self._object.get(Range=range_string)['Body']
 
     def read(self, size=-1):
-        if self.position == self._content_length:
+        if self._position >= self._content_length:
             return b''
-        if size <= 0:
-            end = None
+        if size == -1:
+            binary = self._body.read()
         else:
-            end = min(self._content_length, self.position + size)
-        range_string = _range_string(self.position, stop=end)
-        logger.debug('range_string: %r', range_string)
-        body = self._object.get(Range=range_string)['Body'].read()
-        self.position += len(body)
-        return body
+            binary = self._body.read(size)
+        self._position += len(binary)
+        return binary
 
 
 class BufferedInputBase(io.BufferedIOBase):
@@ -290,9 +303,10 @@ def seek(self, offset, whence=START):
         else:
             new_position = self._content_length + offset
         new_position = _clamp(new_position, 0, self._content_length)
+        self._current_pos = new_position
+        self._raw_reader.seek(new_position)
+        logger.debug('new_position: %r', self._current_pos)
 
-        logger.debug('new_position: %r', new_position)
-        self._current_pos = self._raw_reader.position = new_position
         self._buffer = b""
         self._eof = self._current_pos == self._content_length
         return self._current_pos