do not start multipart upload for small writes

fs_s3fs/_s3fs_file.py

@@ -139,12 +139,12 @@ def __init__(
 
         self._object = s3_object
         self._min_part_size = min_part_size
-        self._mp = self._object.initiate_multipart_upload(**self._upload_kwargs)
-
-        self._buf = b''
+        self._mp = None
+        self._buf = bytearray()
         self._total_bytes = 0
         self._total_parts = 0
         self._parts = []
+        self._closed = False
 
         #
         # This member is part of the io.BufferedIOBase interface.
@@ -156,7 +156,7 @@ def flush(self):
 
     @property
     def closed(self):
-        return self._mp is None
+        return self._closed
 
     def writable(self):
         """Return True if the stream supports writing."""
@@ -180,10 +180,7 @@ def write(self, b):
         There's buffering happening under the covers, so this may not actually
         do any HTTP transfer right away."""
 
-        if self._buf:
-            self._buf += b
-        else:
-            self._buf = b
+        self._buf.extend(b)
 
         length = len(b)
         self._total_bytes += length
@@ -199,36 +196,41 @@ def close(self):
 
         if tuple(sys.exc_info()) != (None, None, None):
             self.terminate()
+            self._closed = True
             return
 
-        if self._buf:
-            self._upload_next_part()
-
-        if self._total_bytes:
-            self._mp.complete(MultipartUpload={"Parts": self._parts})
-            logger.debug("completed multipart upload")
-        else:
+        if self._total_bytes < self._min_part_size:
+            # if we wrote less than min_part_size bytes
+            # then directly put buffer contents instead of starting
+            # multipart upload. It also fixes following:
             #
             # AWS complains with "The XML you provided was not well-formed or
             # did not validate against our published schema" when the input is
             # completely empty => abort the upload, no file created.
             #
-            # We work around this by creating an empty file explicitly.
-            #
-            logger.debug("empty input, ignoring multipart upload")
-            self.terminate()
+            assert not self._mp
+            logger.debug("small input, ignoring multipart upload")
             self._object.put(Body=b"", **self._upload_kwargs)
+        else:
+            if self._buf:
+                self._upload_next_part()
+            self._mp.complete(MultipartUpload={"Parts": self._parts})
+            logger.debug("completed multipart upload")
+
         self._mp = None
+        self._closed = True
         logger.debug("successfully closed")
 
     @check_if_open
     def terminate(self):
-        """Cancel the underlying multipart upload."""
-        assert self._mp, "no multipart upload in progress"
-        self._mp.abort()
-        self._mp = None
+        """Cancel the underlying multipart upload if any"""
+        if self._mp:
+            self._mp.abort()
+            self._mp = None
 
     def _upload_next_part(self):
+        if not self._mp:
+            self._mp = self._object.initiate_multipart_upload(**self._upload_kwargs)
         part_num = self._total_parts + 1
         logger.info(
             "uploading part #%i, %i bytes (total %.3fGB)",
@@ -242,4 +244,4 @@ def _upload_next_part(self):
         logger.debug("upload of part #%i finished" % part_num)
 
         self._total_parts += 1
-        self._buf = bytes()
+        self._buf.clear()