SFTP: add a comment about a subtle interaction between OverwriteableFileConsumer and GeneralSFTPFile, and test the case it is commenting on.

src/allmydata/frontends/sftpd.py

@@ -461,6 +461,12 @@ class OverwriteableFileConsumer(PrefixingLogMixin):
         The caller must perform no more overwrites until the Deferred has fired."""
 
         if noisy: self.log(".read(%r, %r), current_size = %r" % (offset, length, self.current_size), level=NOISY)
+
+        # Note that the overwrite method is synchronous. When a write request is processed
+        # (e.g. a writeChunk request on the async queue of GeneralSFTPFile), overwrite will
+        # be called and will update self.current_size if necessary before returning. Therefore,
+        # self.current_size will be up-to-date for a subsequent call to this read method, and
+        # so it is correct to do the check for a read past the end-of-file here.
         if offset >= self.current_size:
             def _eof(): raise EOFError("read past end of file")
             return defer.execute(_eof)

src/allmydata/test/test_sftp.py

@@ -913,6 +913,9 @@ class Handler(GridTestMixin, ShouldFailMixin, ReallyEqualMixin, unittest.TestCas
                       self.handler.openFile("small", sftp.FXF_READ | sftp.FXF_WRITE, {}))
         def _read_write(rwf):
             d2 = rwf.writeChunk(8, "0123")
+            # test immediate read starting after the old end-of-file
+            d2.addCallback(lambda ign: rwf.readChunk(11, 1))
+            d2.addCallback(lambda data: self.failUnlessReallyEqual(data, "3"))
             d2.addCallback(lambda ign: rwf.readChunk(0, 100))
             d2.addCallback(lambda data: self.failUnlessReallyEqual(data, "012345670123"))
             d2.addCallback(lambda ign: rwf.close())