borg serve: fix transmission data loss of pipe writes, fixes #1268

This problem was found on cygwin/windows due to its small pipe buffer size of 64kiB.

Due to that, bigger (like >64kiB) writes are always only partially done and os.write() returns
the amount of data that was actually sent. the code previously did not use that return value
and assumed that always all is sent, which led to a loss of the remainder of transmission data
and usually some "unexpected RPC data format" error on the client side.

Neither Linux nor *BSD ever do partial writes on blocking pipes, unless interrupted by a
signal, in which case serve() would terminate.

borg/remote.py

@@ -7,6 +7,7 @@ import shlex
 import sys
 import tempfile
 import textwrap
+import time
 from subprocess import Popen, PIPE
 
 from . import __version__
@@ -28,6 +29,23 @@ BUFSIZE = 10 * 1024 * 1024
 MAX_INFLIGHT = 100
 
 
+def os_write(fd, data):
+    """os.write wrapper so we do not lose data for partial writes."""
+    # This is happening frequently on cygwin due to its small pipe buffer size of only 64kiB
+    # and also due to its different blocking pipe behaviour compared to Linux/*BSD.
+    # Neither Linux nor *BSD ever do partial writes on blocking pipes, unless interrupted by a
+    # signal, in which case serve() would terminate.
+    amount = remaining = len(data)
+    while remaining:
+        count = os.write(fd, data)
+        remaining -= count
+        if not remaining:
+            break
+        data = data[count:]
+        time.sleep(count * 1e-09)
+    return amount
+
+
 class ConnectionClosed(Error):
     """Connection closed by remote host"""
 
@@ -106,7 +124,7 @@ class RepositoryServer:  # pragma: no cover
                     if self.repository is not None:
                         self.repository.close()
                     else:
-                        os.write(stderr_fd, "Borg {}: Got connection close before repository was opened.\n"
+                        os_write(stderr_fd, "Borg {}: Got connection close before repository was opened.\n"
                                  .format(__version__).encode())
                     return
                 unpacker.feed(data)
@@ -133,9 +151,9 @@ class RepositoryServer:  # pragma: no cover
                             logging.exception('Borg %s: exception in RPC call:', __version__)
                             logging.error(sysinfo())
                         exc = "Remote Exception (see remote log for the traceback)"
-                        os.write(stdout_fd, msgpack.packb((1, msgid, e.__class__.__name__, exc)))
+                        os_write(stdout_fd, msgpack.packb((1, msgid, e.__class__.__name__, exc)))
                     else:
-                        os.write(stdout_fd, msgpack.packb((1, msgid, None, res)))
+                        os_write(stdout_fd, msgpack.packb((1, msgid, None, res)))
             if es:
                 self.repository.close()
                 return