migration: ensure qemu_fflush() always writes full data amount

The QEMUFile writev_buffer / put_buffer functions are expected
to write out the full set of requested data, blocking until
complete. The qemu_fflush() caller does not expect to deal with
partial writes. Clarify the function comments and add a sanity
check to the code to catch mistaken implementations.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <1461751518-12128-5-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
index 3f6b4ed..5909ff0 100644
--- a/include/migration/qemu-file.h
+++ b/include/migration/qemu-file.h
@@ -28,7 +28,8 @@
 
 /* This function writes a chunk of data to a file at the given position.
  * The pos argument can be ignored if the file is only being used for
- * streaming.  The handler should try to write all of the data it can.
+ * streaming.  The handler must write all of the data or return a negative
+ * errno value.
  */
 typedef ssize_t (QEMUFilePutBufferFunc)(void *opaque, const uint8_t *buf,
                                         int64_t pos, size_t size);
@@ -54,7 +55,8 @@ typedef int (QEMUFileCloseFunc)(void *opaque);
 typedef int (QEMUFileGetFD)(void *opaque);
 
 /*
- * This function writes an iovec to file.
+ * This function writes an iovec to file. The handler must write all
+ * of the data or return a negative errno value.
  */
 typedef ssize_t (QEMUFileWritevBufferFunc)(void *opaque, struct iovec *iov,
                                            int iovcnt, int64_t pos);