block-raw: Allow pread beyond the end of growable images

When using O_DIRECT, qcow2 snapshots didn't work any more for me. In the
process of creating the snapshot, qcow2 tries to pwrite some new information
(e.g. new L1 table) which will often end up being after the old end of the
image file. Now pwrite tries to align things and reads the old contents of the
file, read returns 0 because there is nothing to read after the end of file and
pwrite is stuck in an endless loop.

This patch allows to pread beyond the end of an image file. Whenever the
given offset is after the end of the image file, the read succeeds and fills
the buffer with zeros.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block/raw-posix.c

@@ -117,6 +117,7 @@ typedef struct BDRVRawState {
 static int posix_aio_init(void);
 
 static int fd_open(BlockDriverState *bs);
+static int64_t raw_getlength(BlockDriverState *bs);
 
 #if defined(__FreeBSD__)
 static int cdrom_reopen(BlockDriverState *bs);
@@ -231,6 +232,16 @@ static int raw_pread_aligned(BlockDriverState *bs, int64_t offset,
     if (ret == count)
         goto label__raw_read__success;
 
+    /* Allow reads beyond the end (needed for pwrite) */
+    if ((ret == 0) && bs->growable) {
+        int64_t size = raw_getlength(bs);
+        if (offset >= size) {
+            memset(buf, 0, count);
+            ret = count;
+            goto label__raw_read__success;
+        }
+    }
+
     DEBUG_BLOCK_PRINT("raw_pread(%d:%s, %" PRId64 ", %p, %d) [%" PRId64
                       "] read failed %d : %d = %s\n",
                       s->fd, bs->filename, offset, buf, count,