linux-aio: Allow reads beyond the end of growable images

This is the linux-aio version of commits 22afa7b (raw-posix, synchronous) and
ba1d1af (posix-aio-compat). Reads now produce zeros after the end of file
instead of failing or resulting in short reads, making linux-aio compatible
with the behaviour of synchronous raw-posix requests and posix-aio-compat.

The problem can be reproduced like this:

dd if=/dev/zero of=/tmp/test.raw bs=1 count=1234
./qemu-io -k -n -g -c 'read -p 1024 512' /tmp/test.raw

Previously, the result of this was 'read failed: Invalid argument', now the
read completes successfully.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

linux-aio.c

@@ -31,6 +31,8 @@ struct qemu_laiocb {
     struct iocb iocb;
     ssize_t ret;
     size_t nbytes;
+    QEMUIOVector *qiov;
+    bool is_read;
     QLIST_ENTRY(qemu_laiocb) node;
 };
 
@@ -57,10 +59,17 @@ static void qemu_laio_process_completion(struct qemu_laio_state *s,
 
     ret = laiocb->ret;
     if (ret != -ECANCELED) {
-        if (ret == laiocb->nbytes)
+        if (ret == laiocb->nbytes) {
             ret = 0;
-        else if (ret >= 0)
-            ret = -EINVAL;
+        } else if (ret >= 0) {
+            /* Short reads mean EOF, pad with zeros. */
+            if (laiocb->is_read) {
+                qemu_iovec_memset_skip(laiocb->qiov, 0,
+                    laiocb->qiov->size - ret, ret);
+            } else {
+                ret = -EINVAL;
+            }
+        }
 
         laiocb->common.cb(laiocb->common.opaque, ret);
     }
@@ -162,6 +171,8 @@ BlockDriverAIOCB *laio_submit(BlockDriverState *bs, void *aio_ctx, int fd,
     laiocb->nbytes = nb_sectors * 512;
     laiocb->ctx = s;
     laiocb->ret = -EINPROGRESS;
+    laiocb->is_read = (type == QEMU_AIO_READ);
+    laiocb->qiov = qiov;
 
     iocbs = &laiocb->iocb;