tests: virtio-9p: add FLUSH operation test

The idea is to send a victim request that will possibly block in the
server and to send a flush request to cancel the victim request.

This patch adds two test to verifiy that:
- the server does not reply to a victim request that was actually
  cancelled
- the server replies to the flush request after replying to the
  victim request if it could not cancel it

9p request cancellation reference:

http://man.cat-v.org/plan_9/5/flush

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
(groug, change the test to only write a single byte to avoid
        any alignment or endianess consideration)

hw/9pfs/9p-synth.c

@@ -521,6 +521,20 @@ static ssize_t v9fs_synth_qtest_write(void *buf, int len, off_t offset,
     return 1;
 }
 
+static ssize_t v9fs_synth_qtest_flush_write(void *buf, int len, off_t offset,
+                                            void *arg)
+{
+    bool should_block = !!*(uint8_t *)buf;
+
+    if (should_block) {
+        /* This will cause the server to call us again until we're cancelled */
+        errno = EINTR;
+        return -1;
+    }
+
+    return 1;
+}
+
 static int synth_init(FsContext *ctx, Error **errp)
 {
     QLIST_INIT(&synth_root.child);
@@ -557,6 +571,12 @@ static int synth_init(FsContext *ctx, Error **errp)
         ret = qemu_v9fs_synth_add_file(NULL, 0, QTEST_V9FS_SYNTH_WRITE_FILE,
                                        NULL, v9fs_synth_qtest_write, ctx);
         assert(!ret);
+
+        /* File for FLUSH test */
+        ret = qemu_v9fs_synth_add_file(NULL, 0, QTEST_V9FS_SYNTH_FLUSH_FILE,
+                                       NULL, v9fs_synth_qtest_flush_write,
+                                       ctx);
+        assert(!ret);
     }
 
     return 0;