tests: virtio-9p: use the synth backend

The purpose of virtio-9p-test is to test the virtio-9p device, especially
the 9p server state machine. We don't really care what fsdev backend we're
using. Moreover, if we want to be able to test the flush request or a
device reset with in-flights I/O, it is close to impossible to achieve
with a physical backend because we cannot ask it reliably to put an I/O
on hold at a specific point in time.

Fortunately, we can do that with the synthetic backend, which allows to
register callbacks on read/write accesses to a specific file. This will
be used by a later patch to test the 9P flush request.

The walk request test is converted to using the synth backend.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/9pfs/9p-synth.c

@@ -19,6 +19,7 @@
 #include "qemu/rcu.h"
 #include "qemu/rcu_queue.h"
 #include "qemu/cutils.h"
+#include "sysemu/qtest.h"
 
 /* Root node for synth file system */
 static V9fsSynthNode synth_root = {
@@ -527,6 +528,21 @@ static int synth_init(FsContext *ctx, Error **errp)
 
     /* Mark the subsystem is ready for use */
     synth_fs = 1;
+
+    if (qtest_enabled()) {
+        V9fsSynthNode *node = NULL;
+        int i, ret;
+
+        /* Directory hierarchy for WALK test */
+        for (i = 0; i < P9_MAXWELEM; i++) {
+            char *name = g_strdup_printf(QTEST_V9FS_SYNTH_WALK_FILE, i);
+
+            ret = qemu_v9fs_synth_mkdir(node, 0700, name, &node);
+            assert(!ret);
+            g_free(name);
+        }
+    }
+
     return 0;
 }