[Qemu-devel] [PATCH 3/4] nbd: Support auto-read-only option

[Kevin Wolf <kwolf@redhat.com>]

If read-only=off, but auto-read-only=on are given, open a read-write NBD
connection if the server provides a read-write export, but instead of
erroring out for read-only exports, just degrade to read-only.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/nbd-client.h |  1 +
 block/nbd-client.c | 14 +++++++++++---
 block/nbd.c        |  4 +++-
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/block/nbd-client.h b/block/nbd-client.h
index cfc90550b9..1d14e78e92 100644
--- a/block/nbd-client.h
+++ b/block/nbd-client.h
@@ -46,6 +46,7 @@ int nbd_client_init(BlockDriverState *bs,
                     QCryptoTLSCreds *tlscreds,
                     const char *hostname,
                     const char *x_dirty_bitmap,
+                    bool auto_readonly,
                     Error **errp);
 void nbd_client_close(BlockDriverState *bs);
 
diff --git a/block/nbd-client.c b/block/nbd-client.c
index 9686ecbd5e..6a1e634bec 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -971,6 +971,7 @@ int nbd_client_init(BlockDriverState *bs,
                     QCryptoTLSCreds *tlscreds,
                     const char *hostname,
                     const char *x_dirty_bitmap,
+                    bool auto_readonly,
                     Error **errp)
 {
     NBDClientSession *client = nbd_get_client_session(bs);
@@ -994,9 +995,16 @@ int nbd_client_init(BlockDriverState *bs,
     }
     if (client->info.flags & NBD_FLAG_READ_ONLY &&
         !bdrv_is_read_only(bs)) {
-        error_setg(errp,
-                   "request for write access conflicts with read-only export");
-        return -EACCES;
+        if (auto_readonly) {
+            ret = bdrv_set_read_only(bs, true, errp);
+            if (ret < 0) {
+                return ret;
+            }
+        } else {
+            error_setg(errp, "request for write access conflicts with "
+                             "read-only export");
+            return -EACCES;
+        }
     }
     if (client->info.flags & NBD_FLAG_SEND_FUA) {
         bs->supported_write_flags = BDRV_REQ_FUA;
diff --git a/block/nbd.c b/block/nbd.c
index e87699fb73..8c352a45cd 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -445,7 +445,9 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
 
     /* NBD handshake */
     ret = nbd_client_init(bs, sioc, s->export, tlscreds, hostname,
-                          qemu_opt_get(opts, "x-dirty-bitmap"), errp);
+                          qemu_opt_get(opts, "x-dirty-bitmap"),
+                          (flags & BDRV_O_AUTO_RDONLY),
+                          errp);
  error:
     if (sioc) {
         object_unref(OBJECT(sioc));
-- 
2.13.6