[Libguestfs] [nbdkit PATCH 1/3] connections: Don't advertise TRIM on readonly connection

[Eric Blake]

A client cannot attempt TRIM on a read-only connection (we already
filter it in connections.c with EROFS); as such, it makes no sense
to query the plugin's .can_trim nor to advertise TRIM support to
the client on such a connection, similar to how we do not
advertise WRITE_ZEROES.

FUA only has defined meanings on write operations, but the NBD
protocol allows us to advertise it on reads and to ignore clients
that send it on read (older qemu did so), so we can continue to
advertise it regardless of readonly state.  Likewise, while flush
is unlikely to make a difference on a read-only connection, there
is no reason why it shouldn't be advertised.

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 src/connections.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/connections.c b/src/connections.c
index 8e110a5..75c2c2d 100644
--- a/src/connections.c
+++ b/src/connections.c
@@ -427,6 +427,14 @@ compute_eflags (struct connection *conn, uint16_t *flags)
   }
   if (!conn->readonly) {
     eflags |= NBD_FLAG_SEND_WRITE_ZEROES;
+
+    fl = backend->can_trim (backend, conn);
+    if (fl == -1)
+      return -1;
+    if (fl) {
+      eflags |= NBD_FLAG_SEND_TRIM;
+      conn->can_trim = 1;
+    }
   }

   fl = backend->can_flush (backend, conn);
@@ -445,14 +453,6 @@ compute_eflags (struct connection *conn, uint16_t *flags)
     conn->is_rotational = 1;
   }

-  fl = backend->can_trim (backend, conn);
-  if (fl == -1)
-    return -1;
-  if (fl) {
-    eflags |= NBD_FLAG_SEND_TRIM;
-    conn->can_trim = 1;
-  }
-
   *flags = eflags;
   return 0;
 }
-- 
2.14.3