Re: [Qemu-devel] [Nbd] [PATCH] Further tidy-up on block status

[Alex Bligh <alex@alex.org.uk>]

> On 14 Dec 2016, at 18:18, Alex Bligh <alex@alex.org.uk> wrote:
> 
> Let me have a go at a change.

OK I've pushed a change. I reordered a few bits (to put the
base:allocation next to the stuff that talks about metadata
queries at the start), but the main change is below.

-- 
Alex Bligh



@@ -970,15 +1029,38 @@ of the newstyle negotiation.
        - String, query to list a subset of the available metadata
          contexts.

-    If zero queries are sent, then the server MUST return all
-    the metadata contexts that are available to the client to select
-    on the given export with `NBD_OPT_SET_META_CONTEXT`.
-
     For details on the query string, see under `NBD_OPT_SET_META_CONTEXT`.

     The server MUST either reply with an error (for instance `EINVAL`
     if the option is not supported), or reply with a list of
     `NBD_REP_META_CONTEXT` replies followed by `NBD_REP_ACK`.
+
+    If zero queries are sent, then the server MUST return all
+    the metadata contexts that are available to the client to select
+    on the given export with `NBD_OPT_SET_META_CONTEXT`, save that:
+
+    If one or more queries are sent, then the server MUST return
+    those metadata contexts that are available to the client to
+    select on the given export with `NBD_OPT_SET_META_CONTEXT`,
+    and which match one or more of the queries given. The
+    support of wildcarding within the leaf-name portion of
+    the query string is dependent upon the namespace.
+
+    In either case, however:
+
+    * the server MAY return an incomplete list if returning a
+      complete list would require undue resources.
+
+    * the server MAY return a context consisting of a namespace and
+      a colon only (i.e. omitting the leaf-name) to indicate that
+      the namespace contains a large number of possible contexts
+      within that namespace (for instance a namespace `X-backup` with
+      contexts that indicate whether blocks were written after
+      a given date might accept queries of the form
+      `'X-backup:modifiedtime>[unixdate]'` where `[unixdate]` is an
+      arbitrary integer, and in this case it might simply
+      return `X-backup:`)
+
     The metadata context ID in these replies is reserved and SHOULD be
     set to zero; clients MUST disregard it.

@@ -1009,7 +1091,9 @@ of the newstyle negotiation.

     If zero queries are sent, the server MUST select no metadata contexts.

-    The server MUST reply with a number of `NBD_REP_META_CONTEXT`
+    The server MAY return `NBD_REP_ERR_TOO_BIG` if a request
+    seeks to select too many contexts. Otherwise
+    the server MUST reply with a number of `NBD_REP_META_CONTEXT`
     replies, one for each selected metadata context, each with a unique
     metadata context ID, followed by `NBD_REP_ACK`. The metadata context
     ID is transient and may vary across calls to `NBD_OPT_SET_META_CONTEXT`;