[Qemu-block] [PATCHv2 for-2.4] block: Auto-generate node_names for each BDS entry

[Eric Blake]

From: Jeff Cody <address@hidden>

Currently, node_name is only filled in when done so explicitly by the
user.  If no node_name is specified, then the node name field is not
populated.

If node_names are automatically generated when not specified, that means
that all block job operations can be done by reference to the unique
node_name field.  This eliminates ambiguity in resolving filenames
(relative filenames, or file descriptors, symlinks, mounts, etc..) that
qemu currently needs to deal with.

If a node name is specified, then it will not be automatically
generated for that BDS entry.

If it is automatically generated, it will be prefaced with "__qemu##",
followed by 8 characters of a unique number, followed by 8 random
ASCII characters in the range of 'A-Z'.  Some sample generated node-name
strings:
    __qemu##00000000IAIYNXXR
    __qemu##00000002METXTRBQ
    __qemu##00000001FMBORDWG

The prefix is to aid in identifying it as a qemu-generated name, the
numeric portion is to guarantee uniqueness in a given qemu session, and
the random characters are to further avoid any accidental collisions
with user-specified node-names.

Reviewed-by: Eric Blake <address@hidden>
Signed-off-by: Jeff Cody <address@hidden>
Message-Id: <address@hidden>
[id_wellformed() rejects generated names, which means we can't collide]
Signed-off-by: Eric Blake <address@hidden>
---

v2: revive this patch; very little needed changing

I just posted a libvirt patch series that depends on this patch:
https://www.redhat.com/archives/libvir-list/2015-June/msg01111.html

As the original was posted by Jeff (nearly a year ago!), it counts
as a patch that was submitted prior to soft freeze :)

[I will be on vacation the next 3 weeks, so I may be slow to respond
on replies to this message]

 block.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/block.c b/block.c
index dd4f58d..2532b70 100644
--- a/block.c
+++ b/block.c
@@ -765,16 +765,28 @@ static int bdrv_open_flags(BlockDriverState *bs, int 
flags)
     return open_flags;
 }

+#define GEN_NODE_NAME_PREFIX    "__qemu##"
+#define GEN_NODE_NAME_MAX_LEN   (sizeof(GEN_NODE_NAME_PREFIX) + 8 + 8)
 static void bdrv_assign_node_name(BlockDriverState *bs,
                                   const char *node_name,
                                   Error **errp)
 {
+    char gen_node_name[GEN_NODE_NAME_MAX_LEN];
+    static uint32_t counter; /* simple counter to guarantee uniqueness */
+
+    /* if node_name is NULL, auto-generate a node name */
     if (!node_name) {
-        return;
-    }
-
-    /* Check for empty string or invalid characters */
-    if (!id_wellformed(node_name)) {
+        int len;
+        snprintf(gen_node_name, GEN_NODE_NAME_MAX_LEN,
+                 "%s%08x", GEN_NODE_NAME_PREFIX, counter++);
+        len = strlen(gen_node_name);
+        while (len < GEN_NODE_NAME_MAX_LEN - 1) {
+            gen_node_name[len++] = g_random_int_range('A', 'Z');
+        }
+        gen_node_name[GEN_NODE_NAME_MAX_LEN - 1] = '\0';
+        node_name = gen_node_name;
+    } else if (!id_wellformed(node_name)) {
+        /* Check for empty string or invalid characters */
         error_setg(errp, "Invalid node name");
         return;
     }
-- 
2.4.3