[RFC libnbd PATCH] info: Add support for new qemu:joint-allocation

[Eric Blake]

Qemu is adding qemu:joint-allocation as a single context combining the
two bits of base:allocation and a compression of qemu:allocation-depth
into two bits [1].  Decoding the bits makes it easier for humans to
see the result of that context.

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg02446.html
---

Obviously, this libnbd patch should only go in if the qemu RFC is
accepted favorably.  With this patch applied, the example listed in my
qemu patch 2/2 commit message [2] becomes

$ ~/libnbd/run nbdinfo --map=qemu:joint-allocation nbd://localhost
         0       65536    3  hole,zero,unallocated
     65536       65536    4  allocated,local
    131072       65536    7  hole,zero,local
    196608       65536    3  hole,zero,unallocated

[2] https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg02448.html

For what it's worth, you can also play with the qemu+libnbd patches at:
https://repo.or.cz/qemu/ericb.git/ master
https://repo.or.cz/libnbd/ericb.git/ master

(I sometimes rewind those branches, but they'll be stable for at least
a few days after this email)

 info/map.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/info/map.c b/info/map.c
index ae6d4fe..21e8657 100644
--- a/info/map.c
+++ b/info/map.c
@@ -226,6 +226,27 @@ extent_description (const char *metacontext, uint32_t type)
       return ret;
     }
   }
+  else if (strcmp (metacontext, "qemu:joint-allocation") == 0) {
+    /* Combo of base:allocation and stripped-down qemu:allocation-depth */
+    const char *base, *depth;
+    switch (type & 3) {
+    case 0: base = "allocated"; break;
+    case 1: base = "hole"; break;
+    case 2: base = "zero"; break;
+    case 3: base = "hole,zero"; break;
+    }
+    switch (type & 0xc) {
+    case 0: depth = "unallocated"; break;
+    case 4: depth = "local"; break;
+    case 8: depth = "backing"; break;
+    case 12: depth = "<unexpected depth>"; break;
+    }
+    if (asprintf (&ret, "%s,%s", base, depth) == -1) {
+      perror ("asprintf");
+      exit (EXIT_FAILURE);
+    }
+    return ret;
+  }

   return NULL;   /* Don't know - description field will be omitted. */
 }
-- 
2.31.1