[Qemu-block] [RFC PATCH 1/2] util - add automated ID generation utility

[Jeff Cody]

Multiple sub-systems in QEMU may find it useful to generated IDs
for objects that a user may reference via QMP or HMP.  This patch
presents a standardized way to do it, so that automatic ID generation
follows the same rules.

This patch enforces the following rules when generating an ID:

1.) Guarantee no collisions with a user-specified ID
2.) Identify the sub-system the ID belongs to
3.) Guarantee of uniqueness
4.) Spoiling predictibility, to avoid creating an assumption
    of object ordering and parsing (i.e., we don't want users to think
    they can guess the next ID based on prior behavior).

The scheme for this is as follows (no spaces):

                # subsys D RR
Reserved char --|    |   | |
Subsytem String -----|   | |
Unique number (64-bit) --| |
Two-digit random number ---|

For example, a generated node-name for the block sub-system may take the
look like this:

    #block076

The caller of id_generate() is responsible for freeing the generated
node name string with g_free().

Signed-off-by: Jeff Cody <address@hidden>
---
 include/qemu-common.h |  8 ++++++++
 util/id.c             | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/qemu-common.h b/include/qemu-common.h
index bbaffd1..f6b0105 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -237,6 +237,14 @@ int64_t strtosz_suffix_unit(const char *nptr, char **end,
 #define STR_OR_NULL(str) ((str) ? (str) : "null")
 
 /* id.c */
+
+typedef enum IdSubSystems {
+    ID_QDEV,
+    ID_BLOCK,
+    ID_MAX      /* last element, used as array size */
+} IdSubSystems;
+
+char *id_generate(IdSubSystems);
 bool id_wellformed(const char *id);
 
 /* path.c */
diff --git a/util/id.c b/util/id.c
index 09b22fb..48e2935 100644
--- a/util/id.c
+++ b/util/id.c
@@ -26,3 +26,38 @@ bool id_wellformed(const char *id)
     }
     return true;
 }
+
+#define ID_SPECIAL_CHAR '#'
+
+/* Generates an ID of the form:
+ *
+ * "#block146",
+ *
+ *  where:
+ *      - "#" is always the reserved character '#'
+ *      - "block" refers to the subsystem identifed via IdSubSystems
+ *        and id_subsys_str[]
+ *      - "1" is a unique number (up to a uint64_t) for the subsystem,
+ *      - "46" is a pseudo-random numer to create uniqueness
+ *
+ * The caller is responsible for freeing the returned string with g_free()
+ */
+char *id_generate(IdSubSystems id)
+{
+    const char *id_subsys_str[] = {
+        [ID_QDEV]  = "qdev",
+        [ID_BLOCK] = "block",
+    };
+
+    static uint64_t id_counters[ID_MAX];
+    uint32_t rnd;
+
+    assert(id < ID_MAX);
+
+    rnd = g_random_int_range(0, 99);
+
+    return g_strdup_printf("%c%s%" PRIu64 "%" PRId32, ID_SPECIAL_CHAR,
+                                                      id_subsys_str[id],
+                                                      id_counters[id]++,
+                                                      rnd);
+}
-- 
1.9.3