keyval: Fix and clarify grammar

The grammar has a few issues:

* key-fragment = / [^=,.]* /

  Prose restricts key fragments: they "must be valid QAPI names or
  consist only of decimal digits".  Technically, '' consists only of
  decimal digits.  The code rejects that.  Fix the grammar.

* val          = { / [^,]* / | ',,' }

  Use + instead of *.  Accepts the same language.

* val-no-key   = / [^=,]* /

  The code rejects an empty value.  Fix the grammar.

* Section "Additional syntax for use with an implied key" is
  confusing.  Rewrite it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201011073505.1185335-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

util/keyval.c

@@ -16,8 +16,8 @@
  *   key-vals     = [ key-val { ',' key-val } [ ',' ] ]
  *   key-val      = key '=' val
  *   key          = key-fragment { '.' key-fragment }
- *   key-fragment = / [^=,.]* /
- *   val          = { / [^,]* / | ',,' }
+ *   key-fragment = / [^=,.]+ /
+ *   val          = { / [^,]+ / | ',,' }
  *
  * Semantics defined by reduction to JSON:
  *
@@ -71,12 +71,16 @@
  * Awkward.  Note that we carefully restrict alternate types to avoid
  * similar ambiguity.
  *
- * Additional syntax for use with an implied key:
+ * Alternative syntax for use with an implied key:
  *
- *   key-vals-ik  = val-no-key [ ',' key-vals ]
- *   val-no-key   = / [^=,]* /
+ *   key-vals     = [ key-val-1st { ',' key-val } [ ',' ] ]
+ *   key-val-1st  = val-no-key | key-val
+ *   val-no-key   = / [^=,]+ /
  *
- * where no-key is syntactic sugar for implied-key=val-no-key.
+ * where val-no-key is syntactic sugar for implied-key=val-no-key.
+ *
+ * Note that you can't use the sugared form when the value contains
+ * '=' or ','.
  */
 
 #include "qemu/osdep.h"