qapi: Implement boxed types for commands/events

Turn on the ability to pass command and event arguments in
a single boxed parameter, which must name a non-empty type
(although the type can be a struct with all optional members).
For structs, it makes it possible to pass a single qapi type
instead of a breakout of all struct members (useful if the
arguments are already in a struct or if the number of members
is large); for other complex types, it is now possible to use
a union or alternate as the data for a command or event.

The empty type may be technically feasible if needed down the
road, but it's easier to forbid it now and relax things to allow
it later, than it is to allow it now and have to special case
how the generated 'q_empty' type is handled (see commit 7ce106a9
for reasons why nothing is generated for the empty type).  An
alternate type is never considered empty, but now that a boxed
type can be either an object or an alternate, we have to provide
a trivial QAPISchemaAlternateType.is_empty().  The new call to
arg_type.is_empty() during QAPISchemaCommand.check() requires
that we first check the type in question; but there is no chance
of introducing a cycle since objects do not refer back to commands.

We still have a split in syntax checking between ad-hoc parsing
up front (merely validates that 'boxed' has a sane value) and
during .check() methods (if 'boxed' is set, then 'data' must name
a non-empty user-defined type).

Generated code is unchanged, as long as no client uses the
new feature.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1468468228-27827-10-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Test files renamed to *-boxed-*]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 25 insertions, 2 deletions

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index 48b0b31f2e..de298dcaec 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -410,7 +410,7 @@ following example objects:
 === Commands ===
 
 Usage: { 'command': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
-         '*returns': TYPE-NAME,
+         '*returns': TYPE-NAME, '*boxed': true,
          '*gen': false, '*success-response': false }
 
 Commands are defined by using a dictionary containing several members,
@@ -461,6 +461,20 @@ which would validate this Client JSON Protocol transaction:
  => { "execute": "my-second-command" }
  <= { "return": [ { "value": "one" }, { } ] }
 
+The generator emits a prototype for the user's function implementing
+the command.  Normally, 'data' is a dictionary for an anonymous type,
+or names a struct type (possibly empty, but not a union), and its
+members are passed as separate arguments to this function.  If the
+command definition includes a key 'boxed' with the boolean value true,
+then 'data' is instead the name of any non-empty complex type
+(struct, union, or alternate), and a pointer to that QAPI type is
+passed as a single argument.
+
+The generator also emits a marshalling function that extracts
+arguments for the user's function out of an input QDict, calls the
+user's function, and if it succeeded, builds an output QObject from
+its return value.
+
 In rare cases, QAPI cannot express a type-safe representation of a
 corresponding Client JSON Protocol command.  You then have to suppress
 generation of a marshalling function by including a key 'gen' with
@@ -484,7 +498,8 @@ use of this member.
 
 === Events ===
 
-Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT }
+Usage: { 'event': STRING, '*data': COMPLEX-TYPE-NAME-OR-DICT,
+         '*boxed': true }
 
 Events are defined with the keyword 'event'.  It is not allowed to
 name an event 'MAX', since the generator also produces a C enumeration
@@ -505,6 +520,14 @@ Resulting in this JSON object:
   "data": { "b": "test string" },
   "timestamp": { "seconds": 1267020223, "microseconds": 435656 } }
 
+The generator emits a function to send the event.  Normally, 'data' is
+a dictionary for an anonymous type, or names a struct type (possibly
+empty, but not a union), and its members are passed as separate
+arguments to this function.  If the event definition includes a key
+'boxed' with the boolean value true, then 'data' is instead the name of
+any non-empty complex type (struct, union, or alternate), and a
+pointer to that QAPI type is passed as a single argument.
+
 
 == Client JSON Protocol introspection ==