qapi: Replace uncommon use of the error API by the common one

We commonly use the error API like this:

    err = NULL;
    foo(..., &err);
    if (err) {
        goto out;
    }
    bar(..., &err);

Every error source is checked separately.  The second function is only
called when the first one succeeds.  Both functions are free to pass
their argument to error_set().  Because error_set() asserts no error
has been set, this effectively means they must not be called with an
error set.

The qapi-generated code uses the error API differently:

    // *errp was initialized to NULL somewhere up the call chain
    frob(..., errp);
    gnat(..., errp);

Errors accumulate in *errp: first error wins, subsequent errors get
dropped.  To make this work, the second function does nothing when
called with an error set.  Requires non-null errp, or else the second
function can't see the first one fail.

This usage has also bled into visitor tests, and two device model
object property getters rtc_get_date() and balloon_stats_get_all().

With the "accumulate" technique, you need fewer error checks in
callers, and buy that with an error check in every callee.  Can be
nice.

However, mixing the two techniques is confusing.  You can't use the
"accumulate" technique with functions designed for the "check
separately" technique.  You can use the "check separately" technique
with functions designed for the "accumulate" technique, but then
error_set() can't catch you setting an error more than once.

Standardize on the "check separately" technique for now, because it's
overwhelmingly prevalent.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

2 files changed, 128 insertions, 100 deletions

diff --git a/scripts/qapi-commands.py b/scripts/qapi-commands.py
index 341dba27a8..386f17ef44 100644
--- a/scripts/qapi-commands.py
+++ b/scripts/qapi-commands.py
@@ -2,16 +2,19 @@
 # QAPI command marshaller generator
 #
 # Copyright IBM, Corp. 2011
+# Copyright (C) 2014 Red Hat, Inc.
 #
 # Authors:
 #  Anthony Liguori <aliguori@us.ibm.com>
 #  Michael Roth    <mdroth@linux.vnet.ibm.com>
+#  Markus Armbruster <armbru@redhat.com>
 #
 # This work is licensed under the terms of the GNU GPL, version 2.
 # See the COPYING file in the top-level directory.
 
 from ordereddict import OrderedDict
 from qapi import *
+import re
 import sys
 import os
 import getopt
@@ -37,6 +40,15 @@ def generate_command_decl(name, args, ret_type):
 ''',
                  ret_type=c_type(ret_type), name=c_fun(name), args=arglist).strip()
 
+def gen_err_check(errvar):
+    if errvar:
+        return mcgen('''
+if (local_err) {
+    goto out;
+}
+''')
+    return ''
+
 def gen_sync_call(name, args, ret_type, indent=0):
     ret = ""
     arglist=""
@@ -49,15 +61,14 @@ def gen_sync_call(name, args, ret_type, indent=0):
         arglist += "%s, " % (c_var(argname))
     push_indent(indent)
     ret = mcgen('''
-%(retval)sqmp_%(name)s(%(args)serrp);
+%(retval)sqmp_%(name)s(%(args)s&local_err);
 
 ''',
                 name=c_fun(name), args=arglist, retval=retval).rstrip()
     if ret_type:
+        ret += "\n" + gen_err_check('local_err')
         ret += "\n" + mcgen(''''
-if (!error_is_set(errp)) {
-    %(marshal_output_call)s
-}
+%(marshal_output_call)s
 ''',
                             marshal_output_call=gen_marshal_output_call(name, ret_type)).rstrip()
     pop_indent(indent)
@@ -67,7 +78,7 @@ if (!error_is_set(errp)) {
 def gen_marshal_output_call(name, ret_type):
     if not ret_type:
         return ""
-    return "qmp_marshal_output_%s(retval, ret, errp);" % c_fun(name)
+    return "qmp_marshal_output_%s(retval, ret, &local_err);" % c_fun(name)
 
 def gen_visitor_input_containers_decl(args, obj):
     ret = ""
@@ -109,7 +120,8 @@ bool has_%(argname)s = false;
 
 def gen_visitor_input_block(args, dealloc=False):
     ret = ""
-    errparg = 'errp'
+    errparg = '&local_err'
+    errarg = 'local_err'
 
     if len(args) == 0:
         return ret
@@ -118,6 +130,7 @@ def gen_visitor_input_block(args, dealloc=False):
 
     if dealloc:
         errparg = 'NULL'
+        errarg = None;
         ret += mcgen('''
 qmp_input_visitor_cleanup(mi);
 md = qapi_dealloc_visitor_new();
@@ -132,15 +145,20 @@ v = qmp_input_get_visitor(mi);
         if optional:
             ret += mcgen('''
 visit_optional(v, &has_%(c_name)s, "%(name)s", %(errp)s);
-if (has_%(c_name)s) {
 ''',
                          c_name=c_var(argname), name=argname, errp=errparg)
+            ret += gen_err_check(errarg)
+            ret += mcgen('''
+if (has_%(c_name)s) {
+''',
+                         c_name=c_var(argname))
             push_indent()
         ret += mcgen('''
 %(visitor)s(v, &%(c_name)s, "%(name)s", %(errp)s);
 ''',
                      c_name=c_var(argname), name=argname, argtype=argtype,
                      visitor=type_visitor(argtype), errp=errparg)
+        ret += gen_err_check(errarg)
         if optional:
             pop_indent()
             ret += mcgen('''
@@ -161,15 +179,20 @@ def gen_marshal_output(name, args, ret_type, middle_mode):
     ret = mcgen('''
 static void qmp_marshal_output_%(c_name)s(%(c_ret_type)s ret_in, QObject **ret_out, Error **errp)
 {
+    Error *local_err = NULL;
     QmpOutputVisitor *mo = qmp_output_visitor_new();
     QapiDeallocVisitor *md;
     Visitor *v;
 
     v = qmp_output_get_visitor(mo);
-    %(visitor)s(v, &ret_in, "unused", errp);
-    if (!error_is_set(errp)) {
-        *ret_out = qmp_output_get_qobject(mo);
+    %(visitor)s(v, &ret_in, "unused", &local_err);
+    if (local_err) {
+        goto out;
     }
+    *ret_out = qmp_output_get_qobject(mo);
+
+out:
+    error_propagate(errp, local_err);
     qmp_output_visitor_cleanup(mo);
     md = qapi_dealloc_visitor_new();
     v = qapi_dealloc_get_visitor(md);
@@ -196,13 +219,12 @@ def gen_marshal_input(name, args, ret_type, middle_mode):
     ret = mcgen('''
 %(header)s
 {
+    Error *local_err = NULL;
 ''',
                 header=hdr)
 
     if middle_mode:
         ret += mcgen('''
-    Error *local_err = NULL;
-    Error **errp = &local_err;
     QDict *args = (QDict *)qdict;
 ''')
 
@@ -229,20 +251,23 @@ def gen_marshal_input(name, args, ret_type, middle_mode):
                      visitor_input_block=gen_visitor_input_block(args))
     else:
         ret += mcgen('''
+
     (void)args;
 ''')
 
     ret += mcgen('''
-    if (error_is_set(errp)) {
-        goto out;
-    }
 %(sync_call)s
 ''',
                  sync_call=gen_sync_call(name, args, ret_type, indent=4))
-    ret += mcgen('''
+    if re.search('^ *goto out\\;', ret, re.MULTILINE):
+        ret += mcgen('''
 
 out:
 ''')
+    if not middle_mode:
+        ret += mcgen('''
+    error_propagate(errp, local_err);
+''')
     ret += mcgen('''
 %(visitor_input_block_cleanup)s
 ''',
diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
index 1ef753a9b9..06a79f1631 100644
--- a/scripts/qapi-visit.py
+++ b/scripts/qapi-visit.py
@@ -2,16 +2,19 @@
 # QAPI visitor generator
 #
 # Copyright IBM, Corp. 2011
+# Copyright (C) 2014 Red Hat, Inc.
 #
 # Authors:
 #  Anthony Liguori <aliguori@us.ibm.com>
 #  Michael Roth    <mdroth@linux.vnet.ibm.com>
+#  Markus Armbruster <armbru@redhat.com>
 #
 # This work is licensed under the terms of the GNU GPL, version 2.
 # See the COPYING file in the top-level directory.
 
 from ordereddict import OrderedDict
 from qapi import *
+import re
 import sys
 import os
 import getopt
@@ -32,9 +35,7 @@ static void visit_type_implicit_%(c_type)s(Visitor *m, %(c_type)s **obj, Error *
 
     visit_start_implicit_struct(m, (void **)obj, sizeof(%(c_type)s), &err);
     if (!err) {
-        visit_type_%(c_type)s_fields(m, obj, &err);
-        error_propagate(errp, err);
-        err = NULL;
+        visit_type_%(c_type)s_fields(m, obj, errp);
         visit_end_implicit_struct(m, &err);
     }
     error_propagate(errp, err);
@@ -64,12 +65,9 @@ def generate_visit_struct_fields(name, field_prefix, fn_prefix, members, base =
 
 static void visit_type_%(full_name)s_field_%(c_name)s(Visitor *m, %(name)s **obj, Error **errp)
 {
-    Error *err = NULL;
 ''',
                          name=name, full_name=full_name, c_name=c_var(argname))
-            push_indent()
             ret += generate_visit_struct_body(full_name, argname, argentry)
-            pop_indent()
             ret += mcgen('''
 }
 ''')
@@ -89,6 +87,9 @@ static void visit_type_%(full_name)s_fields(Visitor *m, %(name)s ** obj, Error *
     if base:
         ret += mcgen('''
 visit_type_implicit_%(type)s(m, &(*obj)->%(c_prefix)s%(c_name)s, &err);
+if (err) {
+    goto out;
+}
 ''',
                      c_prefix=c_var(field_prefix),
                      type=type_name(base), c_name=c_var('base'))
@@ -97,7 +98,7 @@ visit_type_implicit_%(type)s(m, &(*obj)->%(c_prefix)s%(c_name)s, &err);
         if optional:
             ret += mcgen('''
 visit_optional(m, &(*obj)->%(c_prefix)shas_%(c_name)s, "%(name)s", &err);
-if ((*obj)->%(prefix)shas_%(c_name)s) {
+if (!err && (*obj)->%(prefix)shas_%(c_name)s) {
 ''',
                          c_prefix=c_var(field_prefix), prefix=field_prefix,
                          c_name=c_var(argname), name=argname)
@@ -121,10 +122,19 @@ visit_type_%(type)s(m, &(*obj)->%(c_prefix)s%(c_name)s, "%(name)s", &err);
             ret += mcgen('''
 }
 ''')
+        ret += mcgen('''
+if (err) {
+    goto out;
+}
+''')
 
     pop_indent()
-    ret += mcgen('''
+    if re.search('^ *goto out\\;', ret, re.MULTILINE):
+        ret += mcgen('''
 
+out:
+''')
+    ret += mcgen('''
     error_propagate(errp, err);
 }
 ''')
@@ -133,9 +143,9 @@ visit_type_%(type)s(m, &(*obj)->%(c_prefix)s%(c_name)s, "%(name)s", &err);
 
 def generate_visit_struct_body(field_prefix, name, members):
     ret = mcgen('''
-if (!error_is_set(errp)) {
+    Error *err = NULL;
+
 ''')
-    push_indent()
 
     if not field_prefix:
         full_name = name
@@ -144,36 +154,26 @@ if (!error_is_set(errp)) {
 
     if len(field_prefix):
         ret += mcgen('''
-visit_start_struct(m, NULL, "", "%(name)s", 0, &err);
+    visit_start_struct(m, NULL, "", "%(name)s", 0, &err);
 ''',
                 name=name)
     else:
         ret += mcgen('''
-Error *err = NULL;
-visit_start_struct(m, (void **)obj, "%(name)s", name, sizeof(%(name)s), &err);
+    visit_start_struct(m, (void **)obj, "%(name)s", name, sizeof(%(name)s), &err);
 ''',
                 name=name)
 
     ret += mcgen('''
-if (!err) {
-    if (*obj) {
-        visit_type_%(name)s_fields(m, obj, &err);
-        error_propagate(errp, err);
-        err = NULL;
+    if (!err) {
+        if (*obj) {
+            visit_type_%(name)s_fields(m, obj, errp);
+        }
+        visit_end_struct(m, &err);
     }
+    error_propagate(errp, err);
 ''',
         name=full_name)
 
-    ret += mcgen('''
-    /* Always call end_struct if start_struct succeeded.  */
-    visit_end_struct(m, &err);
-}
-error_propagate(errp, err);
-''')
-    pop_indent()
-    ret += mcgen('''
-}
-''')
     return ret
 
 def generate_visit_struct(expr):
@@ -191,9 +191,7 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
 ''',
                 name=name)
 
-    push_indent()
     ret += generate_visit_struct_body("", name, members)
-    pop_indent()
 
     ret += mcgen('''
 }
@@ -205,24 +203,26 @@ def generate_visit_list(name, members):
 
 void visit_type_%(name)sList(Visitor *m, %(name)sList ** obj, const char *name, Error **errp)
 {
-    GenericList *i, **prev = (GenericList **)obj;
     Error *err = NULL;
+    GenericList *i, **prev;
 
-    if (!error_is_set(errp)) {
-        visit_start_list(m, name, &err);
-        if (!err) {
-            for (; (i = visit_next_list(m, prev, &err)) != NULL; prev = &i) {
-                %(name)sList *native_i = (%(name)sList *)i;
-                visit_type_%(name)s(m, &native_i->value, NULL, &err);
-            }
-            error_propagate(errp, err);
-            err = NULL;
-
-            /* Always call end_list if start_list succeeded.  */
-            visit_end_list(m, &err);
-        }
-        error_propagate(errp, err);
+    visit_start_list(m, name, &err);
+    if (err) {
+        goto out;
+    }
+
+    for (prev = (GenericList **)obj;
+         !err && (i = visit_next_list(m, prev, &err)) != NULL;
+         prev = &i) {
+        %(name)sList *native_i = (%(name)sList *)i;
+        visit_type_%(name)s(m, &native_i->value, NULL, &err);
     }
+
+    error_propagate(errp, err);
+    err = NULL;
+    visit_end_list(m, &err);
+out:
+    error_propagate(errp, err);
 }
 ''',
                 name=name)
@@ -244,10 +244,15 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
 {
     Error *err = NULL;
 
-    if (!error_is_set(errp)) {
-        visit_start_implicit_struct(m, (void**) obj, sizeof(%(name)s), &err);
-        visit_get_next_type(m, (int*) &(*obj)->kind, %(name)s_qtypes, name, &err);
-        switch ((*obj)->kind) {
+    visit_start_implicit_struct(m, (void**) obj, sizeof(%(name)s), &err);
+    if (err) {
+        goto out;
+    }
+    visit_get_next_type(m, (int*) &(*obj)->kind, %(name)s_qtypes, name, &err);
+    if (err) {
+        goto out_end;
+    }
+    switch ((*obj)->kind) {
 ''',
     name=name)
 
@@ -262,22 +267,24 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
 
         enum_full_value = generate_enum_full_value(disc_type, key)
         ret += mcgen('''
-        case %(enum_full_value)s:
-            visit_type_%(c_type)s(m, &(*obj)->%(c_name)s, name, &err);
-            break;
+    case %(enum_full_value)s:
+        visit_type_%(c_type)s(m, &(*obj)->%(c_name)s, name, &err);
+        break;
 ''',
                 enum_full_value = enum_full_value,
                 c_type = type_name(members[key]),
                 c_name = c_fun(key))
 
     ret += mcgen('''
-        default:
-            abort();
-        }
-        error_propagate(errp, err);
-        err = NULL;
-        visit_end_implicit_struct(m, &err);
+    default:
+        abort();
     }
+out_end:
+    error_propagate(errp, err);
+    err = NULL;
+    visit_end_implicit_struct(m, &err);
+out:
+    error_propagate(errp, err);
 }
 ''')
 
@@ -324,19 +331,20 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
 {
     Error *err = NULL;
 
-    if (!error_is_set(errp)) {
-        visit_start_struct(m, (void **)obj, "%(name)s", name, sizeof(%(name)s), &err);
-        if (!err) {
-            if (*obj) {
+    visit_start_struct(m, (void **)obj, "%(name)s", name, sizeof(%(name)s), &err);
+    if (err) {
+        goto out;
+    }
+    if (*obj) {
 ''',
                  name=name)
 
-    push_indent()
-    push_indent()
-
     if base:
         ret += mcgen('''
         visit_type_%(name)s_fields(m, obj, &err);
+        if (err) {
+            goto out_obj;
+        }
 ''',
             name=name)
 
@@ -346,8 +354,10 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
         disc_key = discriminator
     ret += mcgen('''
         visit_type_%(disc_type)s(m, &(*obj)->kind, "%(disc_key)s", &err);
-        if (!err) {
-            switch ((*obj)->kind) {
+        if (err) {
+            goto out_obj;
+        }
+        switch ((*obj)->kind) {
 ''',
                  disc_type = disc_type,
                  disc_key = disc_key)
@@ -360,32 +370,25 @@ void visit_type_%(name)s(Visitor *m, %(name)s ** obj, const char *name, Error **
 
         enum_full_value = generate_enum_full_value(disc_type, key)
         ret += mcgen('''
-            case %(enum_full_value)s:
-                ''' + fmt + '''
-                break;
+        case %(enum_full_value)s:
+            ''' + fmt + '''
+            break;
 ''',
                 enum_full_value = enum_full_value,
                 c_type=type_name(members[key]),
                 c_name=c_fun(key))
 
     ret += mcgen('''
-            default:
-                abort();
-            }
+        default:
+            abort();
         }
+out_obj:
         error_propagate(errp, err);
         err = NULL;
-''')
-    pop_indent()
-    pop_indent()
-
-    ret += mcgen('''
-            }
-            /* Always call end_struct if start_struct succeeded.  */
-            visit_end_struct(m, &err);
-        }
-        error_propagate(errp, err);
     }
+    visit_end_struct(m, &err);
+out:
+    error_propagate(errp, err);
 }
 ''')
 
@@ -505,7 +508,7 @@ fdecl.write(mcgen('''
 /* THIS FILE IS AUTOMATICALLY GENERATED, DO NOT MODIFY */
 
 /*
- * schema-defined QAPI visitor function
+ * schema-defined QAPI visitor functions
  *
  * Copyright IBM, Corp. 2011
  *