[Qemu-devel] [PATCH v13 14/14] qapi: Detect base class loops

[Eric Blake <eblake@redhat.com>]

It should be fairly obvious that qapi base classes need to
form an acyclic graph, since QMP cannot specify the same
key more than once, while base classes are included as flat
members alongside other members added by the child.  But the
old check_member_clash() parser function was not prepared to
check for this, and entered an infinite recursion (at least
until Python gives up, complaining about nesting too deep).

Now that check_member_clash() has been recently removed,
attempts at self-inheritance trigger an assertion failure
introduced by commit ac88219a.  The obvious fix is to turn
the assertion into a conditional.

This patch includes both the tests (base-cycle-direct and
base-cycle-indirect) and the fix, since the .err file output
for the unfixed case is not useful (particularly when it was
warning about unbounded recursion, as that limit may be
platform-specific).

We don't need to worry about cycles in flat unions (neither
the base type nor the type of a variant can be a union) nor
in alternates (alternate branches cannot themselves be an
alternate).  But if we later allow a union type as a variant,
we will still be okay, as QAPISchemaObjectTypeVariants.check()
triggers the same QAPISchemaObjectType.check() that will
detect any loops.

Likewise, we need not worry about the case of diamond
inheritance where the same class is used for a flat union base
class and one of its variants; either both uses will introduce
a collision in trying to insert the same member name twice, or
the shared type is empty and changes nothing.

Signed-off-by: Eric Blake <eblake@redhat.com>

---
v13 (no v12): simplify error message, use cuter name 'Loopy' in
testsuite
v11 (no v10): rename base-cycle to base-cycle-indirect, and add
base-cycle-direct; touch up commit message
v9: no change
v8: improve commit message
v7: improve commit message
v6: rebase to earlier info changes
---
 scripts/qapi.py                            | 4 +++-
 tests/Makefile                             | 2 ++
 tests/qapi-schema/base-cycle-direct.err    | 1 +
 tests/qapi-schema/base-cycle-direct.exit   | 1 +
 tests/qapi-schema/base-cycle-direct.json   | 2 ++
 tests/qapi-schema/base-cycle-direct.out    | 0
 tests/qapi-schema/base-cycle-indirect.err  | 1 +
 tests/qapi-schema/base-cycle-indirect.exit | 1 +
 tests/qapi-schema/base-cycle-indirect.json | 3 +++
 tests/qapi-schema/base-cycle-indirect.out  | 0
 10 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 tests/qapi-schema/base-cycle-direct.err
 create mode 100644 tests/qapi-schema/base-cycle-direct.exit
 create mode 100644 tests/qapi-schema/base-cycle-direct.json
 create mode 100644 tests/qapi-schema/base-cycle-direct.out
 create mode 100644 tests/qapi-schema/base-cycle-indirect.err
 create mode 100644 tests/qapi-schema/base-cycle-indirect.exit
 create mode 100644 tests/qapi-schema/base-cycle-indirect.json
 create mode 100644 tests/qapi-schema/base-cycle-indirect.out

diff --git a/scripts/qapi.py b/scripts/qapi.py
index fdf6720..4197b30 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -942,7 +942,9 @@ class QAPISchemaObjectType(QAPISchemaType):
         self.members = None

     def check(self, schema):
-        assert self.members is not False        # not running in cycles
+        if self.members is False:               # check for cycles
+            raise QAPIExprError(self.info,
+                                "Object %s contains itself" % self.name)
         if self.members:
             return
         self.members = False                    # mark as being checked
diff --git a/tests/Makefile b/tests/Makefile
index 64cca48..8ccd11e 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -257,6 +257,8 @@ qapi-schema += bad-ident.json
 qapi-schema += bad-type-bool.json
 qapi-schema += bad-type-dict.json
 qapi-schema += bad-type-int.json
+qapi-schema += base-cycle-direct.json
+qapi-schema += base-cycle-indirect.json
 qapi-schema += command-int.json
 qapi-schema += comments.json
 qapi-schema += double-data.json
diff --git a/tests/qapi-schema/base-cycle-direct.err b/tests/qapi-schema/base-cycle-direct.err
new file mode 100644
index 0000000..9c68f65
--- /dev/null
+++ b/tests/qapi-schema/base-cycle-direct.err
@@ -0,0 +1 @@
+tests/qapi-schema/base-cycle-direct.json:2: Object Loopy contains itself
diff --git a/tests/qapi-schema/base-cycle-direct.exit b/tests/qapi-schema/base-cycle-direct.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/qapi-schema/base-cycle-direct.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/base-cycle-direct.json b/tests/qapi-schema/base-cycle-direct.json
new file mode 100644
index 0000000..4fc66d0
--- /dev/null
+++ b/tests/qapi-schema/base-cycle-direct.json
@@ -0,0 +1,2 @@
+# we reject a loop in base classes
+{ 'struct': 'Loopy', 'base': 'Loopy', 'data': {} }
diff --git a/tests/qapi-schema/base-cycle-direct.out b/tests/qapi-schema/base-cycle-direct.out
new file mode 100644
index 0000000..e69de29
diff --git a/tests/qapi-schema/base-cycle-indirect.err b/tests/qapi-schema/base-cycle-indirect.err
new file mode 100644
index 0000000..fc92fe4
--- /dev/null
+++ b/tests/qapi-schema/base-cycle-indirect.err
@@ -0,0 +1 @@
+tests/qapi-schema/base-cycle-indirect.json:2: Object Base1 contains itself
diff --git a/tests/qapi-schema/base-cycle-indirect.exit b/tests/qapi-schema/base-cycle-indirect.exit
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/tests/qapi-schema/base-cycle-indirect.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/base-cycle-indirect.json b/tests/qapi-schema/base-cycle-indirect.json
new file mode 100644
index 0000000..2866772
--- /dev/null
+++ b/tests/qapi-schema/base-cycle-indirect.json
@@ -0,0 +1,3 @@
+# we reject a loop in base classes
+{ 'struct': 'Base1', 'base': 'Base2', 'data': {} }
+{ 'struct': 'Base2', 'base': 'Base1', 'data': {} }
diff --git a/tests/qapi-schema/base-cycle-indirect.out b/tests/qapi-schema/base-cycle-indirect.out
new file mode 100644
index 0000000..e69de29
-- 
2.4.3