python/notmuch2: do not destroy messages owned by a query

Any messages retrieved from a query - either directly via
search_messages() or indirectly via thread objects - are owned by that
query. Retrieving the same message (i.e. corresponding to the same
message ID / database object) several times will always yield the same
C object.

The caller is allowed to destroy message objects owned by a query before
the query itself - which can save memory for long-lived queries.
However, that message must then never be retrieved again from that
query.

The python-notmuch2 bindings will currently destroy every message object
in Message._destroy(), which will lead to an invalid free if the same
message is then retrieved again. E.g. the following python program leads
to libtalloc abort()ing:

import notmuch2
db   = notmuch2.Database(mode = notmuch2.Database.MODE.READ_ONLY)
t    = next(db.threads('*'))
msgs = list(zip(t.toplevel(), t.toplevel()))
msgs = list(zip(t.toplevel(), t.toplevel()))

Fix this issue by creating a subclass of Message, which is used for
"standalone" message which have to be freed by the caller. Message class
is then used only for messages descended from a query, which do not need
to be freed by the caller.

1 files changed, 3 insertions, 3 deletions

diff --git a/bindings/python-cffi/notmuch2/_database.py b/bindings/python-cffi/notmuch2/_database.py
index 3c06402d..fc55fea8 100644
--- a/bindings/python-cffi/notmuch2/_database.py
+++ b/bindings/python-cffi/notmuch2/_database.py
@@ -400,7 +400,7 @@ class Database(base.NotmuchObject):
               capi.lib.NOTMUCH_STATUS_DUPLICATE_MESSAGE_ID]
         if ret not in ok:
             raise errors.NotmuchError(ret)
-        msg = message.Message(self, msg_pp[0], db=self)
+        msg = message.StandaloneMessage(self, msg_pp[0], db=self)
         if sync_flags:
             msg.tags.from_maildir_flags()
         return self.AddedMessage(
@@ -469,7 +469,7 @@ class Database(base.NotmuchObject):
         msg_p = msg_pp[0]
         if msg_p == capi.ffi.NULL:
             raise LookupError
-        msg = message.Message(self, msg_p, db=self)
+        msg = message.StandaloneMessage(self, msg_p, db=self)
         return msg
 
     def get(self, filename):
@@ -502,7 +502,7 @@ class Database(base.NotmuchObject):
         msg_p = msg_pp[0]
         if msg_p == capi.ffi.NULL:
             raise LookupError
-        msg = message.Message(self, msg_p, db=self)
+        msg = message.StandaloneMessage(self, msg_p, db=self)
         return msg
 
     @property