[PATCH] lib: fix memory error in notmuch_config_list_value

[David Bremner]

The documentation for notmuch_config_list_key warns that that the
returned value will be destroyed by the next call to
notmuch_config_list_key, but it neglected to mention that calling
notmuch_config_list_value would also destroy it (by calling
notmuch_config_list_key). This is surprising, and caused a use after
free bug in _setup_user_query_fields (first noticed by an OpenBSD
porter, so kudos to the OpenBSD malloc implementation).  This change
fixes that use-after-free bug.
---
 lib/config.cc | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/config.cc b/lib/config.cc
index da71c16e..2cd8a0b6 100644
--- a/lib/config.cc
+++ b/lib/config.cc
@@ -150,13 +150,17 @@ notmuch_config_list_valid (notmuch_config_list_t *metadata)
     return true;
 }
 
+static inline char * _key_from_iterator (notmuch_config_list_t *list) {
+    return talloc_strdup (list, (*list->iterator).c_str () + CONFIG_PREFIX.length ());
+}
+
 const char *
 notmuch_config_list_key (notmuch_config_list_t *list)
 {
     if (list->current_key)
 	talloc_free (list->current_key);
 
-    list->current_key = talloc_strdup (list, (*list->iterator).c_str () + CONFIG_PREFIX.length ());
+    list->current_key = _key_from_iterator (list);
 
     return list->current_key;
 }
@@ -166,7 +170,7 @@ notmuch_config_list_value (notmuch_config_list_t *list)
 {
     std::string strval;
     notmuch_status_t status;
-    const char *key = notmuch_config_list_key (list);
+    char *key  = _key_from_iterator (list);
 
     /* TODO: better error reporting?? */
     status = _metadata_value (list->notmuch, key, strval);
@@ -177,6 +181,7 @@ notmuch_config_list_value (notmuch_config_list_t *list)
 	talloc_free (list->current_val);
 
     list->current_val = talloc_strdup (list, strval.c_str ());
+    talloc_free (key);
     return list->current_val;
 }
 
-- 
2.24.0