Handle new Context creation API on the FFI

This adopts the FFI API to use the new Context::new() and
Contest::drop() Rust-style APIs while preserving the semantics of the
existing FFI API.  It introduces a wrapper around the context which
keeps an optional reference to the actual context, changing dc_open()
and dc_close() to create/destroy the actual reference.

This changes the events emitted on closing slightly: they only get
emitted when dc_close() was called while the context was open.  While
dc_close() remains idempotent in that it can still be called safely
multiple times, the close events will only occur if the context is
actually closed.

deltachat-ffi/deltachat.h

@@ -393,8 +393,9 @@ int             dc_set_config                (dc_context_t* context, const char*
  * @memberof dc_context_t
  * @param context The context object as created by dc_context_new(). For querying system values, this can be NULL.
  * @param key The key to query.
- * @return Returns current value of "key", if "key" is unset, the default value is returned.
- *     The returned value must be free()'d, NULL is never returned.
+ * @return Returns current value of "key", if "key" is unset, the default
+ *     value is returned.  The returned value must be free()'d, NULL is never
+ *     returned.  If there is an error an empty string will be returned.
  */
 char*           dc_get_config                (dc_context_t* context, const char* key);
 

python/tests/test_lowlevel.py

@@ -17,11 +17,19 @@ def test_callback_None2int():
     clear_context_callback(ctx)
 
 
-def test_dc_close_events():
-    ctx = capi.lib.dc_context_new(capi.lib.py_dc_callback, ffi.NULL, ffi.NULL)
+def test_dc_close_events(tmpdir):
+    ctx = ffi.gc(
+        capi.lib.dc_context_new(capi.lib.py_dc_callback, ffi.NULL, ffi.NULL),
+        lib.dc_context_unref,
+    )
     evlog = EventLogger(ctx)
     evlog.set_timeout(5)
-    set_context_callback(ctx, lambda ctx, evt_name, data1, data2: evlog(evt_name, data1, data2))
+    set_context_callback(
+        ctx,
+        lambda ctx, evt_name, data1, data2: evlog(evt_name, data1, data2)
+    )
+    p = tmpdir.join("hello.db")
+    lib.dc_open(ctx, p.strpath.encode("ascii"), ffi.NULL)
     capi.lib.dc_close(ctx)
 
     def find(info_string):

src/stock.rs

@@ -237,7 +237,6 @@ mod tests {
     use super::*;
     use crate::test_utils::*;
 
-
     use crate::constants::DC_CONTACT_ID_SELF;
     use crate::types::uintptr_t;