feat(ffi-refactor): replace sequence id with pointer to proposals (8/8) (#1221)

This is the final PR in the series to refactor the FFI layer to be more
rust and also a bit more Gooey.

This replaces the serial atomic id for proposals with a pointer to the
actual structures. The wrapper that includes the db handle is only
necessary in order to do the cached view shenanigans. I will come back
to that later.

Author: demosdemon

ffi/firewood.go

@@ -152,8 +152,7 @@ func (db *Database) Propose(keys, vals [][]byte) (*Proposal, error) {
 		return nil, err
 	}
 
-	val := C.fwd_propose_on_db(db.handle, kvp)
-	return newProposal(db.handle, &val)
+	return getProposalFromProposalResult(C.fwd_propose_on_db(db.handle, kvp), db)
 }
 
 // Get retrieves the value for the given key. It always returns a nil error.
@@ -231,3 +230,25 @@ func (db *Database) Revision(root []byte) (*Revision, error) {
 
 	return &Revision{database: db, root: root}, nil
 }
+
+// Close releases the memory associated with the Database.
+//
+// This is not safe to call while there are any outstanding Proposals. All proposals
+// must be freed or committed before calling this.
+//
+// This is safe to call if the pointer is nil, in which case it does nothing. The
+// pointer will be set to nil after freeing to prevent double free. However, it is
+// not safe to call this method concurrently from multiple goroutines.
+func (db *Database) Close() error {
+	if db.handle == nil {
+		return nil
+	}
+
+	if err := getErrorFromVoidResult(C.fwd_close_db(db.handle)); err != nil {
+		return fmt.Errorf("unexpected error when closing database: %w", err)
+	}
+
+	db.handle = nil // Prevent double free
+
+	return nil
+}