libcamera: signal: Disconnect signal automatically on slot deletion

When a signal is connected to a member function slot, the slot is not
disconnected when the slot object is deleted. This can lead to calling a
member function of a deleted object if the signal isn't disconnected
manually by the slot object's destructor.

Make signal handling easier by implementing a base Object class that
tracks all connected signals and disconnects from them automatically
when the object is deleted, using template specialization resolution in
the Signal class.

As inheriting from the Object class may to a too harsh requirement for
Signal usage in applications, keep the existing behaviour working if the
slot doesn't inherit from the Object class. We may reconsider this later
and require all slot objects to inherit from the Object class.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

3 files changed, 56 insertions, 0 deletions

diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
index c5354c13..8384cd0a 100644
--- a/src/libcamera/meson.build
+++ b/src/libcamera/meson.build
@@ -10,6 +10,7 @@ libcamera_sources = files([
     'log.cpp',
     'media_device.cpp',
     'media_object.cpp',
+    'object.cpp',
     'pipeline_handler.cpp',
     'request.cpp',
     'signal.cpp',
diff --git a/src/libcamera/object.cpp b/src/libcamera/object.cpp
new file mode 100644
index 00000000..826eed6f
--- /dev/null
+++ b/src/libcamera/object.cpp
@@ -0,0 +1,50 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * object.cpp - Base object
+ */
+
+#include <libcamera/object.h>
+#include <libcamera/signal.h>
+
+/**
+ * \file object.h
+ * \brief Base object to support automatic signal disconnection
+ */
+
+namespace libcamera {
+
+/**
+ * \class Object
+ * \brief Base object to support automatic signal disconnection
+ *
+ * The Object class simplifies signal/slot handling for classes implementing
+ * slots. By inheriting from Object, an object is automatically disconnected
+ * from all connected signals when it gets destroyed.
+ *
+ * \sa Signal
+ */
+
+Object::~Object()
+{
+	for (SignalBase *signal : signals_)
+		signal->disconnect(this);
+}
+
+void Object::connect(SignalBase *signal)
+{
+	signals_.push_back(signal);
+}
+
+void Object::disconnect(SignalBase *signal)
+{
+	for (auto iter = signals_.begin(); iter != signals_.end(); ) {
+		if (*iter == signal)
+			iter = signals_.erase(iter);
+		else
+			iter++;
+	}
+}
+
+}; /* namespace libcamera */
diff --git a/src/libcamera/signal.cpp b/src/libcamera/signal.cpp
index 8d62b5be..cb7daa11 100644
--- a/src/libcamera/signal.cpp
+++ b/src/libcamera/signal.cpp
@@ -47,6 +47,11 @@ namespace libcamera {
  * \brief Connect the signal to a member function slot
  * \param object The slot object pointer
  * \param func The slot member function
+ *
+ * If the typename T inherits from Object, the signal will be automatically
+ * disconnected from the \a func slot of \a object when \a object is destroyed.
+ * Otherwise the caller shall disconnect signals manually before destroying \a
+ * object.
  */
 
 /**