Add ChildProcessManager

The new class hold a list of ChildProcesses and periodically checks
their return value until they report termination. On Linux this check
is necessary to avoid leaving zombie processes behind.

modules/juce_core/detail/juce_CallbackListenerList.h

@@ -0,0 +1,69 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   By using JUCE, you agree to the terms of both the JUCE 7 End-User License
+   Agreement and JUCE Privacy Policy.
+
+   End User License Agreement: www.juce.com/juce-7-licence
+   Privacy Policy: www.juce.com/juce-privacy-policy
+
+   Or: You may also use this code under the terms of the GPL v3 (see
+   www.gnu.org/licenses).
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce::detail
+{
+
+template <typename... Ts>
+constexpr bool isValueOrLvalueReferenceToConst()
+{
+    return ((   (! std::is_reference_v<Ts>)
+             || (std::is_lvalue_reference_v<Ts> && std::is_const_v<std::remove_reference_t<Ts>>)) && ...);
+}
+
+template <typename... Args>
+class CallbackListenerList
+{
+public:
+    static_assert (isValueOrLvalueReferenceToConst<Args...>(),
+                   "CallbackListenerList can only forward values or const lvalue references");
+
+    using Callback = std::function<void (Args...)>;
+
+    ErasedScopeGuard addListener (Callback callback)
+    {
+        jassert (callback != nullptr);
+
+        const auto it = callbacks.insert (callbacks.end(), std::move (callback));
+        listeners.add (&*it);
+
+        return ErasedScopeGuard { [this, it]
+        {
+            listeners.remove (&*it);
+            callbacks.erase (it);
+        } };
+    }
+
+    void call (Args... args)
+    {
+        listeners.call ([&] (auto& l) { l (std::forward<Args> (args)...); });
+    }
+
+private:
+    std::list<Callback> callbacks;
+    ListenerList<Callback> listeners;
+};
+
+} // namespace juce::detail

modules/juce_core/juce_core.h

@@ -354,6 +354,8 @@ JUCE_END_IGNORE_WARNINGS_MSVC
 #include "files/juce_AndroidDocument.h"
 #include "streams/juce_AndroidDocumentInputSource.h"
 
+#include "detail/juce_CallbackListenerList.h"
+
 #if JUCE_CORE_INCLUDE_OBJC_HELPERS && (JUCE_MAC || JUCE_IOS)
  #include "native/juce_CFHelpers_mac.h"
  #include "native/juce_ObjCHelpers_mac.h"

modules/juce_events/interprocess/juce_ChildProcessManager.cpp

@@ -0,0 +1,52 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   By using JUCE, you agree to the terms of both the JUCE 7 End-User License
+   Agreement and JUCE Privacy Policy.
+
+   End User License Agreement: www.juce.com/juce-7-licence
+   Privacy Policy: www.juce.com/juce-privacy-policy
+
+   Or: You may also use this code under the terms of the GPL v3 (see
+   www.gnu.org/licenses).
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce
+{
+
+void ChildProcessManager::checkProcesses()
+{
+    for (auto it = processes.begin(); it != processes.end();)
+    {
+        auto processPtr = *it;
+
+        if (! processPtr->isRunning())
+        {
+            listeners.call (processPtr.get());
+            it = processes.erase (it);
+        }
+        else
+        {
+            ++it;
+        }
+    }
+
+    if (processes.empty())
+        timer.stopTimer();
+}
+
+JUCE_IMPLEMENT_SINGLETON (ChildProcessManager)
+
+} // namespace juce

modules/juce_events/interprocess/juce_ChildProcessManager.h

@@ -0,0 +1,104 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   By using JUCE, you agree to the terms of both the JUCE 7 End-User License
+   Agreement and JUCE Privacy Policy.
+
+   End User License Agreement: www.juce.com/juce-7-licence
+   Privacy Policy: www.juce.com/juce-privacy-policy
+
+   Or: You may also use this code under the terms of the GPL v3 (see
+   www.gnu.org/licenses).
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce
+{
+    /** Manages a set of ChildProcesses and periodically checks their return value. Upon completion
+        it calls listeners added with addChildProcessExitedListener().
+
+        This class is mostly aimed for usage on Linux, where terminated child processes are only
+        cleaned up if their return code is read after termination. In order to ensure this one needs
+        to call ChildProcess::isFinished() until it returns false or
+        ChildProcess::waitForProcessToFinish() until it returns true.
+
+        This class will keep querying the return code on a Timer thread until the process
+        terminates. This can be handy if one wants to start and stop multiple ChildProcesses on
+        Linux that could take a long time to complete.
+
+        Since this class uses a Timer to check subprocess status, it's generally only safe to
+        access the returned ChildProcesses from the message thread.
+
+        @see ChildProcessManagerSingleton
+
+        @tags{Events}
+    */
+    class JUCE_API  ChildProcessManager final : private DeletedAtShutdown
+    {
+    public:
+       #ifndef DOXYGEN
+        JUCE_DECLARE_SINGLETON_SINGLETHREADED_MINIMAL (ChildProcessManager)
+       #endif
+
+        /** Creates a new ChildProcess and starts it with the provided arguments.
+
+            The arguments are the same as the overloads to ChildProcess::start().
+
+            The manager will keep the returned ChildProcess object alive until it terminates and its
+            return value has been queried. Calling ChildProcess::kill() on the returned object will
+            eventually cause its removal from the ChildProcessManager after it terminates.
+        */
+        template <typename... Args>
+        std::shared_ptr<ChildProcess> createAndStartManagedChildProcess (Args&&... args)
+        {
+            auto p = std::make_shared<ChildProcess>();
+
+            if (! p->start (std::forward<Args> (args)...))
+                return nullptr;
+
+            processes.insert (p);
+            timer.startTimer (1000);
+
+            return p;
+        }
+
+        /** Registers a callback function that is called for every ChildProcess that terminated.
+
+            This registration is deleted when the returned ErasedScopedGuard is deleted.
+        */
+        auto addChildProcessExitedListener (std::function<void (ChildProcess*)> listener)
+        {
+            return listeners.addListener (std::move (listener));
+        }
+
+        /** Returns true if the ChildProcessManager contains any running ChildProcesses that it's
+            monitoring.
+        */
+        auto hasRunningProcess() const
+        {
+            return timer.isTimerRunning();
+        }
+
+    private:
+        ChildProcessManager() = default;
+        ~ChildProcessManager() override  { clearSingletonInstance(); }
+
+        void checkProcesses();
+
+        std::set<std::shared_ptr<ChildProcess>> processes;
+        detail::CallbackListenerList<ChildProcess*> listeners;
+        TimedCallback timer { [this] { checkProcesses(); } };
+    };
+
+} // namespace juce

modules/juce_events/juce_events.cpp

@@ -64,6 +64,7 @@
 #include "broadcasters/juce_ChangeBroadcaster.cpp"
 #include "timers/juce_MultiTimer.cpp"
 #include "timers/juce_Timer.cpp"
+#include "interprocess/juce_ChildProcessManager.cpp"
 #include "interprocess/juce_InterprocessConnection.cpp"
 #include "interprocess/juce_InterprocessConnectionServer.cpp"
 #include "interprocess/juce_ConnectedChildProcess.cpp"

modules/juce_events/juce_events.h

@@ -86,7 +86,9 @@
 #include "broadcasters/juce_ChangeListener.h"
 #include "broadcasters/juce_ChangeBroadcaster.h"
 #include "timers/juce_Timer.h"
+#include "timers/juce_TimedCallback.h"
 #include "timers/juce_MultiTimer.h"
+#include "interprocess/juce_ChildProcessManager.h"
 #include "interprocess/juce_InterprocessConnection.h"
 #include "interprocess/juce_InterprocessConnectionServer.h"
 #include "interprocess/juce_ConnectedChildProcess.h"

modules/juce_events/timers/juce_TimedCallback.h

@@ -0,0 +1,63 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   By using JUCE, you agree to the terms of both the JUCE 7 End-User License
+   Agreement and JUCE Privacy Policy.
+
+   End User License Agreement: www.juce.com/juce-7-licence
+   Privacy Policy: www.juce.com/juce-privacy-policy
+
+   Or: You may also use this code under the terms of the GPL v3 (see
+   www.gnu.org/licenses).
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce
+{
+
+/** Utility class wrapping a single non-null callback called by a Timer.
+
+    You can use the usual Timer functions to start and stop the TimedCallback. Deleting the
+    TimedCallback will automatically stop the underlying Timer.
+
+    With this class you can use the Timer facility without inheritance.
+
+    @see Timer
+    @tags{Events}
+*/
+class TimedCallback final : private Timer
+{
+public:
+    /** Constructor. The passed in callback must be non-null. */
+    explicit TimedCallback (std::function<void()> callbackIn)
+        : callback (std::move (callbackIn))
+    {
+        jassert (callback);
+    }
+
+    /** Destructor. */
+    ~TimedCallback() noexcept override { stopTimer(); }
+
+    using Timer::startTimer;
+    using Timer::startTimerHz;
+    using Timer::stopTimer;
+    using Timer::isTimerRunning;
+
+private:
+    void timerCallback() override { callback(); }
+
+    std::function<void()> callback;
+};
+
+} // namespace juce