QProcess/Unix: introduce setChildProcessModifier()

[ChangeLog][Source-Incompatible Changes] QProcess::setupChildProcess()
was removed. To execute code in a child process, use
QProcess::setChildProcessModifier()

[ChangeLog][QtCore][QProcess] Added setChildProcessModifier() function
with which one can provide code to be run in the Unix child process
between fork() and execve(). With this function, it is no longer
necessary to derive from QProcess in order to execute actions in the
child process.

Another reason is that we can tell whether the std::function carries a
valid target much more easily than we can tell whether QProcess was
overridden.

The setupChildProcess() virtual function does not need to be marked
final, since no overrider could ever return an inaccessible private
class. This also makes sure the error presented to the user is about the
return type, not about attempting to override a final.

Clang:
 error: virtual function 'f' has a different return type ('void') than the function it overrides (which has return type 'QProcess::Use_setChildProcessModifier_Instead')

GCC:
 error: conflicting return type specified for 'virtual void MyProcess::setupChildProcess()'
 note: overridden function is 'virtual QProcess::Use_setChildProcessModifier_Instead QProcess::setupChildProcess()'

ICC:
 error: return type is neither identical to nor covariant with return type "QProcess::Use_setChildProcessModifier_Instead" of overridden virtual function "QProcess::setupChildProcess"

MSVC is not relevant since it doesn't compile to Unix.

Change-Id: Ia8b65350cd5d49debca9fffd15f801161363aea7
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>

4 files changed, 78 insertions, 22 deletions

diff --git a/src/corelib/io/qprocess.cpp b/src/corelib/io/qprocess.cpp
index 8e8664fda4..483c99ed5f 100644
--- a/src/corelib/io/qprocess.cpp
+++ b/src/corelib/io/qprocess.cpp
@@ -1529,7 +1529,7 @@ QProcess::CreateProcessArgumentModifier QProcess::createProcessArgumentsModifier
     \note This function is available only on the Windows platform and requires
     C++11.
 
-    \sa QProcess::CreateProcessArgumentModifier
+    \sa QProcess::CreateProcessArgumentModifier, setChildProcessModifier()
 */
 void QProcess::setCreateProcessArgumentsModifier(CreateProcessArgumentModifier modifier)
 {
@@ -1539,6 +1539,59 @@ void QProcess::setCreateProcessArgumentsModifier(CreateProcessArgumentModifier m
 
 #endif
 
+#if defined(Q_OS_UNIX) || defined(Q_QDOC)
+/*!
+    \since 6.0
+
+    Returns the modifier function previously set by calling
+    setChildProcessModifier().
+
+    \note This function is only available on Unix platforms.
+
+    \sa setChildProcessModifier()
+*/
+std::function<void(void)> QProcess::childProcessModifier() const
+{
+    Q_D(const QProcess);
+    return d->childProcessModifier;
+}
+
+/*!
+    \since 6.0
+
+    Sets the \a modifier function for the child process, for Unix systems
+    (including \macos; for Windows, see setCreateProcessArgumentsModifier()).
+    The function contained by the \a modifier argument will be invoked in the
+    child process after \c{fork()} is completed and QProcess has set up the
+    standard file descriptors for the child process, but before \c{execve()},
+    inside start(). The modifier is useful to change certain properties of the
+    child process, such as setting up additional file descriptors or closing
+    others, changing the nice level, disconnecting from the controlling TTY,
+    etc.
+
+    The following shows an example of setting up a child process to run without
+    privileges:
+
+    \snippet code/src_corelib_io_qprocess.cpp 4
+
+    If the modifier function needs to exit the process, remember to use
+    \c{_exit()}, not \c{exit()}.
+
+    \note In multithreaded applications, this function must be careful not to
+    call any functions that may lock mutexes that may have been in use in
+    other threads (in general, using only functions defined by POSIX as
+    "async-signal-safe" is advised). Most of the Qt API is unsafe inside this
+    callback, including qDebug(), and may lead to deadlocks.
+
+    \sa childProcessModifier()
+*/
+void QProcess::setChildProcessModifier(const std::function<void(void)> &modifier)
+{
+    Q_D(QProcess);
+    d->childProcessModifier = modifier;
+}
+#endif
+
 /*!
     If QProcess has been assigned a working directory, this function returns
     the working directory that the QProcess will enter before the program has
@@ -1843,25 +1896,16 @@ void QProcess::setProcessState(ProcessState state)
     emit stateChanged(state, QPrivateSignal());
 }
 
+#if QT_VERSION < QT_VERSION_CHECK(7,0,0)
 /*!
-  This function is called in the child process context just before the
-    program is executed on Unix or \macos (i.e., after \c fork(), but before
-    \c execve()). Reimplement this function to do last minute initialization
-    of the child process. Example:
-
-    \snippet code/src_corelib_io_qprocess.cpp 4
-
-    You cannot exit the process (by calling exit(), for instance) from
-    this function. If you need to stop the program before it starts
-    execution, your workaround is to emit finished() and then call
-    exit().
-
-    \warning This function is called by QProcess on Unix and \macos
-    only. On Windows and QNX, it is not called.
+    \internal
 */
-void QProcess::setupChildProcess()
+auto QProcess::setupChildProcess() -> Use_setChildProcessModifier_Instead
 {
+    Q_UNREACHABLE();
+    return {};
 }
+#endif
 
 /*! \reimp
 */
diff --git a/src/corelib/io/qprocess.h b/src/corelib/io/qprocess.h
index 9e497c3871..77980e012c 100644
--- a/src/corelib/io/qprocess.h
+++ b/src/corelib/io/qprocess.h
@@ -205,6 +205,10 @@ public:
     CreateProcessArgumentModifier createProcessArgumentsModifier() const;
     void setCreateProcessArgumentsModifier(CreateProcessArgumentModifier modifier);
 #endif // Q_OS_WIN || Q_CLANG_QDOC
+#if defined(Q_OS_UNIX) || defined(Q_CLANG_QDOC)
+    std::function<void(void)> childProcessModifier() const;
+    void setChildProcessModifier(const std::function<void(void)> &modifier);
+#endif
 
     QString workingDirectory() const;
     void setWorkingDirectory(const QString &dir);
@@ -263,8 +267,6 @@ Q_SIGNALS:
 protected:
     void setProcessState(ProcessState state);
 
-    virtual void setupChildProcess();
-
     // QIODevice
     qint64 readData(char *data, qint64 maxlen) override;
     qint64 writeData(const char *data, qint64 len) override;
@@ -273,6 +275,15 @@ private:
     Q_DECLARE_PRIVATE(QProcess)
     Q_DISABLE_COPY(QProcess)
 
+#if QT_VERSION < QT_VERSION_CHECK(7,0,0)
+    // ### Qt7: Remove this struct and the virtual function; they're here only
+    // to cause build errors in Qt 5 code that wasn't updated to Qt 6's
+    // setChildProcessModifier()
+    struct Use_setChildProcessModifier_Instead {};
+    QT_DEPRECATED_X("Use setChildProcessModifier() instead")
+    virtual Use_setChildProcessModifier_Instead setupChildProcess();
+#endif
+
     Q_PRIVATE_SLOT(d_func(), bool _q_canReadStandardOutput())
     Q_PRIVATE_SLOT(d_func(), bool _q_canReadStandardError())
     Q_PRIVATE_SLOT(d_func(), bool _q_canWrite())
diff --git a/src/corelib/io/qprocess_p.h b/src/corelib/io/qprocess_p.h
index 2587530c09..7cb70cc95c 100644
--- a/src/corelib/io/qprocess_p.h
+++ b/src/corelib/io/qprocess_p.h
@@ -322,6 +322,8 @@ public:
 #if defined(Q_OS_WIN)
     QString nativeArguments;
     QProcess::CreateProcessArgumentModifier modifyCreateProcessArgs;
+#else
+    std::function<void(void)> childProcessModifier;
 #endif
     QProcessEnvironment environment;
 
diff --git a/src/corelib/io/qprocess_unix.cpp b/src/corelib/io/qprocess_unix.cpp
index 50390e57f5..f14e78f94e 100644
--- a/src/corelib/io/qprocess_unix.cpp
+++ b/src/corelib/io/qprocess_unix.cpp
@@ -459,7 +459,7 @@ void QProcessPrivate::startProcess()
     // safe with vfork semantics: suspend the parent execution until the child
     // either execve()s or _exit()s.
     int ffdflags = FFD_CLOEXEC;
-    if (typeid(*q) != typeid(QProcess))
+    if (childProcessModifier)
         ffdflags |= FFD_USE_FORK;
     pid_t childPid;
     forkfd = ::forkfd(ffdflags , &childPid);
@@ -544,7 +544,6 @@ void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp
 {
     ::signal(SIGPIPE, SIG_DFL);         // reset the signal that we ignored
 
-    Q_Q(QProcess);
     ChildError error = { 0, {} };       // force zeroing of function[8]
 
     // copy the stdin socket if asked to (without closing on exec)
@@ -574,8 +573,8 @@ void QProcessPrivate::execChild(const char *workingDir, char **argv, char **envp
         goto report_errno;
     }
 
-    // this is a virtual call, and it base behavior is to do nothing.
-    q->setupChildProcess();
+    if (childProcessModifier)
+        childProcessModifier();
 
     // execute the process
     if (!envp) {