Fix bug 4463. The crash was due to a call to X from inside a child process.

The main change is to support/ForkedCalls.{h,cpp}. We introduce a static variable IAmAChild and a corresponding accessor. This is set to true in a new fork() method, in the branch taken by the child. (Note: fork() is safe cross-platform, as it just returns -1 if we don't have fork().) This ForkedProcess::iAmAChild() method is then used to protect GuiView::message().

As Abdel has pointed out, there may be other such calls to be protected, e.g., the emission of the Buffer::changed() signal. Those are not addressed here.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@22587 a592a061-630c-0410-9148-cb99ea01b6c8

src/Buffer.cpp

@@ -2260,14 +2260,12 @@ private:
 };
 
 
-#if !defined (HAVE_FORK)
-# define fork() -1
-#endif
-
 int AutoSaveBuffer::generateChild()
 {
 	// tmp_ret will be located (usually) in /tmp
 	// will that be a problem?
+	// Note that this calls ForkedCalls::fork(), so it's
+	// ok cross-platform.
 	pid_t const pid = fork();
 	// If you want to debug the autosave
 	// you should set pid to -1, and comment out the fork.

src/frontends/qt4/GuiView.cpp

@@ -54,6 +54,7 @@
 #include "support/FileFilterList.h"
 #include "support/FileName.h"
 #include "support/filetools.h"
+#include "support/ForkedCalls.h"
 #include "support/lstrings.h"
 #include "support/os.h"
 #include "support/Package.h"
@@ -459,6 +460,9 @@ void GuiView::dropEvent(QDropEvent* event)
 
 void GuiView::message(docstring const & str)
 {
+	if (ForkedProcess::iAmAChild())
+		return;
+
 	statusBar()->showMessage(toqstr(str));
 	d.statusbar_timer_.stop();
 	d.statusbar_timer_.start(3000);

src/support/ForkedCalls.cpp

@@ -108,6 +108,9 @@ ForkedProcess::ForkedProcess()
 {}
 
 
+bool ForkedProcess::IAmAChild = false;
+
+
 void ForkedProcess::emitSignal()
 {
 	if (signal_.get()) {
@@ -123,6 +126,10 @@ int ForkedProcess::run(Starttype type)
 	pid_ = generateChild();
 	if (pid_ <= 0) { // child or fork failed.
 		retval_ = 1;
+		if (pid_ == 0)
+			//we also do this in fork(), too, but maybe someone will try
+			//to bypass that
+			IAmAChild = true; 
 		return retval_;
 	}
 
@@ -184,6 +191,18 @@ void ForkedProcess::kill(int tol)
 }
 
 
+pid_t ForkedProcess::fork() {
+#if !defined (HAVE_FORK)
+	return -1;
+#else
+	pid_t pid = ::fork();
+	if (pid == 0)
+		IAmAChild = true;
+	return pid;
+#endif
+}
+
+
 // Wait for child process to finish. Returns returncode from child.
 int ForkedProcess::waitForChild()
 {

src/support/ForkedCalls.h

@@ -43,7 +43,7 @@ public:
 	///
 	virtual boost::shared_ptr<ForkedProcess> clone() const = 0;
 
-	/** A SignalType signal is can be emitted once the forked process
+	/** A SignalType signal can be emitted once the forked process
 	 *  has finished. It passes:
 	 *  the PID of the child and;
 	 *  the return value from the child.
@@ -97,12 +97,21 @@ public:
 	 */
 	void kill(int tolerance = 5);
 
+	/// Returns true if this is a child process
+	static bool iAmAChild() { return IAmAChild; }
+
 protected:
 	/** Spawn the child process.
 	 *  Returns returncode from child.
 	 */
 	int run(Starttype type);
 
+	/// implement our own version of fork()
+	/// it just returns -1 if ::fork() is not defined
+	/// otherwise, it forks and sets the global child-process
+	/// boolean IAmAChild
+	pid_t fork();
+
 	/// Callback function
 	SignalTypePtr signal_;
 
@@ -118,6 +127,9 @@ private:
 	/// generate child in background
 	virtual int generateChild() = 0;
 
+	///
+	static bool IAmAChild;
+
 	/// Wait for child process to finish. Updates returncode from child.
 	int waitForChild();
 };