Significantly refactor fatal error handling

Because new glibc has changed `MINSIGSTKSZ` to be a syscall instead
of being constant, the signal posix handling needed changes, as it
used the value in constexpr context, for deciding size of an array.
It would be simple to fix it by having the handler determine the
signal handling stack size and allocate the memory every time the
handler is being installed, but that would add another allocation
and a syscall every time a test case is entered.

Instead, I split apart the idea of preparing fatal error handlers,
and engaging them, so that the memory can be allocated only once
and still be guarded by RAII.

Also turns out that Catch2's use of `MINSIGSTKSZ` was wrong, and
we should've been using `SIGSTKSZ` the whole time, which we use now.

Closes #2178

include/internal/catch_fatal_condition.h

@@ -11,59 +11,60 @@
 
 #include "catch_platform.h"
 #include "catch_compiler_capabilities.h"
-#include "catch_windows_h_proxy.h"
 
-
-#if defined( CATCH_CONFIG_WINDOWS_SEH )
+#include <cassert>
 
 namespace Catch {
 
-    struct FatalConditionHandler {
-
-        static LONG CALLBACK handleVectoredException(PEXCEPTION_POINTERS ExceptionInfo);
-        FatalConditionHandler();
-        static void reset();
-        ~FatalConditionHandler();
-
-    private:
-        static bool isSet;
-        static ULONG guaranteeSize;
-        static PVOID exceptionHandlerHandle;
-    };
-
-} // namespace Catch
-
-#elif defined ( CATCH_CONFIG_POSIX_SIGNALS )
-
-#include <signal.h>
-
-namespace Catch {
-
-    struct FatalConditionHandler {
-
-        static bool isSet;
-        static struct sigaction oldSigActions[];
-        static stack_t oldSigStack;
-        static char altStackMem[];
-
-        static void handleSignal( int sig );
+    /**
+     * Wrapper for platform-specific fatal error (signals/SEH) handlers
+     *
+     * Tries to be cooperative with other handlers, and not step over
+     * other handlers. This means that unknown structured exceptions
+     * are passed on, previous signal handlers are called, and so on.
+     *
+     * Can only be instantiated once, and assumes that once a signal
+     * is caught, the binary will end up terminating. Thus, there
+     */
+    class FatalConditionHandler {
+        bool m_started = false;
 
+        // Install/disengage implementation for specific platform.
+        // Should be if-defed to work on current platform, can assume
+        // engage-disengage 1:1 pairing.
+        void engage_platform();
+        void disengage_platform();
+    public:
+        // Should also have platform-specific implementations as needed
         FatalConditionHandler();
         ~FatalConditionHandler();
-        static void reset();
+
+        void engage() {
+            assert(!m_started && "Handler cannot be installed twice.");
+            m_started = true;
+            engage_platform();
+        }
+
+        void disengage() {
+            assert(m_started && "Handler cannot be uninstalled without being installed first");
+            m_started = false;
+            disengage_platform();
+        }
     };
 
-} // namespace Catch
-
-
-#else
-
-namespace Catch {
-    struct FatalConditionHandler {
-        void reset();
+    //! Simple RAII guard for (dis)engaging the FatalConditionHandler
+    class FatalConditionHandlerGuard {
+        FatalConditionHandler* m_handler;
+    public:
+        FatalConditionHandlerGuard(FatalConditionHandler* handler):
+            m_handler(handler) {
+            m_handler->engage();
+        }
+        ~FatalConditionHandlerGuard() {
+            m_handler->disengage();
+        }
     };
-}
 
-#endif
+} // end namespace Catch
 
 #endif // TWOBLUECUBES_CATCH_FATAL_CONDITION_H_INCLUDED