Fix for sigsegv stack overflow behavior

Also stops Catch from assuming its the only signal user in the binary,
and makes it restore the signal handlers it has replaced. Same goes for
the signal stack.

The signal stack itself probably shouldn't be always reallocated for
fragmentation reasons, but that can be fixed later on.

include/internal/catch_fatal_condition.hpp

@@ -37,6 +37,8 @@ namespace Catch {
 #else // Not Windows - assumed to be POSIX compatible //////////////////////////
 
 #include <signal.h>
+#include <memory>
+#include <array>
 
 namespace Catch {
 
@@ -60,22 +62,39 @@ namespace Catch {
             fatal( "<unknown signal>", -sig );
         }
 
-        FatalConditionHandler() : m_isSet( true ) {
+        FatalConditionHandler(): m_isSet(true), m_altStackMem(new char[SIGSTKSZ]) {
-            for( std::size_t i = 0; i < sizeof(signalDefs)/sizeof(SignalDefs); ++i )
+            stack_t sigStack;
-                signal( signalDefs[i].id, handleSignal );
+            sigStack.ss_sp = m_altStackMem.get();
+            sigStack.ss_size = SIGSTKSZ;
+            sigStack.ss_flags = 0;
+            sigaltstack(&sigStack, &m_oldSigActions);
+            struct sigaction sa = { 0 };
+
+            sa.sa_handler = handleSignal;
+            sa.sa_flags = SA_ONSTACK;
+            for (std::size_t i = 0; i < sizeof(signalDefs)/sizeof(SignalDefs); ++i) {
+                sigaction(signalDefs[i].id, &sa, &m_oldSigActions[i]);
+            }
         }
         ~FatalConditionHandler() {
             reset();
         }
         void reset() {
             if( m_isSet ) {
-                for( std::size_t i = 0; i < sizeof(signalDefs)/sizeof(SignalDefs); ++i )
+                // Set signals back to previous values -- hopefully nobody overwrote them in the meantime
-                    signal( signalDefs[i].id, SIG_DFL );
+                for( std::size_t i = 0; i < sizeof(signalDefs)/sizeof(SignalDefs); ++i ) {
+                    sigaction(signalDefs[i].id, &m_oldSigActions[i], nullptr);
+                }
+                // Return the old stack
+                sigaltstack(&m_oldSigStack, nullptr);
                 m_isSet = false;
             }
         }
 
         bool m_isSet;
+        std::unique_ptr<char[]> m_altStackMem;
+        std::array<struct sigaction, sizeof(signalDefs)/sizeof(SignalDefs)> m_oldSigActions;
+        stack_t m_oldSigStack;
     };
 
 } // namespace Catch