Don't use an alternative stack for SIGSEGV & co.

This commit reverts most of c5757662, in
order to use back main stack for signal handling.

The main reason is that otherwise it is completely pointless that we do
a lot of efforts to print the stack trace on crash, and the content of
the stack and registers as well. Using an alternate stack broken this
feature completely.

src/redis.c

@@ -62,9 +62,6 @@ double R_Zero, R_PosInf, R_NegInf, R_Nan;
 
 /*================================= Globals ================================= */
 
-/* Alternate stack for SIGSEGV/etc handlers */
-char altstack[SIGSTKSZ];
-
 /* Global vars */
 struct redisServer server; /* server global state */
 struct redisCommand *commandTable;
@@ -2351,13 +2348,6 @@ static void sigtermHandler(int sig) {
 
 void setupSignalHandlers(void) {
     struct sigaction act;
-    stack_t stack;
-
-    stack.ss_sp = altstack;
-    stack.ss_flags = 0;
-    stack.ss_size = SIGSTKSZ;
-
-    sigaltstack(&stack, NULL);
 
     /* When the SA_SIGINFO flag is set in sa_flags then sa_sigaction is used.
      * Otherwise, sa_handler is used. */
@@ -2367,10 +2357,8 @@ void setupSignalHandlers(void) {
     sigaction(SIGTERM, &act, NULL);
 
 #ifdef HAVE_BACKTRACE
-    /* Use alternate stack so we don't clobber stack in case of segv, or when we run out of stack ..
-     * also resethand & nodefer so we can get interrupted (and killed) if we cause SEGV during SEGV handler */
     sigemptyset(&act.sa_mask);
-    act.sa_flags = SA_NODEFER | SA_ONSTACK | SA_RESETHAND | SA_SIGINFO;
+    act.sa_flags = SA_NODEFER | SA_RESETHAND | SA_SIGINFO;
     act.sa_sigaction = sigsegvHandler;
     sigaction(SIGSEGV, &act, NULL);
     sigaction(SIGBUS, &act, NULL);