serverPanic(): allow printf() alike formatting.

This is of great interest because allows us to print debugging
informations that could be of useful when debugging, like in the
following example:

    serverPanic("Unexpected encoding for object %d, %d",
        obj->type, obj->encoding);

src/debug.c

@@ -266,6 +266,8 @@ void debugCommand(client *c) {
         blen++; addReplyStatus(c,
         "segfault -- Crash the server with sigsegv.");
         blen++; addReplyStatus(c,
+        "panic -- Crash the server simulating a panic.");
+        blen++; addReplyStatus(c,
         "restart  -- Graceful restart: save config, db, restart.");
         blen++; addReplyStatus(c,
         "crash-and-recovery <milliseconds> -- Hard crash and restart after <milliseconds> delay.");
@@ -300,6 +302,8 @@ void debugCommand(client *c) {
         setDeferredMultiBulkLength(c,blenp,blen);
     } else if (!strcasecmp(c->argv[1]->ptr,"segfault")) {
         *((char*)-1) = 'x';
+    } else if (!strcasecmp(c->argv[1]->ptr,"panic")) {
+        serverPanic("DEBUG PANIC called at Unix time %ld", time(NULL));
     } else if (!strcasecmp(c->argv[1]->ptr,"restart") ||
                !strcasecmp(c->argv[1]->ptr,"crash-and-recover"))
     {
@@ -615,11 +619,17 @@ void _serverAssertWithInfo(const client *c, const robj *o, const char *estr, con
     _serverAssert(estr,file,line);
 }
 
-void _serverPanic(const char *msg, const char *file, int line) {
+void _serverPanic(const char *file, int line, const char *msg, ...) {
+    va_list ap;
+    va_start(ap,msg);
+    char fmtmsg[256];
+    vsnprintf(fmtmsg,sizeof(fmtmsg),msg,ap);
+    va_end(ap);
+
     bugReportStart();
     serverLog(LL_WARNING,"------------------------------------------------");
     serverLog(LL_WARNING,"!!! Software Failure. Press left mouse button to continue");
-    serverLog(LL_WARNING,"Guru Meditation: %s #%s:%d",msg,file,line);
+    serverLog(LL_WARNING,"Guru Meditation: %s #%s:%d",fmtmsg,file,line);
 #ifdef HAVE_BACKTRACE
     serverLog(LL_WARNING,"(forcing SIGSEGV in order to print the stack trace)");
 #endif

src/server.h

@@ -435,7 +435,7 @@ typedef long long mstime_t; /* millisecond time type. */
 /* We can print the stacktrace, so our assert is defined this way: */
 #define serverAssertWithInfo(_c,_o,_e) ((_e)?(void)0 : (_serverAssertWithInfo(_c,_o,#_e,__FILE__,__LINE__),_exit(1)))
 #define serverAssert(_e) ((_e)?(void)0 : (_serverAssert(#_e,__FILE__,__LINE__),_exit(1)))
-#define serverPanic(_e) _serverPanic(#_e,__FILE__,__LINE__),_exit(1)
+#define serverPanic(...) _serverPanic(__FILE__,__LINE__,__VA_ARGS__),_exit(1)
 
 /*-----------------------------------------------------------------------------
  * Data types
@@ -1960,7 +1960,7 @@ void *realloc(void *ptr, size_t size) __attribute__ ((deprecated));
 /* Debugging stuff */
 void _serverAssertWithInfo(const client *c, const robj *o, const char *estr, const char *file, int line);
 void _serverAssert(const char *estr, const char *file, int line);
-void _serverPanic(const char *msg, const char *file, int line);
+void _serverPanic(const char *file, int line, const char *msg, ...);
 void bugReportStart(void);
 void serverLogObjectDebugInfo(const robj *o);
 void sigsegvHandler(int sig, siginfo_t *info, void *secret);