Sentinel: test for writable config file.

This commit introduces a funciton called when Sentinel is ready for
normal operations to avoid putting Sentinel specific stuff in redis.c.

src/redis.c

@@ -3012,7 +3012,7 @@ int main(int argc, char **argv) {
         if (server.sofd > 0)
             redisLog(REDIS_NOTICE,"The server is now ready to accept connections at %s", server.unixsocket);
     } else {
-        redisLog(REDIS_WARNING,"Sentinel runid is %s", server.runid);
+        sentinelIsRunning();
     }
 
     /* Warning the user about suspicious maxmemory setting. */

src/redis.h

@@ -1187,6 +1187,7 @@ void initSentinelConfig(void);
 void initSentinel(void);
 void sentinelTimer(void);
 char *sentinelHandleConfiguration(char **argv, int argc);
+void sentinelIsRunning(void);
 
 /* Scripting */
 void scriptingInit(void);

src/sentinel.c

@@ -415,6 +415,17 @@ void initSentinel(void) {
     sentinel.scripts_queue = listCreate();
 }
 
+/* This function gets called when the server is in Sentinel mode, started,
+ * loaded the configuration, and is ready for normal operations. */
+void sentinelIsRunning(void) {
+    redisLog(REDIS_WARNING,"Sentinel runid is %s", server.runid);
+
+    if (server.configfile == NULL || access(server.configfile,W_OK) == -1) {
+        redisLog(REDIS_WARNING,"Sentinel started without a config file, or config file not writable. Exiting...");
+        exit(1);
+    }
+}
+
 /* ============================== sentinelAddr ============================== */
 
 /* Create a sentinelAddr object and return it on success.
@@ -2071,7 +2082,7 @@ int sentinelSendHello(sentinelRedisInstance *ri) {
 
     /* Try to obtain our own IP address. */
     if (anetSockName(ri->cc->c.fd,ip,sizeof(ip),NULL) == -1) return REDIS_ERR;
-    if (ri->flags & SRI_DISCONNECTED) return;
+    if (ri->flags & SRI_DISCONNECTED) return REDIS_ERR;
 
     /* Format and send the Hello message. */
     snprintf(payload,sizeof(payload),