Retry first connection to redis in db_connect. (#112)

* Retry first connection to redis in db_connect.

* Declare usleep.

* Formatting.

src/common/state/redis.c

@@ -2,8 +2,8 @@
 
 #include <assert.h>
 #include <stdbool.h>
-
 #include <stdlib.h>
+#include <unistd.h>
 /* Including hiredis here is necessary on Windows for typedefs used in ae.h. */
 #include "hiredis/hiredis.h"
 #include "hiredis/adapters/ae.h"
@@ -20,6 +20,11 @@
 #include "redis.h"
 #include "io.h"
 
+#ifndef _WIN32
+/* This function is actually not declared in standard POSIX, so declare it. */
+extern int usleep(useconds_t usec);
+#endif
+
 #define CHECK_REDIS_CONNECT(CONTEXT_TYPE, context, M, ...) \
   do {                                                     \
     CONTEXT_TYPE *_context = (context);                    \
@@ -119,8 +124,21 @@ db_handle *db_connect(const char *address,
   db_handle *db = malloc(sizeof(db_handle));
   /* Sync connection for initial handshake */
   redisReply *reply;
+  int connection_attempts = 0;
   redisContext *context = redisConnect(address, port);
-  CHECK_REDIS_CONNECT(redisContext, context, "could not connect to redis %s:%d",
+  while (context == NULL || context->err) {
+    if (connection_attempts >= REDIS_DB_CONNECT_RETRIES) {
+      break;
+    }
+    LOG_WARN("Failed to connect to Redis, retrying.");
+    /* Sleep for a little. */
+    usleep(REDIS_DB_CONNECT_WAIT_MS * 1000);
+    context = redisConnect(address, port);
+    connection_attempts += 1;
+  }
+  CHECK_REDIS_CONNECT(redisContext, context,
+                      "could not establish synchronous connection to redis "
+                      "%s:%d",
                       address, port);
   /* Enable keyspace events. */
   reply = redisCommand(context, "CONFIG SET notify-keyspace-events AKE");
@@ -161,12 +179,16 @@ db_handle *db_connect(const char *address,
   /* Establish async connection */
   db->context = redisAsyncConnect(address, port);
   CHECK_REDIS_CONNECT(redisAsyncContext, db->context,
-                      "could not connect to redis %s:%d", address, port);
+                      "could not establish asynchronous connection to redis "
+                      "%s:%d",
+                      address, port);
   db->context->data = (void *) db;
   /* Establish async connection for subscription */
   db->sub_context = redisAsyncConnect(address, port);
   CHECK_REDIS_CONNECT(redisAsyncContext, db->sub_context,
-                      "could not connect to redis %s:%d", address, port);
+                      "could not establish asynchronous subscription "
+                      "connection to redis %s:%d",
+                      address, port);
   db->sub_context->data = (void *) db;
 
   return db;

src/common/state/redis.h

@@ -10,6 +10,10 @@
 #include "uthash.h"
 #include "utarray.h"
 
+/* Allow up to 5 seconds for connecting to Redis. */
+#define REDIS_DB_CONNECT_RETRIES 50
+#define REDIS_DB_CONNECT_WAIT_MS 100
+
 #define LOG_REDIS_ERROR(context, M, ...) \
   LOG_ERROR("Redis error %d %s; %s", context->err, context->errstr, M)