Allow setting up replication without purging data in the slave first.

patch-keep-slave-data.diff

diff --git a/redis.conf b/redis.conf
index 44fb536..505cb6e 100644
--- a/redis.conf
+++ b/redis.conf
@@ -150,6 +150,13 @@ slave-serve-stale-data yes
 #
 # repl-timeout 60
 
+# When setting up replication, the slave will purge the current data set before
+# loading the data sync from the master. Setting this will prevent that from
+# happening. This is not recommended. Only use this if you know what you are
+# doing.
+#
+keep-slave-data no
+
 ################################## SECURITY ###################################
 
 # Require clients to issue AUTH <PASSWORD> before processing any other
diff --git a/src/config.c b/src/config.c
index 1be88f5..0f7c8a1 100644
--- a/src/config.c
+++ b/src/config.c
@@ -213,6 +213,10 @@ void loadServerConfig(char *filename) {
             if ((server.repl_serve_stale_data = yesnotoi(argv[1])) == -1) {
                 err = "argument must be 'yes' or 'no'"; goto loaderr;
             }
+        } else if (!strcasecmp(argv[0],"keep-slave-data") && argc == 2) {
+            if ((server.keep_slave_data = yesnotoi(argv[1])) == -1) {
+                err = "argument must be 'yes' or 'no'"; goto loaderr;
+            }
         } else if (!strcasecmp(argv[0],"glueoutputbuf")) {
             redisLog(REDIS_WARNING, "Deprecated configuration directive: \"%s\"", argv[0]);
         } else if (!strcasecmp(argv[0],"rdbcompression") && argc == 2) {
diff --git a/src/rdb.c b/src/rdb.c
index 7d11a11..61b431e 100644
--- a/src/rdb.c
+++ b/src/rdb.c
@@ -1000,7 +1000,11 @@ int rdbLoad(char *filename) {
             continue;
         }
         /* Add the new object in the hash table */
-        dbAdd(db,key,val);
+        if (lookupKeyWrite(db,key) == NULL) {
+            dbAdd(db,key,val);
+        } else {
+            dbOverwrite(db,key,val);
+        }
 
         /* Set the expire time if needed */
         if (expiretime != -1) setExpire(db,key,expiretime);
diff --git a/src/redis.c b/src/redis.c
index 8dda301..1d6a571 100644
--- a/src/redis.c
+++ b/src/redis.c
@@ -868,6 +868,7 @@ void initServerConfig() {
     server.repl_syncio_timeout = REDIS_REPL_SYNCIO_TIMEOUT;
     server.repl_serve_stale_data = 1;
     server.repl_down_since = -1;
+    server.keep_slave_data = 0;
 
     /* Double constants initialization */
     R_Zero = 0.0;
diff --git a/src/redis.h b/src/redis.h
index 6090860..95e73d8 100644
--- a/src/redis.h
+++ b/src/redis.h
@@ -470,6 +470,7 @@ struct redisServer {
     time_t repl_transfer_lastio; /* unix time of the latest read, for timeout */
     int repl_serve_stale_data; /* Serve stale data when link is down? */
     time_t repl_down_since; /* unix time at which link with master went down */
+    int keep_slave_data; /* Don't trash db when setting up replication. */
     /* Limits */
     unsigned int maxclients;
     unsigned long long maxmemory;
diff --git a/src/replication.c b/src/replication.c
index afe86d1..44e4c00 100644
--- a/src/replication.c
+++ b/src/replication.c
@@ -339,7 +339,13 @@ void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) {
             return;
         }
         redisLog(REDIS_NOTICE, "MASTER <-> SLAVE sync: Loading DB in memory");
-        emptyDb();
+
+        /* Only purge data in the slave if we haven't specified that we want to
+         * keep it. */
+        if (!server.keep_slave_data) {
+            emptyDb();
+        }
+
         /* Before loading the DB into memory we need to delete the readable
          * handler, otherwise it will get called recursively since
          * rdbLoad() will call the event loop to process events from time to