Replication: showLatestBacklog() refactored out.

src/networking.c

@@ -396,31 +396,7 @@ void addReplyErrorLength(client *c, const char *s, size_t len) {
         if (ctype == CLIENT_TYPE_MASTER && server.repl_backlog &&
             server.repl_backlog_histlen > 0)
         {
-            long long dumplen = 256;
+            showLatestBacklog();
-            if (server.repl_backlog_histlen < dumplen)
-                dumplen = server.repl_backlog_histlen;
-
-            /* Identify the first byte to dump. */
-            long long idx =
-              (server.repl_backlog_idx + (server.repl_backlog_size - dumplen)) %
-               server.repl_backlog_size;
-
-            /* Scan the circular buffer to collect 'dumplen' bytes. */
-            sds dump = sdsempty();
-            while(dumplen) {
-                long long thislen =
-                    ((server.repl_backlog_size - idx) < dumplen) ?
-                    (server.repl_backlog_size - idx) : dumplen;
-
-                dump = sdscatrepr(dump,server.repl_backlog+idx,thislen);
-                dumplen -= thislen;
-                idx = 0;
-            }
-
-            /* Finally log such bytes: this is vital debugging info to
-             * understand what happened. */
-            serverLog(LL_WARNING,"Latest backlog is: '%s'", dump);
-            sdsfree(dump);
         }
         server.stat_unexpected_error_replies++;
     }

src/replication.c

@@ -307,6 +307,40 @@ void replicationFeedSlaves(list *slaves, int dictid, robj **argv, int argc) {
     }
 }
 
+/* This is a debugging function that gets called when we detect something
+ * wrong with the replication protocol: the goal is to peek into the
+ * replication backlog and show a few final bytes to make simpler to
+ * guess what kind of bug it could be. */
+void showLatestBacklog(void) {
+    if (server.repl_backlog == NULL) return;
+
+    long long dumplen = 256;
+    if (server.repl_backlog_histlen < dumplen)
+        dumplen = server.repl_backlog_histlen;
+
+    /* Identify the first byte to dump. */
+    long long idx =
+      (server.repl_backlog_idx + (server.repl_backlog_size - dumplen)) %
+       server.repl_backlog_size;
+
+    /* Scan the circular buffer to collect 'dumplen' bytes. */
+    sds dump = sdsempty();
+    while(dumplen) {
+        long long thislen =
+            ((server.repl_backlog_size - idx) < dumplen) ?
+            (server.repl_backlog_size - idx) : dumplen;
+
+        dump = sdscatrepr(dump,server.repl_backlog+idx,thislen);
+        dumplen -= thislen;
+        idx = 0;
+    }
+
+    /* Finally log such bytes: this is vital debugging info to
+     * understand what happened. */
+    serverLog(LL_WARNING,"Latest backlog is: '%s'", dump);
+    sdsfree(dump);
+}
+
 /* This function is used in order to proxy what we receive from our master
  * to our sub-slaves. */
 #include <ctype.h>

src/server.h

@@ -1810,6 +1810,7 @@ void clearReplicationId2(void);
 void chopReplicationBacklog(void);
 void replicationCacheMasterUsingMyself(void);
 void feedReplicationBacklog(void *ptr, size_t len);
+void showLatestBacklog(void);
 void rdbPipeReadHandler(struct aeEventLoop *eventLoop, int fd, void *clientData, int mask);
 void rdbPipeWriteHandlerConnRemoved(struct connection *conn);