Dump recent backlog on master query generating errors.

src/networking.c

@@ -393,6 +393,35 @@ void addReplyErrorLength(client *c, const char *s, size_t len) {
         serverLog(LL_WARNING,"== CRITICAL == This %s is sending an error "
                              "to its %s: '%s' after processing the command "
                              "'%s'", from, to, s, cmdname);
+        if (ctype == CLIENT_TYPE_MASTER && server.repl_backlog &&
+            server.repl_backlog_histlen > 0)
+        {
+            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);
+        }
         server.stat_unexpected_error_replies++;
     }
 }