BLPOP inside MULTI/EXEC block no longer crashes, instead if the list is empty...

BLPOP inside MULTI/EXEC block no longer crashes, instead if the list is empty the behavior is like if the timeout is reached. This fixes Issue 285

src/t_list.c

@@ -823,6 +823,13 @@ void blockingPopGenericCommand(redisClient *c, int where) {
             }
         }
     }
+    /* If we are inside a MULTI/EXEC and the list is empty the only thing
+     * we can do is treating it as a timeout (even with timeout 0). */
+    if (c->flags & REDIS_MULTI) {
+        addReply(c,shared.nullmultibulk);
+        return;
+    }
+
     /* If the list is empty or the key does not exists we must block */
     timeout = strtol(c->argv[c->argc-1]->ptr,NULL,10);
     if (timeout > 0) timeout += time(NULL);

tests/unit/type/list.tcl

@@ -172,6 +172,17 @@ start_server {
         }
     }
 
+    test {BLPOP inside a transaction} {
+        r del xlist
+        r lpush xlist foo
+        r lpush xlist bar
+        r multi
+        r blpop xlist 0
+        r blpop xlist 0
+        r blpop xlist 0
+        r exec
+    } {{xlist bar} {xlist foo} {}}
+
     test {LPUSHX, RPUSHX - generic} {
         r del xlist
         assert_equal 0 [r lpushx xlist a]