Repository: cunla/fakeredis-py

Issue:
Using a blocking XREAD on an empty stream returns a list instead of blocking 
**Describe the bug**
When using `xread` on an empty or non-existant stream with a blocking argument, the function incorrectly returns an empty list instead of blocking.

This is a regression of behavior from the real Redis client.

The source of the bug seems to be in `_xread` in `streammixins.py`. If the stream is empty, the function will return `res` which is an empty list.

When we call `xread` with a blocking argument, it wraps the function `_xread` with `_blocking` from `_basefakesocket.py`. Since the `_xread` returns `list()` instead of `None`, the blocking wrapper will not retry, as it only yields if the function returns a `None`.

Having `_xread` return `None` in the case of an empty stream could probably fix this issue, but I haven't verified if this doesn't create a regression elsewhere.


**To Reproduce**
Step to reproduce the behavior:
```
client.xread(streams={"empty-stream": 0}, count=1, block=0)
```
In fakeredis, this code immediately returns an empty list. The same is also true for the `async` version.

**Expected behavior**
The same line of code in `redis-py` yields forever until an event is added to the stream, and returns it.

This is the proper behavior since count is `1` and we specified for it to block infinitely. It should never return a result without exactly one event.

**Desktop:**
 - OS: Ubuntu 22
 - Python: 3.11
 - Fakeredis version: 2.20.1
 - Redis-Py version: 5.0.1


Discussion:

Patch:
diff --git a/fakeredis/commands_mixins/streams_mixin.py b/fakeredis/commands_mixins/streams_mixin.py
index e14d811..4d37900 100644
--- a/fakeredis/commands_mixins/streams_mixin.py
+++ b/fakeredis/commands_mixins/streams_mixin.py
@@ -104,15 +104,19 @@ class StreamsCommandsMixin:
         (count,), _ = extract_args(args, ("+count",))
         return self._xrange(key.value, _max, _min, True, count)
 
-    def _xread(self, stream_start_id_list: List, count: int, first_pass: bool):
+    def _xread(self, stream_start_id_list: List[Tuple[bytes, StreamRangeTest]], count: int, blocking: bool,
+               first_pass: bool):
         max_inf = StreamRangeTest.decode(b"+")
         res = list()
-        for item, start_id in stream_start_id_list:
+        for stream_name, start_id in stream_start_id_list:
+            item = CommandItem(stream_name, self._db, item=self._db.get(stream_name), default=None)
             stream_results = self._xrange(item.value, start_id, max_inf, False, count)
             if first_pass and (count is None):
                 return None
             if len(stream_results) > 0:
                 res.append([item.key, stream_results])
+        if blocking and count and len(res) == 0:
+            return None
         return res
 
     def _xreadgroup(
@@ -135,6 +139,8 @@ class StreamsCommandsMixin:
     @staticmethod
     def _parse_start_id(key: CommandItem, s: bytes) -> StreamRangeTest:
         if s == b"$":
+            if key.value is None:
+                return StreamRangeTest.decode(b"0-0")
             return StreamRangeTest.decode(key.value.last_item_key(), exclusive=True)
         return StreamRangeTest.decode(s, exclusive=True)
 
@@ -146,23 +152,16 @@ class StreamsCommandsMixin:
         left_args = left_args[1:]
         num_streams = int(len(left_args) / 2)
 
-        stream_start_id_list = list()
+        stream_start_id_list: List[Tuple[bytes, StreamRangeTest]] = list()  # (name, start_id)
         for i in range(num_streams):
-            item = CommandItem(
-                left_args[i], self._db, item=self._db.get(left_args[i]), default=None
-            )
+            item = CommandItem(left_args[i], self._db, item=self._db.get(left_args[i]), default=None)
             start_id = self._parse_start_id(item, left_args[i + num_streams])
-            stream_start_id_list.append(
-                (
-                    item,
-                    start_id,
-                )
-            )
+            stream_start_id_list.append((left_args[i], start_id))
         if timeout is None:
-            return self._xread(stream_start_id_list, count, False)
+            return self._xread(stream_start_id_list, count, blocking=False, first_pass=False)
         else:
             return self._blocking(
-                timeout / 1000.0, functools.partial(self._xread, stream_start_id_list, count)
+                timeout / 1000.0, functools.partial(self._xread, stream_start_id_list, count, True)
             )
 
     @command(name="XREADGROUP", fixed=(bytes, bytes, bytes), repeat=(bytes,))


Test patch:
diff --git a/test/test_mixins/test_streams_commands.py b/test/test_mixins/test_streams_commands.py
index b74b333..cb76857 100644
--- a/test/test_mixins/test_streams_commands.py
+++ b/test/test_mixins/test_streams_commands.py
@@ -1,3 +1,4 @@
+import threading
 import time
 from typing import List
 
@@ -744,3 +745,28 @@ def test_xclaim(r: redis.Redis):
         stream, group, consumer1,
         min_idle_time=0, message_ids=(message_id,), justid=True,
     ) == [message_id, ]
+
+
+def test_xread_blocking(create_redis):
+    # thread with xread block 0 should hang
+    # putting data in the stream should unblock it
+    event = threading.Event()
+    event.clear()
+
+    def thread_func():
+        while not event.is_set():
+            time.sleep(0.1)
+        r = create_redis(db=1)
+        r.xadd("stream", {"x": "1"})
+        time.sleep(0.1)
+
+    t = threading.Thread(target=thread_func)
+    t.start()
+    r1 = create_redis(db=1)
+    event.set()
+    result = r1.xread({"stream": "$"}, block=0, count=1)
+    event.clear()
+    t.join()
+    assert result[0][0] == b"stream"
+    assert result[0][1][0][1] == {b'x': b'1'}
+    pass