has_message_available() with timestamp seek returns True on first call when reaching end of the topic

[abeliangroupie]

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Ran into this while trying to upgrade to the v3.8.0 Python client. Tested on a Pulsar v4.0.4 cluster, which is after this issue was fixed in Pulsar v4.0.1.

Possibly related to apache/pulsar #23917?

---

It seems that if you:

• Have a reader
• Seek by timestamp to a point where there's messages after it
• read_next() until there's no more messages

Then the next has_message_available() call returns True, even though there's no more messages afterwards. Additionally, calling has_message_available() again correctly returns False, meaning that this bug only happens the first time.

This is an issue because a loop like:

while reader.has_message_available():
    msg = reader.read_next()

will end up hanging when it reaches the end of the topic, due to the incorrectly returned True value from has_message_available().

---

Tested by having a producer that publishes one message per second:

import json
import time

import pulsar

client = pulsar.Client("pulsar://localhost:6650")

producer = client.create_producer(
    "persistent://foo/bar/baz",
)

while True:
    data = json.dumps({"ts": time.time()}).encode("utf-8")
    print("Sending data:", data, flush=True)
    producer.send(data)
    time.sleep(1)

Then start a reader like so:

import time

import pulsar

client = pulsar.Client("pulsar://localhost:6650")

reader = client.create_reader(
    "persistent://foo/bar/baz",
    start_message_id=pulsar.MessageId.latest,
)
reader.seek(int(time.time() - 10) * 1000)  # Seek to 10s ago.

while True:
    print(reader.has_message_available(), flush=True)
    print(reader.has_message_available(), flush=True)
    print(reader.has_message_available(), flush=True)
    print("="*20)
    msg = reader.read_next()

Observed output:

• Initially there's a lot of True True True, for messages in the past while the reader is catching up
• Once the reader catches up, the outputs become True False False, indicating that the first has_message_available() call each time is incorrectly giving True

Expected output:

• Once the reader has caught up, the outputs should be False False False, since there's no message available and read_next() needs to block until a new message comes in.

Notes:

• The bug doesn't seem to happen if there's no seek by timestamp done at the start.
• The bug doesn't seem to happen if there's a small delay after the read_next() (e.g. time.sleep(0.1)) -- for this it's easier to observe if you make the producer publish slower.
• The bug was probably introduced in v3.5.0, since it does not occur with the v3.4.0 client but it does with the v3.5.0 client.