After #11599 it is possible that messages of the
ReadBufferFromKafkaConsumer will be cleaned-up right in
read_kafka_message callback (from KafkaBlockInputStream) if the stop
flag isset (i.e. DROP TABLE is waiting the consumer), and if
read_kafka_message already processed some rows it will not return 0 and
the loop after will try to get current topic from the buffer, which uses
messages in the underlying and this will got SIGSEGV:

    12:14:56.173262 [ 55421 ] {f7930856-d478-4e41-af56-24ce7b693e95} <Debug> executeQuery: (from 0.0.0.0:0, user: ) DROP TABLE IF EXISTS data.queue
    12:14:56.173285 [ 55421 ] {f7930856-d478-4e41-af56-24ce7b693e95} <Trace> StorageKafka (newly_queue): Waiting for cleanup
    12:14:56.180016 [ 55390 ] {} <Trace> BaseDaemon: Received signal 11
    12:14:56.180267 [ 4914 ] {} <Fatal> BaseDaemon: ########################################
    12:14:56.181879 [ 4914 ] {} <Fatal> BaseDaemon: (version 20.6.1.1, build id: 4CE0298F08583658) (from thread 55468) (no query) Received signal Segmentation fault (11)
    12:14:56.181900 [ 4914 ] {} <Fatal> BaseDaemon: Address: 0x8 Access: read. Address not mapped to object.
    12:14:56.181909 [ 4914 ] {} <Fatal> BaseDaemon: Stack trace:
    12:14:56.184676 [ 4914 ] {} <Fatal> BaseDaemon: 3. /ch/contrib/cppkafka/include/cppkafka/message.h:111: DB::KafkaBlockInputStream::readImpl() @ 0xe343f1c in /usr/lib/debug/usr/bin/clickhouse
    12:14:56.185553 [ 4914 ] {} <Fatal> BaseDaemon: 4. /ch/contrib/libcxx/include/vector:1003: DB::IBlockInputStream::read() @ 0xd9d95bd in /usr/lib/debug/usr/bin/clickhouse
    12:14:56.188238 [ 4914 ] {} <Fatal> BaseDaemon: 5. /ch/src/DataStreams/copyData.cpp:26: DB::copyData() @ 0xd9f712a in /usr/lib/debug/usr/bin/clickhouse
    12:14:56.188780 [ 4914 ] {} <Fatal> BaseDaemon: 6. /ch/contrib/libcxx/include/vector:1532: DB::StorageKafka::streamToViews() @ 0xe335e73 in /usr/lib/debug/usr/bin/clickhouse
    12:14:56.189331 [ 4914 ] {} <Fatal> BaseDaemon: 7. /ch/src/Storages/Kafka/StorageKafka.cpp:491: DB::StorageKafka::threadFunc() @ 0xe336738 in /usr/lib/debug/usr/bin/clickhouse

55421 thread (shows that it still waiting for deactivation):

    5  std::__1::lock_guard<>::lock_guard () at ../contrib/libcxx/include/__mutex_base:90
    6  DB::BackgroundSchedulePoolTaskInfo::deactivate (this=0x7fc7e4465f20) at ../src/Core/BackgroundSchedulePool.cpp:59
    7  DB::StorageKafka::shutdown (this=0x7fc7e45e4600) at ../contrib/libcxx/include/memory:3821

And just in case thread where read_kafka_message is called:

    0  DB::ReadBufferFromKafkaConsumer::nextImpl (this=0x7fd4901d4118) at ../contrib/libcxx/include/atomic:1491
    1  DB::ReadBuffer::next (this=0x7fd4901d4118) at ../src/IO/ReadBuffer.h:59
    2  DB::ReadBuffer::eof (this=0x7fd4901d4118) at ../src/IO/ReadBuffer.h:81
    3  DB::skipWhitespaceIfAny (buf=...) at ../src/IO/ReadHelpers.h:945
    4  DB::JSONEachRowRowInputFormat::readRow (ext=..., columns=..., this=0x7fd499a7a020) at ../src/Processors/Formats/Impl/JSONEachRowRowInputFormat.cpp:222
    5  DB::JSONEachRowRowInputFormat::readRow (this=0x7fd499a7a020, columns=..., ext=...) at ../src/Processors/Formats/Impl/JSONEachRowRowInputFormat.cpp:218
    6  DB::IRowInputFormat::generate (this=0x7fd499a7a020) at ../src/Processors/Formats/IRowInputFormat.cpp:64
    7  DB::ISource::work (this=0x7fd499a7a020) at ../src/Processors/ISource.cpp:48
    8  DB::KafkaBlockInputStream::<lambda()>::operator()(void) const () at ../contrib/libcxx/include/memory:3826
    9  DB::KafkaBlockInputStream::readImpl (this=0x7fd46e718820) at ../contrib/libcxx/include/new:340

Cc: @filimonov