catch2/docs/thread-safety.md

Thread safety in Catch2

Contents
Using assertion macros from spawned threads
Assertion-like message macros and spawned threads
Message macros and spawned threads
examples
STATIC_REQUIRE and STATIC_CHECK
Fatal errors and multiple threads
Performance overhead

Thread safe assertions were introduced in Catch2 3.9.0

Thread safety in Catch2 is currently limited to all the assertion macros, and to message or message-adjacent macros (e.g. INFO or WARN).

Interacting with benchmark macros, sections macros, generator macros, or test case macros is not thread-safe. The way sections define paths through the test is incompatible with user spawning threads arbitrarily, so this limitation is here to stay.

Important: thread safety in Catch2 is opt-in

Using assertion macros from spawned threads

The full set of Catch2's runtime assertion macros is thread-safe. However, it is important to keep in mind that their semantics might not support being used from user-spawned threads.

Specifically, the REQUIRE family of assertion macros have semantics of stopping the test execution on failure. This is done by throwing an exception, but since the user-spawned thread will not have the test-level try-catch block ready to catch the test failure exception, failing a REQUIRE assertion inside user-spawned thread will terminate the process.

The CHECK family of assertions does not have this issue, because it does not try to stop the test execution.

Note that CHECKED_IF and CHECKED_ELSE are also thread safe (internally they are assertion macro + an if).

Assertion-like message macros and spawned threads

Similarly to assertion macros, not all assertion-like message macros can be used from spawned thread.

SKIP and FAIL macros stop the test execution. Just like with REQUIRE, this means that they cannot be used inside user-spawned threads. SUCCEED, FAIL_CHECK and WARN do not attempt to stop the test execution and thus can be used from any thread.

Message macros and spawned threads

Macros that add extra messages to following assertion, such as INFO or CAPTURE, are all thread safe and can be used in any thread. Note that these messages are per-thread, and thus INFO inside a user-spawned thread will not be seen by the main thread, and vice versa.

examples

REQUIRE from the main thread, CHECK from spawned threads

TEST_CASE( "Failed REQUIRE in the main thread is fine" ) {
    std::vector<std::jthread> threads;
    for ( size_t t = 0; t < 16; ++t) {
        threads.emplace_back( []() {
            for (size_t i = 0; i < 10'000; ++i) {
                CHECK( true );
                CHECK( false );
            }
        } );
    }

    REQUIRE( false );
}

This will work as expected, that is, the process will finish running normally, the test case will fail and there will be the correct count of passing and failing assertions (160000 and 160001 respectively). However, it is important to understand that when the main thread fails its assertion, the spawned threads will keep running.

REQUIRE from spawned threads

TEST_CASE( "Successful REQUIRE in spawned thread is fine" ) {
    std::vector<std::jthread> threads;
    for ( size_t t = 0; t < 16; ++t) {
        threads.emplace_back( []() {
            for (size_t i = 0; i < 10'000; ++i) {
                REQUIRE( true );
            }
        } );
    }
}

This will also work as expected, because the REQUIRE is successful.

TEST_CASE( "Failed REQUIRE in spawned thread kills the process" ) {
    std::vector<std::jthread> threads;
    for ( size_t t = 0; t < 16; ++t) {
        threads.emplace_back( []() {
            for (size_t i = 0; i < 10'000; ++i) {
                REQUIRE( false );
            }
        } );
    }
}

This will fail catastrophically and terminate the process.

INFO across threads

TEST_CASE( "messages don't cross threads" ) {
    std::jthread t1( [&]() {
        for ( size_t i = 0; i < 100; ++i ) {
            INFO( "spawned thread #1" );
            CHECK( 1 == 1 );
        }
    } );

    std::thread t2( [&]() {
        for (size_t i = 0; i < 100; ++i) {
            UNSCOPED_INFO( "spawned thread #2" );
        }
    } );

    for (size_t i = 0; i < 100; ++i) {
        CHECK( 1 == 2 );
    }
}

None of the failed checks will show the "spawned thread #1" message, as that message is for the t1 thread. If the reporter shows passing assertions (e.g. due to the tests being run with -s), you will see the "spawned thread #1" message alongside the passing CHECK( 1 == 1 ) assertion.

The message "spawned thread #2" will never be shown, because there are no assertions in t2.

FAIL/SKIP from the main thread

TEST_CASE( "FAIL in the main thread is fine" ) {
    std::vector<std::jthread> threads;
    for ( size_t t = 0; t < 16; ++t) {
        threads.emplace_back( []() {
            for (size_t i = 0; i < 10; ++i) {
                CHECK( true );
                CHECK( false );
            }
        } );
    }

    FAIL();
}

This will work as expected, that is, the process will finish running normally, the test case will fail and there will be 321 total assertions, 160 passing and 161 failing (FAIL counts as failed assertion).

However, when the main thread hits FAIL, it will wait for the other threads to finish due to std::jthread's destructor joining the spawned thread. Due to this, using SKIP is not recommended once more threads are spawned; while the main thread will bail from the test execution, the spawned threads will keep running and may fail the test case.

FAIL/SKIP from spawned threads

TEST_CASE( "FAIL/SKIP in spawned thread kills the process" ) {
    std::vector<std::jthread> threads;
    for ( size_t t = 0; t < 16; ++t) {
        threads.emplace_back( []() {
            for (size_t i = 0; i < 10'000; ++i) {
                FAIL();
            }
        } );
    }
}

As with failing REQUIRE, both FAIL and SKIP in spawned threads terminate the process.

STATIC_REQUIRE and STATIC_CHECK

All of STATIC_REQUIRE, STATIC_REQUIRE_FALSE, STATIC_CHECK, and STATIC_CHECK_FALSE are thread safe in the delayed evaluation configuration.

Fatal errors and multiple threads

By default, Catch2 tries to catch fatal errors (POSIX signals/Windows Structured Exceptions) and report something useful to the user. This always happened on a best-effort basis, but in presence of multiple threads and locks the chance of it working decreases. If this starts being an issue for you, you can disable it.

Performance overhead

In the worst case, which is optimized build and assertions using the fast path for successful assertions, the performance overhead of using the thread-safe assertion implementation can reach 40%. In other cases, the overhead will be smaller, between 4% and 20%.

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