Prior to this commit throwing error_already_set was expensive due to the
eager construction of the error string (which required traversing the
Python stack). See pybind#1853 for more context and an alternative take on the
issue.

Note that error_already_set no longer inherits from std::runtime_error
because the latter has no default constructor.

This is not supported on PyPy-2.7 5.8.0.

This is faster than dynamically looking up __name__ via GetAttrString.
Note though that the runtime of the code throwing an error_already_set
will be dominated by stack unwinding so the improvement will not be
noticeable.

Before:

396 ns ± 0.913 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

After:

277 ns ± 0.549 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

Benchmark:

const std::string foo() {
    PyErr_SetString(PyExc_KeyError, "");
    const std::string &s = py::detail::error_string();
    PyErr_Clear();
    return s;
}

PYBIND11_MODULE(foo, m) {
    m.def("foo", &::foo);
}

The implementation of __name__ is slightly more complex than that.
It handles the module name prefix, and heap-allocated types. We could
port it to pybind11 later on but for now it seems like an overkill.

This reverts commit f1435c7.

Note that detail::error_string() no longer calls PyException_SetTraceback
as it is unncessary for pretty-printing the exception.

…-error-string

…-error-string

…-error-string

…o lazy-error-string

…-error-string

…ng" and terminate process.

…vate

…-error-string

…ind11 into lazy-error-string

…h comments

It sure would need to be guarded by `if (m_type)`, otherwise `what()` fails and masks that no error was set (see update unit test). But since `error_already_set` is copyable, there is no point in releasing m_type, m_value, m_trace, therefore we can just as well avoid the runtime overhead of force-building `m_lazy_what`, it may never be used.

…flection of what we know.

…r indicator not set. The net result is that a std::runtime_error is thrown instead of error_already_set, but all tests pass as is.

…3.7 test failures expected. This is meant for benchmarking, to determine if it is worth the trouble looking into the failures.

… long comment.

…sable`

…xception, if & when needed. Consistently document the side-effect.

          function                   #calls  test time [s]  μs / call
master    pure_unwind                729540      1.061      14.539876
          err_set_unwind_err_clear   681476      1.040      15.260282
          err_set_error_already_set  508038      1.049      20.640525
          error_already_set_restore  555578      1.052      18.933288
          pr1895_original_foo        244113      1.050      43.018168
                                                                       PR / master
PR pybind#1895  pure_unwind                736981      1.054      14.295685       98.32%
          err_set_unwind_err_clear   685820      1.045      15.237399       99.85%
          err_set_error_already_set  661374      1.046      15.811879       76.61%
          error_already_set_restore  669881      1.048      15.645176       82.63%
          pr1895_original_foo        318243      1.059      33.290806       77.39%

master @ commit ad146b2

Running tests in directory "/usr/local/google/home/rwgk/forked/pybind11/tests":
============================= test session starts ==============================
platform linux -- Python 3.9.10, pytest-6.2.3, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /usr/local/google/home/rwgk/forked/pybind11/tests, configfile: pytest.ini
collecting ... collected 5 items

test_perf_error_already_set.py::test_perf[pure_unwind]
PERF pure_unwind,729540,1.061,14.539876
PASSED
test_perf_error_already_set.py::test_perf[err_set_unwind_err_clear]
PERF err_set_unwind_err_clear,681476,1.040,15.260282
PASSED
test_perf_error_already_set.py::test_perf[err_set_error_already_set]
PERF err_set_error_already_set,508038,1.049,20.640525
PASSED
test_perf_error_already_set.py::test_perf[error_already_set_restore]
PERF error_already_set_restore,555578,1.052,18.933288
PASSED
test_perf_error_already_set.py::test_perf[pr1895_original_foo]
PERF pr1895_original_foo,244113,1.050,43.018168
PASSED

============================== 5 passed in 12.38s ==============================

pr1895 @ commit 8dff51d

Running tests in directory "/usr/local/google/home/rwgk/forked/pybind11/tests":
============================= test session starts ==============================
platform linux -- Python 3.9.10, pytest-6.2.3, py-1.10.0, pluggy-0.13.1 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /usr/local/google/home/rwgk/forked/pybind11/tests, configfile: pytest.ini
collecting ... collected 5 items

test_perf_error_already_set.py::test_perf[pure_unwind]
PERF pure_unwind,736981,1.054,14.295685
PASSED
test_perf_error_already_set.py::test_perf[err_set_unwind_err_clear]
PERF err_set_unwind_err_clear,685820,1.045,15.237399
PASSED
test_perf_error_already_set.py::test_perf[err_set_error_already_set]
PERF err_set_error_already_set,661374,1.046,15.811879
PASSED
test_perf_error_already_set.py::test_perf[error_already_set_restore]
PERF error_already_set_restore,669881,1.048,15.645176
PASSED
test_perf_error_already_set.py::test_perf[pr1895_original_foo]
PERF pr1895_original_foo,318243,1.059,33.290806
PASSED

============================== 5 passed in 12.40s ==============================

clang++ -o pybind11/tests/test_perf_error_already_set.os -c -std=c++17 -fPIC -fvisibility=hidden -Os -flto -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wnon-virtual-dtor -Wunused-result -isystem /usr/include/python3.9 -isystem /usr/include/eigen3 -DPYBIND11_STRICT_ASSERTS_CLASS_HOLDER_VS_TYPE_CASTER_MIX -DPYBIND11_TEST_BOOST -Ipybind11/include -I/usr/local/google/home/rwgk/forked/pybind11/include -I/usr/local/google/home/rwgk/clone/pybind11/include /usr/local/google/home/rwgk/forked/pybind11/tests/test_perf_error_already_set.cpp

clang++ -o lib/pybind11_tests.so -shared -fPIC -Os -flto -shared ...

Debian clang version 13.0.1-3+build2
Target: x86_64-pc-linux-gnu
Thread model: posix

… testing.

…ind11 into lazy-error-string

…try-catch.

…ceback()

…move ctor

…s not make sense anymore with the lazy what)

…765113).

…ote that "method" is Python terminology).

…xperimenting).

… MSVC test failures.

…Skylion007 IIUC).

…ses the copy ctor.

… (The tests in the previous commit exposed that those are broken.)