ctest: Allow passing -j without value to choose a contextual default

Under job server integration, added by commit 80fe56c481 (ctest: Add
support for running under a make job server on POSIX systems,
2023-11-15, v3.29.0-rc1~324^2), use a very high default so that
parallelism is effectively limited only by available job server tokens.

Otherwise, choose a default limit based on the number of processors.

Also allow passing `0` to specify unbounded parallelism.

Fixes: #25739

1 files changed, 11 insertions, 0 deletions

diff --git a/Help/envvar/CTEST_PARALLEL_LEVEL.rst b/Help/envvar/CTEST_PARALLEL_LEVEL.rst
index 126bdb3..0ef01d5 100644
--- a/Help/envvar/CTEST_PARALLEL_LEVEL.rst
+++ b/Help/envvar/CTEST_PARALLEL_LEVEL.rst
@@ -8,4 +8,15 @@ For example, if ``CTEST_PARALLEL_LEVEL`` is set to 8, CTest will run
 up to 8 tests concurrently as if ``ctest`` were invoked with the
 :option:`--parallel 8 <ctest --parallel>` option.
 
+.. versionchanged:: 3.29
+
+  The value may be empty, or ``0``, to let ctest use a default level of
+  parallelism, or unbounded parallelism, respectively, as documented by
+  the :option:`ctest --parallel` option.
+
+  On Windows, environment variables cannot be set to an empty string.
+  CTest will interpret a whitespace-only string as empty.
+
+  In CMake 3.28 and earlier, an empty or ``0`` value was equivalent to ``1``.
+
 See :manual:`ctest(1)` for more information on parallel test execution.