ctest_build: Add the PARALLEL_LEVEL argument Fixes: #19712

diff --git a/Help/command/ctest_build.rst b/Help/command/ctest_build.rst
index 4d6dc5a..e05df1a 100644
--- a/Help/command/ctest_build.rst
+++ b/Help/command/ctest_build.rst

@@ -7,6 +7,7 @@
 
   ctest_build([BUILD <build-dir>] [APPEND]
               [CONFIGURATION <config>]
+              [PARALLEL_LEVEL <parallel>]
               [FLAGS <flags>]
               [PROJECT_NAME <project-name>]
               [TARGET <target-name>]
@@ -42,6 +43,13 @@
   Otherwise the ``-C <cfg>`` option given to the :manual:`ctest(1)`
   command will be used, if any.
 
+``PARALLEL_LEVEL <parallel>``
+  .. versionadded:: 3.21
+
+  Specify the parallel level of the underlying build system.  If not
+  specified, the :envvar:`CMAKE_BUILD_PARALLEL_LEVEL` environment
+  variable will be checked.
+
 ``FLAGS <flags>``
   Pass additional arguments to the underlying build command.
   If not specified the ``CTEST_BUILD_FLAGS`` variable will be checked.

diff --git a/Help/release/dev/generate-cmake-build-command-parallel.rst b/Help/release/dev/generate-cmake-build-command-parallel.rst
index 8d3b7d7..1b8dd40 100644
--- a/Help/release/dev/generate-cmake-build-command-parallel.rst
+++ b/Help/release/dev/generate-cmake-build-command-parallel.rst

@@ -2,3 +2,5 @@
 -------------------------------------
 
 * The :command:`build_command` command gained a ``PARALLEL_LEVEL`` option.
+
+* The :command:`ctest_build` command gained a ``PARALLEL_LEVEL`` option.

diff --git a/Source/CTest/cmCTestBuildCommand.cxx b/Source/CTest/cmCTestBuildCommand.cxx
index 099d1e2..483c316 100644
--- a/Source/CTest/cmCTestBuildCommand.cxx
+++ b/Source/CTest/cmCTestBuildCommand.cxx

@@ -27,6 +27,7 @@
   this->Bind("CONFIGURATION"_s, this->Configuration);
   this->Bind("FLAGS"_s, this->Flags);
   this->Bind("PROJECT_NAME"_s, this->ProjectName);
+  this->Bind("PARALLEL_LEVEL"_s, this->ParallelLevel);
 }
 
 cmCTestBuildCommand::~cmCTestBuildCommand() = default;
@@ -98,7 +99,7 @@
       std::string dir = this->CTest->GetCTestConfiguration("BuildDirectory");
       std::string buildCommand =
         this->GlobalGenerator->GenerateCMakeBuildCommand(
-          cmakeBuildTarget, cmakeBuildConfiguration, "",
+          cmakeBuildTarget, cmakeBuildConfiguration, this->ParallelLevel,
           cmakeBuildAdditionalFlags, this->Makefile->IgnoreErrorsCMP0061());
       cmCTestOptionalLog(this->CTest, HANDLER_VERBOSE_OUTPUT,
                          "SetMakeCommand:" << buildCommand << "\n",

diff --git a/Source/CTest/cmCTestBuildCommand.h b/Source/CTest/cmCTestBuildCommand.h
index 00dbcc4..1254dad 100644
--- a/Source/CTest/cmCTestBuildCommand.h
+++ b/Source/CTest/cmCTestBuildCommand.h

@@ -60,4 +60,5 @@
   std::string Configuration;
   std::string Flags;
   std::string ProjectName;
+  std::string ParallelLevel;
 };

diff --git a/Tests/RunCMake/ctest_build/ParallelLevel-check.cmake b/Tests/RunCMake/ctest_build/ParallelLevel-check.cmake
new file mode 100644
index 0000000..f45d2a2
--- /dev/null
+++ b/Tests/RunCMake/ctest_build/ParallelLevel-check.cmake

@@ -0,0 +1,11 @@
+file(GLOB build_xml_file "${RunCMake_TEST_BINARY_DIR}/Testing/*/Build.xml")
+if(build_xml_file)
+  file(STRINGS "${build_xml_file}" build_cmd LIMIT_COUNT 1 REGEX "<BuildCommand>")
+  if(NOT build_cmd MATCHES [[ --parallel "1"]])
+    set(RunCMake_TEST_FAILED
+      "Build.xml does not have expected build command with --parallel flag"
+      )
+  endif()
+else()
+  set(RunCMake_TEST_FAILED "Build.xml not found")
+endif()

diff --git a/Tests/RunCMake/ctest_build/RunCMakeTest.cmake b/Tests/RunCMake/ctest_build/RunCMakeTest.cmake
index 072fbac..511cd71 100644
--- a/Tests/RunCMake/ctest_build/RunCMakeTest.cmake
+++ b/Tests/RunCMake/ctest_build/RunCMakeTest.cmake

@@ -10,6 +10,7 @@
 endfunction()
 
 run_ctest_build(BuildQuiet QUIET)
+run_ctest_build(ParallelLevel PARALLEL_LEVEL 1)
 
 function(run_BuildFailure)
   set(CASE_CMAKELISTS_SUFFIX_CODE [[