CMake: set ALWAYS_EMIT_SHARED_TEST_TARGETS when using ninja

cmake/macros/macro_deal_ii_pickup_tests.cmake

@@ -66,6 +66,11 @@
 #          * "medium": workstation, >=8 physical cores, >=32GB RAM
 #          * "heavy":  compute node, >=32 physical cores, >=128GB RAM
 #
+#   ALWAYS_EMIT_SHARED_TEST_TARGETS
+#     - If set to true then we: (a) always split out compiling and linking
+#       of an executable into a separate "test_dependency/" test, and (b)
+#       break up implicit target dependencies between the two.
+#
 # numdiff is used for the comparison of test results. Its location can be
 # specified with NUMDIFF_DIR.
 #
@@ -256,6 +261,28 @@ macro(deal_ii_pickup_tests)
   set_if_empty(TESTING_ENVIRONMENT "$ENV{TESTING_ENVIRONMENT}")
   set_if_empty(TESTING_ENVIRONMENT "light")
 
+  if(CMAKE_GENERATOR MATCHES "Ninja")
+    #
+    # When running tests for in parallel ctest issues multiple concurrent
+    # calls to the underlying build system. For performance reasons the
+    # ninja build system only ensures that ".ninja_log" is not corrupted
+    # in such a scenario, but doesn't ensure that it is in fact complete.
+    #
+    # We thus need to set ALWAYS_EMIT_SHARED_TEST_TARGETS to true, which
+    # splits all test targets and drops implicit target dependencies
+    # between (a) compilation and linkage, and (b) execution and output
+    # comparison.
+    #
+    # That way a parallel $ ctest invocation does what one expects, i.e.,
+    # trying to compile a test executable precisely once. Without this
+    # workaround multiple ninja instances might try to compile the same
+    # test executable multiple times leading to race conditions.
+    #
+    set_if_empty(ALWAYS_EMIT_SHARED_TEST_TARGETS TRUE)
+  else()
+    set_if_empty(ALWAYS_EMIT_SHARED_TEST_TARGETS FALSE)
+  endif()
+
   #
   # ... and finally pick up tests:
   #