tests/unit: fix cmake custom command race condition

CMake's documentation says [1]:

> Do not list the output in more than one independent target that may
> build in parallel or the two instances of the rule may conflict
> (instead use the add_custom_target() command to drive the command and
> make the other targets depend on that one).

Because gschemas.compiled will be referenced multiple times
independently, a level of redirection is needed to prevent CMake from
generating the rule for the file multiple times.

This is encountered in ayatana-indicator-power as well, and fixed in a
similar way [2].

[1] https://cmake.org/cmake/help/latest/command/add_custom_command.html#generating-files
[2] https://github.com/AyatanaIndicators/ayatana-indicator-power/commit/9a90f932e51db486b166dd38f00e4186e40a1aee

1 files changed, 8 insertions, 1 deletions

diff --git a/tests/unit/CMakeLists.txt b/tests/unit/CMakeLists.txt
index a70362a..7d42693 100644
--- a/tests/unit/CMakeLists.txt
+++ b/tests/unit/CMakeLists.txt
@@ -17,11 +17,17 @@ add_definitions(-DSCHEMA_DIR="${SCHEMA_DIR}")
 execute_process (COMMAND ${PKG_CONFIG_EXECUTABLE} gio-2.0 --variable glib_compile_schemas
                  OUTPUT_VARIABLE COMPILE_SCHEMA_EXECUTABLE
                  OUTPUT_STRIP_TRAILING_WHITESPACE)
+# Because gschemas.compiled will be referenced multiple times independently, a level of
+# redirection is needed to prevent CMake from generating the rule for the file multiple
+# times, causing a race condition.
+# https://discourse.cmake.org/t/race-condition-when-multi-add-custom-target-deps-on-same-file-generated-by-add-custom-command/2358
+# https://bugs.debian.org/892091 (bug for ayatana-indicators-power, but applies here as well)
 add_custom_command (OUTPUT gschemas.compiled
                     DEPENDS ${CMAKE_BINARY_DIR}/data/org.ayatana.display.gschema.xml
                     COMMAND mkdir -p ${SCHEMA_DIR}
                     COMMAND cp -f ${DISPLAY_SCHEMA} ${SCHEMA_DIR}
                     COMMAND ${COMPILE_SCHEMA_EXECUTABLE} ${SCHEMA_DIR})
+add_custom_target(gschemas-compiled DEPENDS gschemas.compiled)
 
 set(SERVICE_LINK_LIBRARIES
     ${SERVICE_LIB}
@@ -46,9 +52,10 @@ add_definitions(
 function(add_test_by_name name)
   set(TEST_NAME ${name})
   set(COVERAGE_TEST_TARGETS ${COVERAGE_TEST_TARGETS} ${TEST_NAME} PARENT_SCOPE)
-  add_executable (${TEST_NAME} ${TEST_NAME}.cpp gschemas.compiled)
+  add_executable (${TEST_NAME} ${TEST_NAME}.cpp)
   target_link_options(${TEST_NAME} PRIVATE -no-pie)
   add_test(${TEST_NAME} ${TEST_NAME})
+  add_dependencies(${TEST_NAME} gschemas-compiled)
   set_property(TEST ${TEST_NAME} APPEND PROPERTY ENVIRONMENT ${CTEST_ENVIRONMENT})
   target_link_libraries(${TEST_NAME} ${SERVICE_LINK_LIBRARIES} ${TEST_LINK_LIBRARIES} ${THREAD_LINK_LIBRARIES})
 endfunction()