fix: Ensure that coverage file generation is atomic. (#5426)

Running unit tests in parallel and multiple threads can write into one file can corrupt output files, and then gcovr won't be able to parse the corrupted file. This change adds -fprofile-update=atomic as instructed by https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68080.

.github/workflows/nix.yml

@@ -247,7 +247,7 @@ jobs:
           mkdir -p ~/.conan
           tar -xzf conan.tar -C ~/.conan
       - name: install gcovr
-        run: pip install "gcovr>=7,<8"
+        run: pip install "gcovr>=7,<9"
       - name: check environment
         run: |
           echo ${PATH} | tr ':' '\n'

cmake/CodeCoverage.cmake

@@ -98,6 +98,9 @@
 # 2024-04-03, Bronek Kozicki
 #     - add support for output formats: jacoco, clover, lcov
 #
+# 2025-05-12, Jingchen Wu
+#     - add -fprofile-update=atomic to ensure atomic profile generation
+#
 # USAGE:
 #
 # 1. Copy this file into your cmake modules path.
@@ -200,15 +203,27 @@ set(COVERAGE_COMPILER_FLAGS "-g --coverage"
     CACHE INTERNAL "")
 if(CMAKE_CXX_COMPILER_ID MATCHES "(GNU|Clang)")
     include(CheckCXXCompilerFlag)
+    include(CheckCCompilerFlag)
+
     check_cxx_compiler_flag(-fprofile-abs-path HAVE_cxx_fprofile_abs_path)
     if(HAVE_cxx_fprofile_abs_path)
         set(COVERAGE_CXX_COMPILER_FLAGS "${COVERAGE_COMPILER_FLAGS} -fprofile-abs-path")
     endif()
-    include(CheckCCompilerFlag)
+
     check_c_compiler_flag(-fprofile-abs-path HAVE_c_fprofile_abs_path)
     if(HAVE_c_fprofile_abs_path)
         set(COVERAGE_C_COMPILER_FLAGS "${COVERAGE_COMPILER_FLAGS} -fprofile-abs-path")
     endif()
+
+    check_cxx_compiler_flag(-fprofile-update HAVE_cxx_fprofile_update)
+    if(HAVE_cxx_fprofile_update)
+        set(COVERAGE_CXX_COMPILER_FLAGS "${COVERAGE_COMPILER_FLAGS}  -fprofile-update=atomic")
+    endif()
+
+    check_c_compiler_flag(-fprofile-update HAVE_c_fprofile_update)
+    if(HAVE_c_fprofile_update)
+        set(COVERAGE_C_COMPILER_FLAGS "${COVERAGE_COMPILER_FLAGS} -fprofile-update=atomic")
+    endif()
 endif()
 
 set(CMAKE_Fortran_FLAGS_COVERAGE