Windows: Prevent auto exports to be regenerated on every build

Check modified time stamps of input files against an existing exports file
before generating the auto exports.

Fixes: #19650

Help/release/dev/windows-auto-export-incremental-build.rst

@@ -0,0 +1,6 @@
+windows-auto-export-incremental-build
+-------------------------------------
+
+* On Windows, existing auto generated exports are now only updated if the
+  modified time stamp of the exports is not newer than any modified time stamp
+  of the input files.

Source/cmcmd.cxx

@@ -562,19 +562,36 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string> const& args)
                      "objlistfile [-nm=nm-path]\n";
         return 1;
       }
-      FILE* fout = cmsys::SystemTools::Fopen(args[2].c_str(), "w+");
-      if (!fout) {
-        std::cerr << "could not open output .def file: " << args[2].c_str()
-                  << "\n";
-        return 1;
-      }
       cmsys::ifstream fin(args[3].c_str(), std::ios::in | std::ios::binary);
       if (!fin) {
         std::cerr << "could not open object list file: " << args[3].c_str()
                   << "\n";
         return 1;
       }
-      std::string file;
+      std::vector<std::string> files;
+      {
+        std::string file;
+        cmFileTime outTime;
+        bool outValid = outTime.Load(args[2]);
+        while (cmSystemTools::GetLineFromStream(fin, file)) {
+          files.push_back(file);
+          if (outValid) {
+            cmFileTime inTime;
+            outValid = inTime.Load(file) && inTime.Older(outTime);
+          }
+        }
+        if (outValid) {
+          // The def file already exists and all input files are older than the
+          // existing def file.
+          return 0;
+        }
+      }
+      FILE* fout = cmsys::SystemTools::Fopen(args[2].c_str(), "w+");
+      if (!fout) {
+        std::cerr << "could not open output .def file: " << args[2].c_str()
+                  << "\n";
+        return 1;
+      }
       bindexplib deffile;
       if (args.size() >= 5) {
         auto a = args[4];
@@ -585,7 +602,7 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string> const& args)
           std::cerr << "unknown argument: " << a << "\n";
         }
       }
-      while (cmSystemTools::GetLineFromStream(fin, file)) {
+      for (auto const& file : files) {
         std::string const& ext = cmSystemTools::GetFilenameLastExtension(file);
         if (cmSystemTools::LowerCase(ext) == ".def") {
           if (!deffile.AddDefinitionFile(file.c_str())) {

Tests/RunCMake/AutoExportDll/RunCMakeTest.cmake

@@ -7,9 +7,13 @@ file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
 run_cmake(AutoExport)
 unset(RunCMake_TEST_OPTIONS)
 # don't run this test on Watcom or Borland make as it is not supported
-if("${RunCMake_GENERATOR}" MATCHES "Watcom WMake|Borland Makefiles")
+if(RunCMake_GENERATOR MATCHES "Watcom WMake|Borland Makefiles")
   return()
 endif()
+if(RunCMake_GENERATOR MATCHES "Ninja|Visual Studio" AND
+   CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
+  set(EXPORTS TRUE)
+endif()
 # we build debug so the say.exe will be found in Debug/say.exe for
 # Visual Studio generators
 if(RunCMake_GENERATOR_IS_MULTI_CONFIG)
@@ -18,9 +22,36 @@ endif()
 # build AutoExport
 run_cmake_command(AutoExportBuild ${CMAKE_COMMAND} --build
   ${RunCMake_TEST_BINARY_DIR} --config Debug --clean-first)
+# save the current timestamp of exports.def
+if(EXPORTS)
+  set(EXPORTS_DEF "${RunCMake_TEST_BINARY_DIR}/say.dir/${INTDIR}exports.def")
+  if(NOT EXISTS "${EXPORTS_DEF}")
+    set(EXPORTS_DEF
+      "${RunCMake_TEST_BINARY_DIR}/CMakeFiles/say.dir/${INTDIR}exports.def")
+  endif()
+  file(TIMESTAMP "${EXPORTS_DEF}" timestamp)
+  if(NOT timestamp)
+    message(SEND_ERROR "Could not get timestamp for \"${EXPORTS_DEF}\"")
+  endif()
+endif()
 # run the executable that uses symbols from the dll
 if(WIN32)
   set(EXE_EXT ".exe")
 endif()
 run_cmake_command(AutoExportRun
-  ${RunCMake_BINARY_DIR}/AutoExport-build/bin/${INTDIR}say${EXE_EXT})
+  ${RunCMake_TEST_BINARY_DIR}/bin/${INTDIR}say${EXE_EXT})
+# build AutoExport again without modification
+run_cmake_command(AutoExportBuildAgain ${CMAKE_COMMAND} --build
+  ${RunCMake_TEST_BINARY_DIR} --config Debug)
+# compare timestamps of exports.def to make sure it has not been updated
+if(EXPORTS)
+  file(TIMESTAMP "${EXPORTS_DEF}" timestamp_after)
+  if(NOT timestamp_after)
+    message(SEND_ERROR "Could not get timestamp for \"${EXPORTS_DEF}\"")
+  endif()
+  if (timestamp_after STREQUAL timestamp)
+    message(STATUS "AutoExportTimeStamp - PASSED")
+  else()
+    message(SEND_ERROR "\"${EXPORTS_DEF}\" has been updated.")
+  endif()
+endif()

Tests/RunCMake/CMakeLists.txt

@@ -543,7 +543,7 @@ set(cpack_tests
 add_RunCMake_test_group(CPack "${cpack_tests}")
 # add a test to make sure symbols are exported from a shared library
 # for MSVC compilers CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS property is used
-add_RunCMake_test(AutoExportDll)
+add_RunCMake_test(AutoExportDll -DCMAKE_CXX_COMPILER_ID=${CMAKE_CXX_COMPILER_ID})
 
 add_RunCMake_test(AndroidMK)