file(GENERATE): Only write the file if content is different.

No policy is used to control this behavior for now.

Source/cmGeneratorExpressionEvaluationFile.cxx

@@ -13,6 +13,7 @@
 #include "cmGeneratorExpressionEvaluationFile.h"
 
 #include "cmMakefile.h"
+#include "cmGeneratedFileStream.h"
 #include <cmsys/FStream.hxx>
 
 #include <assert.h>
@@ -79,19 +80,9 @@ void cmGeneratorExpressionEvaluationFile::Generate(const std::string& config,
   this->Files.push_back(outputFileName);
   outputFiles[outputFileName] = outputContent;
 
-  cmsys::ofstream fout(outputFileName.c_str());
-
-  if(!fout)
-    {
-    cmOStringStream e;
-    e << "Evaluation file \"" << outputFileName << "\" cannot be written.";
-    this->Makefile->IssueMessage(cmake::FATAL_ERROR, e.str());
-    return;
-    }
-
+  cmGeneratedFileStream fout(outputFileName.c_str());
+  fout.SetCopyIfDifferent(true);
   fout << outputContent;
-
-  fout.close();
 }
 
 //----------------------------------------------------------------------------

Tests/RunCMake/File_Generate/RunCMakeTest.cmake

@@ -8,3 +8,30 @@ run_cmake(EmptyCondition1)
 run_cmake(EmptyCondition2)
 run_cmake(BadCondition)
 run_cmake(DebugEvaluate)
+
+set(timeformat "%Y%j%H%M%S")
+
+file(REMOVE "${RunCMake_BINARY_DIR}/WriteIfDifferent-build/output_file.txt")
+set(RunCMake_TEST_FILE "WriteIfDifferent")
+set(RunCMake_TEST_BINARY_DIR "${RunCMake_BINARY_DIR}/WriteIfDifferent-build")
+run_cmake(WriteIfDifferent-prepare)
+unset(RunCMake_TEST_FILE)
+unset(RunCMake_TEST_BINARY_DIR)
+file(TIMESTAMP "${RunCMake_BINARY_DIR}/WriteIfDifferent-build/output_file.txt" timestamp ${timeformat})
+if(NOT timestamp)
+  message(SEND_ERROR "Could not get timestamp for \"${RunCMake_BINARY_DIR}/WriteIfDifferent-build/output_file.txt\"")
+endif()
+
+execute_process(COMMAND ${CMAKE_COMMAND} -E sleep 1)
+
+set(RunCMake_TEST_NO_CLEAN ON)
+run_cmake(WriteIfDifferent)
+file(TIMESTAMP "${RunCMake_BINARY_DIR}/WriteIfDifferent-build/output_file.txt" timestamp_after ${timeformat})
+if(NOT timestamp_after)
+  message(SEND_ERROR "Could not get timestamp for \"${RunCMake_BINARY_DIR}/WriteIfDifferent-build/output_file.txt\"")
+endif()
+unset(RunCMake_TEST_NO_CLEAN)
+
+if (NOT timestamp_after STREQUAL timestamp)
+  message(SEND_ERROR "WriteIfDifferent changed output file.")
+endif()

Tests/RunCMake/File_Generate/WriteIfDifferent-result.txt

@@ -0,0 +1 @@
+0

Tests/RunCMake/File_Generate/WriteIfDifferent-stderr.txt

@@ -0,0 +1 @@
+^$

Tests/RunCMake/File_Generate/WriteIfDifferent.cmake

@@ -0,0 +1,5 @@
+
+file(GENERATE
+  OUTPUT output_file.txt
+  CONTENT "123"
+)