From 42e1cd137c0eff0297adaea5b34abca0da254eba Mon Sep 17 00:00:00 2001
From: Stephen Kelly <steveire@gmail.com>
Date: Fri, 25 Apr 2014 20:11:05 +0200
Subject: [PATCH] file(GENERATE): Only write the file if content is different.

No policy is used to control this behavior for now.
---
 .../cmGeneratorExpressionEvaluationFile.cxx   | 15 +++--------
 .../RunCMake/File_Generate/RunCMakeTest.cmake | 27 +++++++++++++++++++
 .../File_Generate/WriteIfDifferent-result.txt |  1 +
 .../File_Generate/WriteIfDifferent-stderr.txt |  1 +
 .../File_Generate/WriteIfDifferent.cmake      |  5 ++++
 5 files changed, 37 insertions(+), 12 deletions(-)
 create mode 100644 Tests/RunCMake/File_Generate/WriteIfDifferent-result.txt
 create mode 100644 Tests/RunCMake/File_Generate/WriteIfDifferent-stderr.txt
 create mode 100644 Tests/RunCMake/File_Generate/WriteIfDifferent.cmake

diff --git a/Source/cmGeneratorExpressionEvaluationFile.cxx b/Source/cmGeneratorExpressionEvaluationFile.cxx
index 95a946a82..01d31d3b8 100644
--- a/Source/cmGeneratorExpressionEvaluationFile.cxx
+++ b/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();
 }
 
 //----------------------------------------------------------------------------
diff --git a/Tests/RunCMake/File_Generate/RunCMakeTest.cmake b/Tests/RunCMake/File_Generate/RunCMakeTest.cmake
index f07431c0e..f23fa1983 100644
--- a/Tests/RunCMake/File_Generate/RunCMakeTest.cmake
+++ b/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()
diff --git a/Tests/RunCMake/File_Generate/WriteIfDifferent-result.txt b/Tests/RunCMake/File_Generate/WriteIfDifferent-result.txt
new file mode 100644
index 000000000..573541ac9
--- /dev/null
+++ b/Tests/RunCMake/File_Generate/WriteIfDifferent-result.txt
@@ -0,0 +1 @@
+0
diff --git a/Tests/RunCMake/File_Generate/WriteIfDifferent-stderr.txt b/Tests/RunCMake/File_Generate/WriteIfDifferent-stderr.txt
new file mode 100644
index 000000000..10f32932e
--- /dev/null
+++ b/Tests/RunCMake/File_Generate/WriteIfDifferent-stderr.txt
@@ -0,0 +1 @@
+^$
diff --git a/Tests/RunCMake/File_Generate/WriteIfDifferent.cmake b/Tests/RunCMake/File_Generate/WriteIfDifferent.cmake
new file mode 100644
index 000000000..d1d832a11
--- /dev/null
+++ b/Tests/RunCMake/File_Generate/WriteIfDifferent.cmake
@@ -0,0 +1,5 @@
+
+file(GENERATE
+  OUTPUT output_file.txt
+  CONTENT "123"
+)