execute_process: Improve stdout/stderr merging

Use the KWSys Process "MergeOutput" option to give the child process
the same pipe (or file) for both stdout and stderr.  This allows
natural merging of stdout and stderr together instead of merging
on arbitrary buffered read boundaries as before.

Help/command/execute_process.rst

@@ -57,7 +57,8 @@ OUTPUT_VARIABLE, ERROR_VARIABLE
 INPUT_FILE, OUTPUT_FILE, ERROR_FILE
  The file named will be attached to the standard input of the first
  process, standard output of the last process, or standard error of
- all processes, respectively.
+ all processes, respectively.  If the same file is named for both
+ output and error then it will be used for both.
 
 OUTPUT_QUIET, ERROR_QUIET
  The standard output or standard error results will be quietly ignored.

Help/release/dev/execute_process-merge-output.rst

@@ -0,0 +1,5 @@
+execute_process-merge-output
+----------------------------
+
+* The :command:`execute_process` command learned to support specifying
+  the same file for ``OUTPUT_FILE`` and ``ERROR_FILE``.

Source/cmExecuteProcessCommand.cxx

@@ -255,7 +255,7 @@ bool cmExecuteProcessCommand
   cmsysProcess_SetOption(cp, cmsysProcess_Option_HideWindow, 1);
 
   // Check the output variables.
-  bool merge_output = (output_variable == error_variable);
+  bool merge_output = false;
   if(!input_file.empty())
     {
     cmsysProcess_SetPipeFile(cp, cmsysProcess_Pipe_STDIN, input_file.c_str());
@@ -267,8 +267,23 @@ bool cmExecuteProcessCommand
     }
   if(!error_file.empty())
     {
-    cmsysProcess_SetPipeFile(cp, cmsysProcess_Pipe_STDERR,
-                             error_file.c_str());
+    if (error_file == output_file)
+      {
+      merge_output = true;
+      }
+    else
+      {
+      cmsysProcess_SetPipeFile(cp, cmsysProcess_Pipe_STDERR,
+                               error_file.c_str());
+      }
+    }
+  if (!output_variable.empty() && output_variable == error_variable)
+    {
+    merge_output = true;
+    }
+  if (merge_output)
+    {
+    cmsysProcess_SetOption(cp, cmsysProcess_Option_MergeOutput, 1);
     }
 
   // Set the timeout if any.
@@ -289,8 +304,7 @@ bool cmExecuteProcessCommand
   while((p = cmsysProcess_WaitForData(cp, &data, &length, 0), p))
     {
     // Put the output in the right place.
-    if((p == cmsysProcess_Pipe_STDOUT && !output_quiet) ||
-       (p == cmsysProcess_Pipe_STDERR && !error_quiet && merge_output))
+    if (p == cmsysProcess_Pipe_STDOUT && !output_quiet)
       {
       if(output_variable.empty())
         {

Tests/RunCMake/CMakeLists.txt

@@ -132,6 +132,7 @@ add_RunCMake_test(add_custom_command)
 add_RunCMake_test(add_custom_target)
 add_RunCMake_test(add_dependencies)
 add_RunCMake_test(build_command)
+add_RunCMake_test(execute_process)
 add_RunCMake_test(export)
 add_RunCMake_test(cmake_minimum_required)
 add_RunCMake_test(continue)

Tests/RunCMake/execute_process/MergeOutput-stdout.txt

@@ -0,0 +1,10 @@
+^-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr$

Tests/RunCMake/execute_process/MergeOutput.cmake

@@ -0,0 +1,4 @@
+foreach(i RANGE 1 5)
+  message(STATUS "Output on stdout")
+  message("Output on stderr")
+endforeach()

Tests/RunCMake/execute_process/MergeOutputFile-stderr.txt

@@ -0,0 +1,10 @@
+^-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr$

Tests/RunCMake/execute_process/MergeOutputFile.cmake

@@ -0,0 +1,7 @@
+execute_process(
+  COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_LIST_DIR}/MergeOutput.cmake
+  OUTPUT_FILE out.txt
+  ERROR_FILE out.txt
+  )
+file(READ out.txt out)
+message("${out}")

Tests/RunCMake/execute_process/MergeOutputVars-stderr.txt

@@ -0,0 +1,10 @@
+^-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr
+-- Output on stdout
+Output on stderr$

Tests/RunCMake/execute_process/MergeOutputVars.cmake

@@ -0,0 +1,6 @@
+execute_process(
+  COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_LIST_DIR}/MergeOutput.cmake
+  OUTPUT_VARIABLE out
+  ERROR_VARIABLE out
+  )
+message("${out}")

Tests/RunCMake/execute_process/RunCMakeTest.cmake

@@ -0,0 +1,8 @@
+include(RunCMake)
+
+set(RunCMake_TEST_OUTPUT_MERGE 1)
+run_cmake_command(MergeOutput ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/MergeOutput.cmake)
+unset(RunCMake_TEST_OUTPUT_MERGE)
+
+run_cmake_command(MergeOutputFile ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/MergeOutputFile.cmake)
+run_cmake_command(MergeOutputVars ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/MergeOutputVars.cmake)