From fe19df49d48680cbfd8e1023d4a93b39f12d4b64 Mon Sep 17 00:00:00 2001
From: Brad King <brad.king@kitware.com>
Date: Tue, 12 May 2020 07:48:17 -0400
Subject: Initialize CMAKE_EXPORT_COMPILE_COMMANDS only if not set already

Also de-duplicate its initialization code.

Fixes: #16588
---
 Modules/CMakeGenericSystem.cmake                               | 8 +-------
 Tests/RunCMake/CMakeLists.txt                                  | 3 +++
 Tests/RunCMake/ExportCompileCommands/BeforeProject-check.cmake | 4 ++++
 Tests/RunCMake/ExportCompileCommands/BeforeProject.cmake       | 3 +++
 Tests/RunCMake/ExportCompileCommands/BeforeProjectBEFORE.cmake | 1 +
 Tests/RunCMake/ExportCompileCommands/CMakeLists.txt            | 3 +++
 Tests/RunCMake/ExportCompileCommands/RunCMakeTest.cmake        | 3 +++
 Tests/RunCMake/ExportCompileCommands/empty.c                   | 0
 8 files changed, 18 insertions(+), 7 deletions(-)
 create mode 100644 Tests/RunCMake/ExportCompileCommands/BeforeProject-check.cmake
 create mode 100644 Tests/RunCMake/ExportCompileCommands/BeforeProject.cmake
 create mode 100644 Tests/RunCMake/ExportCompileCommands/BeforeProjectBEFORE.cmake
 create mode 100644 Tests/RunCMake/ExportCompileCommands/CMakeLists.txt
 create mode 100644 Tests/RunCMake/ExportCompileCommands/RunCMakeTest.cmake
 create mode 100644 Tests/RunCMake/ExportCompileCommands/empty.c

diff --git a/Modules/CMakeGenericSystem.cmake b/Modules/CMakeGenericSystem.cmake
index f539b46..000fba1 100644
--- a/Modules/CMakeGenericSystem.cmake
+++ b/Modules/CMakeGenericSystem.cmake
@@ -50,15 +50,9 @@ if(CMAKE_GENERATOR MATCHES "Make")
   if(DEFINED CMAKE_TARGET_MESSAGES)
     set_property(GLOBAL PROPERTY TARGET_MESSAGES ${CMAKE_TARGET_MESSAGES})
   endif()
-  if(CMAKE_GENERATOR MATCHES "Unix Makefiles")
-    set(CMAKE_EXPORT_COMPILE_COMMANDS "$ENV{CMAKE_EXPORT_COMPILE_COMMANDS}"
-      CACHE BOOL "Enable/Disable output of compile commands during generation."
-      )
-    mark_as_advanced(CMAKE_EXPORT_COMPILE_COMMANDS)
-  endif()
 endif()
 
-if(CMAKE_GENERATOR MATCHES "Ninja")
+if(NOT DEFINED CMAKE_EXPORT_COMPILE_COMMANDS AND CMAKE_GENERATOR MATCHES "Ninja|Unix Makefiles")
   set(CMAKE_EXPORT_COMPILE_COMMANDS "$ENV{CMAKE_EXPORT_COMPILE_COMMANDS}"
     CACHE BOOL "Enable/Disable output of compile commands during generation."
     )
diff --git a/Tests/RunCMake/CMakeLists.txt b/Tests/RunCMake/CMakeLists.txt
index cc88868..8aeb412 100644
--- a/Tests/RunCMake/CMakeLists.txt
+++ b/Tests/RunCMake/CMakeLists.txt
@@ -197,6 +197,9 @@ endif()
 add_RunCMake_test(CompilerNotFound)
 add_RunCMake_test(Configure -DMSVC_IDE=${MSVC_IDE})
 add_RunCMake_test(DisallowedCommands)
+if("${CMAKE_GENERATOR}" MATCHES "Unix Makefiles|Ninja")
+  add_RunCMake_test(ExportCompileCommands)
+endif()
 add_RunCMake_test(ExternalData)
 add_RunCMake_test(FeatureSummary)
 add_RunCMake_test(FPHSA)
diff --git a/Tests/RunCMake/ExportCompileCommands/BeforeProject-check.cmake b/Tests/RunCMake/ExportCompileCommands/BeforeProject-check.cmake
new file mode 100644
index 0000000..87058e2
--- /dev/null
+++ b/Tests/RunCMake/ExportCompileCommands/BeforeProject-check.cmake
@@ -0,0 +1,4 @@
+if(NOT EXISTS "${RunCMake_TEST_BINARY_DIR}/compile_commands.json")
+  set(RunCMake_TEST_FAILED "compile_commands.json not generated")
+  return()
+endif()
diff --git a/Tests/RunCMake/ExportCompileCommands/BeforeProject.cmake b/Tests/RunCMake/ExportCompileCommands/BeforeProject.cmake
new file mode 100644
index 0000000..b8cbdef
--- /dev/null
+++ b/Tests/RunCMake/ExportCompileCommands/BeforeProject.cmake
@@ -0,0 +1,3 @@
+enable_language(C)
+add_library(empty STATIC empty.c)
+message(STATUS "CMAKE_EXPORT_COMPILE_COMMANDS='${CMAKE_EXPORT_COMPILE_COMMANDS}'")
diff --git a/Tests/RunCMake/ExportCompileCommands/BeforeProjectBEFORE.cmake b/Tests/RunCMake/ExportCompileCommands/BeforeProjectBEFORE.cmake
new file mode 100644
index 0000000..87f9c87
--- /dev/null
+++ b/Tests/RunCMake/ExportCompileCommands/BeforeProjectBEFORE.cmake
@@ -0,0 +1 @@
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
diff --git a/Tests/RunCMake/ExportCompileCommands/CMakeLists.txt b/Tests/RunCMake/ExportCompileCommands/CMakeLists.txt
new file mode 100644
index 0000000..b7117bd
--- /dev/null
+++ b/Tests/RunCMake/ExportCompileCommands/CMakeLists.txt
@@ -0,0 +1,3 @@
+cmake_minimum_required(VERSION 3.17)
+project(${RunCMake_TEST} NONE)
+include(${RunCMake_TEST}.cmake)
diff --git a/Tests/RunCMake/ExportCompileCommands/RunCMakeTest.cmake b/Tests/RunCMake/ExportCompileCommands/RunCMakeTest.cmake
new file mode 100644
index 0000000..b540a04
--- /dev/null
+++ b/Tests/RunCMake/ExportCompileCommands/RunCMakeTest.cmake
@@ -0,0 +1,3 @@
+include(RunCMake)
+
+run_cmake_with_options(BeforeProject -DCMAKE_PROJECT_INCLUDE_BEFORE=BeforeProjectBEFORE.cmake)
diff --git a/Tests/RunCMake/ExportCompileCommands/empty.c b/Tests/RunCMake/ExportCompileCommands/empty.c
new file mode 100644
index 0000000..e69de29
-- 
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