[Mac/iOS/GN] Place a version in the command of toolchain tools implemented using scripts.

When a tool is implemented as a script, modification of the script does not
cause any dependent build steps to be dirtied and rebuilt, since the script
isn't listed as an input. To hack around this, use another script to get the
tool script's modification time and use that as the command line tool version.
This still won't cause rebuild if just the script changes, but when the build
files are regenerated (like after apply_patch and generate_build_files on the
bots), the dependent build steps will get rebuilt.

BUG=619083
R=brettw@chromium.org, sdefresne@chromium.org

Review URL: https://codereview.chromium.org/2075703002 .

Cr-Original-Commit-Position: refs/heads/master@{#400519}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f2eb108ab1e65962104bfde275743369b0e7665b

toolchain/mac/BUILD.gn

@@ -38,6 +38,30 @@ gyp_mac_tool_source =
     rebase_path("//tools/gyp/pylib/gyp/mac_tool.py", root_build_dir)
 exec_script("setup_toolchain.py", [ gyp_mac_tool_source ])
 
+# When implementing tools using Python scripts, a TOOL_VERSION=N env
+# variable is placed in front of the command. The N should be incremented
+# whenever the script is changed, so that the build system rebuilds all
+# edges that utilize the script. Ideally this should be changed to use
+# proper input-dirty checking, but that could be expensive. Instead, use a
+# script to get the tool scripts' modification time to use as the version.
+# This won't cause a re-generation of GN files when the tool script changes
+# but it will cause edges to be marked as dirty if the ninja files are
+# regenerated. See https://crbug.com/619083 for details. A proper fix
+# would be to have inputs to tools (https://crbug.com/621119).
+tool_versions =
+    exec_script("get_tool_mtime.py",
+                rebase_path([
+                              "//build/toolchain/mac/compile_xcassets.py",
+                              "//build/toolchain/mac/copy_bundle_data.py",
+                              "//build/toolchain/mac/filter_libtool.py",
+                              "//build/toolchain/mac/linker_driver.py",
+                            ],
+                            root_build_dir),
+                "trim scope")
+
+# Work around for unused variable warning in template https://crbug.com/395883.
+assert(tool_versions != "")
+
 # Shared toolchain definition. Invocations should set toolchain_os to set the
 # build args in this definition.
 template("mac_toolchain") {
@@ -60,6 +84,7 @@ template("mac_toolchain") {
     ld = invoker.ld
 
     linker_driver =
+        "TOOL_VERSION=${tool_versions.linker_driver} " +
         rebase_path("//build/toolchain/mac/linker_driver.py", root_build_dir)
 
     # Make these apply to all tools below.
@@ -136,7 +161,7 @@ template("mac_toolchain") {
     tool("alink") {
       script =
           rebase_path("//build/toolchain/mac/filter_libtool.py", root_build_dir)
-      command = "rm -f {{output}} && python $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
+      command = "rm -f {{output}} && TOOL_VERSION=${tool_versions.filter_libtool} python $script libtool -static {{arflags}} -o {{output}} {{inputs}}"
       description = "LIBTOOL-STATIC {{output}}"
       outputs = [
         "{{output_dir}}/{{target_output_name}}{{output_extension}}",
@@ -283,7 +308,7 @@ template("mac_toolchain") {
       }
       _tool = rebase_path("//build/toolchain/mac/copy_bundle_data.py",
                           root_build_dir)
-      command = "python $_tool ${_extra_args} {{source}} {{output}}"
+      command = "TOOL_VERSION=${tool_versions.copy_bundle_data} python $_tool ${_extra_args} {{source}} {{output}}"
       description = "COPY_BUNDLE_DATA {{source}} {{output}}"
       pool = ":bundle_pool($default_toolchain)"
     }
@@ -298,6 +323,7 @@ template("mac_toolchain") {
         _min_deployment_target = mac_deployment_target
       }
       command = "rm -f {{output}} && " +
+                "TOOL_VERSION=${tool_versions.compile_xcassets} " +
                 "python $_tool -p $_sdk_name -t $_min_deployment_target " +
                 "-o {{output}} {{inputs}}"
 

toolchain/mac/get_tool_mtime.py

@@ -0,0 +1,17 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import os
+import sys
+
+# Usage: python get_tool_mtime.py path/to/file1.py path/to/file2.py
+#
+# Prints a GN scope with the variable name being the basename sans-extension
+# and the value being the file modification time. A variable is emitted for
+# each file argument on the command line.
+
+if __name__ == '__main__':
+  for f in sys.argv[1:]:
+    variable = os.path.splitext(os.path.basename(f))[0]
+    print '%s = %d' % (variable, os.path.getmtime(f))