Add a flag to zero the mtime in OSO fields for deterministic builds on macOS.

This requires a sufficiently new version of ld64. I confirmed that the one that
comes bundled with Xcode 10.12.1 is new enough. There's no harm in setting this
for older versions of ld64.

This is currently gated behind the gn arg mac_deterministic_build because lldb
has a bug that causes it to fail to load object files when the binary is built
deterministically.

Bug: 330262
Change-Id: I9802420a46f487888f4768c01168dae1b8d88359
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1711105
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#687360}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7dc7c87cff652ab9a63f83029682c488e3ba515d

toolchain/mac/BUILD.gn

@@ -27,6 +27,12 @@ declare_args() {
   # tools as they can cause lots of I/O contention when invoking ninja with a
   # large number of parallel jobs (e.g. when using distributed build like goma).
   bundle_pool_depth = -1
+
+  # This makes the linker set timestamps in Mach-O files to 0. This isn't
+  # enabled by default because this breaks Xcode's lldb. This has been fixed in
+  # https://reviews.llvm.org/rL368199 but that has not yet made it into a public
+  # lldb release.
+  mac_deterministic_build = false
 }
 
 if (current_toolchain == default_toolchain) {
@@ -150,6 +156,10 @@ template("mac_toolchain") {
     _strippath = "$mac_bin_path" + "strip"
     linker_driver += " -Wcrl,strippath," + _strippath
 
+    if (mac_deterministic_build) {
+      linker_driver += " --deterministic"
+    }
+
     # On iOS, the final applications are assembled using lipo (to support fat
     # builds). The correct flags are passed to the linker_driver.py script
     # directly during the lipo call.

toolchain/mac/linker_driver.py

@@ -15,6 +15,10 @@ import sys
 DSYMUTIL_INVOKE = ['xcrun', 'dsymutil']
 STRIP_INVOKE = ['xcrun', 'strip']
 
+# Setting this flag will emit a deterministic binary by stripping dates from the
+# N_OSO field.
+DETERMINISTIC_FLAG = '--deterministic'
+
 # The linker_driver.py is responsible for forwarding a linker invocation to
 # the compiler driver, while processing special arguments itself.
 #
@@ -69,20 +73,28 @@ def Main(args):
   # the arguments being passed to the compiler driver.
   linker_driver_actions = {}
   compiler_driver_args = []
+  deterministic = False
   for arg in args[1:]:
     if arg.startswith(_LINKER_DRIVER_ARG_PREFIX):
       # Convert driver actions into a map of name => lambda to invoke.
       driver_action = ProcessLinkerDriverArg(arg)
       assert driver_action[0] not in linker_driver_actions
       linker_driver_actions[driver_action[0]] = driver_action[1]
+    elif arg == DETERMINISTIC_FLAG:
+      deterministic = True
     else:
       compiler_driver_args.append(arg)
 
   linker_driver_outputs = [_FindLinkerOutput(compiler_driver_args)]
 
   try:
+    # Zero the mtime in OSO fields for deterministic builds.
+    # https://crbug.com/330262.
+    env = os.environ.copy()
+    if deterministic:
+      env['ZERO_AR_DATE'] = '1'
     # Run the linker by invoking the compiler driver.
-    subprocess.check_call(compiler_driver_args)
+    subprocess.check_call(compiler_driver_args, env=env)
 
     # Run the linker driver actions, in the order specified by the actions list.
     for action in _LINKER_DRIVER_ACTIONS: