Add support for lldbinit to macOS.

The build-flag `strip_absolute_paths_from_debug_symbols = 1` produces
deterministic outputs and has better goma caching, but breaks lldb. This CL adds
machinery to help users get functional symbols in lldb while still using that
compile flag.

Bug: 330262
Change-Id: I42f1e6c9c76296d451730ea481d7e1c3c0a69476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775227Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691393}

base/BUILD.gn

@@ -2070,6 +2070,8 @@ buildflag_header("debugging_buildflags") {
   header_dir = "base/debug"
   enable_gdbinit_warning =
       is_debug && (strip_absolute_paths_from_debug_symbols || use_custom_libcxx)
+  enable_lldbinit_warning =
+      is_debug && strip_absolute_paths_from_debug_symbols && is_mac
 
   flags = [
     "ENABLE_LOCATION_SOURCE=$enable_location_source",
@@ -2078,6 +2080,7 @@ buildflag_header("debugging_buildflags") {
     "UNSAFE_DEVELOPER_BUILD=$is_unsafe_developer_build",
     "CAN_UNWIND_WITH_CFI_TABLE=$can_unwind_with_cfi_table",
     "ENABLE_GDBINIT_WARNING=$enable_gdbinit_warning",
+    "ENABLE_LLDBINIT_WARNING=$enable_lldbinit_warning",
   ]
 }
 

base/debug/debugger.h

@@ -40,7 +40,8 @@ BASE_EXPORT bool IsDebugUISuppressed();
 
 // If a debugger is present, verifies that it is properly set up, and DCHECK()s
 // if misconfigured.  Currently only verifies that //tools/gdb/gdbinit has been
-// sourced when using gdb on Linux.
+// sourced when using gdb on Linux and //tools/lldb/lldbinit.py has been sourced
+// when using lldb on macOS.
 BASE_EXPORT void VerifyDebugger();
 
 }  // namespace debug

base/debug/debugger_posix.cc

@@ -128,7 +128,21 @@ bool BeingDebugged() {
   return being_debugged;
 }
 
-void VerifyDebugger() {}
+void VerifyDebugger() {
+#if BUILDFLAG(ENABLE_LLDBINIT_WARNING)
+  // Quick check before potentially slower GetDebuggerProcess().
+  if (Environment::Create()->HasVar("CHROMIUM_LLDBINIT_SOURCED"))
+    return;
+  DCHECK(false)
+      << "Detected lldb without sourcing //tools/lldb/lldbinit.py. lldb may "
+         "not be able to find debug symbols. Please see debug instructions for "
+         "using //tools/lldb/lldbinit.py:\n"
+         "https://chromium.googlesource.com/chromium/src/+/master/docs/"
+         "lldbinit.md\n"
+         "To continue anyway, type 'continue' in lldb. To always skip this "
+         "check, define an environment variable CHROMIUM_LLDBINIT_SOURCED=1";
+#endif
+}
 
 #elif defined(OS_LINUX) || defined(OS_ANDROID) || defined(OS_AIX)
 

docs/lldbinit.md

+# Usage of tools/lldb/lldbinit.py
+
+Usage of Chromium's [lldbinit.py](../tools/lldb/lldbinit.py) is recommended when
+debugging with lldb. This is necessary for source-level debugging when
+`strip_absolute_paths_from_debug_symbols` is enabled.
+
+To use, add the following to your `~/.lldbinit`
+
+```
+# So that lldbinit.py takes precedence.
+script sys.path[:0] = ['<.../path/to/chromium/src/tools/lldb>']
+script import lldbinit
+```

tools/lldb/lldbinit.py

+# Copyright 2019 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.
+
+# The GN arg `strip_absolute_paths_from_debug_symbols = 1` uses relative paths
+# for debug symbols. This confuses lldb. We explicitly set the source-map to
+# point at the root directory of the chromium checkout.
+import os
+import lldb
+this_dir = os.path.dirname(os.path.abspath(__file__))
+source_dir = os.path.join(os.path.join(this_dir, os.pardir), os.pardir)
+
+lldb.debugger.HandleCommand(
+    'settings set target.source-map ../.. ' + source_dir)
+lldb.debugger.HandleCommand(
+    'settings set target.env-vars CHROMIUM_LLDBINIT_SOURCED=1')