GN: Propagate top-level host_toolchain through to all toolchains

build/config/BUILDCONFIG.gn

 # Copyright (c) 2013 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.
 
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
 #
 # There are two main things to set: "os" and "cpu". The "toolchain" is the name
 # of the GN thing that encodes combinations of these things.
@@ skipping 111 matching lines @@
 
   # Set to true when compiling with the Clang compiler. Typically this is used
   # to configure warnings.
   is_clang = current_os == "mac" || current_os == "ios" ||
              current_os == "linux" || current_os == "chromeos"
 
   # Allows the path to a custom target toolchain to be injected as a single
   # argument, and set as the default toolchain.
   custom_toolchain = ""
 
+  # This should not normally be set as a build argument.  It's here so that
+  # every toolchain can pass through the "global" value via toolchain_args().
+  host_toolchain = ""
+
   # DON'T ADD MORE FLAGS HERE. Read the comment above.
 }
 
 # ==============================================================================
 # TOOLCHAIN SETUP
 # ==============================================================================
 #
 # Here we set the default toolchain, as well as the variable host_toolchain
 # which will identify the toolchain corresponding to the local system when
 # doing cross-compiles. When not cross-compiling, this will be the same as the
 # default toolchain.
 #
 # We do this before anything else to make sure we complain about any
 # unsupported os/cpu combinations as early as possible.
 
-if (host_os == "linux") {
-  if (target_os != "linux") {
-    # TODO(dpranke) - is_clang normally applies only to the target
-    # build, and there is no way to indicate that you want to override
-    # it for both the target build *and* the host build. Do we need to
-    # support this?
-    host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
-  } else if (is_clang) {
-    host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
+if (host_toolchain == "") {
+  # This should only happen in the top level context.
+  # In a specific toolchain context, the toolchain_args()
+  # block should have propagated a value down.

[Dirk Pranke, 2015/11/25 03:48:05]

In the top level context (the first time through BUILDCONFIG.gn), current_cpu ==
target_cpu and current_os == target_os (see line 53).

I think the logic you implemented breaks on, e.g., android and ios.

+  assert(current_os == host_os,
+         "All toolchains must plumb through host_toolchain in toolchain_args()")
+  if (host_os == "win") {
+    # TODO(crbug.com/467159): win cross-compiles don't actually work yet, so
+    # use the target_cpu instead of the host_cpu.
+    assert(
+        current_cpu == target_cpu,
+        "All toolchains must plumb through host_toolchain in toolchain_args()")
   } else {
-    host_toolchain = "//build/toolchain/linux:$host_cpu"
+    assert(
+        current_cpu == host_cpu,
+        "All toolchains must plumb through host_toolchain in toolchain_args()")
   }
-} else if (host_os == "mac") {
-  host_toolchain = "//build/toolchain/mac:clang_$host_cpu"
-} else if (host_os == "win") {
-  # TODO(crbug.com/467159): win cross-compiles don't actually work yet, so
-  # use the target_cpu instead of the host_cpu.
-  if (is_clang) {
-    host_toolchain = "//build/toolchain/win:clang_$target_cpu"
+
+  if (host_os == "linux") {
+    if (is_clang) {
+      host_toolchain = "//build/toolchain/linux:clang_$host_cpu"
+    } else {
+      host_toolchain = "//build/toolchain/linux:$host_cpu"
+    }
+  } else if (host_os == "mac") {
+    host_toolchain = "//build/toolchain/mac:clang_$host_cpu"
+  } else if (host_os == "win") {
+    # TODO(crbug.com/467159): win cross-compiles don't actually work yet, so
+    # use the target_cpu instead of the host_cpu.
+    if (is_clang) {
+      host_toolchain = "//build/toolchain/win:clang_$target_cpu"
+    } else {
+      host_toolchain = "//build/toolchain/win:$target_cpu"
+    }
   } else {
-    host_toolchain = "//build/toolchain/win:$target_cpu"
+    assert(false, "Unsupported host_os: $host_os")
   }
-} else {
-  assert(false, "Unsupported host_os: $host_os")
 }
 
 _default_toolchain = ""
 
 if (target_os == "android") {
   assert(host_os == "linux" || host_os == "mac",
          "Android builds are only supported on Linux and Mac hosts.")
   if (is_clang) {
     _default_toolchain = "//build/toolchain/android:clang_$target_cpu"
   } else {
@@ skipping 400 matching lines @@
     forward_variables_from(invoker, "*")
 
     # All shared libraries must have the sanitizer deps to properly link in
     # asan mode (this target will be empty in other cases).
     if (!defined(deps)) {
       deps = []
     }
     deps += [ "//build/config/sanitizers:deps" ]
   }
 }