GN: Propagate top-level host_toolchain through to all toolchains

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 3eed120832f10c091058706af881c439adc0454c..ef8753b4090a2c27397c9988a9c3e767471aea67 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -50,6 +50,13 @@ if (target_cpu == "") {
   }
 }
 
+# The current_cpu and current_os variables are empty on the first
+# run through this file.  When this file is evaluated again in a
+# specific toolchain context, the toolchain_args() block will have
+# set these explicitly.  So them being empty is the best available
+# proxy for a "is this the first time through" predicate.
+buildconfig_in_default_toolchain = current_cpu == "" && current_os == ""

[brettw, 2015/12/01 22:08:19]

I'm not super fond of this hack and I'd prefer to avoid this type of thing. I
don't actually think it will be a big problem.

 I have some ideas of how toolchain setup can be improved. If forgetting the
host toolchain ends up being a problem in practice, we can implement that for a
precise way to tell this.

[Roland McGrath, 2015/12/01 22:12:18]

My concern is that when it goes wrong the failure modes are likely to be quite
subtle.  I'd really like to have something to ensure that the host_toolchain
calculation is never carried out a second time inside a toolchain context.  If
there is nothing reliable we can do now but there will be something eventually,
then I'm fine with just replacing the assert with a TODO.  But I'm not at all
sanguine about just dropping the assert and hand-waving that it won't bite us in
the future.

+
 if (current_cpu == "") {
   current_cpu = target_cpu
 }
@@ -129,6 +136,10 @@ declare_args() {
   # 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.
 }
 
@@ -144,30 +155,32 @@ declare_args() {
 # 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"
-  } 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_$target_cpu"
+if (host_toolchain == "") {
+  # This should only happen in the top-level context.
+  # In a specific toolchain context, the toolchain_args()
+  # block should have propagated a value down.
+  assert(buildconfig_in_default_toolchain,
+         "All toolchains must plumb through host_toolchain in toolchain_args()")
+
+  if (host_os == "linux") {
+    if (is_clang) {
+      host_toolchain = "//build/toolchain/linux:clang_$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 = ""
@@ -200,7 +213,7 @@ if (target_os == "android") {
   assert(host_os == "mac", "Mac cross-compiles are unsupported.")
   _default_toolchain = host_toolchain
 } else if (target_os == "win") {
-  # On windows we use the same toolchain for host and target by default.
+  # On Windows we use the same toolchain for host and target by default.
   assert(target_os == host_os, "Win cross-compiles only work on win hosts.")
   if (is_clang) {
     _default_toolchain = "//build/toolchain/win:clang_$target_cpu"