Add thin_archive config to global build config

To enable thin archives on posix, the current plan is to:
1. Land this CL.
2. Remove thin_archive from third_party build files as appropriate.
3. Uncomment the arflags = [ "-T" ] that this CL adds.

If you want a standalone static library, you now need to both set
complete_static_lib=true and remove the thin_archive config. See the comment in
build/config/compiler/BUILD.gn for details.

BUG=801925
R=thakis,dpranke
TBR=sky

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Icd1d3389da13106f46de3468475f0102a88c6211
Reviewed-on: https://chromium-review.googlesource.com/954344Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544221}

build/config/BUILDCONFIG.gn

@@ -532,6 +532,7 @@ default_compiler_configs = [
   "//build/config/compiler:no_exceptions",
   "//build/config/compiler:no_rtti",
   "//build/config/compiler:runtime_library",
+  "//build/config/compiler:thin_archive",
   "//build/config/coverage:default_coverage",
   "//build/config/sanitizers:default_sanitizer_flags",
 ]

build/config/compiler/BUILD.gn

@@ -1503,6 +1503,31 @@ config("no_rtti") {
   }
 }
 
+# thin_archive -----------------------------------------------------------------
+#
+# Enables thin archives on posix.  Regular archives directly include the object
+# fiels used to generate it.  Thin archives merely reference the object files.
+# This makes building them faster since it requires less disk IO, but is in
+# appropriate if you wish to redistribute your static library.
+# This config is added to the global config, so thin archives should already be
+# enabled.  If you want to make a distributable static library, you need to do 2
+# things:
+# 1. Set complete_static_lib so that all dependencies of the library make it
+#    into the library. See `gn help complete_static_lib` for details.
+# 2. Remove the thin_archive config, so that the .a file actually contains all
+#    .o files, instead of just references to .o files in the build directoy
+config("thin_archive") {
+  if (is_posix && !is_nacl) {
+    # TODO(thomasanderson): Remove !is_clang conditional once llvm-ar supports
+    # multiple dashed flags.
+    if (!is_clang) {
+      # TODO(thomasanderson): Introduce -T once :thin_archive is removed from
+      # all third_party repos where appropriate.
+      # arflags = [ "-T" ]
+    }
+  }
+}
+
 # exceptions -------------------------------------------------------------------
 #
 # Allows turning Exceptions on or off.

chrome/installer/gcapi_mac/BUILD.gn

@@ -21,6 +21,7 @@ config("gcapi_config") {
 
 static_library("gcapi_lib") {
   complete_static_lib = true
+  configs -= [ "//build/config/compiler:thin_archive" ]
   sources = [
     "gcapi.h",
     "gcapi.mm",

components/cronet/ios/BUILD.gn

@@ -174,6 +174,7 @@ static_library("cronet_static") {
 static_library("cronet_deps_complete") {
   visibility = [ ":*" ]
   complete_static_lib = true
+  configs -= [ "//build/config/compiler:thin_archive" ]
   deps = [
     ":cronet_static",
   ]

media/cast/BUILD.gn

@@ -546,6 +546,7 @@ if (is_linux && !is_chromeos) {
 
 static_library("cast_sender") {
   complete_static_lib = true
+  configs -= [ "//build/config/compiler:thin_archive" ]
   deps = [
     ":sender",
   ]
@@ -553,6 +554,7 @@ static_library("cast_sender") {
 
 static_library("cast_receiver") {
   complete_static_lib = true
+  configs -= [ "//build/config/compiler:thin_archive" ]
   deps = [
     ":receiver",
   ]

ppapi/cpp/BUILD.gn

@@ -42,6 +42,7 @@ target(cpp_target_type, "cpp") {
       # This library is distributed as a part of the SDK and as such has to
       # be a static library rather than a source set.
       complete_static_lib = true
+      configs -= [ "//build/config/compiler:thin_archive" ]
     }
   }
 }