[imagediff] During cross builds, build for swarm host

Specific tools need to run on the test-initiating machines instead of
the build host or the build target. Since GN does not provide a third
toolchain designation for this, it needs to be calculated in the
exceptional cases that need it.

This CL adds this logic for imagediff.

Bug: 893460
Change-Id: I07f7a39971f47517550fc4edc0922f4b15eb87e5
Reviewed-on: https://chromium-review.googlesource.com/c/1489484
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636602}

BUILD.gn

@@ -207,7 +207,7 @@ group("gn_all") {
       "//third_party/blink/renderer/platform/heap:blink_heap_unittests",
       "//third_party/catapult/telemetry:bitmaptools($host_toolchain)",
       "//third_party/smhasher:pmurhash",
-      "//tools/imagediff($host_toolchain)",
+      "//tools/imagediff",
       "//ui/display:display_unittests",
       "//ui/events:events_unittests",
       "//ui/latency:latency_unittests",

tools/imagediff/BUILD.gn

@@ -4,7 +4,35 @@
 
 import("//build/symlink.gni")
 
-if (current_toolchain == host_toolchain) {
+# There are three machines involved with building and running tests:
+# * the build host, where GN, ninja, and the build tools run
+# * the test host, where the swarming task runs
+# * the test target, where the test collateral runs
+#
+# image_diff runs on the test host.
+#
+# Normally all three machines can be built for using the same toolchain, but
+# cross-compilation is an important exception. A few of these are:
+# * Linux build host + Windows test host + Windows test target
+# * Linux build host + Linux test host + Android test target
+#
+# Since the concept of a 'test host' toolchain is very rarely needed, GN does
+# not provide it. Instead, determine it here.
+if (target_os == "win" && host_os != "win") {
+  # When targeting Windows from not Windows, use the test target toolchain.
+  image_diff_toolchain = default_toolchain
+} else if (target_os != host_os) {
+  # In common cross-compilation scenarios, the build host matches the test host.
+  # TODO: In chrome/android-on-mac cross builds, image_diff would still have to
+  # be built for Linux and this line is wrong.
+  imagediff_toolchain = host_toolchain
+} else {
+  # When not cross-compiling, use the test target toolchain.
+  imagediff_toolchain = default_toolchain
+}
+
+# If the current toolchain is the test host toolchain, build the tool.
+if (current_toolchain == imagediff_toolchain) {
   executable("imagediff") {
     output_name = "image_diff"  # Different than dir name for historical reasons.
     sources = [
@@ -22,13 +50,21 @@ if (current_toolchain == host_toolchain) {
       "//third_party/zlib",
     ]
   }
-} else {
+  # Otherwise, if the current toolchain is the test target toolchain, make a
+  # symlink to the test host toolchain output so the tests can find it.
+  #
+  # Note that Windows does not follow links when searching for DLLs, so
+  # image_diff.exe from component builds won't run via symlink. Fortunately,
+  # Windows builds use the default_toolchain and so avoid making the link.
+} else if (current_toolchain == default_toolchain &&
+           default_toolchain != imagediff_toolchain) {
   binary_symlink("imagediff") {
-    binary_label = ":$target_name($host_toolchain)"
+    binary_label = ":$target_name($imagediff_toolchain)"
     binary_output_name = "image_diff"
 
-    # The 'executable' target does this automatically.
-    if (is_win) {
+    # For Windows builds, the test host and test target are both Windows and so
+    # binary_symlink needs to use the Windows binary suffix.
+    if (target_os == "win") {
       binary_output_name += ".exe"
     }
   }