Proof of concept for test selection via GN

This CL implements gn based unittest filtering. The files in the rts_exclude_file are manually selected for this example, but will later be populated using the smart logic in rts-chromium.
These two cases (source_set and test template) cover most of the test files. I haven't seen any other templates with unittests, but even if there are some this would be a great start. This also doesn't cover webtests, but I think there's enough exclusion power here to save a lot of compute.
This also adds a linux-rts builder to the mb config to prepare the experimental builder.

Bug: 1145216
Change-Id: I7343249f3df0739e39cfe351e5714d7978059d86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2551276
Commit-Queue: Gregory Guterman <guterman@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841248}

build/config/BUILDCONFIG.gn

@@ -162,6 +162,17 @@ declare_args() {
   is_component_build = is_debug && current_os != "ios"
 }
 
+declare_args() {
+  # Regression Test Selection (RTS).
+  # https://chromium.googlesource.com/chromium/src/+/master/docs/testing/regression-test-selection.md
+  #
+  # Exclude unittest/browsertest files listed in this file.
+  #
+  # Entries should be newline separated.
+  # Each entry much be an source-absolute path starting with //.
+  rts_exclude_file = ""
+}
+
 assert(!(is_debug && is_official_build), "Can't do official debug builds")
 
 # ==============================================================================
@@ -575,3 +586,32 @@ set_defaults("component") {
     configs = default_compiler_configs
   }
 }
+
+# ==============================================================================
+# Regression Test Selection (RTS)
+# https://chromium.googlesource.com/chromium/src/+/master/docs/testing/regression-test-selection.md
+# ==============================================================================
+if (rts_exclude_file != "") {
+  rts_exclusions = read_file(rts_exclude_file, "list lines")
+
+  # Many tests are included in source_sets
+  template("source_set") {
+    _target_name = target_name
+    target("source_set", _target_name) {
+      forward_variables_from(invoker, "*", TESTONLY_AND_VISIBILITY)
+      forward_variables_from(invoker, TESTONLY_AND_VISIBILITY)
+
+      if (defined(sources) && sources != []) {
+        # Normalize paths
+        abs_paths = get_path_info(sources, "abspath")
+
+        # Filter
+        filtered_sources = filter_exclude(abs_paths, rts_exclusions)
+
+        # Do the replacement
+        sources = []
+        sources = filtered_sources
+      }
+    }
+  }
+}

docs/testing/regression-test-selection.md

+# Regression test selection (RTS)
+
+Regression Test Selection (RTS) is a technique to intellegently select tests to
+run, without spending too many resources on testing, but still detecting bad
+code changes.
+
+[TOC]
+
+## Current strategy
+
+The current strategy is to skip files which we deem unaffected by the CL's
+changelist. Affectedness is estimated using the heuristic that dependent files
+are committed together. We look at git history and measure how often each file
+A appears together each file B in the same commit. If A affects B,
+and B affects C, we assume A also affects C. We represent these transitive
+relationships in a graph where files are the nodes and the probability that the
+two files affect each other are the weighted edges. The distance between two
+nodes represents the (inverse) probability that these two files affect
+each other. Finally, we skip tests in files that are farther than a
+certain distance from the files in the CL.
+
+## Skipping mechanism
+
+Test skipping happens at the GN level in
+[source_set](/build/config/BUILDCONFIG.gn) and [test](/testing/test.gni)
+GN targets.
+
+## Known failure mode
+
+Consider a test file A that contains unit tests, as well as some variables
+used in another file B. When our RTS strategy excludes A, but not B, a
+compilation error will occur.
+
+## Design Docs
+
+- [File-level RTS](http://doc/1KWG82gNpkaRAchlp3jtENFdlefGvJxMHkAszlu9fo1c)
+- [Chrome RTS](http://doc/10RP1XRw8ZSrvgVky1flH7ykAaIaG15RymM4gW7bFURQ)
+

testing/test.gni

@@ -41,6 +41,20 @@ if (is_android) {
 #      Similar to the GN arg 'enable_run_ios_unittests_with_xctest' but
 #      for build targets.
 template("test") {
+  # RTS
+  if (rts_exclude_file != "" && defined(invoker.sources) &&
+      invoker.sources != []) {
+    # Normalize paths
+    abs_paths = get_path_info(invoker.sources, "abspath")
+
+    # Filter
+    filtered_sources = filter_exclude(abs_paths, rts_exclusions)
+
+    # Do the replacement
+    invoker.sources = []
+    invoker.sources = filtered_sources
+  }
+
   testonly = true
   if (!is_ios) {
     assert(!defined(invoker.is_xctest) || !invoker.is_xctest,

tools/mb/mb_config.pyl

@@ -994,6 +994,7 @@
       'linux-perfetto-rel': 'perfetto_release_trybot',
       'linux-rel': 'gpu_tests_release_trybot_no_symbols_use_dummy_lastchange_code_coverage',
       'linux-rel-builderful': 'gpu_tests_release_trybot_no_symbols_use_dummy_lastchange_code_coverage',
+      'linux-rts': 'release_trybot_rts',
       'linux-trusty-rel': 'gpu_tests_release_trybot_no_symbols_use_dummy_lastchange',
       'linux-viz-rel': 'release_trybot',
       'linux-warmed': 'release_trybot_no_symbols_use_dummy_lastchange_code_coverage',
@@ -2592,6 +2593,10 @@
       'no_goma',
     ],
 
+    'release_trybot_rts': [
+      'release_bot', 'rts_bot',
+    ],
+
     'tsan_disable_nacl_debug_bot': [
       'tsan', 'disable_nacl', 'debug_bot',
     ],
@@ -3238,6 +3243,10 @@
       'gn_args': 'enable_resource_allowlist_generation=true',
     },
 
+    'rts_bot': {
+      'gn_args': 'rts_exclude_file="//testing/rts_exclude_file.txt"',
+    },
+
     'shared': {
       'gn_args': 'is_component_build=true',
     },

tools/mb/mb_config_expectations/tryserver.chromium.linux.json

@@ -554,6 +554,14 @@
       "use_goma": true
     }
   },
+  "linux-rts": {
+    "gn_args": {
+      "is_component_build": false,
+      "is_debug": false,
+      "rts_exclude_file": "//testing/rts_exclude_file.txt",
+      "use_goma": true
+    }
+  },
   "linux-trusty-rel": {
     "gn_args": {
       "dcheck_always_on": true,