Add the notion of an OS category to the os enum.

This reduces the maintenance burden of maintaining OS-specific defaults
(e.g. builderless) and enables simplifying a proposed change that goma
wishes to make for their builders.

Change-Id: Icc68cfcbbf536d28e3e0df6d7eedfb85266a4db5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1995464
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730914}

infra/config/lib/builders.star

@@ -31,28 +31,40 @@ cpu = struct(
 )
 
 
+# The category for an os: a more generic grouping than specific OS versions that
+# can be used for computing defaults
+os_category = struct(
+    ANDROID = 'Android',
+    LINUX = 'Linux',
+    MAC = 'Mac',
+    WINDOWS = 'Windows',
+)
+
 # The os constants to be used for the os parameter of the builder function
 # The *_DEFAULT members enable distinguishing between a use that runs the
 # "current" version of the OS and a use that runs against a specific version
 # that happens to be the "current" version
-os = struct(
-    ANDROID = 'Android',
+def os_enum(dimension, category):
+  return struct(dimension=dimension, category=category)
 
-    LINUX_TRUSTY = 'Ubuntu-14.04',
-    LINUX_XENIAL = 'Ubuntu-16.04',
-    LINUX_DEFAULT = 'Ubuntu-16.04',
-
-    MAC_10_12 = 'Mac-10.12',
-    MAC_10_13 = 'Mac-10.13',
-    MAC_10_14 = 'Mac-10.14',
-    MAC_DEFAULT = 'Mac-10.13',
-    MAC_ANY = 'Mac',
-
-    WINDOWS_7 = 'Windows-7',
-    WINDOWS_8_1 = 'Windows-8.1',
-    WINDOWS_10 = 'Windows-10',
-    WINDOWS_DEFAULT = 'Windows-10',
-    WINDOWS_ANY = 'Windows',
+os = struct(
+    ANDROID = os_enum('Android', os_category.ANDROID),
+
+    LINUX_TRUSTY = os_enum('Ubuntu-14.04', os_category.LINUX),
+    LINUX_XENIAL = os_enum('Ubuntu-16.04', os_category.LINUX),
+    LINUX_DEFAULT = os_enum('Ubuntu-16.04', os_category.LINUX),
+
+    MAC_10_12 = os_enum('Mac-10.12', os_category.MAC),
+    MAC_10_13 = os_enum('Mac-10.13', os_category.MAC),
+    MAC_10_14 = os_enum('Mac-10.14', os_category.MAC),
+    MAC_DEFAULT = os_enum('Mac-10.13', os_category.MAC),
+    MAC_ANY = os_enum('Mac', os_category.MAC),
+
+    WINDOWS_7 = os_enum('Windows-7', os_category.WINDOWS),
+    WINDOWS_8_1 = os_enum('Windows-8.1', os_category.WINDOWS),
+    WINDOWS_10 = os_enum('Windows-10', os_category.WINDOWS),
+    WINDOWS_DEFAULT = os_enum('Windows-10', os_category.WINDOWS),
+    WINDOWS_ANY = os_enum('Windows', os_category.WINDOWS),
 )
 
 
@@ -108,7 +120,7 @@ goma = struct(
 # Implementation details                                                       #
 ################################################################################
 
-_DEFAULT_BUILDERLESS_OSES = [os.LINUX_TRUSTY, os.LINUX_XENIAL]
+_DEFAULT_BUILDERLESS_OS_CATEGORIES = [os_category.LINUX]
 
 
 def _sentinel(tag):
@@ -319,11 +331,11 @@ def builder(
 
   os = _default('os', os)
   if os:
-    dimensions['os'] = os
+    dimensions['os'] = os.dimension
 
   builderless = _default('builderless', builderless)
   if builderless == _COMPUTE:
-    builderless = os in _DEFAULT_BUILDERLESS_OSES
+    builderless = os != None and os.category in _DEFAULT_BUILDERLESS_OS_CATEGORIES
   if builderless:
     dimensions['builderless'] = '1'