swarmbucket: fix empty dimension value

appengine/cr-buildbucket/proto/project_config.proto

 // Copyright 2015 The Swarming Authors. All rights reserved.
 // Use of this source code is governed by the Apache v2.0 license that can be
 // found in the LICENSE file.
 
 // Schemas for project configs.
 
 syntax = "proto2";
 
 package buildbucket;
 
@@ skipping 43 matching lines @@
     // Name of the builder. Will be propagated to "builder" build tag and
     // "buildername" recipe property.
     optional string name = 1;
     // Builder category. Will be used for visual grouping, for example in Code Review.
     optional string category = 6;
     // Will be become to swarming task tags.
     // Each tag will end up in "swarming_tag" buildbucket tag, for example
     // "swarming_tag:builder:release"
     repeated string swarming_tags = 2;
     // Colon-delimited key-value pair of task dimensions.
+    //
+    // If value is not specified ("<key>:"), then it excludes a value for the
+    // same key specified in common_dimensions.
     repeated string dimensions = 3;
     // Specifies that a recipe to run.
     optional Recipe recipe = 4;
     // Swarming task priority.
     optional int32 priority = 5;
     // Maximum build execution time. Not to be confused with pending time.
     // If not set, defaults to bucket's common_execution_timeout_secs.
     optional int32 execution_timeout_secs = 7;
   }
 
   // Hostname of the swarming instance, e.g. "chromium-swarm.appspot.com".
   optional string hostname = 1;
   // Used to generate a URL for Build, may contain parameters
   // {swarming_hostname}, {task_id}, {bucket} and {builder}. Defaults to:
   // https://{swarming_hostname}/user/task/{task_id}
   optional string url_format = 2;
 
   // Will be put to all swarming tasks.
   repeated string common_swarming_tags = 3;
 
   // Colon-delimited key-value pair of common task dimensions.
+  //
+  // If value is not specified ("<key>:"), then it excludes a value for the
+  // same key specified in swarming task template.
   repeated string common_dimensions = 4;
 
   // Configuration for each builder.
   // Swarming tasks are created only for builds for builders that are not
   // explicitly specified.
   repeated Builder builders = 5;
 
   // Defines a default recipe for all builders in this bucket.
   // A builder may override it or parts of it with something else.
   optional Recipe common_recipe = 6;
@@ skipping 13 matching lines @@
   repeated Acl acls = 2;
   // Buildbucket-swarming integration.
   optional Swarming swarming = 3;
 }
 
 // Schema of buildbucket.cfg file, a project config.
 message BuildbucketCfg {
   // All buckets defined for this project.
   repeated Bucket buckets = 1;
 }