milo: Add SwarmBucket templating.

swarming/tasktemplate/template.go

Index: swarming/tasktemplate/template.go
diff --git a/swarming/tasktemplate/template.go b/swarming/tasktemplate/template.go
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--- /dev/null
+++ b/swarming/tasktemplate/template.go
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+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// Package tasktemplate resolves Swarming task templates.
+//
+// Templated strings may be passed to Swarming tasks in order to have them
+// resolve task-side parameters. See Params for more information.
+package tasktemplate
+
+import (
+	"github.com/luci/luci-go/common/data/text/stringtemplate"
+)
+
+// Params contains supported Swarming task string substitution parameters.
+//
+// A template string can be resolved by passing it to a Params instance's
+// Resolve method.
+type Params struct {
+	// SwarmingRunID is the substitution to use for the "swarming_run_id" template
+	// parameter.
+	//
+	// Note that this is the Swarming Run ID, not Task ID. The Run ID is the
+	// combination of the Task ID with the try number.
+	SwarmingRunID string
+}
+
+func (p *Params) substMap() map[string]string {
+	res := make(map[string]string, 1)
+	if p.SwarmingRunID != "" {
+		res["swarming_run_id"] = p.SwarmingRunID
+	}
+	return res
+}
+
+// Resolve resolves v against p's parameters, populating any template fields
+// with their respective parameter value.
+//
+// If the string is invalid, or if it references an undefined template
+// parameter, an error will be returned.
+func (p *Params) Resolve(v string) (string, error) {
+	return stringtemplate.Resolve(v, p.substMap())
+}