milo: Add SwarmBucket templating.

common/data/text/stringtemplate/template.go

Index: common/data/text/stringtemplate/template.go
diff --git a/common/data/text/stringtemplate/template.go b/common/data/text/stringtemplate/template.go
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index 0000000000000000000000000000000000000000..1140c2dc4e10578a6c9cd3deeb3789ce18297145
--- /dev/null
+++ b/common/data/text/stringtemplate/template.go
@@ -0,0 +1,105 @@
+// 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 stringtemplate implements Python string.Template-like substitution.
+package stringtemplate
+
+import (
+	"regexp"
+	"strings"
+
+	"github.com/luci/luci-go/common/errors"
+)
+
+const idPattern = "[_a-z][_a-z0-9]*"
+
+// We're looking for:
+//
+// Submatch indices:
+// [0:1] Full match
+// [2:3] $$ (escaped)
+// [4:5] $key (without braces)
+// [6:7] ${key} (with braces)
+// [8:9] $... (Invalid)
+var namedFormatMatcher = regexp.MustCompile(
+	`\$(?:` +
+		`(\$)|` + // Escaped ($$)
+		`(` + idPattern + `)|` + // Without braces: $key
+		`(?:\{(` + idPattern + `)\})|` + // With braces: ${key}
+		`(.*)` + // Invalid
+		`)`)
+
+// Resolve resolves substitutions in v using the supplied substitution map,
+// subst.
+//
+// A substitution can have the form:
+//
+//	$key
+//	${key}
+//
+// The substitution can also be escaped using a second "$", such as "$$".
+//
+// If the string includes an erroneous substitution, or if a referenced
+// template variable isn't included in the "substitutions" map, Resolve will
+// return an error.
+func Resolve(v string, subst map[string]string) (string, error) {
+	smi := namedFormatMatcher.FindAllStringSubmatchIndex(v, -1)
+	if len(smi) == 0 {
+		// No substitutions.
+		return v, nil
+	}
+
+	var (
+		parts = make([]string, 0, (len(smi)*2)+1)
+		pos   = 0
+	)
+
+	for _, match := range smi {
+		key := ""
+		switch {
+		case match[8] >= 0:
+			// Invalid.
+			return "", errors.Reason("invalid template: %(template)q").
+				D("template", v).
+				Err()
+
+		case match[2] >= 0:
+			// Escaped.
+			parts = append(parts, v[pos:match[2]])
+			pos = match[3]
+			continue
+
+		case match[4] >= 0:
+			// Key (without braces)
+			key = v[match[4]:match[5]]
+		case match[6] >= 0:
+			// Key (with braces)
+			key = v[match[6]:match[7]]
+
+		default:
+			panic("impossible")
+		}
+
+		// Add anything in between the previous match and the current. If our match
+		// includes a non-escape character, add that too.
+		if pos < match[0] {
+			parts = append(parts, v[pos:match[0]])
+		}
+		pos = match[1]
+
+		subst, ok := subst[key]
+		if !ok {
+			return "", errors.Reason("no substitution for %(key)q").
+				D("key", key).
+				Err()
+		}
+		parts = append(parts, subst)
+	}
+
+	// Append any trailing string.
+	parts = append(parts, v[pos:])
+
+	// Join the parts.
+	return strings.Join(parts, ""), nil
+}