milo: Add SwarmBucket templating.

common/data/text/stringtemplate/template.go

+// 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.

[nodir, 2017/02/18 07:00:54]

this is a nice package, thanks

+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]]
+                }

[nodir, 2017/02/18 07:00:54]

nit: can you add `panic("impossible") in the default case, to have a peace of
mind for future modifications of this code

[dnj, 2017/02/18 07:42:48]

Done.

+
+                // 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
+}