server/config: Generic caching backend.

server/config/caching/codec.go

Index: server/config/caching/codec.go
diff --git a/server/config/caching/codec.go b/server/config/caching/codec.go
new file mode 100644
index 0000000000000000000000000000000000000000..bf3f13851eb9d48d2efff00132518a65f4b351eb
--- /dev/null
+++ b/server/config/caching/codec.go
@@ -0,0 +1,83 @@
+// Copyright 2015 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 caching
+
+import (
+	"bytes"
+	"compress/zlib"
+	"crypto/sha256"
+	"encoding/binary"
+	"encoding/json"
+	"strings"
+
+	"github.com/luci/luci-go/common/errors"
+)
+
+// Encode is a convenience method for generating a ZLIB-compressed JSON-encoded
+// object.
+func Encode(v interface{}) ([]byte, error) {

[iannucci, 2017/01/07 20:53:16]

maybe EncodeJSONZlib / DecodeJSONZlib?

[dnj, 2017/01/10 03:29:17]

i was thinking I'd leave it ambiguous in function name in case we want to change
it in the future. The contract is that Decode works on Encode :)

+	var buf bytes.Buffer
+	zw := zlib.NewWriter(&buf)
+	enc := json.NewEncoder(zw)
+
+	if err := enc.Encode(v); err != nil {
+		zw.Close()
+		return nil, errors.Annotate(err).Reason("failed to JSON-encode").Err()
+	}
+	if err := zw.Close(); err != nil {
+		return nil, errors.Annotate(err).Reason("failed to Close zlib Writer").Err()
+	}
+	return buf.Bytes(), nil
+}
+
+// Decode is a convenience method for decoding a ZLIB-compressed JSON-encoded
+// object encoded by Encode.
+func Decode(d []byte, v interface{}) error {
+	zr, err := zlib.NewReader(bytes.NewReader(d))
+	if err != nil {
+		return errors.Annotate(err).Reason("failed to create zlib Reader").Err()
+	}
+	defer zr.Close()
+
+	if err := json.NewDecoder(zr).Decode(v); err != nil {
+		return errors.Annotate(err).Reason("failed to JSON-decode").Err()
+	}
+	return nil
+}
+
+// HashParams is a convenience method for hashing a series of strings into a
+// unique hash key for that series.

[iannucci, 2017/01/07 20:53:16]

I would document that the output has a fixed size of 32 bytes.

[dnj, 2017/01/10 03:29:17]

Done.

+func HashParams(params ...string) []byte {
+	size := 0
+	for i := range params {
+		size += len(params[i]) + 1
+	}
+	size += binary.MaxVarintLen64 + 1
+
+	var buf bytes.Buffer
+	buf.Grow(size)
+
+	// Prefix the hash input with the number of elements. This will prevent some
+	// inputs that include "0x00" from from stomping over other inputs.
+	sizeBuf := [binary.MaxVarintLen64]byte{}
+	_, _ = buf.Write(sizeBuf[:binary.PutUvarint(sizeBuf[:], uint64(len(params)))])
+	buf.WriteByte(0x00)
+
+	// Write each string to the Buffer.
+	for i, s := range params {
+		// First, scan through the string to see if it has any "0x00". It probably
+		// won't! But if it does, it could create a conflict. If we observe this,
+		// we'll escape "0x00".
+		if idx := strings.Index(s, "\x00"); idx >= 0 {
+			_, _ = buf.Write(bytes.Replace([]byte(params[i]), []byte{0x00}, []byte{0x00, 0x00}, -1))
+		} else {
+			_, _ = buf.WriteString(s)
+		}
+		buf.WriteByte(0x00)
+	}
+
+	hash := sha256.Sum256(buf.Bytes())
+	return hash[:]
+}