server/config: Generic caching backend.

server/config/caching/codec.go

+// 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[:]
+}