Add sha512 support

cache/hashing/BUILD.bazel

@@ -7,6 +7,7 @@ go_library(
         "hashing.go",
         "sha1.go",
         "sha256.go",
+        "sha512.go",
     ],
     importpath = "github.com/buchgr/bazel-remote/v2/cache/hashing",
     visibility = ["//visibility:public"],

cache/hashing/sha512.go

@@ -0,0 +1,66 @@
+package hashing
+
+import (
+	"crypto/sha512"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"hash"
+	"regexp"
+
+	pb "github.com/buchgr/bazel-remote/v2/genproto/build/bazel/remote/execution/v2"
+)
+
+func init() {
+	hasher := &sha512Hasher{}
+	register(hasher)
+}
+
+var sha512Regex = regexp.MustCompile("^[a-f0-9]{128}$")
+
+type sha512Hasher struct{}
+
+func (d *sha512Hasher) New() hash.Hash {
+	return sha512.New()
+}
+
+func (d *sha512Hasher) Hash(data []byte) string {
+	sum := sha512.Sum512(data)
+	return hex.EncodeToString(sum[:])
+}
+
+func (d *sha512Hasher) DigestFunction() pb.DigestFunction_Value {
+	return pb.DigestFunction_SHA512
+}
+
+func (d *sha512Hasher) Dir() string {
+	return "sha512"
+}
+
+func (d *sha512Hasher) Empty() string {
+	return "cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e"
+}
+
+func (d *sha512Hasher) Size() int {
+	return sha512.Size
+}
+
+func (d *sha512Hasher) Validate(value string) error {
+	if d.Size()*2 != len(value) {
+		return fmt.Errorf("Invalid sha256 hash length %d: expected %d", len(value), d.Size())
+	}
+	if !sha512Regex.MatchString(value) {
+		return errors.New("Malformed sha1 hash " + value)
+	}
+	return nil
+}
+
+func (d *sha512Hasher) ValidateDigest(hash string, size int64) error {
+	if size == int64(0) {
+		if hash == d.Empty() {
+			return nil
+		}
+		return fmt.Errorf("Invalid zero-length %s hash", d.DigestFunction())
+	}
+	return d.Validate(hash)
+}