Accpet HMAC key from file

Fixes saracen#15

README.md

@@ -33,3 +33,13 @@ git config lfs.url http://localhost:9876/
 # you can confirm the Endpoint that will be used by running
 git lfs env | grep Endpoint
 ```
+
+When running multiple instances of lfscache, you must use a shared hmac key so
+signatures from one instance can be verified by the others with a shared key.
+This key is not used for storage, so it's safe to rotate the key and restart all
+instances.
+
+```
+dd if=/dev/urandom bs=1 count=64 > hmac-key
+$ ./lfscache --hmac-key-file hmac-key ...
+```

main.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"net/url"
@@ -30,6 +31,7 @@ func main() {
 		tlsCert      = flag.String("tls-cert", "", "HTTPS TLS certificate filepath")
 		lfsServerURL = flag.String("url", "", "LFS server URL")
 		directory    = flag.String("directory", "./objects", "cache directory")
+		hmacKeyFile  = flag.String("hmac-key-file", "", "file containing 64 byte HMAC key for request signing")
 		printVersion = flag.Bool("v", false, "print version")
 	)
 
@@ -56,7 +58,18 @@ func main() {
 		os.Exit(1)
 	}
 
-	s, err := server.New(logger, addr.String(), *directory)
+	var hmacKey []byte = nil
+	if *hmacKeyFile != "" {
+		hmac, err := ioutil.ReadFile(*hmacKeyFile)
+		if err != nil {
+			level.Error(logger).Log("err", err)
+			os.Exit(1)
+		}
+		hmacKey = hmac
+	}
+
+
+	s, err := server.New(logger, addr.String(), *directory, hmacKey)
 	if err != nil {
 		panic(err)
 	}

server/server.go

@@ -98,16 +98,16 @@ type Server struct {
 }
 
 // New returns a new LFS proxy caching server.
-func New(logger log.Logger, upstream, directory string) (*Server, error) {
-	return newServer(logger, upstream, directory, true)
+func New(logger log.Logger, upstream, directory string, hmacKey []byte) (*Server, error) {
+	return newServer(logger, upstream, directory, true, hmacKey)
 }
 
 // NewNoCache returns a new LFS proxy server, with no caching.
 func NewNoCache(logger log.Logger, upstream string) (*Server, error) {
-	return newServer(logger, upstream, "", false)
+	return newServer(logger, upstream, "", false, nil)
 }
 
-func newServer(logger log.Logger, upstream, directory string, cacheEnabled bool) (*Server, error) {
+func newServer(logger log.Logger, upstream, directory string, cacheEnabled bool, hmacKey []byte) (*Server, error) {
 	var fs *cache.FilesystemCache
 	var err error
 	if cacheEnabled {
@@ -135,10 +135,14 @@ func newServer(logger log.Logger, upstream, directory string, cacheEnabled bool)
 		ObjectBatchActionURLRewriter: DefaultObjectBatchActionURLRewriter,
 	}
 
-	_, err = rand.Read(s.hmacKey[:])
-	if err != nil {
-		return nil, err
-	}
+        if hmacKey != nil {
+		copy(s.hmacKey[:], hmacKey)
+        } else {
+		_, err = rand.Read(s.hmacKey[:])
+		if err != nil {
+			return nil, err
+		}
+        }
 
 	if s.upstream, err = url.Parse(upstream); err != nil {
 		return nil, err

server/server_test.go

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"crypto/rand"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -47,11 +48,27 @@ func server() (*httptest.Server, *Server, string, error) {
 		return ts, nil, dir, err
 	}
 
-	s, err := New(log.NewNopLogger(), ts.URL, dir)
+	s, err := New(log.NewNopLogger(), ts.URL, dir, nil)
 
 	return ts, s, dir, err
 }
 
+func TestHmac(t *testing.T) {
+	var hmac [64]byte
+	_, err := rand.Read(hmac[:])
+	require.NoError(t, err)
+
+	s, err := New(log.NewNopLogger(), "http://example.com", "", hmac[:])
+	require.NoError(t, err)
+	assert.Equal(t, s.hmacKey, hmac)
+}
+
+func TestNoHmac(t *testing.T) {
+	s, err := New(log.NewNopLogger(), "http://example.com", "", nil)
+	require.NoError(t, err)
+	assert.NotEmpty(t, s.hmacKey)
+}
+
 func TestProxy(t *testing.T) {
 	ts, s, dir, err := server()
 	defer os.RemoveAll(dir)