address review comments; fix compile error.

mount/file_test.go

@@ -35,10 +35,12 @@ func TestFileMount(t *testing.T) {
 	require.True(t, stat.Exists)
 	require.EqualValues(t, size, stat.Size)
 
+	// check URL.
+	require.Equal(t, mnt.Path, mnt.Serialize().Host)
+
 	info := mnt.Info()
 	require.True(t, info.AccessSequential && info.AccessSeek && info.AccessRandom) // all flags true
 	require.Equal(t, KindLocal, info.Kind)
-	require.Equal(t, "file://"+mnt.Path, info.URL.String())
 
 	reader, err := mnt.Fetch(context.Background())
 	require.NoError(t, err)

shard.go

@@ -11,9 +11,8 @@ import (
 // waiter encapsulates a context passed by the user, and the channel they want
 // the result returned to.
 type waiter struct {
-	// context governing the operation if this is an external op.
-	ctx   context.Context
-	outCh chan ShardResult
+	ctx   context.Context  // governs the op if it's external
+	outCh chan ShardResult // to send back the result
 }
 
 func (w waiter) deliver(res *ShardResult) {
@@ -27,15 +26,17 @@ func (w waiter) deliver(res *ShardResult) {
 type Shard struct {
 	lk sync.RWMutex
 
-	// IMMUTABLE FIELDS: safe to read outside the event loop without a lock.
-	d     *DAGStore
-	key   shard.Key
-	mount *mount.Upgrader
-
-	// MUTABLE FIELDS: cannot read/write outside event loop.
-	state   ShardState
-	err     error // populated if shard state is errored.
-	indexed bool
+	// IMMUTABLE FIELDS
+	// safe to read outside the event loop without a lock
+	d     *DAGStore       // backreference
+	key   shard.Key       // persisted in PersistedShard.Key
+	mount *mount.Upgrader // persisted in PersistedShard.URL (underlying)
+
+	// MUTABLE FIELDS
+	// cannot read/write outside event loop.
+	state   ShardState // persisted in PersistedShard.State
+	err     error      // populated if shard state is errored; persisted in PersistedShard.Error
+	indexed bool       // persisted in PersistedShard.Indexed
 
 	wRegister *waiter
 	wAcquire  []*waiter

shard_persist.go

@@ -22,7 +22,8 @@ type PersistedShard struct {
 }
 
 // MarshalJSON returns a serialized representation of the state. It must be
-// called from inside the event loop, as it accesses mutable state.
+// called from inside the event loop, as it accesses mutable state, or under a
+// shard read lock.
 func (s *Shard) MarshalJSON() ([]byte, error) {
 	u, err := s.d.mounts.Represent(s.mount)
 	if err != nil {