[COMMENT] TSDB: Document assumptions about what needs locking in memS…

…eries

Signed-off-by: Bryan Boreham <[email protected]>

tsdb/head.go

@@ -2120,8 +2120,7 @@ func (s sample) Type() chunkenc.ValueType {
 // memSeries is the in-memory representation of a series. None of its methods
 // are goroutine safe and it is the caller's responsibility to lock it.
 type memSeries struct {
-	sync.Mutex
-
+	// Members up to the Mutex are not changed after construction, so can be accessed without a lock.
 	ref  chunks.HeadSeriesRef
 	lset labels.Labels
 	meta *metadata.Metadata
@@ -2133,6 +2132,10 @@ type memSeries struct {
 	// Value returned by secondary hash function.
 	secondaryHash uint32
 
+	sync.Mutex
+
+	// Everything after here should only be accessed with the lock.
+
 	// Immutable chunks on disk that have not yet gone into a block, in order of ascending time stamps.
 	// When compaction runs, chunks get moved into a block and all pointers are shifted like so:
 	//