Remove unnecessary chunk fetch in Head queries

`safeChunk` is only obtained from the `headChunkReader.Chunk` call where
the chunk is already fetched and stored with the `safeChunk`. So, when
getting the iterator for the `safeChunk`, we don't need to get the chunk again.

Also removed a couple of unnecessary fields from `safeChunk` as a part of this.

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

tsdb/head_read.go

@@ -308,8 +308,6 @@ func (h *headChunkReader) Chunk(meta chunks.Meta) (chunkenc.Chunk, error) {
 		s:        s,
 		cid:      cid,
 		isoState: h.isoState,
-		chunkDiskMapper: h.head.chunkDiskMapper,
-		memChunkPool:    &h.head.memChunkPool,
 	}, nil
 }
 
@@ -603,40 +601,21 @@ type safeChunk struct {
 	s        *memSeries
 	cid      chunks.HeadChunkID
 	isoState *isolationState
-	chunkDiskMapper *chunks.ChunkDiskMapper
-	memChunkPool    *sync.Pool
 }
 
 func (c *safeChunk) Iterator(reuseIter chunkenc.Iterator) chunkenc.Iterator {
 	c.s.Lock()
-	it := c.s.iterator(c.cid, c.isoState, c.chunkDiskMapper, c.memChunkPool, reuseIter)
+	it := c.s.iterator(c.cid, c.Chunk, c.isoState, reuseIter)
 	c.s.Unlock()
 	return it
 }
 
 // iterator returns a chunk iterator for the requested chunkID, or a NopIterator if the requested ID is out of range.
 // It is unsafe to call this concurrently with s.append(...) without holding the series lock.
-func (s *memSeries) iterator(id chunks.HeadChunkID, isoState *isolationState, chunkDiskMapper *chunks.ChunkDiskMapper, memChunkPool *sync.Pool, it chunkenc.Iterator) chunkenc.Iterator {
-	c, garbageCollect, err := s.chunk(id, chunkDiskMapper, memChunkPool)
-	// TODO(fabxc): Work around! An error will be returns when a querier have retrieved a pointer to a
-	// series's chunk, which got then garbage collected before it got
-	// accessed.  We must ensure to not garbage collect as long as any
-	// readers still hold a reference.
-	if err != nil {
-		return chunkenc.NewNopIterator()
-	}
-	defer func() {
-		if garbageCollect {
-			// Set this to nil so that Go GC can collect it after it has been used.
-			// This should be done always at the end.
-			c.chunk = nil
-			memChunkPool.Put(c)
-		}
-	}()
-
+func (s *memSeries) iterator(id chunks.HeadChunkID, c chunkenc.Chunk, isoState *isolationState, it chunkenc.Iterator) chunkenc.Iterator {
 	ix := int(id) - int(s.firstChunkID)
 
-	numSamples := c.chunk.NumSamples()
+	numSamples := c.NumSamples()
 	stopAfter := numSamples
 
 	if isoState != nil && !isoState.IsolationDisabled() {
@@ -681,9 +660,9 @@ func (s *memSeries) iterator(id chunks.HeadChunkID, isoState *isolationState, ch
 		return chunkenc.NewNopIterator()
 	}
 	if stopAfter == numSamples {
-		return c.chunk.Iterator(it)
+		return c.Iterator(it)
 	}
-	return makeStopIterator(c.chunk, it, stopAfter)
+	return makeStopIterator(c, it, stopAfter)
 }
 
 // stopIterator wraps an Iterator, but only returns the first

tsdb/head_test.go

@@ -537,11 +537,18 @@ func TestHead_ReadWAL(t *testing.T) {
 				require.NoError(t, c.Err())
 				return x
 			}
-			require.Equal(t, []sample{{100, 2, nil, nil}, {101, 5, nil, nil}}, expandChunk(s10.iterator(0, nil, head.chunkDiskMapper, nil, nil)))
-			require.Equal(t, []sample{{101, 6, nil, nil}}, expandChunk(s50.iterator(0, nil, head.chunkDiskMapper, nil, nil)))
+
+			c, _, err := s10.chunk(0, head.chunkDiskMapper, &head.memChunkPool)
+			require.NoError(t, err)
+			require.Equal(t, []sample{{100, 2, nil, nil}, {101, 5, nil, nil}}, expandChunk(c.chunk.Iterator(nil)))
+			c, _, err = s50.chunk(0, head.chunkDiskMapper, &head.memChunkPool)
+			require.NoError(t, err)
+			require.Equal(t, []sample{{101, 6, nil, nil}}, expandChunk(c.chunk.Iterator(nil)))
 			// The samples before the new series record should be discarded since a duplicate record
 			// is only possible when old samples were compacted.
-			require.Equal(t, []sample{{101, 7, nil, nil}}, expandChunk(s100.iterator(0, nil, head.chunkDiskMapper, nil, nil)))
+			c, _, err = s100.chunk(0, head.chunkDiskMapper, &head.memChunkPool)
+			require.NoError(t, err)
+			require.Equal(t, []sample{{101, 7, nil, nil}}, expandChunk(c.chunk.Iterator(nil)))
 
 			q, err := head.ExemplarQuerier(context.Background())
 			require.NoError(t, err)
@@ -2566,7 +2573,13 @@ func TestIteratorSeekIntoBuffer(t *testing.T) {
 		require.True(t, ok, "sample append failed")
 	}
 
-	it := s.iterator(s.headChunkID(len(s.mmappedChunks)), nil, chunkDiskMapper, nil, nil)
+	c, _, err := s.chunk(0, chunkDiskMapper, &sync.Pool{
+		New: func() interface{} {
+			return &memChunk{}
+		},
+	})
+	require.NoError(t, err)
+	it := c.chunk.Iterator(nil)
 
 	// First point.
 	require.Equal(t, chunkenc.ValFloat, it.Seek(0))