Fix panic in ARM builds of Prometheus (#6110)

An extra sync.Pool was added during a refactor which caused some 64 bit,
atomically accessed variables to no longer be 64 bit aligned. By moving
all atomically accessed variables to the beginning of the struct they
are guaranteed to be 64 bit aligned.

Signed-off-by: Chris Marchbanks <csmarchbanks@gmail.com>

storage/remote/ewma.go

@@ -21,12 +21,15 @@ import (
 
 // ewmaRate tracks an exponentially weighted moving average of a per-second rate.
 type ewmaRate struct {
+	// Keep all 64bit atomically accessed variables at the top of this struct.
+	// See https://golang.org/pkg/sync/atomic/#pkg-note-BUG for more info.
 	newEvents int64
-	alpha     float64
-	interval  time.Duration
-	lastRate  float64
-	init      bool
-	mutex     sync.Mutex
+
+	alpha    float64
+	interval time.Duration
+	lastRate float64
+	init     bool
+	mutex    sync.Mutex
 }
 
 // newEWMARate always allocates a new ewmaRate, as this guarantees the atomically

storage/remote/intern.go

@@ -40,8 +40,11 @@ type pool struct {
 }
 
 type entry struct {
-	s    string
+	// Keep all 64bit atomically accessed variables at the top of this struct.
+	// See https://golang.org/pkg/sync/atomic/#pkg-note-BUG for more info.
 	refs int64
+
+	s string
 }
 
 func newPool() *pool {

tsdb/db.go

@@ -216,7 +216,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 		db.mtx.RLock()
 		defer db.mtx.RUnlock()
 		if len(db.blocks) == 0 {
-			return float64(db.head.minTime)
+			return float64(db.head.MinTime())
 		}
 		return float64(db.blocks[0].meta.MinTime)
 	})

tsdb/head.go

@@ -61,18 +61,20 @@ var (
 
 // Head handles reads and writes of time series data within a time window.
 type Head struct {
-	chunkRange int64
+	// Keep all 64bit atomically accessed variables at the top of this struct.
+	// See https://golang.org/pkg/sync/atomic/#pkg-note-BUG for more info.
+	chunkRange       int64
+	numSeries        uint64
+	minTime, maxTime int64 // Current min and max of the samples included in the head.
+	minValidTime     int64 // Mint allowed to be added to the head. It shouldn't be lower than the maxt of the last persisted block.
+	lastSeriesID     uint64
+
 	metrics    *headMetrics
 	wal        *wal.WAL
 	logger     log.Logger
 	appendPool sync.Pool
 	seriesPool sync.Pool
 	bytesPool  sync.Pool
-	numSeries  uint64
-
-	minTime, maxTime int64 // Current min and max of the samples included in the head.
-	minValidTime     int64 // Mint allowed to be added to the head. It shouldn't be lower than the maxt of the last persisted block.
-	lastSeriesID     uint64
 
 	// All series addressable by their ID or hash.
 	series *stripeSeries