Use simpler locking in the Go 1.17 collector (#975)

A previous PR made it so that the Go 1.17 collector locked only around
uses of rmSampleBuf, but really that means that Metric values may be
sent over the channel containing some values from future metrics.Read
calls. While generally-speaking this isn't a problem, we lose any
consistency guarantees provided by the runtime/metrics package.

Also, that optimization to not just lock around all of Collect was
premature. Truthfully, Collect is called relatively infrequently, and
its critical path is fairly fast (10s of µs). To prove it, this change
also adds a benchmark.

name            old time/op  new time/op  delta
GoCollector-16  43.7µs ± 2%  43.2µs ± 2%   ~     (p=0.190 n=9+9)

Note that because the benchmark is single-threaded it actually looks
like it might be getting *slightly* faster, because all those Collect
calls for the Metrics are direct calls instead of interface calls.

Signed-off-by: Michael Anthony Knyszek <mknyszek@google.com>

prometheus/go_collector_go117.go

@@ -31,8 +31,11 @@ import (
 type goCollector struct {
 	base baseGoCollector
 
+	// mu protects updates to all fields ensuring a consistent
+	// snapshot is always produced by Collect.
+	mu sync.Mutex
+
 	// rm... fields all pertain to the runtime/metrics package.
-	rmSampleMu  sync.Mutex
 	rmSampleBuf []metrics.Sample
 	rmSampleMap map[string]*metrics.Sample
 	rmMetrics   []collectorMetric
@@ -135,10 +138,16 @@ func (c *goCollector) Collect(ch chan<- Metric) {
 	// rmSampleBuf. Just read into rmSampleBuf but write all the data
 	// we get into our Metrics or MemStats.
 	//
-	// Note that we cannot simply read and then clone rmSampleBuf
-	// because we'd need to perform a deep clone of it, which is likely
-	// not worth it.
-	c.rmSampleMu.Lock()
+	// This lock also ensures that the Metrics we send out are all from
+	// the same updates, ensuring their mutual consistency insofar as
+	// is guaranteed by the runtime/metrics package.
+	//
+	// N.B. This locking is heavy-handed, but Collect is expected to be called
+	// relatively infrequently. Also the core operation here, metrics.Read,
+	// is fast (O(tens of microseconds)) so contention should certainly be
+	// low, though channel operations and any allocations may add to that.
+	c.mu.Lock()
+	defer c.mu.Unlock()
 
 	// Populate runtime/metrics sample buffer.
 	metrics.Read(c.rmSampleBuf)
@@ -157,10 +166,13 @@ func (c *goCollector) Collect(ch chan<- Metric) {
 			if v1 > v0 {
 				m.Add(unwrapScalarRMValue(sample.Value) - m.get())
 			}
+			m.Collect(ch)
 		case *gauge:
 			m.Set(unwrapScalarRMValue(sample.Value))
+			m.Collect(ch)
 		case *batchHistogram:
 			m.update(sample.Value.Float64Histogram(), c.exactSumFor(sample.Name))
+			m.Collect(ch)
 		default:
 			panic("unexpected metric type")
 		}
@@ -169,17 +181,6 @@ func (c *goCollector) Collect(ch chan<- Metric) {
 	// populate the old metrics from it.
 	var ms runtime.MemStats
 	memStatsFromRM(&ms, c.rmSampleMap)
-
-	c.rmSampleMu.Unlock()
-
-	// Export all the metrics to ch.
-	// At this point we must not access rmSampleBuf or rmSampleMap, because
-	// a concurrent caller could use it. It's safe to Collect all our Metrics,
-	// however, because they're updated in a thread-safe way while MemStats
-	// is local to this call of Collect.
-	for _, m := range c.rmMetrics {
-		m.Collect(ch)
-	}
 	for _, i := range c.msMetrics {
 		ch <- MustNewConstMetric(i.desc, i.valType, i.eval(&ms))
 	}

prometheus/go_collector_go117_test.go

@@ -292,7 +292,7 @@ func TestGoCollectorConcurrency(t *testing.T) {
 		go func() {
 			ch := make(chan Metric)
 			go func() {
-				// Drain all metrics recieved until the
+				// Drain all metrics received until the
 				// channel is closed.
 				for range ch {
 				}

prometheus/go_collector_test.go

@@ -154,3 +154,20 @@ func TestGoCollectorGC(t *testing.T) {
 		break
 	}
 }
+
+func BenchmarkGoCollector(b *testing.B) {
+	c := NewGoCollector().(*goCollector)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		ch := make(chan Metric, 8)
+		go func() {
+			// Drain all metrics received until the
+			// channel is closed.
+			for range ch {
+			}
+		}()
+		c.Collect(ch)
+		close(ch)
+	}
+}