Fix a possible nil-dereference crash (#478)

* Test for a panic inside global internal meter instrument's Unbind

* Fix a possible nil-dereference crash

There is a nil dereference crash if we perform some operations in
certain order:

- get a global meter
- create an instrument
- bind it
- set the delegate
- unbind the instrument
- call some recording function on the not-really-bound-anymore
  instrument

Unbind will run the no op run-once initialization routine, so the
follow-up RecordOne call will not run it's initialization
routine. Which RecordOne's initialization routine being skipped, the
delegate to bounded instrument is not set, but the code is still
trying to get a pointer to it and then unconditionally dereference it.

Add an extra check for a nil pointer - if this is true, then Unbind
was first and RecordOne should effectively be a no op.

Co-authored-by: Joshua MacDonald <jmacd@users.noreply.github.com>

api/global/internal/meter.go

@@ -233,6 +233,12 @@ func (bound *instHandle) RecordOne(ctx context.Context, number core.Number) {
 	if implPtr == nil {
 		implPtr = (*metric.BoundInstrumentImpl)(atomic.LoadPointer(&bound.delegate))
 	}
+	// This may still be nil if instrument was created and bound
+	// without a delegate, then the instrument was set to have a
+	// delegate and unbound.
+	if implPtr == nil {
+		return
+	}
 	(*implPtr).RecordOne(ctx, number)
 }
 

api/global/internal/meter_test.go

@@ -225,3 +225,23 @@ func TestDefaultSDK(t *testing.T) {
 	require.Equal(t, `{"updates":[{"name":"test.builtin{A=B}","sum":1}]}
 `, <-ch)
 }
+
+func TestUnbindThenRecordOne(t *testing.T) {
+	internal.ResetForTest()
+
+	// Note: this test uses oppsite Float64/Int64 number kinds
+	// vs. the above, to cover all the instruments.
+	ctx := context.Background()
+	sdk := metrictest.NewProvider()
+	meter := global.MeterProvider().Meter("test")
+	counter := meter.NewInt64Counter("test.counter")
+	boundC := counter.Bind(meter.Labels())
+	global.SetMeterProvider(sdk)
+	boundC.Unbind()
+
+	require.NotPanics(t, func() {
+		boundC.Add(ctx, 1)
+	})
+	mock := global.MeterProvider().Meter("test").(*metrictest.Meter)
+	require.Equal(t, 0, len(mock.MeasurementBatches))
+}