Fix registration of multiple callbacks when using the global meter pr…

…ovider (#4945)

CHANGELOG.md

@@ -17,6 +17,10 @@ The next release will require at least [Go 1.21].
 - Add exemplar support to `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc`. (#4900)
 - Add exemplar support to `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp`. (#4900)
 
+### Fixed
+
+- Fix registration of multiple callbacks when using the global meter provider from `go.opentelemetry.io/otel`. (#4945)
+
 ## [1.23.1] 2024-02-07
 
 ### Fixed

internal/global/meter.go

@@ -130,9 +130,11 @@ func (m *meter) setDelegate(provider metric.MeterProvider) {
 		inst.setDelegate(meter)
 	}
 
-	for e := m.registry.Front(); e != nil; e = e.Next() {
+	var n *list.Element
+	for e := m.registry.Front(); e != nil; e = n {
 		r := e.Value.(*registration)
 		r.setDelegate(meter)
+		n = e.Next()
 		m.registry.Remove(e)
 	}
 

internal/global/meter_test.go

@@ -354,20 +354,27 @@ func TestRegistrationDelegation(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, 1, mImpl.registry.Len(), "second callback not registered")
 
-	mp := &testMeterProvider{}
+	var called2 bool
+	_, err = m.RegisterCallback(func(context.Context, metric.Observer) error {
+		called2 = true
+		return nil
+	}, actr)
+	require.NoError(t, err)
+	require.Equal(t, 2, mImpl.registry.Len(), "third callback not registered")
 
-	// otel.SetMeterProvider(mp)
+	mp := &testMeterProvider{}
 	globalMeterProvider.setDelegate(mp)
 
 	testCollect(t, m) // This is a hacky way to emulate a read from an exporter
 	require.False(t, called0, "pre-delegation unregistered callback called")
-	require.True(t, called1, "callback not called")
+	require.True(t, called1, "second callback not called")
+	require.True(t, called2, "third callback not called")
 
-	called1 = false
 	assert.NoError(t, reg1.Unregister(), "unregister second callback")
-
-	testCollect(t, m) // This is a hacky way to emulate a read from an exporter
-	assert.False(t, called1, "unregistered callback called")
+	called1, called2 = false, false // reset called capture
+	testCollect(t, m)               // This is a hacky way to emulate a read from an exporter
+	assert.False(t, called1, "unregistered second callback called")
+	require.True(t, called2, "third callback not called")
 
 	assert.NotPanics(t, func() {
 		assert.NoError(t, reg1.Unregister(), "duplicate unregister calls")