add ability to override ints in compose files with 0

Signed-off-by: Marco Spiess <marco.spiess@hotmail.de>

cli/compose/loader/merge.go

@@ -61,6 +61,7 @@ func mergeServices(base, override []types.ServiceConfig) ([]types.ServiceConfig,
 			reflect.TypeOf([]types.ServiceVolumeConfig{}):    mergeSlice(toServiceVolumeConfigsMap, toServiceVolumeConfigsSlice),
 			reflect.TypeOf(types.ShellCommand{}):             mergeShellCommand,
 			reflect.TypeOf(&types.ServiceNetworkConfig{}):    mergeServiceNetworkConfig,
+			reflect.PointerTo(reflect.TypeOf(uint64(1))):     mergeUint64,
 		},
 	}
 	for name, overrideService := range overrideServices {
@@ -259,6 +260,14 @@ func mergeServiceNetworkConfig(dst, src reflect.Value) error {
 	return nil
 }
 
+//nolint:unparam
+func mergeUint64(dst, src reflect.Value) error {
+	if !src.IsNil() {
+		dst.Elem().Set(src.Elem())
+	}
+	return nil
+}
+
 func getLoggingDriver(v reflect.Value) string {
 	return v.FieldByName("Driver").String()
 }

cli/compose/loader/merge_test.go

@@ -1237,3 +1237,60 @@ func TestMergeServiceNetworkConfig(t *testing.T) {
 		},
 	)
 }
+
+// issue #3293
+func TestMergeServiceOverrideReplicasZero(t *testing.T) {
+	base := types.ServiceConfig{
+		Name: "someService",
+		Deploy: types.DeployConfig{
+			Replicas: uint64Ptr(3),
+		},
+	}
+	override := types.ServiceConfig{
+		Name: "someService",
+		Deploy: types.DeployConfig{
+			Replicas: uint64Ptr(0),
+		},
+	}
+	services, err := mergeServices([]types.ServiceConfig{base}, []types.ServiceConfig{override})
+	assert.NilError(t, err)
+	assert.Equal(t, len(services), 1)
+	actual := services[0]
+	assert.DeepEqual(
+		t,
+		actual,
+		types.ServiceConfig{
+			Name: "someService",
+			Deploy: types.DeployConfig{
+				Replicas: uint64Ptr(0),
+			},
+		},
+	)
+}
+
+func TestMergeServiceOverrideReplicasNotNil(t *testing.T) {
+	base := types.ServiceConfig{
+		Name: "someService",
+		Deploy: types.DeployConfig{
+			Replicas: uint64Ptr(3),
+		},
+	}
+	override := types.ServiceConfig{
+		Name:   "someService",
+		Deploy: types.DeployConfig{},
+	}
+	services, err := mergeServices([]types.ServiceConfig{base}, []types.ServiceConfig{override})
+	assert.NilError(t, err)
+	assert.Equal(t, len(services), 1)
+	actual := services[0]
+	assert.DeepEqual(
+		t,
+		actual,
+		types.ServiceConfig{
+			Name: "someService",
+			Deploy: types.DeployConfig{
+				Replicas: uint64Ptr(3),
+			},
+		},
+	)
+}