control: convert single tier to metered+per_unit (#267)

This changes how Tier interprets and converts tiers to Stripe prices.

Previously, a single tier in a pricing.json would convert to a metered
price with one or two tiers depending on if the tier had an inf limit or
not. If it did not have an inf limit, a catch-all was created to satisfy
Stripe. This produced awkward invoices.

This commit changes Tier to convert a feature that has a single tier
without a base price to be a metered+per_unit price in Stripe, which
shows up less awkwardly on invoices.

This also opens us up to support pricing transforms in Stripe for these
types of prices.

control/client.go

@@ -110,11 +110,11 @@ type Feature struct {
 
 // IsMetered reports if the feature is metered.
 func (f *Feature) IsMetered() bool {
-	// checking the mode is more reliable than checking the existence of
-	// tiers because not all responses from stripe containing prices
+	// checking the aggregate is more reliable than checking the existence
+	// of tiers because not all responses from stripe containing prices
 	// include tiers, but they all include the mode, which is empty for
 	// license prices.
-	return f.Mode != ""
+	return f.Aggregate != ""
 }
 
 func (f *Feature) ID() string {
@@ -276,11 +276,24 @@ func (c *Client) pushFeature(ctx context.Context, f Feature) (providerID string,
 	data.Set("recurring", "interval", interval)
 	data.Set("recurring", "interval_count", 1) // TODO: support user-defined interval count
 
-	if len(f.Tiers) == 0 {
+	numTiers := len(f.Tiers)
+	switch {
+	case numTiers == 0:
 		data.Set("recurring", "usage_type", "licensed")
 		data.Set("billing_scheme", "per_unit")
 		data.Set("unit_amount_decimal", f.Base)
-	} else {
+	case numTiers == 1 && f.Tiers[0].Base == 0:
+		t := f.Tiers[0]
+		data.Set("recurring", "usage_type", "metered")
+		data.Set("billing_scheme", "per_unit")
+		aggregate := aggregateToStripe[f.Aggregate]
+		if aggregate == "" {
+			return "", fmt.Errorf("unknown aggregate: %q", f.Aggregate)
+		}
+		data.Set("recurring", "aggregate_usage", aggregate)
+		data.Set("unit_amount_decimal", t.Price)
+		data.Set("metadata", "tier.limit", t.Upto)
+	default:
 		data.Set("recurring", "usage_type", "metered")
 		data.Set("billing_scheme", "tiered")
 		data.Set("tiers_mode", f.Mode)
@@ -357,8 +370,17 @@ func stripePriceToFeature(p stripePrice) Feature {
 		Interval:    intervalFromStripe[p.Recurring.Interval],
 		Mode:        p.TiersMode,
 		Aggregate:   aggregateFromStripe[p.Recurring.AggregateUsage],
-		Base:        p.UnitAmount,
 	}
+
+	if len(p.Tiers) == 0 && p.Recurring.UsageType == "metered" {
+		f.Tiers = append(f.Tiers, Tier{
+			Upto:  parseLimit(p.Metadata.Limit),
+			Price: p.UnitAmount,
+		})
+	} else {
+		f.Base = p.UnitAmount
+	}
+
 	for i, t := range p.Tiers {
 		f.Tiers = append(f.Tiers, Tier{
 			Upto:  t.Upto,

control/client_test.go

@@ -9,7 +9,6 @@ import (
 	"golang.org/x/exp/slices"
 	"kr.dev/diff"
 	"tier.run/refs"
-	"tier.run/stripe"
 	"tier.run/stripe/stroke"
 )
 
@@ -36,20 +35,20 @@ func TestRoundTrip(t *testing.T) {
 
 	want := []Feature{
 		{
-			FeaturePlan: refs.MustParseFeaturePlan("feature:decimal@fractionalBase"),
+			FeaturePlan: refs.MustParseFeaturePlan("feature:licensed:base:decimal@0"),
 			Interval:    "@daily",
 			Currency:    "eur",
 			Base:        0.1,
 		},
 		{
-			FeaturePlan: refs.MustParseFeaturePlan("feature:test@plan:free@3"),
+			FeaturePlan: refs.MustParseFeaturePlan("feature:licensed:base:int@0"),
 			Interval:    "@daily",
 			Currency:    "eur",
 			Title:       "Test2",
 			Base:        1000,
 		},
 		{
-			FeaturePlan: refs.MustParseFeaturePlan("feature:test@plan:free@theVersion"),
+			FeaturePlan: refs.MustParseFeaturePlan("feature:metered:tiers:many@0"),
 			PlanTitle:   "PlanTitle",
 			Interval:    "@yearly",
 			Currency:    "usd",
@@ -62,6 +61,22 @@ func TestRoundTrip(t *testing.T) {
 				{Upto: 3, Price: 300, Base: 3},
 			},
 		},
+		{
+			FeaturePlan: refs.MustParseFeaturePlan("feature:metered:tiers:one@0"),
+			PlanTitle:   "PlanTitle",
+			Interval:    "@yearly",
+			Currency:    "usd",
+			Title:       "FeatureTitle",
+
+			// a single tier is turned into per_unit + metered, so
+			// tiers_mode isn't needed or even set
+			Mode: "",
+
+			Aggregate: "perpetual",
+			Tiers: []Tier{
+				{Upto: 1, Price: 100, Base: 0},
+			},
+		},
 	}
 
 	if !slices.IsSortedFunc(want, func(a, b Feature) bool {
@@ -86,21 +101,10 @@ func TestRoundTrip(t *testing.T) {
 
 	diff.Test(t, t.Errorf, got, want,
 		diff.ZeroFields[Feature]("ProviderID"))
-
-	t.Run("product title", func(t *testing.T) {
-		var got struct {
-			Name string
-		}
-		if err := cc.Stripe.Do(ctx, "GET", "/v1/products/tier__feature-test-plan-free-theVersion", stripe.Form{}, &got); err != nil {
-			t.Fatal(err)
-		}
-		const want = "PlanTitle - FeatureTitle"
-		if got.Name != want {
-			t.Errorf("got %q, want %q", got.Name, want)
-		}
-	})
 }
 
+// TODO(bmizerany): add TestTitle
+
 func TestPushPlanInvalidDecimal(t *testing.T) {
 	cc := newTestClient(t) // TODO(bmizerany): use a client without creating an account
 	ctx := context.Background()

control/schedule_test.go

@@ -302,7 +302,6 @@ func TestScheduleFreeTrials(t *testing.T) {
 	s.checkInvoices("org:paid", []Invoice{{
 		Lines: []InvoiceLineItem{
 			lineItem(featureX, 2, 2),
-			zero,
 		},
 		SubtotalPreTax: 2,
 		Subtotal:       2,