Add idempotency key to Stripe PaymentIntent creation

src/main/java/com/stripe/mpp/methods/stripe/StripeApi.java

@@ -3,6 +3,7 @@
 import com.stripe.StripeClient;
 import com.stripe.exception.StripeException;
 import com.stripe.model.PaymentIntent;
+import com.stripe.net.RequestOptions;
 import com.stripe.param.PaymentIntentCreateParams;
 import com.stripe.mpp.error.VerificationFailedException;
 
@@ -54,7 +55,8 @@ Result createAndConfirm(
         String currency,
         String spt,
         List<String> paymentMethodTypes,
-        Map<String, String> metadata
+        Map<String, String> metadata,
+        String challengeId
     ) {
         try {
             StripeClient client = new StripeClient(secretKey);
@@ -70,11 +72,19 @@ Result createAndConfirm(
                 builder.putAllMetadata(metadata);
             }
 
-            PaymentIntent pi = client.paymentIntents().create(builder.build());
+            RequestOptions options = RequestOptions.builder()
+                .setIdempotencyKey(buildIdempotencyKey(challengeId, spt))
+                .build();
+
+            PaymentIntent pi = client.paymentIntents().create(builder.build(), options);
             return new Result(pi.getId(), pi.getStatus());
 
         } catch (StripeException e) {
             throw new VerificationFailedException(e.getMessage());
         }
     }
+
+    static String buildIdempotencyKey(String challengeId, String spt) {
+        return "mpp-java_" + challengeId + "_" + spt;
+    }
 }

src/main/java/com/stripe/mpp/methods/stripe/StripeChargeIntent.java

@@ -99,8 +99,9 @@ public Receipt verify(Credential credential, Map<String, Object> request) {
         Map<String, Object> methodDetails = methodDetails(request);
         List<String> paymentMethodTypes = paymentMethodTypes(methodDetails);
         Map<String, String> metadata = extractMetadata(methodDetails);
+        String challengeId = credential.challenge().id();
         StripeApi.Result result = stripeApi.createAndConfirm(
-            secretKey, amountMinorUnits, currency, spt, paymentMethodTypes, metadata);
+            secretKey, amountMinorUnits, currency, spt, paymentMethodTypes, metadata, challengeId);
 
         if ("requires_action".equals(result.status())) {
             throw new com.stripe.mpp.error.PaymentActionRequiredException(

src/test/java/com/stripe/mpp/methods/stripe/StripeChargeIntentTest.java

@@ -58,6 +58,7 @@ static class StubStripeApi extends StripeApi {
         String lastCurrency;
         List<String> lastPaymentMethodTypes;
         Map<String, String> lastMetadata;
+        String lastChallengeId;
 
         StubStripeApi(StripeApi.Result result) {
             this.result = result;
@@ -66,13 +67,15 @@ static class StubStripeApi extends StripeApi {
         @Override
         StripeApi.Result createAndConfirm(
             String secretKey, long amountMinorUnits, String currency,
-            String spt, List<String> paymentMethodTypes, Map<String, String> metadata
+            String spt, List<String> paymentMethodTypes, Map<String, String> metadata,
+            String challengeId
         ) {
             this.lastSpt      = spt;
             this.lastAmount   = amountMinorUnits;
             this.lastCurrency = currency;
             this.lastPaymentMethodTypes = paymentMethodTypes;
             this.lastMetadata = metadata;
+            this.lastChallengeId = challengeId;
             return result;
         }
     }
@@ -188,7 +191,8 @@ void stripeApiExceptionBecomesVerificationFailed() {
             @Override
             StripeApi.Result createAndConfirm(
                 String secretKey, long amountMinorUnits, String currency,
-                String spt, List<String> paymentMethodTypes, Map<String, String> metadata
+                String spt, List<String> paymentMethodTypes, Map<String, String> metadata,
+                String challengeId
             ) {
                 throw new VerificationFailedException("card_declined");
             }
@@ -208,6 +212,41 @@ void metadataPassedToStripeApi() {
         assertThat(api.lastMetadata).containsEntry("orderId", "order-42");
     }
 
+    @Test
+    void challengeIdPassedForIdempotency() {
+        StubStripeApi api = new StubStripeApi(new StripeApi.Result("pi_abc123", "succeeded"));
+        intent(api).verify(credential("spt_xxx"), REQUEST);
+
+        assertThat(api.lastChallengeId).isEqualTo("chal-id");
+    }
+
+    @Test
+    void idempotencyKeyDerivedFromChallengeIdAndSpt() {
+        String key = StripeApi.buildIdempotencyKey("chal-id", "spt_xxx");
+        assertThat(key).isEqualTo("mpp-java_chal-id_spt_xxx");
+    }
+
+    @Test
+    void idempotencyKeyDeterministic() {
+        String key1 = StripeApi.buildIdempotencyKey("chal-abc", "spt_123");
+        String key2 = StripeApi.buildIdempotencyKey("chal-abc", "spt_123");
+        assertThat(key1).isEqualTo(key2);
+    }
+
+    @Test
+    void idempotencyKeyDiffersForDifferentChallenges() {
+        String key1 = StripeApi.buildIdempotencyKey("chal-abc", "spt_123");
+        String key2 = StripeApi.buildIdempotencyKey("chal-def", "spt_123");
+        assertThat(key1).isNotEqualTo(key2);
+    }
+
+    @Test
+    void idempotencyKeyDiffersForDifferentSpts() {
+        String key1 = StripeApi.buildIdempotencyKey("chal-abc", "spt_123");
+        String key2 = StripeApi.buildIdempotencyKey("chal-abc", "spt_456");
+        assertThat(key1).isNotEqualTo(key2);
+    }
+
     @Test
     void missingPaymentMethodTypesThrows() {
         StubStripeApi api = new StubStripeApi(new StripeApi.Result("pi_abc123", "succeeded"));