feat: allow you to gradually scale back constraint usage (#7622)

This PR updates the limit validation for constraint numbers on a single
strategy. In cases where you're already above the limit, it allows you
to still update the strategy as long as you don't add any **new**
constraints (that is: the number of constraints doesn't increase).

A discussion point: I've only tested this with unit tests of the method
directly. I haven't tested that the right parameters are passed in from
calling functions. The main reason being that that would involve
updating the fake strategy and feature stores to sync their flag lists
(or just checking that the thrown error isn't a limit exceeded error),
because right now the fake strategy store throws an error when it
doesn't find the flag I want to update.

src/lib/features/feature-toggle/createFeatureToggleService.ts

@@ -219,5 +219,10 @@ export const createFakeFeatureToggleService = (config: IUnleashConfig) => {
         dependentFeaturesService,
         featureLifecycleReadModel,
     );
-    return { featureToggleService, featureToggleStore, projectStore };
+    return {
+        featureToggleService,
+        featureToggleStore,
+        projectStore,
+        featureStrategiesStore,
+    };
 };

src/lib/features/feature-toggle/feature-toggle-service.ts

@@ -390,11 +390,17 @@ class FeatureToggleService {
         }
     }
 
-    validateConstraintsLimit(updatedConstrains: IConstraint[]) {
+    private validateConstraintsLimit(constraints: {
+        updated: IConstraint[];
+        existing: IConstraint[];
+    }) {
         if (!this.flagResolver.isEnabled('resourceLimits')) return;
 
         const constraintsLimit = this.resourceLimits.constraints;
-        if (updatedConstrains.length > constraintsLimit) {
+        if (
+            constraints.updated.length > constraintsLimit &&
+            constraints.updated.length > constraints.existing.length
+        ) {
             throwExceedsLimitError(this.eventBus, {
                 resource: `constraints`,
                 limit: constraintsLimit,
@@ -402,7 +408,7 @@ class FeatureToggleService {
         }
 
         const constraintValuesLimit = this.resourceLimits.constraintValues;
-        const constraintOverLimit = updatedConstrains.find(
+        const constraintOverLimit = constraints.updated.find(
             (constraint) =>
                 Array.isArray(constraint.values) &&
                 constraint.values?.length > constraintValuesLimit,
@@ -661,7 +667,10 @@ class FeatureToggleService {
             strategyConfig.constraints &&
             strategyConfig.constraints.length > 0
         ) {
-            this.validateConstraintsLimit(strategyConfig.constraints);
+            this.validateConstraintsLimit({
+                updated: strategyConfig.constraints,
+                existing: [],
+            });
             strategyConfig.constraints = await this.validateConstraints(
                 strategyConfig.constraints,
             );
@@ -820,7 +829,10 @@ class FeatureToggleService {
 
         if (existingStrategy.id === id) {
             if (updates.constraints && updates.constraints.length > 0) {
-                this.validateConstraintsLimit(updates.constraints);
+                this.validateConstraintsLimit({
+                    updated: updates.constraints,
+                    existing: existingStrategy.constraints,
+                });
                 updates.constraints = await this.validateConstraints(
                     updates.constraints,
                 );

src/lib/features/feature-toggle/tests/feature-toggle-service.limit.test.ts

@@ -8,6 +8,7 @@ import type {
     IUser,
 } from '../../../types';
 import getLogger from '../../../../test/fixtures/no-logger';
+import { ExceedsLimitError } from '../../../error/exceeds-limit-error';
 
 const alwaysOnFlagResolver = {
     isEnabled() {
@@ -91,6 +92,72 @@ describe('Strategy limits', () => {
         );
     });
 
+    test('Should not throw limit exceeded errors if the new number of constraints is less than or equal to the previous number', async () => {
+        const LIMIT = 1;
+        const { featureToggleService, featureStrategiesStore } =
+            createFakeFeatureToggleService({
+                getLogger,
+                flagResolver: alwaysOnFlagResolver,
+                resourceLimits: {
+                    constraints: LIMIT,
+                },
+            } as unknown as IUnleashConfig);
+
+        const constraints: IConstraint[] = [
+            {
+                values: ['1'],
+                operator: 'IN',
+                contextName: 'appName',
+            },
+            {
+                values: ['2'],
+                operator: 'IN',
+                contextName: 'appName',
+            },
+            {
+                values: ['3'],
+                operator: 'IN',
+                contextName: 'appName',
+            },
+        ];
+
+        const flagName = 'feature';
+
+        await featureStrategiesStore.createFeature({
+            name: flagName,
+            createdByUserId: 1,
+        });
+
+        const strategy = await featureStrategiesStore.createStrategyFeatureEnv({
+            parameters: {},
+            strategyName: 'default',
+            featureName: flagName,
+            constraints: constraints,
+            projectId: 'default',
+            environment: 'default',
+        });
+
+        const updateStrategy = (newConstraints) =>
+            featureToggleService.unprotectedUpdateStrategy(
+                strategy.id,
+                {
+                    constraints: newConstraints,
+                },
+                { projectId: 'default', featureName: 'feature' } as any,
+                {} as IAuditUser,
+            );
+
+        // check that you can save the same amount of constraints
+        await updateStrategy(constraints);
+        // check that you can save fewer constraints but still over the limit
+        await updateStrategy(constraints.slice(0, 2));
+
+        // check that you can't save more constraints
+        await expect(async () =>
+            updateStrategy([...constraints, ...constraints]),
+        ).rejects.toThrow(new ExceedsLimitError('constraints', LIMIT));
+    });
+
     test('Should not allow to exceed constraint values limit', async () => {
         const LIMIT = 3;
         const { featureToggleService, featureToggleStore } =