fix: allow external flag resolver to override false experiments with variants in getAll (#10966)

Fixes a bug / uncovered edge case in the flag resolver in Unleash:
If a local experiment was defined as false (the typical default value),
then that flag could only ever be returned as a boolean from the
`ui-config` endpoint. In other words, even if the external resolver has
a variant for that flag, the UI would never get the variant.

The fix is to not just check `isEnabled` for false flags, but instead:
- use `getVariant`
- then check `variant.enabled` (in which case we have a variant and can
return it)
- else check `variant.feature_enabled`, falling back to `isEnabled` only
if `feature_enabled` is null/undefined.

src/lib/util/flag-resolver.test.ts

@@ -29,12 +29,11 @@ test('should produce UI flags with extra dynamic flags', () => {
 
 test('should use external resolver for dynamic flags', () => {
     const externalResolver = {
-        isEnabled: (name: string) => {
-            if (name === 'extraFlag') {
-                return true;
-            }
-        },
-        getVariant: () => defaultVariant,
+        isEnabled: (name: string) => name === 'extraFlag',
+        getVariant: (name: string) => ({
+            ...defaultVariant,
+            feature_enabled: name === 'extraFlag',
+        }),
         getStaticContext: () => ({}),
     };
 
@@ -225,6 +224,53 @@ test('should call external resolver getVariant when not overridden to be true, e
     );
 });
 
+test('should allow overriding false experiments with externally resolved variants when getting all flags (getAll)', () => {
+    const variant = {
+        enabled: true,
+        name: 'variant',
+    };
+
+    const externalResolver = {
+        isEnabled: () => false,
+        getVariant: () => variant,
+        getStaticContext: () => ({}),
+    };
+
+    const config = {
+        flags: { willStayBool: true, willBeVariant: false },
+        externalResolver,
+    };
+
+    const resolver = new FlagResolver(config as IExperimentalOptions);
+    const flags = resolver.getAll() as typeof config.flags;
+
+    expect(flags.willStayBool).toStrictEqual(true);
+    expect(flags.willBeVariant).toStrictEqual(variant);
+});
+
+test('should fall back to isEnabled if variant.feature_enabled is not defined in getAll', () => {
+    const variant = {
+        enabled: false,
+        name: 'variant',
+    };
+
+    const externalResolver = {
+        isEnabled: () => true,
+        getVariant: () => variant,
+        getStaticContext: () => ({}),
+    };
+
+    const config = {
+        flags: { flag: false },
+        externalResolver,
+    };
+
+    const resolver = new FlagResolver(config as IExperimentalOptions);
+    const flags = resolver.getAll() as typeof config.flags;
+
+    expect(flags.flag).toStrictEqual(true);
+});
+
 test('should call external resolver getStaticContext ', () => {
     const variant = {
         enabled: true,

src/lib/util/flag-resolver.ts

@@ -27,10 +27,17 @@ export default class FlagResolver implements IFlagResolver {
             const flag = flags[flagName];
             if (typeof flag === 'boolean') {
                 if (!flag) {
-                    flags[flagName] = this.externalResolver.isEnabled(
+                    const variant = this.externalResolver.getVariant(
                         flagName,
                         context,
                     );
+                    if (variant.enabled) {
+                        flags[flagName] = variant;
+                    } else {
+                        flags[flagName] =
+                            variant.feature_enabled ??
+                            this.externalResolver.isEnabled(flagName, context);
+                    }
                 }
             } else {
                 if (!flag?.enabled) {