Fix a circular import in the prebuilt iOS headers (#58635)

Summary:

Each module ships a small header that acts as its entry point. Those were
being packaged into the React framework, but the code that includes them
lives in the shared headers bundle the framework is built on top of. The two
ended up pointing at each other and the iOS build failed.

Ship the entry-point headers with the shared headers instead. They still
resolve the same way for anyone including them, and no other header moves.

Changelog: [Internal]

Differential Revision: D121187558

Author: j-piasecki

packages/react-native/scripts/ios-prebuild/__docs__/headers-rules.md

@@ -8,7 +8,7 @@ scripts under `scripts/ios-prebuild/`:
 | Script                 | Role                                                                                                                                                          |
 | ---------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `headers-inventory.js` | **Discover + classify** every shipped header (the facts)                                                                                                      |
-| `headers-spec.js`      | **The rules** (R1–R11) — turns the inventory into a layout plan + module maps                                                                                 |
+| `headers-spec.js`      | **The rules** (R1–R12) — turns the inventory into a layout plan + module maps                                                                                 |
 | `headers-compose.js`   | **Emit** — projects the plan into `React.xcframework` and `ReactNativeHeaders.xcframework`                                                                    |
 | `headers-verify.js`    | **Gate** — generator-time verification: include-health ratchet, structural byte-compare, consumer-shaped compile smokes (runs in the prebuild CI compose job) |
 
@@ -35,7 +35,7 @@ everything else. No overlay, no include rewriting, no consumer flags.
 podspecs ──► headers-inventory.js ──► inventory (facts per header)
                                           │
                                           ▼
-                              headers-spec.js  (rules R1–R10)
+                              headers-spec.js  (rules R1–R12)
                                           │  plan: what goes where + module maps
                                           ▼
                               headers-compose.js (emission)
@@ -233,6 +233,28 @@ emitted as a one-line redirect shim (`#import <owner>`). Shims that are
 namespace-module members are fine: they import the owning module, so
 declarations stay single-owned.
 
+**R12 — a namespace module's own umbrella stays in ReactNativeHeaders.** The C++
+stable API ships one umbrella per module, physically nested inside it
+(`ReactCommon/react/debug/React/Debug.h`), so its natural path is
+`React/Debug.h` and R1 would hoist it into the framework. It must not be:
+**ReactNativeHeaders is the lower layer** — `React.framework` imports it
+(`RCTCallInvoker.h` → `<ReactCommon/CallInvoker.h>`). A framework-owned umbrella
+makes every `#include <React/X.h>` inside a lowercase-namespace header an import
+of module `React`, closing a cycle:
+
+```text
+React -> ReactNativeHeaders_react -> React
+```
+
+Found empirically: `react/timing/primitives.h` -> `<React/Debug.h>`. This is
+the same two-module-ownership failure as `UMBRELLA_CXX_GUARDED_EXCLUSIONS`
+(`RCTFrameTimingsObserver.h`, which reaches the same `primitives.h`), in the
+opposite direction. They are `objc-blocked` by construction (they re-export
+their module's C++ surface), so they were never R4 umbrella or R5 module
+members; `planFromInventory` fails closed if one ever becomes a modular
+candidate, since its R5 module would be named `React` and alias the framework
+module.
+
 ## Stage 3 — Emission (headers-compose.js)
 
 `computeSpecPlan(rnRoot)` = inventory → plan, throwing on R8 collisions. Then:

packages/react-native/scripts/ios-prebuild/__tests__/headers-spec-test.js

@@ -266,6 +266,55 @@ describe('R11 redirect shims for dual-identity headers', () => {
   });
 });
 
+describe('R12 namespace module umbrellas stay in ReactNativeHeaders', () => {
+  test('a module-nested React/ umbrella is not hoisted into the framework', () => {
+    const m = validManifest();
+    m.headers.push(
+      entry(
+        'React/Debug.h',
+        'objc-blocked',
+        'ReactCommon/react/debug/React/Debug.h',
+      ),
+    );
+    const plan = planFromInventoryForTest(m);
+    // Lower layer: including it from a react/-namespace header cannot create a
+    // React module edge (React -> ReactNativeHeaders_react -> React).
+    expect(
+      plan.reactNativeHeaders.find(e => e.naturalPath === 'React/Debug.h')
+        ?.relPath,
+    ).toBe('React/Debug.h');
+    expect(
+      plan.react.find(e => e.naturalPath === 'React/Debug.h'),
+    ).toBeUndefined();
+    expect(plan.umbrella).not.toContain('React/Debug.h');
+    expect(Object.keys(plan.namespaceModules)).not.toContain('React');
+  });
+
+  test('genuine React.framework headers are still hoisted (R1)', () => {
+    const m = validManifest();
+    m.headers.push(
+      entry('React/RCTMessageThread.h', 'cxx', 'React/Base/RCTMessageThread.h'),
+    );
+    const plan = planFromInventoryForTest(m);
+    expect(
+      plan.react.find(e => e.naturalPath === 'React/RCTMessageThread.h')
+        ?.relPath,
+    ).toBe('RCTMessageThread.h');
+  });
+
+  test('fails closed if an R12 umbrella becomes a modular candidate', () => {
+    const m = validManifest();
+    m.headers.push(
+      entry(
+        'React/Debug.h',
+        'objc-modular-candidate',
+        'ReactCommon/react/debug/React/Debug.h',
+      ),
+    );
+    expect(() => planFromInventoryForTest(m)).toThrow(/R12/);
+  });
+});
+
 describe('DEPS_NAMESPACES (R2 — the deps sidecar namespace set)', () => {
   test('includes SocketRocket: one physical home, in the sidecar', () => {
     // Pre-sidecar, SocketRocket was excluded from relocation because a REAL

packages/react-native/scripts/ios-prebuild/headers-spec.js

@@ -79,6 +79,20 @@
  *     one-line redirect shim (`#import <owner>`). Shims that are namespace-
  *     module members are fine: they import the owning module, so declarations
  *     stay single-owned.
+ * R12. A namespace module's OWN umbrella (ReactCommon/<ns...>/React/<Name>.h,
+ *     natural path React/<Name>.h) ships in ReactNativeHeaders — it is NOT
+ *     hoisted into React.framework by R1. ReactNativeHeaders is the LOWER
+ *     layer (React.framework imports it: RCTCallInvoker.h pulls
+ *     <ReactCommon/CallInvoker.h>), so a framework-owned umbrella turns every
+ *     `#include <React/X.h>` inside a lowercase-namespace header into an import
+ *     of module React and closes a cycle:
+ *       React -> ReactNativeHeaders_react -> React
+ *     Found empirically: react/timing/primitives.h -> <React/Debug.h>. Kept in
+ *     ReactNativeHeaders the same spelling still resolves — framework lookup
+ *     misses and clang falls back to the header search path — textually and
+ *     within one artifact, so no module edge is created. These umbrellas are
+ *     objc-blocked by construction (they re-export their module's C++
+ *     surface), so they were never R4 umbrella or R5 module members anyway.
  */
 
 const fs = require('node:fs');
@@ -280,6 +294,13 @@ function renderNamespaceUmbrella(
   return `#ifdef __OBJC__\n#import <UIKit/UIKit.h>\n#endif\n\n${imports}\n`;
 }
 
+// R12: a namespace module's own umbrella — physically nested inside the module
+// it re-exports (ReactCommon/react/debug/React/Debug.h), which is what tells it
+// apart from the ~310 genuine React.framework headers that also carry a
+// `React/` natural path but live under React/, Libraries/, ReactApple/, ...
+const NS_MODULE_UMBRELLA_RE /*: RegExp */ =
+  /^ReactCommon\/.+\/React\/[^/]+\.h$/;
+
 /**
  * Computes the full layout plan from the header inventory manifest
  * (build/header-inventory.json — regenerate with header-inventory.js).
@@ -307,7 +328,13 @@ function planFromInventory(
     let bucketKey;
     let entryList;
     let relPath;
-    if (np.startsWith('React/')) {
+    if (np.startsWith('React/') && NS_MODULE_UMBRELLA_RE.test(source)) {
+      // R12: a namespace module's own umbrella stays in the LOWER layer, so
+      // including it from that namespace cannot create a React module edge.
+      relPath = np;
+      bucketKey = `ReactNativeHeaders/${relPath}`;
+      entryList = reactNativeHeaders;
+    } else if (np.startsWith('React/')) {
       relPath = np.slice(6); // R1: hoist React/ to the framework Headers root
       bucketKey = `React.framework/${relPath}`;
       entryList = react;
@@ -332,8 +359,13 @@ function planFromInventory(
     seen.set(bucketKey, source);
     entryList.push({relPath, source, naturalPath: np});
 
-    // R4: React umbrella membership.
-    if (np.startsWith('React/') && isUmbrellaSafe(h, root)) {
+    // R4: React umbrella membership. Only headers the framework actually ships
+    // (R12 umbrellas carry a React/ natural path but live in ReactNativeHeaders).
+    if (
+      entryList === react &&
+      np.startsWith('React/') &&
+      isUmbrellaSafe(h, root)
+    ) {
       umbrella.push(np);
     }
     // R5: namespace modules (only for ReactNativeHeaders namespaces). Every
@@ -346,6 +378,19 @@ function planFromInventory(
     if (entryList === reactNativeHeaders) {
       const ns = np.split('/')[0];
       if (isUmbrellaSafe(h, root)) {
+        // R12 assert: an R12-routed umbrella lands under the `React` namespace
+        // here, and renderNamespaceModuleMap only renames the lowercase `react`
+        // one — so its module would be named `React` and alias the framework
+        // module, reintroducing the cycle R12 exists to break. These umbrellas
+        // are objc-blocked today so they never reach this branch; fail closed
+        // if that ever changes.
+        if (ns === 'React') {
+          throw new Error(
+            `R12: '${np}' is a modular candidate in ReactNativeHeaders. Its ` +
+              `namespace module would be named 'React' and alias the React ` +
+              `framework module. Keep it out of the modular surface.`,
+          );
+        }
         // R5 exemption assert: a namespace whose name is not a valid module
         // identifier cannot get a module, so a modular-candidate header in it
         // would be silently non-modular — consumers importing it from a