Configure the host Hermes compiler for one architecture

.changeset/prebuilt-hermes-host-compiler.md

@@ -0,0 +1,15 @@
+---
+"react-native-node-api": patch
+---
+
+Fix `prebuilt-hermes` failing to configure the host Hermes compiler. It passed
+`CMAKE_OSX_ARCHITECTURES=arm64;x86_64` to build a universal `hermesc`, but a
+multi-arch host configure makes llvh's feature try-compiles fail — standard
+headers report as missing and the configure dies with "Host compiler appears to
+require libatomic, but cannot find it". The host compiler is now configured the
+way Hermes and React Native configure it, for the host architecture only.
+
+`hermesc` is consequently native to the Mac that built the archive, so the
+archive name now carries the host architecture. An Intel Mac finds no published
+archive for its architecture and builds its own, rather than downloading one
+whose `hermesc` it cannot execute.

.github/workflows/hermes-prebuilt.yml

@@ -43,12 +43,17 @@ jobs:
       - run: pnpm run build
       # Resolved from the test app so the archive is built against the React
       # Native version this repository actually pins.
+      # Each command substitution is assigned before it is echoed: inside
+      # `echo "x=$(cmd)"` the step's exit status is echo's, so a failing cmd
+      # passes the step with an empty value.
       - name: Resolve prebuilt Hermes name
         id: hermes
         working-directory: apps/test-app
         run: |
-          echo "archive=$(pnpm exec react-native-node-api prebuilt-hermes --print name)" >> "$GITHUB_OUTPUT"
-          echo "tag=$(pnpm exec react-native-node-api prebuilt-hermes --print tag)" >> "$GITHUB_OUTPUT"
+          archive=$(pnpm exec react-native-node-api prebuilt-hermes --print name)
+          tag=$(pnpm exec react-native-node-api prebuilt-hermes --print tag)
+          echo "archive=$archive" >> "$GITHUB_OUTPUT"
+          echo "tag=$tag" >> "$GITHUB_OUTPUT"
       - name: Cache prebuilt Hermes
         uses: actions/cache@v6
         with:
@@ -59,7 +64,9 @@ jobs:
       - name: Build prebuilt Hermes
         id: build
         working-directory: apps/test-app
-        run: echo "path=$(pnpm exec react-native-node-api prebuilt-hermes --no-download)" >> "$GITHUB_OUTPUT"
+        run: |
+          path=$(pnpm exec react-native-node-api prebuilt-hermes --no-download)
+          echo "path=$path" >> "$GITHUB_OUTPUT"
       # --latest=false keeps these out of the "latest release" slot, which
       # belongs to the package releases changesets publishes.
       - name: Publish as a release asset

docs/CLI.md

@@ -21,7 +21,9 @@ The archive is looked for in this order, and cached under `~/Library/Caches/reac
 2. The [release asset](https://github.com/callstackincubator/react-native-node-api/releases) published for the pinned commit by the `Hermes prebuilt` workflow, unless `--no-download` is passed.
 3. A local build from the vendored source, unless `--no-build` is passed. This requires macOS and Xcode, and takes a while — but only once per pinned commit.
 
-Its name covers everything that changes its contents: the pinned Hermes commit, the React Native version whose `ReactCommon/jsi` it is compiled against, the build type and the platforms. That makes it usable as a CI cache key.
+Its name covers everything that changes its contents: the pinned Hermes commit, the React Native version whose `ReactCommon/jsi` it is compiled against, the build type, the platforms and the host architecture. That makes it usable as a CI cache key.
+
+The host architecture is part of it because `destroot/bin/hermesc` is a native binary for whichever Mac built the archive. Archives are published from Apple Silicon runners, so an Intel Mac finds none to download and builds its own instead of getting a `hermesc` it cannot execute.
 
 - `[from]` — Path to a file inside the app package. Defaults to the current working directory.
 - `--react-native-package <package-name>` — The React Native package to resolve Hermes for. Defaults to `react-native`.

packages/host/src/node/cli/hermes-prebuilt.ts

@@ -74,7 +74,10 @@ export function getPrebuiltDirectory() {
  * Identifies an archive by everything that changes its contents. The React
  * Native version is part of it because Hermes is compiled against that
  * package's ReactCommon/jsi: a JSI mismatch between the framework and the app
- * linking it is an ABI break.
+ * linking it is an ABI break. The host architecture is part of it because the
+ * hermesc in destroot/bin is a native binary for whichever Mac built it, so a
+ * host of the other architecture has to build its own rather than download one
+ * it cannot execute.
  */
 export function getArchiveName({
   reactNativeVersion,
@@ -89,7 +92,7 @@ export function getArchiveName({
   // GitHub rewrites every character outside [A-Za-z0-9._-] in a release asset
   // name, so the name has to stay within that set to survive a round-trip.
   const platformSuffix = [...platforms].sort().join("-");
-  return `hermes-${shortSha}-rn${reactNativeVersion}-${buildType}-${platformSuffix}.tar.gz`;
+  return `hermes-${shortSha}-rn${reactNativeVersion}-${buildType}-${platformSuffix}-${process.arch}.tar.gz`;
 }
 
 export function getReleaseTag() {
@@ -174,9 +177,11 @@ async function buildArchive({
       },
     });
 
-  // Configured here instead of letting build-apple-framework.sh's
-  // build_host_hermesc do it: that one takes no architectures, and the hermesc
-  // we ship has to run on both Apple Silicon and Intel Macs.
+  // Configured here rather than by build-apple-framework.sh's
+  // build_host_hermesc only so the build type is explicit — the pinned Hermes
+  // hard-errors without one. Do not add CMAKE_OSX_ARCHITECTURES: a multi-arch
+  // host configure makes llvh's try-compiles fail, down to "Host compiler
+  // appears to require libatomic, but cannot find it".
   if (!fs.existsSync(importHostCompilersPath)) {
     await run("cmake", [
       "-S",
@@ -185,7 +190,6 @@ async function buildArchive({
       hermescPath,
       `-DJSI_DIR=${jsiPath}`,
       "-DCMAKE_BUILD_TYPE=Release",
-      "-DCMAKE_OSX_ARCHITECTURES=arm64;x86_64",
     ]);
     await run("cmake", [
       "--build",