[fairysword]

Summary:

downloadHermes always writes to downloadsDir/hermes.tar.gz, so every Hermes version shares a single destination file. The download plugin's ETag store, packages/react-native/ReactAndroid/hermes-engine/build/download-task/etags.json, is keyed by URL only:

{"https://github.com":{"/meta/hermes/tarball/250829098.0.0-stable":{"ETag":"..."},"/meta/hermes/tarball/hermes-v250829098.0.17":{"ETag":"..."}}}

Once a build has downloaded a given Hermes tag, returning to that tag answers 304 Not Modified, so downloadHermes keeps the file that is already on disk. That file now holds a different version's bytes, because the previous tag wrote to the same name. unzipHermes then extracts the wrong Hermes source and the build silently compiles a Hermes version that does not match sdks/.hermesv1version.

This needs no unusual setup. downloadsDir defaults to packages/react-native/sdks/download, which sits outside the Gradle build directory, and CI points REACT_NATIVE_DOWNLOADS_DIR at a shared /opt/react-native-downloads. Anyone who moves between Hermes versions — testing a Hermes bump, switching branches, bisecting — can hit it.

useETag was added in daefeb1 ("Ensure local hermes.tar.gz doesn't get stale between subsequent runs.", Dec 2022) to stop the shared file from going stale when a Hermes commit landed. This change removes the remaining case, where ETag caching itself keeps the shared file stale, by giving each version its own file.

Every other third-party source downloaded into the same directory is already named after its version:

• boost_${BOOST_VERSION}.tar.gz
• double-conversion-${DOUBLE_CONVERSION_VERSION}.tar.gz
• folly-${FOLLY_VERSION}.tar.gz
• fast_float-${FAST_FLOAT_VERSION}.tar.gz
• fmt-${FMT_VERSION}.tar.gz
• glog-${GLOG_VERSION}.tar.gz

Hermes now follows the same convention and becomes hermes-<version>.tar.gz. The hermes-v prefix is stripped from the tag so the name stays hermes-<version>.tar.gz for both a release tag (hermes-v250829098.0.17) and the default value (250829098.0.0-stable); scripts/hermes/bump-hermes-version.js normalizes the same prefix.

Versions no longer overwrite each other, so switching back to a version that is already on disk needs no download at all.

Changelog:

[INTERNAL] [FIXED] - Name the downloaded Hermes source archive after the Hermes version so that switching Hermes versions cannot reuse another version's source archive

Test Plan:

Both runs below use a clean downloadsDir and a clean ETag store, and build Hermes from source:

./gradlew :packages:react-native:ReactAndroid:hermes-engine:downloadHermes \
  -Preact.internal.useHermesStable=false -Preact.internal.useHermesNightly=false

Each step changes only packages/react-native/sdks/.hermesv1version, then re-runs that command. The two Hermes versions used are:

Hermes version	archive md5	size
250829098.0.0-stable (default, no version file)	d212f870733227513a2907dab0a59166	22317635
hermes-v250829098.0.17	e598a0fd04d093365c7f24a31281dc59	22315764

Before

Step	File in downloadsDir	md5
build 250829098.0.0-stable	hermes.tar.gz	d212f870...
switch to hermes-v250829098.0.17	hermes.tar.gz	e598a0fd...
switch back to 250829098.0.0-stable	hermes.tar.gz	e598a0fd...

The third step is the bug: downloadHermes does not run, and the build uses the hermes-v250829098.0.17 source while .hermesv1version asks for 250829098.0.0-stable. Reproduced on a clean checkout with the default downloadsDir and no REACT_NATIVE_DOWNLOADS_DIR set.

After

Step	Files in downloadsDir	md5
build 250829098.0.0-stable	hermes-250829098.0.0-stable.tar.gz	d212f870...
switch to hermes-v250829098.0.17	both archives kept	d212f870... / e598a0fd...
switch back to 250829098.0.0-stable	both archives kept, no download	d212f870... / e598a0fd...

Each version resolves to its own archive, and returning to a version already on disk downloads nothing.

Also verified:

• ./gradlew :packages:react-native:ReactAndroid:hermes-engine:unzipHermes succeeds with the new archive name and extracts the Hermes source tree (CMakeLists.txt, API, android, cmake, ...) into packages/react-native/sdks/hermes.
• The changed lines match ktfmt output. yarn format-check-kotlin skips this file because of the **/hermes-engine/** ignore entry in scripts/format-kotlin.js, so ktfmt was run against the file directly.