[kraenhansen]

Summary:

Fixes #59728.

hermesFatalHandler, which logs the reason for a Hermes fatal error and records it as the abort message, is only installed by HermesExecutorHolder on the legacy path. On the New Architecture nothing sets a handler, so Hermes writes the reason to stderr, which doesn't reach logcat, and crashes with its deliberate null write. The result is a SIGSEGV with no reason.

This installs the same handler from JHermesInstance::initHybrid, once per process. It's kept separate from the handler in reactexecutor/OnLoad.cpp so that the legacy code can be removed on its own.

Changelog:

[ANDROID] [FIXED] - Report the reason for Hermes fatal errors in logcat and tombstones on the New Architecture

Test Plan:

Used the reproducer from the issue, https://github.com/kraenhansen/rn-hermes-fatal-handler-repro, on an arm64 API 37 emulator. It's a stock 0.88.1 release app that exhausts a small-heap runtime after startup. The same change was applied to v0.88.1, ReactAndroid was built from source, and the resulting libhermestooling.so was swapped into the APK. The change itself wasn't built on main.

Before:

I HermesGC: OOM: [repro] reason = Max heap size was exceeded (...)
F libc    : Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 in tid 7064 (Thread-2), pid 6925 (com.fatalrepro)
F DEBUG   : Cause: null pointer dereference

After, in 3 of 3 runs, with the app setting no handler:

I HermesGC: OOM: [repro] reason = Max heap size was exceeded (...)
F Hermes  : OOM: error_code(value = 1, category = vm_allocate_category, message = Max heap size was exceeded)
F libc    : Fatal signal 6 (SIGABRT), code -1 (SI_QUEUE) in tid 8464 (Thread-2), pid 8405 (com.fatalrepro)
F DEBUG   : Abort message: 'OOM: error_code(value = 1, category = vm_allocate_category, message = Max heap size was exceeded)'

Note that fatal errors now end in SIGABRT rather than SIGSEGV, matching the legacy path, which may change how they group in crash reporters. An app that sets its own handler before HermesInstance is created would be replaced, because Hermes has a single handler and no getter.

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