Work around a V8 bug in WasmCodePointerTable::AllocateUninitializedEntry()
that causes SIGSEGV when multiple isolates are created concurrently on
x86_64 Linux (https://github.com/denoland/deno_core/issues/952).
- Add V8_ISOLATE_CREATE_LOCK mutex to serialize JsRuntime::new() calls
- Replace oneshot channel with Arc<Mutex<Option<IsolateHandle>>> shared
between spawn_blocking and an IsolateDropGuard for proper cancellation
- Remove terminate_execution() call on dead isolate handle in error path
(was use-after-free: handle dereferenced after JsRuntime already dropped)
- Clear handle before drop(js_runtime) to prevent guard from terminating
a destroyed isolate
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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