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diff --git a/crates/shirabe/src/util/sync_executor.rs b/crates/shirabe/src/util/sync_executor.rs
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+++ b/crates/shirabe/src/util/sync_executor.rs
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+//! Minimal synchronous future executor used as a drop-in for the `tokio::runtime::Runtime::new()`
+//! + `block_on` sync bridges scattered across the codebase (repository / installer / downloader).
+//!
+//! Those bridges drive `async fn`s whose `.await` points all resolve synchronously: the only async
+//! I/O (reqwest in `CurlDownloader`) is performed through a blocking client, so no reactor is
+//! required. Nesting `tokio` runtimes is forbidden ("Cannot start a runtime from within a runtime"),
+//! which is why this no-reactor executor exists; it can be nested freely.
+//!
+//! TODO(phase-e): remove this module once the async bridges are either made genuinely synchronous or
+//! consolidated onto a single shared runtime driven from `main`.
+
+use std::future::Future;
+use std::task::{Context, Poll};
+
+/// Polls `fut` to completion on the current thread without any tokio runtime.
+///
+/// Relies on the invariant that no awaited future parks on a reactor (it would otherwise spin).
+pub fn block_on<F: Future>(fut: F) -> F::Output {
+    let mut fut = std::pin::pin!(fut);
+    let waker = std::task::Waker::noop();
+    let mut cx = Context::from_waker(waker);
+    loop {
+        if let Poll::Ready(value) = fut.as_mut().poll(&mut cx) {
+            return value;
+        }
+        std::hint::spin_loop();
+    }
+}