pageserver: make bench_ingest build (but panic) on macOS (#8641)

## Problem

Some developers build on MacOS, which doesn't have  io_uring.

## Summary of changes

- Add `io_engine_for_bench`, which on linux will give io_uring or panic
if it's unavailable, and on MacOS will always panic.

We do not want to run such benchmarks with StdFs: the results aren't
interesting, and will actively waste the time of any developers who
start investigating performance before they realize they're using a
known-slow I/O backend.

Why not just conditionally compile this benchmark on linux only? Because
even on linux, I still want it to refuse to run if it can't get
io_uring.

pageserver/src/virtual_file.rs

@@ -30,6 +30,7 @@ use tokio::time::Instant;
 pub use pageserver_api::models::virtual_file as api;
 pub(crate) mod io_engine;
 pub use io_engine::feature_test as io_engine_feature_test;
+pub use io_engine::io_engine_for_bench;
 pub use io_engine::FeatureTestResult as IoEngineFeatureTestResult;
 mod metadata;
 mod open_options;

pageserver/src/virtual_file/io_engine.rs

@@ -328,3 +328,29 @@ pub fn feature_test() -> anyhow::Result<FeatureTestResult> {
     .join()
     .unwrap()
 }
+
+/// For use in benchmark binaries only.
+///
+/// Benchmarks which initialize `virtual_file` need to know what engine to use, but we also
+/// don't want to silently fall back to slower I/O engines in a benchmark: this could waste
+/// developer time trying to figure out why it's slow.
+///
+/// In practice, this method will either return IoEngineKind::TokioEpollUring, or panic.
+pub fn io_engine_for_bench() -> IoEngineKind {
+    #[cfg(not(target_os = "linux"))]
+    {
+        panic!("This benchmark does I/O and can only give a representative result on Linux");
+    }
+    #[cfg(target_os = "linux")]
+    {
+        match feature_test().unwrap() {
+            FeatureTestResult::PlatformPreferred(engine) => engine,
+            FeatureTestResult::Worse {
+                engine: _engine,
+                remark,
+            } => {
+                panic!("This benchmark does I/O can requires the preferred I/O engine: {remark}");
+            }
+        }
+    }
+}