Add regression test for deadlocking rayon

Cargo.toml

@@ -27,6 +27,10 @@ harness = false
 name = "large_image"
 harness = false
 
+[[test]]
+name = "rayon"
+required-features = ["rayon"]
+
 [features]
 default = ["rayon"]
 platform_independent = []

src/worker/multithreaded.rs

@@ -12,12 +12,12 @@ use super::immediate::ImmediateWorker;
 
 pub fn with_multithreading<T>(f: impl FnOnce(&mut dyn Worker) -> T) -> T {
     #[cfg(not(feature = "rayon"))]
-    let mut worker = StdThreadWorker(MpscWorker::default());
+    return self::enter_threads(f);
 
     #[cfg(feature = "rayon")]
-    jpeg_rayon::enter(|mut worker| {
+    return jpeg_rayon::enter(|mut worker| {
         f(&mut worker)
-    })
+    });
 }
 
 enum WorkerMsg {
@@ -128,6 +128,13 @@ fn collect_worker_thread(rx: Receiver<Vec<u8>>) -> Vec<u8> {
     rx.recv().expect("jpeg-decoder worker thread error")
 }
 
+#[allow(dead_code)]
+fn enter_threads<T>(f: impl FnOnce(&mut dyn Worker) -> T) -> T {
+    let mut worker = StdThreadWorker(MpscWorker::default());
+    f(&mut worker)
+}
+
+
 #[cfg(feature = "rayon")]
 mod jpeg_rayon {
     use crate::error::Result;

tests/rayon.rs

@@ -0,0 +1,16 @@
+use std::{fs::File, path::Path};
+use jpeg_decoder::Decoder;
+
+#[test]
+fn decoding_in_limited_threadpool_does_not_deadlock() {
+    let path = Path::new("tests").join("reftest").join("images").join("mozilla").join("jpg-progressive.jpg");
+
+    let pool = rayon::ThreadPoolBuilder::new()
+        .num_threads(1)
+        .build()
+        .unwrap();
+    pool.install(|| {
+        let mut decoder = Decoder::new(File::open(&path).unwrap());
+        let _ = decoder.decode().unwrap();
+    });
+}