wasm-mutate-stats: release worklist mutex before file I/O (#2491)

The writer thread popped from the to_write queue with

    if let Some(wasm) = to_write_clone.lock().unwrap().pop() {
        std::fs::write(filename, &wasm)?;
    }

The temporary MutexGuard returned by .lock() lives for the entire
'if let' / 'while let' body, so the queue mutex stayed held across
each std::fs::write call, blocking the producer thread (and shutdown)
on every file write.

Hoist the .pop() into its own statement so the guard is dropped
before the I/O, then match on the resulting Option in a separate
scope. Cargo check on -p wasm-mutate-stats stays green.

Co-authored-by: SAY-5 <saiasish.cnp@gmail.com>

Author: SAY-5

crates/wasm-mutate-stats/src/bin/wasm-mutate-stats.rs

@@ -453,18 +453,24 @@ impl State {
             let mut counter = 0;
 
             while !finish_writing_wrap_clone.load(Relaxed) {
-                // pop from worklist
-                if let Some(wasm) = to_write_clone.lock().unwrap().pop() {
+                // Pop from the worklist while holding the mutex for as short a window
+                // as possible — the previous form `if let Some(_) = to_write_clone.lock()
+                // .unwrap().pop()` keeps the temporary MutexGuard alive for the entire
+                // body of the `if let`, blocking the producers (and shutdown) on every
+                // file write.
+                let wasm = to_write_clone.lock().unwrap().pop();
+                if let Some(wasm) = wasm {
                     let filename = artifact_folder_cp.join(format!("mutated.{counter}.wasm"));
                     std::fs::write(filename, &wasm).context("Failed to write mutated wasm")?;
                     counter += 1;
                 }
             }
             eprintln!("Writing down pending mutated binaries!");
-            // Then write pending wasms
-            while let Some(wasm) = to_write_clone.lock().unwrap().pop() {
+            // Then write pending wasms — same lock-narrowing as above.
+            loop {
+                let wasm = to_write_clone.lock().unwrap().pop();
+                let Some(wasm) = wasm else { break };
                 let filename = artifact_folder_cp.join(format!("mutated.{counter}.wasm"));
-
                 std::fs::write(filename, &wasm).context("Failed to write mutated wasm")?;
                 counter += 1;
             }