fix(driver): unify buffer pool error to ResourceBusy across backends (compio-rs#961)

Author: Sherlock-Holo

compio-driver/src/sys/buffer_pool/fallback.rs

@@ -22,9 +22,12 @@ impl BufControl {
     }
 
     pub fn pop(&mut self) -> io::Result<u16> {
-        self.queue
-            .pop_front()
-            .ok_or_else(|| io::Error::other("buffer ring has no available buffer"))
+        self.queue.pop_front().ok_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::ResourceBusy,
+                "buffer ring has no available buffer",
+            )
+        })
     }
 
     pub unsafe fn reset(&mut self, buffer_id: u16, _: BufPtr, _: u32) {

compio-driver/src/sys/driver/iour/mod.rs

@@ -462,7 +462,15 @@ fn create_entry(cq_entry: CEntry) -> Entry {
 
 fn create_result(result: i32) -> io::Result<usize> {
     if result < 0 {
-        Err(io::Error::from_raw_os_error(-result))
+        // ENOBUFS indicates the io_uring buffer pool has no available buffer.
+        if -result == libc::ENOBUFS {
+            Err(io::Error::new(
+                io::ErrorKind::ResourceBusy,
+                "buffer ring has no available buffer",
+            ))
+        } else {
+            Err(io::Error::from_raw_os_error(-result))
+        }
     } else {
         Ok(result as _)
     }