MDEV-27900 fixes

* prevent infinite recursion in beyond-EOF reads (when pread returns 0)
* reduce code duplication

followup for d7817382 and f4fb6cb3

tpool/aio_liburing.cc

@@ -161,8 +161,7 @@ class aio_uring final : public tpool::aio
       }
 
       io_uring_cqe_seen(&aio->uring_, cqe);
-      if (iocb->m_ret_len != iocb->m_len && !iocb->m_err)
-        finish_synchronous(iocb);
+      finish_synchronous(iocb);
 
       // If we need to resubmit the IO operation, but the ring is full,
       // we will follow the same path as for any other error codes.

tpool/aio_linux.cc

@@ -128,8 +128,7 @@ class aio_linux final : public aio
           {
             iocb->m_ret_len= event.res;
             iocb->m_err= 0;
-            if (iocb->m_ret_len != iocb->m_len)
-              finish_synchronous(iocb);
+            finish_synchronous(iocb);
           }
           iocb->m_internal_task.m_func= iocb->m_callback;
           iocb->m_internal_task.m_arg= iocb;

tpool/tpool.h

@@ -173,7 +173,17 @@ class aio
 protected:
   static void synchronous(aiocb *cb);
   /** finish a partial read/write callback synchronously */
-  static void finish_synchronous(aiocb *cb);
+  static inline void finish_synchronous(aiocb *cb)
+  {
+    if (!cb->m_err && cb->m_ret_len != cb->m_len)
+    {
+      /* partial read/write */
+      cb->m_buffer= (char *) cb->m_buffer + cb->m_ret_len;
+      cb->m_len-= (unsigned int) cb->m_ret_len;
+      cb->m_offset+= cb->m_ret_len;
+      synchronous(cb);
+    }
+  }
 };
 
 class timer

tpool/tpool_generic.cc

@@ -85,24 +85,11 @@ void aio::synchronous(aiocb *cb)
 #endif
   cb->m_ret_len = ret_len;
   cb->m_err = err;
-  if (!err && cb->m_ret_len != cb->m_len)
+  if (ret_len)
     finish_synchronous(cb);
 }
 
 
-/**
-  A partial read/write has occured, continue synchronously.
-*/
-void aio::finish_synchronous(aiocb *cb)
-{
-  assert(cb->m_ret_len != (unsigned int) cb->m_len && !cb->m_err);
-  /* partial read/write */
-  cb->m_buffer= (char *) cb->m_buffer + cb->m_ret_len;
-  cb->m_len-= (unsigned int) cb->m_ret_len;
-  cb->m_offset+= cb->m_ret_len;
-  synchronous(cb);
-}
-
 /**
   Implementation of generic threadpool.
   This threadpool consists of the following components