cygwin: pread/pwrite: prevent EBADF error after fork()

If the parent process has already used pread() or pwrite(), these
functions fail with EBADF if used on the inherited fd.  Ensure that
fix_after_fork() is called to invalidate the prw_handle.  This issue
has been detected by 'stress-ng --pseek 1'.

Fixes: c36cd56c54 ("* fhandler.cc (fhandler_base::open): Drop local create_options variable.")
Signed-off-by: Christian Franke <christian.franke@t-online.de>

winsup/cygwin/fhandler/disk_file.cc

@@ -1803,6 +1803,9 @@ fhandler_disk_file::prw_open (bool write, void *aio)
       return -1;
     }
 
+  /* prw_handle is invalid after fork. */
+  need_fork_fixup (true);
+
   /* record prw_handle's asyncness for subsequent pread/pwrite operations */
   prw_handle_isasync = !!aio;
   return 0;

winsup/cygwin/release/3.5.5

@@ -7,3 +7,6 @@ Fixes:
 
 - Fix a regression in 3.5.4 that writing to pipe extremely slows down.
   Addresses: https://cygwin.com/pipermail/cygwin/2024-August/256398.html
+
+- Fix pread() and pwrite() EBADF error after fork().
+  Addresses: https://sourceware.org/pipermail/cygwin/2024-September/256468.html