Cygwin: message queues are not devices

Message queues are basically just files and in most cases can be handled
like normal files.  So it was a mistake to set the "on-disk-device" flag
for them to fix the mq_unlink() problem reported in
https://cygwin.com/pipermail/cygwin/2025-January/257119.html

Rather, given that unlink() just checks if the object to be deleted
has an on-disk representation, make sure message queues are added to
the path_conv::isondisk() predicate.

This also reverts commit d870655.

Fixes: d870655 ("Cygwin: path_conv: set on-disk-device flag for message queue files")
Reported-by: Christian Franke <[email protected]>
Signed-off-by: Corinna Vinschen <[email protected]>
(cherry picked from commit b940faa)

Author: github-cygwin

winsup/cygwin/local_includes/devices.h

@@ -377,7 +377,8 @@ class device : private _device
   }
   fh_devices operator = (fh_devices n) {return d.devn_fh_devices = n;}
   inline void setfs (bool x) {dev_on_fs = x;}
-  inline bool isfs () const {return dev_on_fs || d.devn == FH_FS;}
+  inline bool isfs () const {return dev_on_fs || d.devn == FH_FS
+				    || d.devn == FH_MQUEUE;}
   inline bool is_fs_special () const {return dev_on_fs && d.devn != FH_FS;}
   inline bool is_dev_resident () const {return lives_in_dev;}
   inline int exists () const {return exists_func (*this);}

winsup/cygwin/local_includes/path.h

@@ -228,8 +228,7 @@ class path_conv
      /proc. */
   int isspecial () const {return dev.not_device (FH_FS);}
   /* Devices with representation on disk. This includes local sockets, FIFOs,
-     message queues and devices created with mknod.  It does not include
-     the /proc hierarchy. */
+     devices created with mknod.  It does not include the /proc hierarchy. */
   int is_fs_special () const {return dev.is_fs_special ();}
   /* Like is_fs_special but excluding local sockets. */
   int is_lnk_special () const {return is_fs_special () && !issocket ();}

winsup/cygwin/path.cc

@@ -1233,10 +1233,7 @@ path_conv::check (const char *src, unsigned opt,
 
 	  /* FIXME: bad hack alert!!!  We need a better solution */
 	  if (!strncmp (path_copy, MQ_PATH, MQ_LEN) && path_copy[MQ_LEN])
-	    {
-	      dev.parse (FH_MQUEUE);
-	      dev.setfs (1);
-	    }
+	    dev.parse (FH_MQUEUE);
 	}
 
       if (opt & PC_NOFULL)

winsup/cygwin/release/3.5.7

@@ -0,0 +1,5 @@
+Fixes:
+------
+
+- Fix stat() on message queues.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-January/257186.html