[patch, fortran] Emit error for recursive I/O

[Hans-Peter Nilsson]

> Date: Mon, 5 Jan 2026 20:56:34 +0100
> From: Thomas Koenig <tkoenig@netcologne.de>

That patch, committed as r16-6515-ge32c3fb4311bca, broke
build for non-gthreads targets (or possibly
"just"__GTHREADS_CXX0X targets) such as your typical
bare-iron *-elf target, as detailed in PR123446.

Looking at the patch, there's an obvious flaw.

Unconditional access to "self" member (multiple, just one
example):

> diff --git a/libgfortran/io/async.h b/libgfortran/io/async.h
> index 76c087c554a..07f1557a98c 100644
> --- a/libgfortran/io/async.h
> +++ b/libgfortran/io/async.h
> @@ -175,6 +175,11 @@
>      INTERN_UNLOCK (mutex);						\
>    }while (0)
>  
> +#define UNLOCK_UNIT(unit) do { \
> +  unit->self = 0;							\
> +  UNLOCK(&(unit)->lock);						\
> +  } while(0)
> +

Also, unconditional access to __gthread_ functions:

@@ -210,6 +239,12 @@
     DEBUG_PRINTF ("%s" DEBUG_RED "ACQ:" DEBUG_NORM " %-30s %78p\n", aio_prefix, #mutex, mutex); \
   } while (0)
 
+
+#define LOCK_UNIT(unit) do {		\
+    LOCK (&(unit)->lock);		\
+    (unit)->self = __gthread_self ();	\
+  } while (0)
+

Conditional declaration of said member:

> --- a/libgfortran/io/io.h
> +++ b/libgfortran/io/io.h
> @@ -728,6 +728,9 @@ typedef struct gfc_unit
>    int last_char;
>    bool has_size;
>    GFC_IO_INT size_used;
> +#ifdef __GTHREADS_CXX0X
> +  __gthread_t self;
> +#endif

brgds, H-P