I just realized this could be greatly simplified

diff --git a/usr.bin/mg/grep.c b/usr.bin/mg/grep.c
index aa7f9dfd8..cf5337f8d 100644
--- a/usr.bin/mg/grep.c
+++ b/usr.bin/mg/grep.c
@@ -21,6 +21,7 @@

 int     globalwd = FALSE;
 static int      compile_goto_error(int, int);
+int             valid_compile_buffer(void);
 int             next_error(int, int);
 static int      grep(int, int);
 static int      gid(int, int);
@@ -318,9 +319,31 @@ fail:
 }

 int
-next_error(int f, int n)
+valid_compile_buffer(void)
 {
+       struct mgwin *wp;
+
        if (compile_win == NULL || compile_buffer == NULL) {
+               return (FALSE);
+       }
+
+       for (wp = wheadp; wp != NULL; wp = wp->w_wndp) {
+               if (compile_win == wp && compile_win->w_bufp ==
compile_buffer) {
+                       return (TRUE);
+               }
+       }
+
+       compile_win = NULL;
+       compile_buffer = NULL;
+
+       return (FALSE);
+}
+
+
+int
+next_error(int f, int n)
+{
+    if (!valid_compile_buffer()) {
                dobeep();
                ewprintf("No compilation active");
                return (FALSE);

On Sat, 2024-07-27 at 17:10 -0400, Dante Catalfamo wrote:
> Hello tech@,
> 
> There is a bug in mg that will cause a crash if you call `next-error`
> after deleting the compile window or compile buffer. You can easily
> replicate the issue by calling `grep`, changing the buffer in the
> completion window, and calling `next-error`.
> 
> This patch will check if the completion buffer and window are still
> valid before attempting to read from them.
> 
> Thanks
> 
> diff --git a/usr.bin/mg/grep.c b/usr.bin/mg/grep.c
> index aa7f9dfd8..b6db138d3 100644
> --- a/usr.bin/mg/grep.c
> +++ b/usr.bin/mg/grep.c
> @@ -21,6 +21,7 @@
>  
>  int	 globalwd = FALSE;
>  static int	 compile_goto_error(int, int);
> +int		 valid_compile_buffer(void);
>  int		 next_error(int, int);
>  static int	 grep(int, int);
>  static int	 gid(int, int);
> @@ -318,9 +319,47 @@ fail:
>  }
>  
>  int
> -next_error(int f, int n)
> +valid_compile_buffer(void)
>  {
> +	struct mgwin *wp;
> +	struct buffer *bp;
> +	int win_found, buffer_found;
> +
>  	if (compile_win == NULL || compile_buffer == NULL) {
> +		return (FALSE);
> +	}
> +
> +	win_found = buffer_found = 0;
> +
> +	for (wp = wheadp; wp != NULL; wp = wp->w_wndp) {
> +		if (compile_win == wp)
> +			win_found = 1;
> +	}
> +	if (!win_found) {
> +		compile_win = NULL;
> +		return (FALSE);
> +	}
> +
> +	for (bp = bheadp; bp != NULL; bp = bp->b_bufp) {
> +		if (compile_buffer == bp)
> +			buffer_found = 1;
> +	}
> +	if (!buffer_found) {
> +		compile_buffer = NULL;
> +		return (FALSE);
> +	}
> +
> +	if (compile_win->w_bufp != compile_buffer)
> +		return (FALSE);
> +
> +	return (TRUE);
> +}
> +
> +
> +int
> +next_error(int f, int n)
> +{
> +    if (!valid_compile_buffer()) {
>  		dobeep();
>  		ewprintf("No compilation active");
>  		return (FALSE);
>