Re: Improve reporting of I/O, access errors for Emacs

[Paul Eggert]

On 9/12/19 12:34 PM, Eli Zaretskii wrote:
> We should just set
> errno and return an error indication, and then let the caller deal
> with that.

I don't see how that could work. Callers are typically written in Lisp, 
and so can't see errno.

For example, suppose Lisp code calls (file-symlink-p "/a/b/c") and Emacs 
cannot tell whether /a/b/c is a symbolic link because of an I/O error, 
or because of a timeout in network access, or because /a is a symlink 
into an unreadable directory, or whatever. If file-symlink-p merely 
returns nil, there's no way for the Lisp code to know that there was an 
exceptional situation and the Lisp code will continue based on the 
incorrect assumption that /a/b/c was not a symlink.

Instead of signaling these exceptional conditions, we could extend each 
 primitive with a new optional argument saying what to do on failure. 
For example, (file-symlink-p "/a/b/c" t) could signal an exception or 
return a special value for these exceptional cases, whereas 
(file-symlink-p "/a/b/c" nil) would simply return nil. However, such an 
extension would require changes to Lisp code if it is intended to be 
reliable, and most Lisp code would not be changed and would continue to 
be unreliable in these exceptional situations. In contrast, the proposed 
patch means  existing Lisp code will gain reliability without needing to 
be changed.

> That's too small a sample, so it doesn't prove much in my book, sorry.

I've looked at and experimented with a reasonable amount of Elisp code, 
and haven't run into problems with the proposed patch. What's your 
contrary intuition based on? Can you give a realistic example of the 
problems you see from the patch?