bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

[Greg Minshall]

if url-retrieve is called with a url for a non-existent domain, it
neither returns an error (it returns a buffer, as it is documented to
do), nor does it seem to invoke the callback routine passed to it to
report any error (or success, or anything else, for that matter).

the following code uses example.com.  calling
- `(mytry 0)` should work, downloading "index.html"
- `(mytry 1)` should give a 404 -- no such file "index99.html"
- `(mytry 2)` doesn't give anything (but, the code ends after 5 seconds)
  as there is no such host "x.example.com".

with 0 or 1, the callback routine is invoked, but with 2, it isn't.
(after 5 seconds the code finishes, displays whatever might be in the
buffer url-retrieve uses to return the results.)

apologies if i'm missing some channel through which `url-retrieve` is
announcing the failure (like a sentinel?).


>>>>
;;; ...  -*- lexical-binding: t -*-

(defun mysage (&rest args)
    "Like (message), but send ARGS to stdout, even in batch mode.
  \(In batch mode, (message) sends to stderr; the prin1, etc.,
  send to stdout."
    (princ (format "%s\n" (apply 'format args))))

(defun mytry (n)
  "Try url-retrieve on a URL.  N is 0, 1, or 2."
  (let* ((urls '("https://example.com/index.html"
                 "https://example.com/index99.html"
                 "https://x.example.com/index.html"))
         (buffer
         (url-retrieve (nth n urls)
                       (lambda (&rest results)
                         (mysage "called back"))
                       nil)))
    (mysage "url-retrieve returns %s" buffer)
    (sleep-for 5)
    (with-current-buffer buffer
      (mysage "buffer %s contents: %s" buffer (buffer-substring (point-min) (point-max))))))
>>>>

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.42, cairo version 1.18.0) of 2024-08-15 built on archlinuxRepository revision: dcd5b80b09c4679d7cceecc852725dba6a75b5f0
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101013
System Description: Arch Linux

Configured using:
 'configure --with-xwidgets --with-modules --without-dbus CFLAGS=-g3
 'LDFLAGS=-L/usr/local/lib -L/lib -L/usr/X11R6/lib''

Configured features:
ACL CAIRO FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG LCMS2
LIBOTF LIBSYSTEMD LIBXML2 M17N_FLT MODULES NATIVE_COMP NOTIFY INOTIFY
PDUMPER PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS
TREE_SITTER WEBP X11 XDBE XIM XINPUT2 XPM XWIDGETS GTK3 ZLIB

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bug#72652: sentinel

[Greg Minshall]

i guess one can get some notification from a sentinel, though finding
the process itself is a bit kludgy:
````
(defun msg-me (process event)
  (princ
   (format "Process: %s had the event '%s'" process event)))

(let ((uproc (url-retrieve "https://y.example.com/index.html"
                           (lambda (x) (message "callback called: %s" x)))))
  (with-current-buffer uproc
    (set-process-sentinel url-http-process 'msg-me)))
````

gives output
````
Contacting host: y.example.com:443
Process: example.com had the event 'connection broken by remote peer
'
````

the message ("broken by remote peer") isn't as helpful as a "host does
not exist" message might be.

i guess to distinguish a name lookup failure, one could then (or before)
use, e.g., `dns-query-asynchronous`:

````
(dns-query-asynchronous
    "x.example.com"
    (lambda (x) (message "dns callback: %s" x)))
````

gives
````
dns callback: nil
````

(hope this formatting comes through reasonably...)

bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

[Eli Zaretskii]

> From: Greg Minshall <minshall@umich.edu>
> Date: Fri, 16 Aug 2024 07:34:08 +0300
> 
> if url-retrieve is called with a url for a non-existent domain, it
> neither returns an error (it returns a buffer, as it is documented to
> do), nor does it seem to invoke the callback routine passed to it to
> report any error (or success, or anything else, for that matter).
> 
> the following code uses example.com.  calling
> - `(mytry 0)` should work, downloading "index.html"
> - `(mytry 1)` should give a 404 -- no such file "index99.html"
> - `(mytry 2)` doesn't give anything (but, the code ends after 5 seconds)
>   as there is no such host "x.example.com".
> 
> with 0 or 1, the callback routine is invoked, but with 2, it isn't.
> (after 5 seconds the code finishes, displays whatever might be in the
> buffer url-retrieve uses to return the results.)

On MS-Windows (mytry 2) signals an error:

  Debugger entered--Lisp error: (error "x.example.com/443 No such host is known. ")
    make-network-process(:name "x.example.com" :buffer #<buffer  *url-http-temp*> :host "x.example.com" :service 443 :nowait (:nowait t) :noquery nil :tls-parameters (gnutls-x509pki :priority "NORMAL:%DUMBFW" :hostname "x.example.com" :loglevel 0 :min-prime-bits nil :trustfiles nil :crlfiles nil :keylist nil :verify-flags nil :verify-error nil :pass nil :flags nil :callbacks nil) :coding nil)
    open-network-stream("x.example.com" #<buffer  *url-http-temp*> "x.example.com" 443 :nowait (:nowait t) :tls-parameters (gnutls-x509pki :priority "NORMAL:%DUMBFW" :hostname "x.example.com" :loglevel 0 :min-prime-bits nil :trustfiles nil :crlfiles nil :keylist nil :verify-flags nil :verify-error nil :pass nil :flags nil :callbacks nil) :coding nil)
    open-gnutls-stream("x.example.com" #<buffer  *url-http-temp*> "x.example.com" 443 (:type tls :nowait (:nowait t)))
    network-stream-open-tls("x.example.com" #<buffer  *url-http-temp*> "x.example.com" 443 (:type tls :nowait (:nowait t)))
    open-network-stream("x.example.com" #<buffer  *url-http-temp*> "x.example.com" 443 :type tls :nowait (:nowait t))
    url-open-stream("x.example.com" #<buffer  *url-http-temp*> "x.example.com" 443 tls)
    url-http-find-free-connection("x.example.com" 443 tls)
    url-http(#s(url :type "https" :user nil :password nil :host "x.example.com" :portspec nil :filename "/index.html" :target nil :attributes nil :fullness t :silent nil :use-cookies t :asynchronous t) #f(lambda (&rest results) [t] (mysage "called back")) (nil) nil tls)
    url-https(#s(url :type "https" :user nil :password nil :host "x.example.com" :portspec nil :filename "/index.html" :target nil :attributes nil :fullness t :silent nil :use-cookies t :asynchronous t) #f(lambda (&rest results) [t] (mysage "called back")) (nil))
    url-retrieve-internal("https://x.example.com/index.html" #f(lambda (&rest results) [t] (mysage "called back")) (nil) nil nil)
    url-retrieve("https://x.example.com/index.html" #f(lambda (&rest results) [t] (mysage "called back")) nil)
    (let* ((urls '("https://example.com/index.html" "https://example.com/index99.html" "https://x.example.com/index.html")) (buffer (url-retrieve (nth n urls) #'(lambda (&rest results) (mysage "called back")) nil))) (mysage "url-retrieve returns %s" buffer) (sleep-for 5) (save-current-buffer (set-buffer buffer) (mysage "buffer %s contents: %s" buffer (buffer-substring (point-min) (point-max)))))
    mytry(2)
    eval((mytry 2) t)
    #f(compiled-function () #<bytecode 0x127ffea33bfbce22>)()
    #f(compiled-function () #<bytecode -0x5c06e1955cf31d1>)()
    handler-bind-1(#f(compiled-function () #<bytecode -0x5c06e1955cf31d1>) (error) eval-expression--debug)
    eval-expression((mytry 2) nil nil 127)
    funcall-interactively(eval-expression (mytry 2) nil nil 127)
    call-interactively(eval-expression nil nil)
    command-execute(eval-expression)

On GNU/Linux, I get this in the echo-area after about 2 sec:

  Contacting host: x.example.com:443
  url-retrieve returns  *http x.example.com:443*
  buffer  *http x.example.com:443* contents:
  "buffer  *http x.example.com:443* contents:
  "

IOW, I get an empty buffer.  Why is that behavior a problem?

bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

[Greg Minshall]

hi, Eli,

> On MS-Windows (mytry 2) signals an error:

i see.  possibly the Emacs on windows is using a synchronous DNS
resolver (possibly not).  but, an error is better than nothing.

> On GNU/Linux, I get this in the echo-area after about 2 sec:
> 
>   Contacting host: x.example.com:443
>   url-retrieve returns  *http x.example.com:443*
>   buffer  *http x.example.com:443* contents:
>   "buffer  *http x.example.com:443* contents:
>   "
> 
> IOW, I get an empty buffer.  Why is that behavior a problem?

the problem is that there's no indication from url-retrieve that the
requested transfer has failed (or has in any way finished).  the only
reason the message about the buffer was printed out was because the
mytry script times out after 5 seconds, prints out whatever might be in
the buffer.

so, a program that calls url-retrieve for an unknown host is never
notified that anything happened.


if some sort of error is detected synchronous with the call to
url-retrieve (i.e., before url-retrieve returns to its caller),
signaling an error seems fine.

in the case where the error is detected later, then i think (if
possible) url-retrieve should cause a callback to happen, with the
`:error` data in the status parameter specifying something like "unknown
host".

in the case of (mytry 1), there is a callback (to the callback routine
specified in the invocation of url-retrieve) which passes an error
indication:
----
called back: ((:error (error http 500)
----
(one can modify mytry to print out the status given to the
callback routine -- see below.)

hth.

cheers, Greg

----
modified mytry: prints out status parameter to callback:

----
;;; ...  -*- lexical-binding: t -*-

(defun mysage (&rest args)
    "Like (message), but send ARGS to stdout, even in batch mode.
  \(In batch mode, (message) sends to stderr; the prin1, etc.,
  send to stdout."
    (princ (format "%s\n" (apply 'format args))))

(defun mytry (n)
  "Try url-retrieve on a URL.  N is 0, 1, or 2."
  (let* ((urls '("https://example.com/index.html"
                 "https://example.com/index99.html"
                 "https://x.example.com/index.html"))
         (buffer
         (url-retrieve (nth n urls)
                       (lambda (&rest results)
                         (mysage "called back: %s" results))
                       nil)))
    (mysage "url-retrieve returns %s" buffer)
    (sleep-for 5)
    (with-current-buffer buffer
      (mysage "buffer %s contents: %s" buffer (buffer-substring (point-min) (point-max))))))

bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

[Eli Zaretskii]

> cc: 72652@debbugs.gnu.org
> From: Greg Minshall <minshall@umich.edu>
> Comments: In-reply-to Eli Zaretskii <eliz@gnu.org>
>    message dated "Fri, 16 Aug 2024 10:43:30 +0300."
> Date: Fri, 16 Aug 2024 22:39:11 +0300
> 
> > On MS-Windows (mytry 2) signals an error:
> 
> i see.  possibly the Emacs on windows is using a synchronous DNS
> resolver (possibly not).

Yes, I think so.

> the problem is that there's no indication from url-retrieve that the
> requested transfer has failed (or has in any way finished).  the only
> reason the message about the buffer was printed out was because the
> mytry script times out after 5 seconds, prints out whatever might be in
> the buffer.
> 
> so, a program that calls url-retrieve for an unknown host is never
> notified that anything happened.
> 
> 
> if some sort of error is detected synchronous with the call to
> url-retrieve (i.e., before url-retrieve returns to its caller),
> signaling an error seems fine.
> 
> in the case where the error is detected later, then i think (if
> possible) url-retrieve should cause a callback to happen, with the
> `:error` data in the status parameter specifying something like "unknown
> host".
> 
> in the case of (mytry 1), there is a callback (to the callback routine
> specified in the invocation of url-retrieve) which passes an error
> indication:
> ----
> called back: ((:error (error http 500)
> ----
> (one can modify mytry to print out the status given to the
> callback routine -- see below.)

Patches to report an error in the (mytry 2) case are welcome.

bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

[Greg Minshall]

> Patches to report an error in the (mytry 2) case are welcome.

that'd be great.  thanks.

bug#72652: 31.0.50; url-retrieve on non-existent domain gives no indication of error

[Dmitry Gutov]

On 17/08/2024 15:54, Greg Minshall wrote:
>> Patches to report an error in the (mytry 2) case are welcome.
> that'd be great.  thanks.

For those who'd like to try their hand at writing the fix, 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=26835#18 might be 
relevant, or at least point to the problem area.