bug#65274: 29.0.91; [feature request] Readonly argument for emacs built-in sqlite support.

[Eli Zaretskii]

Ping!  Nicolas, could you please try the patch?  I'd like to install
it if it solves your problem.

> Cc: 65274@debbugs.gnu.org
> Date: Sat, 19 Aug 2023 12:21:05 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Nicolas Graves <ngraves@ngraves.fr>
> > Cc: 65274@debbugs.gnu.org
> > Date: Wed, 16 Aug 2023 12:17:00 +0200
> > 
> > My use-case is the package ibrowse.el, which uses sql commands to parse
> > in real time the database of browser (for history or bookmarks,
> > depending on the case)
> > (https://git.sr.ht/~ngraves/ibrowse.el/tree/master/item/ibrowse-sql.el).
> > 
> > Web browsers use very frequent updates and most of the time, even
> > trying to access it with readonly mode is not enough (I don't remember
> > all the details, but it's something along the lines of you can't open
> > even in read-only mode if the database file is currently being
> > updated).
> > 
> > Most other emacs packages which try to do the same thing circumvent the
> > issue by copying the whole database to /tmp and parsing it from
> > there. I've found another solution which doesn't require to copy the
> > file and it's to open it in immutable mode
> > (https://git.sr.ht/~ngraves/ibrowse.el/tree/master/item/ibrowse-sql.el#L58)
> > I remember the following stackoverflow link lead me to try this:
> > https://stackoverflow.com/questions/7857755/is-it-possible-to-open-a-locked-sqlite-database-in-read-only-mode
> > 
> > On the CLI, it's simply done by adding the uri file:...?immutable=1. The SQL
> > manual warns for possible errors if the database is being updated at the
> > same time, but I don't keep an open connection so I don't experience any
> > error whatsoever. One stackoverflow comment also hints about how it
> > could be done, this probably applies to emacs as well:
> > 
> > "To achieve the same thing with most SQLite drivers, because we can't 
> > directly set SQLITE_IOCAP_IMMUTABLE, the best way is to open the connection 
> > with the flag SQLITE_OPEN_READONLY | SQLITE_OPEN_URI to allow passing the 
> > file:...?immutable=1 URI as filename."
> 
> Thanks.  I'm not sure I understood all the intricacies of this use
> case, but could you please see if the patches below make your use case
> possible?
> 
> diff --git a/src/sqlite.c b/src/sqlite.c
> index fd528f2..f68edb9 100644
> --- a/src/sqlite.c
> +++ b/src/sqlite.c
> @@ -277,13 +277,20 @@ check_sqlite (Lisp_Object db, bool is_statement)
>  
>  static int db_count = 0;
>  
> -DEFUN ("sqlite-open", Fsqlite_open, Ssqlite_open, 0, 1, 0,
> +DEFUN ("sqlite-open", Fsqlite_open, Ssqlite_open, 0, 2, 0,
>         doc: /* Open FILE as an sqlite database.
> -If FILE is nil, an in-memory database will be opened instead.  */)
> -  (Lisp_Object file)
> +If FILE is nil, an in-memory database will be opened instead.
> +If READONLY is non-nil, open the database in read-only mode,
> +otherwise open it in read-write mode.  */)
> +  (Lisp_Object file, Lisp_Object readonly)
>  {
>    Lisp_Object name;
> -  int flags = (SQLITE_OPEN_CREATE  | SQLITE_OPEN_READWRITE);
> +  int flags;
> +
> +  if (!NILP (readonly))
> +    flags = SQLITE_OPEN_READONLY;
> +  else
> +    flags = (SQLITE_OPEN_CREATE | SQLITE_OPEN_READWRITE);
>  #ifdef SQLITE_OPEN_FULLMUTEX
>    flags |= SQLITE_OPEN_FULLMUTEX;
>  #endif
> 
> 
> 
>