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

[Eli Zaretskii]

> 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