SQLite Forum

readonly access to WAL database

readonly access to WAL database

(1) By dubiousjim on 2020-05-26 12:39:24 [source]

Hi I'm finding myself unable to open a WAL database readonly. I've read the WAL documentation and this thread.

I observe this problem with the sqlite3 shell as well as with my C code, so let's set the latter aside. The only thing relevant about the C code is that it first creates the database, sets journal_mode = WAL, and calls sqlite3_file_control(...SQLITE_FCNTL_PERSIST_WAL, &1). In my testing, the -wal file is always at 0 bytes.

I can then open the database fine in r/w mode using the sqlite3 shell. (Though doing so then removes the -wal file when I close the database, as the SQLITE_FCNTL_PERSIST_WAL apparently needs to be set every time the file is opened. Don't know how to do that from the shell.)

If I try to open the database -readonly using the shell (whether the -wal file has been removed or not), this is what happens:

$ sqlite3 -readonly test.db
SQLite version 3.31.1 2020-01-27 19:55:54
Enter ".help" for usage hints.
sqlite> .databases
Error: disk I/O error
sqlite> .schema
Error: disk I/O error
sqlite> select count(*) from sqlite_master;
Error: disk I/O error
sqlite> .q

The sqlite3 shell does have Unix r/w permissions to the database file, the -wal file, and the directory that contains them.

My environment is a Mac 10.10.5, using the macports toolchain. But I compiled the sqlite3 shell by hand.

There is no -shm file because I compiled the shell (and also the libraries my C code linked against) using -DSQLITE_DEFAULT_LOCKING_MODE=1.

Here are the shell's config options:

sqlite> pragma compile_options;

If I open the database in r/w mode, set the journal_mode = DELETE, and close it, then I can subsequently open and use the database fine in -readonly mode. But I'd like to understand what I'm doing that producing the inability to access the database with WAL and readonly together, which the documentation seems to say should work in my situation. (If the -wal file still exists, it's supposed to work, or if that file doesn't exist, but the shell has write-access to the directory and so can create it, it's also supposed to work.)

(2) By dubiousjim on 2020-05-26 14:01:52 in reply to 1 [link] [source]

Looks like the problem is that I was defaulting to locking_mode = EXCLUSIVE. If I open the database -readonly but then do pragma locking_mode=normal;, it works as expected.

(But then a -shm file is created, so I expect this wouldn't work if that file didn't already exist and the sqlite process didn't have permissions to create it.)

(3) By Warren Young (wyoung) on 2020-05-26 17:16:31 in reply to 2 [link] [source]

I think you simply shouldn't use the stock SQLite shell in this case. The SQLITE_FCNTL_PERSIST_WAL mode you found is cooperative: all processes requiring that behavior have to apply that mode for it to work as expected.

If you're just using the shell to test things, the solution ends there. If instead you really do need a SQL command shell for your application, I suggest that you hack the stock shell to have the behavior you need.

I'd then install it under a different name (e.g. myappsqlshell) to differentiate it from a stock sqlite3 app.

(4) By dubiousjim on 2020-05-26 18:02:42 in reply to 3 [link] [source]

Thanks. I was just using the shell for testing. And I was only using the SQLITE_FCNTL_PERSIST_WAL mode in order to make it possible to open the database -readonly even when no r/w processes were using it. (Didn't want to rely on the r/o process having write access to the directory containing the database.)

What I've learned from all this is:

  • Though it's possible in recent versions of sqlite to open -wal databases r/o, it still is awkward in a couple of ways, and opening rollback-journal databases r/o is easier.

  • If you do want to open a -wal database r/o, it can't be in locking_mode = EXCLUSIVE. That's ok with rollback-journal databases though.

  • If you do open a rollback-journal database in locking_mode = EXCLUSIVE, while lacking write permissions to the database file or the directory that contains it, you are able to successfully use the database. From my experiments, it looks like the lock is in those cases simply not acquired.