over+msgmap: respect WAL journal_mode if set

WAL actually seems to have ideal locking characteristics given
concurrency problems I'm experiencing with --reindex running
in parallel with expensive read-only SQLite queries:
<https://public-inbox.org/meta/20200825001204.GA840@dcvr/>

Unfortunately, we cannot blindly use WAL while preserving
compatibility with existing setups nor our guarantees that
read-only daemons are indeed "read-only".

However, respect an user's the choice to set WAL on their
own if they're comfortable with giving -nntpd/-httpd/-imapd
processes write permission to the directory storing SQLite DBs.

1 files changed, 22 insertions, 1 deletions

diff --git a/lib/PublicInbox/Over.pm b/lib/PublicInbox/Over.pm
index a2f04117..3e74b7a6 100644
--- a/lib/PublicInbox/Over.pm
+++ b/lib/PublicInbox/Over.pm
@@ -42,7 +42,28 @@ sub dbh_new {
                 $st = pack('dd', $st[0], $st[1]);
         } while ($st ne $self->{st} && $tries++ < 3);
         warn "W: $f: .st_dev, .st_ino unstable\n" if $st ne $self->{st};
-        $dbh->do('PRAGMA synchronous = OFF') if ($rw // 0) > 1;
+
+        if ($rw) {
+                # TRUNCATE reduces I/O compared to the default (DELETE).
+                #
+                # Do not use WAL by default since we expect the case
+                # where any users may read via read-only daemons
+                # (-httpd/-imapd/-nntpd); but only a single user has
+                # write permissions for -watch/-mda.
+                #
+                # Read-only WAL support in SQLite 3.22.0 (2018-01-22)
+                # doesn't do what we need: it is only intended for
+                # immutable read-only media (e.g. CD-ROM) and not
+                # usable for our use case described above.
+                #
+                # If an admin is willing to give read-only daemons R/W
+                # permissions; they can enable WAL manually and we will
+                # respect that by not clobbering it.
+                my $jm = $dbh->selectrow_array('PRAGMA journal_mode');
+                $dbh->do('PRAGMA journal_mode = TRUNCATE') if $jm ne 'wal';
+
+                $dbh->do('PRAGMA synchronous = OFF') if $rw > 1;
+        }
         $dbh;
 }