user/dev discussion of public-inbox itself
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From: Eric Wong <e@80x24.org>
To: meta@public-inbox.org
Subject: [PATCH] grok-pull.post_update_hook: flock(2) before SQLite check
Date: Tue, 25 Aug 2020 10:23:14 +0000
Message-ID: <20200825102314.GA19671@dcvr> (raw)
In-Reply-To: <20200825001204.GA840@dcvr>

Unlike DBD::SQLite, the sqlite3(1) CLI does not have a default
busy timeout enabled, so it easily times out while acquiring a
SHARED lock for read-only queries.  We can avoid battery-wasting
polling from the SQLite timeout handler by relying on flock(2)
as we do in our Perl code.

Furthermore, this avoids triggering some locking problems[1]
from a long "SELECT COUNT(*) ..." query and reindex.

While there may be other SQLite-related parallelism issues[1],
this works around one of them by relying on flock(2).

[1] https://public-inbox.org/meta/20200825001204.GA840@dcvr/
---
 examples/grok-pull.post_update_hook.sh | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/examples/grok-pull.post_update_hook.sh b/examples/grok-pull.post_update_hook.sh
index 1f51140f..77489472 100755
--- a/examples/grok-pull.post_update_hook.sh
+++ b/examples/grok-pull.post_update_hook.sh
@@ -25,11 +25,13 @@ then
 	inbox_dir=$(expr "$full_git_dir" : "$EPOCH2MAIN")
 	inbox_name=$(basename "$inbox_dir")
 	msgmap="$inbox_dir"/msgmap.sqlite3
+	inbox_lock="$inbox_dir"/inbox.lock
 else
 	inbox_fmt=1
 	inbox_dir="$full_git_dir"
 	inbox_name=$(basename "$inbox_dir" .git)
 	msgmap="$inbox_dir"/public-inbox/msgmap.sqlite3
+	inbox_lock="$inbox_dir"/ssoma.lock
 fi
 
 # run public-inbox-init iff unconfigured
@@ -118,7 +120,18 @@ esac
 # don't know what indexlevel a user wants
 if test -f "$msgmap"
 then
-	n=$(echo 'SELECT COUNT(*) FROM msgmap' | sqlite3 -readonly "$msgmap")
+	# We need to use flock(1) (from util-linux) to avoid timeouts
+	# and SQLite locking problems.
+	# FreeBSD has a similar lockf(1) utility, but it unlinks by
+	# default so we use `-k' to keep the lock on the FS.
+	FLOCK=flock
+	case $(uname -s) in
+	FreeBSD) FLOCK='lockf -k' ;;
+	# ... other OSes here
+	esac
+
+	n=$(echo 'SELECT COUNT(*) FROM msgmap' | \
+		$FLOCK $inbox_lock sqlite3 -readonly "$msgmap")
 	case $n in
 	0|'')
 		: v2 inboxes may be init-ed with an empty msgmap

  reply	other threads:[~2020-08-25 10:23 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-25  0:12 anybody hit SQLite "database is locked" errors? Eric Wong
2020-08-25 10:23 ` Eric Wong [this message]
2020-08-25 16:29 ` Eric Wong

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