grok-pull.post_update_hook: flock(2) before SQLite check

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/

diff --git a/examples/grok-pull.post_update_hook.sh b/examples/grok-pull.post_update_hook.sh
index 1f51140ffbd139b57d41065e619a4115173ee841..77489472123192ca1df6522e6a1bbaa8fe6845b5 100755 (executable)
--- 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