#!/usr/bin/env bash
# #6957 v03-0008: WHO assigns the XID to the REPACK backend before the replay?
#
# The Assert in compute_new_xmax_infomask() (heapam.c) requires that, if the
# xmax about to be written is not ours, we must not have a top XID:
#
#   Assert(TransactionIdIsCurrentTransactionId(add_to_xmax) ||
#          !TransactionIdIsValid(GetTopTransactionIdIfAny()))
#
# and it fails during the replay that runs after AccessExclusiveLock is taken.
# Antonin said (2026-09-23) he does not know when that XID gets assigned.  This
# measures it instead of reasoning about it: gdb attaches TO THE REPACK BACKEND
# and dumps a backtrace on every AssignTransactionId(), plus the one at the
# TRAP.  The last backtrace before the TRAP is the one that assigned the XID
# that breaks the Assert.
#
#   catch_xid.sh [build] [repack_snapshot_after]
set -u
N=${1:-p6957a}
AFTER=${2:-50}
BIN=$HOME/pgprog/i-$N/bin
HERE=$(cd "$(dirname "$0")" && pwd)
PGD=$(mktemp -d /tmp/claude-1000/catchxid.XXXX)
PORT=54967
OUT=$HERE/cazar_xid.out
export PGPORT=$PORT PGHOST=/tmp PGDATABASE=postgres

"$BIN/initdb" -D "$PGD" -A trust --no-sync > /dev/null
cat >> "$PGD/postgresql.conf" <<EOF
port = $PORT
unix_socket_directories = '/tmp'
wal_level = logical
max_replication_slots = 10
log_line_prefix = '%m [%p] '
logging_collector = off
restart_after_crash = on
EOF
"$BIN/pg_ctl" -D "$PGD" -l "$PGD/server.log" -w start > /dev/null
P="$BIN/psql -X -q"
$P -c "CREATE TABLE t (id int PRIMARY KEY, v text)" \
   -c "INSERT INTO t SELECT g, repeat('x', 200) FROM generate_series(1, 30000) g"

# The session that runs REPACK: through a fifo, so we have its PID beforehand.
FIFO=$(mktemp -u); mkfifo "$FIFO"
# -At (unaligned, no headers) is required: with the table format, \g also
# writes the "(1 row)" line and the PID comes out with a 1 glued to it.
"$BIN/psql" -X -q -At < "$FIFO" > "$PGD/repack.out" 2>&1 &
exec 3> "$FIFO"
echo "SELECT pg_backend_pid() \\g $PGD/pid" >&3
for _ in $(seq 40); do [ -s "$PGD/pid" ] && break; sleep 0.2; done
PID=$(tr -dc '0-9' < "$PGD/pid")
kill -0 "$PID" 2>/dev/null || { echo "invalid PID: '$PID'"; exit 1; }
echo "== REPACK backend: $PID"

cat > "$PGD/catch.gdb" <<'EOF'
set pagination off
set confirm off
set print frame-arguments all
# Postgres uses SIGUSR1 for IPC all the time: without this, gdb stops at the
# first signal, the trailing `continue` is consumed there, and not a single
# AssignTransactionId is captured (seen: 0 captures and one backtrace sitting
# in epoll_wait).
handle SIGUSR1 nostop noprint pass
handle SIGUSR2 nostop noprint pass
handle SIGALRM nostop noprint pass
handle SIGPIPE nostop noprint pass
break AssignTransactionId
commands
silent
printf "\n=== AssignTransactionId ===\n"
bt 22
continue
end
continue
printf "\n=== THE BACKEND STOPPED (TRAP/signal) ===\n"
bt 30
EOF
gdb -p "$PID" -batch -x "$PGD/catch.gdb" > "$OUT" 2>&1 &
GDB=$!
sleep 3   # let gdb finish attaching before sending anything

# Concurrent load: with no UPDATEs to replay there is no replay to break.
( for i in $(seq 1 300); do
    $P -c "UPDATE t SET v = 'upd' WHERE id = $i * 7" 2> /dev/null
  done ) &
LOAD=$!

[ "$AFTER" = 0 ] || echo "SET repack_snapshot_after = $AFTER;" >&3
echo "REPACK (CONCURRENTLY) t;" >&3
echo "SELECT 'done' \\g $PGD/done" >&3

for _ in $(seq 180); do [ -s "$PGD/done" ] && break; kill -0 "$PID" 2>/dev/null || break; sleep 1; done
wait "$LOAD" 2>/dev/null
exec 3>&- ; rm -f "$FIFO"
wait "$GDB" 2>/dev/null

echo "== TRAPs in the log: $(grep -c '^TRAP' "$PGD/server.log")"
grep -m1 '^TRAP' "$PGD/server.log" | cut -c1-150
echo "== XID assignments captured: $(grep -c '=== AssignTransactionId ===' "$OUT")"
cp "$PGD/server.log" "$HERE/cazar_xid.server.log"
cp "$PGD/repack.out" "$HERE/cazar_xid.repack.out" 2>/dev/null
"$BIN/pg_ctl" -D "$PGD" -m immediate -w stop > /dev/null 2>&1
rm -rf "$PGD"
echo "== backtraces in $OUT"