/lib/systemd/system-generators/postfix-instance-generator is in postfix 3.3.0-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | #!/bin/sh
set -e
WANTDIR="$1/postfix.service.wants"
SERVICEFILE="/lib/systemd/system/postfix@.service"
mkdir -p "$WANTDIR"
ln -s "$SERVICEFILE" "$WANTDIR/postfix@-.service"
for DIR in $(postconf -h multi_instance_directories); do
ln -s "$SERVICEFILE" "$WANTDIR/postfix@$(postconf -hc $DIR multi_instance_name).service"
done
exit 0
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