Init, first working version

Makefile

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+install:
+	mkdir -p $(DESTDIR)/usr/lib/cups/backends
+	install -m0700 cups-backend-copy-raw.sh /usr/lib/cups/backends/bbb-copy-raw

cups-backend-copy-raw.sh

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+#!/usr/bin/env bash
+
+# Put it into /usr/lib/cups/backend/bbb-copy-raw
+
+# This virtual "printer" is run after BigBlueButton collects
+# all artefacts of the meeting to /var/bigbluebutton/recording/raw/$meeting_id
+# and sends all that to a special worker which processes (converts) recordings.
+# All recordings are sent, even those where recording was not turned on,
+# because it may have been not tunred on erroneously and so we still have
+# to keep the recording artefact. The storage of recordings will rotate them.
+# "Printer class" in CUPS may be used to load balance processing of recordings
+# to multiple workers. Printer Device URI can be used to show which "printer"
+# (recordings processing worker) has been chosen by CUPS.
+
+# Authors:
+# - Mikhail Novosyolov <mikhailnov@dumalogiya.ru>, 2021
+
+# Based on examples at https://community.kde.org/Printing/Developer_Tools
+# Docs:
+# - backend(7) https://www.cups.org/doc/man-backend.html
+###############################################################################
+
+set -efu
+set -o pipefail
+
+# jobtitle=meeting ID (/var/bigbluebutton/recording/raw/$meeting_id),
+# we are not interested in all other variables
+readonly jobtitle=${3}
+
+BBB_DIR_RAW="${BBB_DIR_RAW:-/var/bigbluebutton/recording/raw}"
+
+# Exit codes from /usr/include/cups/backend.h
+CUPS_BACKEND_OK=0                  # Job completed successfully
+CUPS_BACKEND_FAILED=1              # Job failed, use error-policy
+CUPS_BACKEND_AUTH_REQUIRED=2       # Job failed, authentication required
+CUPS_BACKEND_HOLD=3                # Job failed, hold job
+CUPS_BACKEND_STOP=4                # Job failed, stop queue
+CUPS_BACKEND_CANCEL=5              # Job failed, cancel job
+CUPS_BACKEND_RETRY=6               # Job failed, retry this job later
+CUPS_BACKEND_RETRY_CURRENT=7       # Job failed, retry this job immediately
+
+input=""
+
+case ${#} in
+	0 )
+		# This is CUPS listing backends
+		# device-class scheme "Unknown" "device-info"
+		echo direct bbb-copy-raw \"Unknown\" \"Collect raw recording of BigBlueButton\"
+		exit
+	;;
+	12 )
+		input=/dev/stdin
+	;;
+	13 )
+		# do not remove {} here!
+		input="${13}"
+	;;
+	* )
+		echo CRIT: unsupported number of arguements! 1>&2
+		exit "$CUPS_BACKEND_FAILED"
+	;;
+esac
+
+if [ "$jobtitle" = "(stdin)" ]; then
+	echo CRIT: printing job must be titled! 1>&2
+	exit "$CUPS_BACKEND_FAILED"
+fi	
+
+if [ -z "$jobtitle" ]; then
+	echo CRIT: empty title of the printing job! 1>&2
+	exit "$CUPS_BACKEND_FAILED"
+fi
+
+# Examples of valid URIs:
+# - local/srv/container2/raw
+#   it means copy from /var/bigbluebutton/recording/raw/$meeting_id to /srv/container2/raw/$meeting_id
+# - ssh/port/user@host/remote_path (has not been tested and used!)
+#   it means rsync -av -e "ssh -p $port" /var/bigbluebutton/recording/raw/$meeting_id user@host:/remote_path/
+DEVICE_URI="$(echo "$DEVICE_URI" | sed -e 's,^bbb-copy-raw://,,' -e 's,^bbb-copy-raw:/,,')"
+case "$DEVICE_URI" in
+	local/* )
+		# https://docstore.mik.ua/orelly/unix3/upt/ch10_13.htm
+		# pack /var/bigbluebutton/recording/raw/$meeting_id into file.tar (root of $meeting_id = root of tarball)
+		# and send to stdin of CUPS
+		target_dir=/"$(echo "$DEVICE_URI" | sed -e 's,^local/,,')"
+		if ! test -d "$target_dir"; then
+			echo CRIT: directory "$target_dir" does not exist! 1>&2
+			exit "$CUPS_BACKEND_FAILED"
+		fi
+		mkdir -p "$target_dir"/"$jobtitle"
+		dd if="$input" | tar -C "$target_dir"/"$jobtitle" xf -
+	;;
+	ssh/* )
+		ssh_port="$(echo "$DEVICE_URI" | awk -F '/' '{print $2}')"
+		ssh_userhost="$(echo "$DEVICE_URI" | awk -F '/' '{print $3}')"
+		# ssh/port/user@host/remote_path/dir1/dir2 -> remote_path/dir1/dir2
+		# https://stackoverflow.com/a/49130247
+		ssh_remote_path="$(echo "$DEVICE_URI" | awk -F '/' '{for(i=4; i<=NF; ++i) printf "%s/", $i;}')"
+		# TODO: run as sudo -u <special user> ssh ...
+		ssh -p "$ssh_port" "$ssh_userhost" mkdir -p "$ssh_remote_path"/"$jobtitle"
+		dd if="$input" | ssh -p "$ssh_port" "$ssh_userhost" tar -C "$ssh_remote_path"/"$jobtitle" xf -
+	;;
+	* )
+		echo CRIT: Device URI is not supported! 1>&1
+		exit "$CUPS_BACKEND_FAILED"
+	;;
+esac