wrapper

#!/usr/bin/env bash

set -o errexit
set -o pipefail
set -o nounset

# Print debug messages
DEBUG=

# This variable may be defined by the user
# in order to orchestrate the jobserver
NUM_SLOTS=

# Get --jobserver-auth and the total
# number of jobs set by the user
JOBSERVER_FLAG=
TOTALJOBS_FLAG=

# Each byte represents a job slot from jobserver pool
# All the bytes taken must be given back at the end
# of the operations
BYTES=

# If --jobserver-auth=fifo:/path/to/fifo, then
# get the filename of the FIFO
FIFO=

# if --jobserver-auth=R,W, then get both file
# descriptors. [0]R=read and [1]W=write
FD=()

# Run command and exit
run_cmd() {
	eval "$@"
	exit
}

# Warning wrapper
warn() {
	echo "wrapper: $@" >&2
}

# Debug wrapper. Print if defined DEBUG
debug() {
	if [[ -n "$DEBUG" ]]; then
		echo "wrapper [debug]: $@" >&2
	fi
}

# It must be trapped in order to cleanup
# all the mess before exiting
cleanup() {
	if [[ -n "$BYTES" ]]; then
		if [[ -n "$FIFO" ]]; then
			debug "Return ${#BYTES} bytes to jobserver-auth=fifo:$FIFO"
			echo -n "$BYTES" > "$FIFO"
		elif [[ -n "$FD" ]]; then
			debug "Return ${#BYTES} bytes to jobserver-auth=${FD[0]},${FD[1]}"
			echo -n "$BYTES" >&"${FD[1]}"
		fi
	fi
}

# Try to allocate slots from jobserver
try_allocate_slots() {
	if [[ "$JOBSERVER_FLAG" =~ --jobserver-auth=fifo:(.+) ]]; then
		# Set FIFO path
		FIFO="${BASH_REMATCH[1]}"

		# Try to consume the number of required jobs from FIFO
		if ! read -n "$(( NUM_SLOTS - 1 ))" BYTES < "$FIFO" 2> /dev/null; then
			warn "Cannot read fifo:$FIFO. It will not consume jobserver slots"
		else
			debug "Read ${#BYTES} bytes from jobserver-auth=fifo:$FIFO"
		fi
	elif [[ "$JOBSERVER_FLAG" =~ --jobserver-auth=(-*[0-9]+),(-*[0-9]+) ]]; then
		# Set the array to both file descriptors
		FD=(${BASH_REMATCH[1]} ${BASH_REMATCH[2]})

		# Check if both file descriptors are valid before consuming the jobs
		# If they are not, do not consume more jobs
		if [[ "${FD[0]}" -ge 0 && "${FD[1]}" -ge 0 ]]; then
			# The FD[0] can be a valid integer and a bad fd at the same time
			if ! read -n "$(( NUM_SLOTS - 1 ))" -u "${FD[0]}" BYTES 2> /dev/null; then
				warn "Cannot read file descriptor. Maybe you should append '+' before your command"
			else
				debug "Read ${#BYTES} bytes from jobserver-auth=${FD[0]},${FD[1]}"
			fi
		else
			warn "File descriptor not available. Maybe you should append '+' before your command"
		fi
	# Something weird happend
	else
		warn "Unknown --jobserver-auth: '$JOBSERVER_FLAG'"
	fi
}

# Parse MAKEFLAGS which is set by make
parse_makeflags() {
	# Make a an array with all CLI flags
	local makeflags_a=($MAKEFLAGS)

	# Look for  the last --jobserver-auth and
	# get the total amount of jobs into the pool
	# with the flag -j
	for flag in "${makeflags_a[@]}"; do
		if [[ "$flag" =~ --jobserver-auth ]]; then
			JOBSERVER_FLAG="$flag"
		elif [[ "$flag" =~ -j[0-9]+ ]]; then
			TOTALJOBS_FLAG="$flag"
		fi
	done

	# Make called with infinite number of jobs `make -j` or with just one
	# `make -j1`  will skip this test
	if [[ -n "$TOTALJOBS_FLAG" && "${TOTALJOBS_FLAG:2}" -gt 1 ]]; then
		# Throw an warning if we cannot define `--jobserver-auth`
		if [[ -z "$JOBSERVER_FLAG" ]]; then
			warn "$TOTALJOBS_FLAG is set, but cannot define --jobserver-auth"
		# Throw an warning if NUM_SLOTS greater than TOTALJOBS_FLAG
		# We cannot consume more jobs than there are in the pool
		elif [[ "$NUM_SLOTS" -gt "${TOTALJOBS_FLAG:2}" ]]; then
			warn "NUM_SLOTS=$NUM_SLOTS greater than --jobs=${TOTALJOBS_FLAG:2}. Run forced"
		# If `make -j N` with (N > 1), --jobserver-auth is defined and
		# NUM_SLOTS <= TOTALJOBS_FLAG, try to allocate more slots
		else
			try_allocate_slots
		fi
	else
		debug 'Make called with `make -j` or `make -j1`'
	fi
}

main() {
	# Remove trailing '-c' if any
	[[ "$1" == '-c' ]] && shift

	# Our hero
	local cmd="$@"

	# Trap the EXIT signal to always release the bytes
	# taken from the jobserver pool
	trap cleanup EXIT

	# Get env variable DEBUG value
	if [[ "$cmd" =~ DEBUG=([A-Za-z0-9]+) ]]; then
		DEBUG="${BASH_REMATCH[1]}"
	fi

	# Get env variable NUM_SLOTS value
	if [[ "$cmd" =~ NUM_SLOTS=([0-9]+) ]]; then
		NUM_SLOTS="${BASH_REMATCH[1]}"
	fi

	# Run command as it is if NUM_SLOTS= or NUM_SLOTS < 2
	if [[ -n "$NUM_SLOTS" && "$NUM_SLOTS" -gt 1 ]]; then
		parse_makeflags
	else
		debug "Do not use jobserver, because NUM_SLOTS not defined or lesser than 2"
	fi

	# Gooooooooo
	run_cmd "$cmd"
}

# Do nothing if the first char is 'n'
[[ "$MAKEFLAGS" =~ ^n  ]] || main "$@"

# FINITO