#/bin/bash

# Usage: replicate <hostname> <zfs filesystem>
remote=$1
remote_fs=$2
remote_pool=${2%%/*}
remote_lockdir="/tmp/zfs-admin-lock"

# Set the name of the local pool used to store the backup of the remote
local_pool=backups

# Set the email address to send notification to
mailto=alan@pippins.net

# The ssh connection doesn't find zfs without this.
zfs=/usr/sbin/zfs

# Make sure we have valid arguments
if [[ -z "$remote" ]] || [[ -z "$remote_fs" ]]; then
  echo "Usage: $0 <hostname> <zfs filesystem>"
  exit 1
fi

# Make sure the local pool and local receiving filesystem exist, or print some errors
zpool list -H "$local_pool" >/dev/null 2>&1
if [ $? != 0 ]; then
  echo >&2 "-E- The local pool, '$local_pool' doesn't seem to exist."
  exit 1
fi
zfs list "$local_pool/$remote_pool" >/dev/null 2>&1
if [ $? != 0 ]; then
  echo >&2 "-I- The local filesystem for the remote pool, '$local_pool/$remote_pool' doesn't seem to exist."
  echo >&2 "    Creating the local filesystem to receive the remote pool into: $local_pool/$remote_pool"
  $zfs create $local_pool/$remote_pool
  if [ $? != 0 ]; then
    echo "-E- remote $zfs create command failed"
    exit 1
  fi
fi

# Obtain the zpool guid for the local pool
local_pool_guid=`zpool get guid $local_pool 2>&1 | grep $local_pool | awk '{ print $3 }'`
zpool get guid $local_pool > /dev/null 2>&1
if [ $? != 0 ]; then
  echo >&2 "-E- Unable to extract the guid for the local pool: $local_pool"
  exit 1
fi

# Turn on shell verbosity
set -x

# Create the remote lockdir before continuing with the replicate
# Spinlock on creating the lock
maxsleeptime=60
maxattempts=100
attempts=0
while true; do
  ssh $remote mkdir "$remote_lockdir" >/dev/null 2>&1
  if [ $? != 0 ]; then
    # Another zfs admin tool is running.
    # Wait a random amount of time and try again
    ransleep=$(($RANDOM % $maxsleeptime))
    sleep $ransleep
    ((attempts=attempts+1))
  else 
    # No other zfs admin tool is running, we can now.
    break
  fi
  if [[ $attempts -gt $maxattempts ]]; then
    # We've exceeded our maximum while loop count
    echo "-W- The zfs filesystem has been locked down. Skipping replicate operation."
    ssh $remote ls -ld $remote_lockdir | /usr/bin/mailx -s "zfs-replicate-all unable to obtain zfs admin lock" $mailto
    exit 1
  fi
done

# Declare a cleanup() method to remove the remote lockdir
cleanup() { ssh $remote rm -rf "$remote_lockdir"; }
trap cleanup EXIT

# Setup our backup marker names
current_backup_marker=${remote_fs}@current-backup-${local_pool_guid}
previous_backup_marker=${remote_fs}@previous-backup-${local_pool_guid}

# List the snapshots on the remote machine.
remote_list=$(mktemp /tmp/replicate.XXXXXX)
ssh $remote \
    $zfs list -H -t snapshot |
    grep ^${remote_fs}@ |
    awk '{print$1}' > $remote_list
if [ $? != 0 ]; then
  echo "-E- remote $zfs list command failed"
  exit 1
fi

# List the snapshots on the local machine.
local_list=$(mktemp /tmp/replicate.XXXXXX)
$zfs list -H -t snapshot |
    grep ^${local_pool}/${remote_fs}@ |
    awk "{gsub(/^$local_pool./,\"\",\$1); print\$1}" > $local_list
if [ $? != 0 ]; then
  echo "-E- local $zfs list command failed"
  exit 1
fi

# Destroy the current backup marker snapshot on the remote system if it exists
grep -q ${current_backup_marker} $remote_list
if [ $? == 0 ]; then
  ssh $remote $zfs destroy ${current_backup_marker} 
  if [ $? != 0 ]; then
    echo "-E- remote $zfs destroy command failed"
    exit 1
  fi
fi

# Create the current backup marker snapshot on the remote system
ssh $remote $zfs snapshot ${current_backup_marker}
if [ $? != 0 ]; then
  echo "-E- remote $zfs snapshot command failed"
  exit 1
fi

# Check to see if the previous backup marker exists in the remote snapshot list.
# Check to see if the previous backup marker exists in the local snapshot list.
# If the previous backup markers exists, perform an incremental replicate. Else:
# 1) check to see if a common snapshot exists, and perform an incremental replicate.
# 2) if no common snapshot exists, destroy the local filesystem, and perform a full replicate.
grep -q ${previous_backup_marker} $remote_list
no_markers=$?
grep -q ${previous_backup_marker} $local_list
no_markers=$(($no_markers || $?))

if [ $no_markers == 0 ]; then
  # We found backup markers, incrementally send the new snaps

  # First, rollback the local pool to the previous backup marker in case the previous
  # backup was interrupted for some reason. If we don't do this, the zfs send -R command
  # below may complain about snaps already existing as it tries to resend from the 
  # previous backup marker again from a previously interrupted replicate.
  $zfs rollback -r ${local_pool}/${previous_backup_marker} 
  if [ $? != 0 ]; then
    echo "-E- remote incremental $zfs rollback command failed"
    exit 1
  fi
  # Now it should be safe to send the snaps
  ssh $remote $zfs send -R -I${previous_backup_marker} ${current_backup_marker} | 
      $zfs receive -vF -d ${local_pool}/${remote_pool}
  if [ $? != 0 ]; then
    echo "-E- remote incremental $zfs send command failed"
    exit 1
  fi
else
  # We didn't find any backup markers, next check to see if we have a common snapshot.

  # See what the most recent snapshot on the remote end is.
  latest=$(tail -n 1 $remote_list)

  # I did this to make sure that diff would always display the most recent common
  # Since we're keying off the context of the diff, we need to ensure we will get context
  # by injecting a known difference in case no others exist in the lists.
  echo bogus.remote >> $remote_list
  echo bogus.local  >> $local_list
  common=$(diff -u $remote_list $local_list | grep '^ ' | tail -n 1)

  if [[ -n "$common" ]]; then
    # We found a common snapshot, incrementally send the new snaps
    ssh $remote $zfs send -R -I${common/*@/@} ${current_backup_marker} |
        $zfs receive -vF -d ${local_pool}/${remote_pool}
    if [ $? != 0 ]; then
      echo "-E- remote incremental $zfs send command failed"
      exit 1
    fi
  else
    # We did not find any markers or a common snapshot
    # At this point, we'll have to send the entire filesystem
    # Destroy the local filesystem if it exists before receving the full replicate
    zfs list ${local_pool}/${remote_fs} > /dev/null 2>&1
    if [ $? == 0 ]; then
      zfs destroy -r ${local_pool}/${remote_fs}
      if [ $? != 0 ]; then
        echo "-E- remote full $zfs destroy command failed"
        exit 1
      fi
    fi
    # Send the full filesystem
    ssh $remote $zfs send -R ${current_backup_marker} |
        $zfs receive -vF -d ${local_pool}/${remote_pool}
    if [ $? != 0 ]; then
      echo "-E- remote full $zfs send command failed"
      exit 1
    fi
  fi
fi
 
# destroy the previous backup markers now that we've replicated past them
# don't check the return codes here because these may not exist, and that is ok
$zfs destroy ${local_pool}/${previous_backup_marker} > /dev/null 2>&1
ssh $remote $zfs destroy ${previous_backup_marker} > /dev/null 2>&1

# Rename the current backup marker to be the previous backup marker
$zfs rename ${local_pool}/${current_backup_marker} ${local_pool}/${previous_backup_marker}
if [ $? != 0 ]; then
  echo "-E- local $zfs rename command failed"
  exit 1
fi
ssh $remote $zfs rename ${current_backup_marker} ${previous_backup_marker}
if [ $? != 0 ]; then
  echo "-E- remote $zfs rename command failed"
  exit 1
fi
   
# Remove tmp files
rm -f $local_list $remote_list