btrbk: add lockfile support, using perl "flock": add configuration option "lockfile <file>", and command line option "--lockfile"'

ChangeLog

@@ -1,5 +1,7 @@
 btrbk-current
 
+  * Added "lockfile" configuration option and --lockfile command line
+    option (close: #81).
   * Bugfix: raw targets: correctly handle multiple backups in same
     target directory (close: #87).
   * Use relative instead of absolute binary calls in btrbk-mail.

btrbk

@@ -96,6 +96,7 @@ my %config_options = (
   rate_limit                  => { default => undef,     accept => [ "no" ], accept_regexp => qr/^[0-9]+[kmgt]?$/, require_bin => 'pv' },
   transaction_log             => { default => undef,     accept_file => { absolute => 1 } },
   transaction_syslog          => { default => undef,     accept => \@syslog_facilities },
+  lockfile                    => { default => undef,     accept_file => { absolute => 1 }, context => [ "root" ] },
 
   raw_target_compress         => { default => undef,     accept => [ "no", "gzip", "bzip2", "xz" ] },
   raw_target_compress_level   => { default => "default", accept => [ "default" ], accept_numeric => 1 },
@@ -204,6 +205,7 @@ my $show_progress = 0;
 my $err = "";
 my $abrt = "";  # last ABORTED() message
 my $output_format;
+my $lockfile;
 my $tlog_fh;
 my $syslog_enabled = 0;
 my $current_transaction;
@@ -3379,7 +3381,7 @@ MAIN:
   @tm_now = localtime($start_time);
 
   my %config_override_cmdline;
-  my ($config_cmdline, $quiet, $verbose, $preserve_backups, $resume_only, $print_schedule);
+  my ($config_cmdline, $quiet, $verbose, $preserve_backups, $resume_only, $print_schedule, $lockfile_cmdline);
   unless(GetOptions(
     'help|h'         => sub { VERSION_MESSAGE(); HELP_MESSAGE(0); exit 0; },
     'version'        => sub { VERSION_MESSAGE(); exit 0; },
@@ -3394,6 +3396,7 @@ MAIN:
     'table|t'        => sub { $output_format = "table" },
     'format=s'       => \$output_format,
     'print-schedule' => \$print_schedule,
+    'lockfile=s'     => \$lockfile_cmdline,
     'override=s'    => \%config_override_cmdline,  # e.g. --override=incremental=no
    ))
   {
@@ -3535,6 +3538,15 @@ MAIN:
       exit 2;
     }
   }
+  if(defined($lockfile_cmdline)) {
+    if($lockfile_cmdline =~ /^($file_match)$/) {
+      $lockfile = $1; # untaint argument
+    } else {
+      ERROR "Option \"--lockfile\" is not a valid file name: \"$lockfile_cmdline\"";
+      HELP_MESSAGE(0);
+      exit 2;
+    }
+  }
 
 
   INFO "$VERSION_INFO  (" . localtime($start_time) . ")";
@@ -3637,6 +3649,22 @@ MAIN:
     exit 2;
   }
 
+  #
+  # try exclusive lock if set in config or command-line option
+  #
+  $lockfile //= config_key($config, "lockfile");
+  if(defined($lockfile) && (not $dryrun)) {
+    unless(open(LOCKFILE, ">>$lockfile")) {
+      # NOTE: the lockfile is never deleted by design
+      ERROR "Failed to open lock file '$lockfile': $!";
+      exit 3;
+    }
+    unless(flock(LOCKFILE, 6)) {  #  exclusive, non-blocking (LOCK_EX | LOCK_NB)
+      ERROR "Failed to take lock (another btrbk instance is running): $lockfile";
+      exit 3;
+    }
+  }
+
 
   if($action_archive)
   {

btrbk.conf.example

@@ -56,6 +56,10 @@ snapshot_dir               _btrbk_snap
 #ssh_compression            no
 #ssh_cipher_spec            default
 
+# Enable lock file support: Ensures that only one instance of btrbk
+# can be run at a time.
+#lockfile                   /var/lock/btrbk.lock
+
 # Don't wait for transaction commit on deletion. Set this to "after"
 # or "each" to make sure the deletion of subvolumes is committed to
 # disk when btrbk terminates.

contrib/cron/btrbk-mail

@@ -111,6 +111,8 @@ exitcode=$?
 case $exitcode in
     0)  status="All backups successful"
 	;;
+    3)  status="Another instance of btrbk is running, no backup tasks performed!"
+	;;
     10) status="ERROR: At least one backup task aborted!"
 	;;
     *)  status="ERROR: btrbk failed with error code $exitcode"

doc/btrbk.1

@@ -1,4 +1,4 @@
-.TH "btrbk" "1" "2016-05-03" "btrbk v0.23.1" ""
+.TH "btrbk" "1" "2016-06-07" "btrbk v0.23.2-dev" ""
 .\" disable hyphenation
 .nh
 .\" disable justification (adjust text to left margin only)
@@ -118,6 +118,14 @@ between different output formats.
 Override a configuration option <config_option> with <value>.
 Globally, for ALL contexts. Use with care!
 .RE
+.PP
+\-\-lockfile <file>
+.RS 4
+Create lockfile <file> on startup; checks lockfile before running any
+btrfs commands (using perl "flock"), and exits if the lock is held by
+another btrbk instance. Overrides configuration option
+"lockfile". Ignored on dryrun (\fI\-n\fR, \fI\-\-dry\-run\fR).
+.RE
 .SH COMMANDS
 .PP
 .B run
@@ -324,6 +332,8 @@ No problems occurred.
 Generic error code.
 .IP "2" 4
 Parse error: when parsing command-line options or configuration file.
+.IP "3" 4
+Lockfile error: if lockfile is present on startup.
 .IP "10" 4
 Backup abort: At least one backup task aborted.
 .IP "255" 4

doc/btrbk.conf.5

@@ -1,4 +1,4 @@
-.TH "btrbk.conf" "5" "2016-05-03" "btrbk v0.23.1" ""
+.TH "btrbk.conf" "5" "2016-06-07" "btrbk v0.23.2-dev" ""
 .\" disable hyphenation
 .nh
 .\" disable justification (adjust text to left margin only)
@@ -231,6 +231,14 @@ suffix of "k", "m", "g", or "t" can be added to denote kilobytes
 (*1024), megabytes, and so on. Defaults to \[lq]no\[rq].
 .RE
 .PP
+\fBlockfile\fR  <file>
+.RS 4
+Create lockfile <file> on startup; checks lockfile before running any
+btrfs commands (using perl "flock"), and exits if the lock is held by
+another btrbk instance. Ignored on dryrun (\fI\-n\fR,
+\fI\-\-dry\-run\fR). See also \fI\-\-lockfile\fR command-line option.
+.RE
+.PP
 \fBbtrfs_commit_delete\fR  after|each|no
 .RS 4
 If set, make sure the deletion of snapshot and backup subvolumes are