Warn if no slaves are found.

bin/pt-table-checksum

@@ -8741,6 +8741,15 @@ sub main {
          } @$slaves;
       }
       PTDEBUG && _d(scalar @$slaves, 'slaves found');
+      if ( !@$slaves && $o->get('recursion-method')->[0] ne 'none' ) {
+         $exit_status |= 1;
+         if ( $o->get('quiet') < 2 ) {
+            warn "Diffs cannot be detected because no slaves were found.  "
+               . "Please read the --recursion-method documentation for "
+               . "information.\n";
+         }
+      }
+
 
       # https://bugs.launchpad.net/percona-toolkit/+bug/938068
       if ( $o->get('check-binlog-format') ) {
@@ -11490,6 +11499,13 @@ type: array; default: processlist,hosts
 
 Preferred recursion method for discovering replicas.  pt-table-checksum
 performs several L<"REPLICA CHECKS"> before and while running.
+
+Although replicas are not required to run pt-table-checksum, the tool
+cannot detect diffs on slaves that it cannot discover.  Therefore,
+a warning is printed and the L<"EXIT STATUS"> is non-zero if no replicas 
+are found and the method is not C<none>.  If this happens, try a different
+recursion method, or use the C<dsn> method to specify the replicas to check.
+
 Possible methods are:
 
   METHOD       USES

t/pt-table-checksum/basics.t

@@ -361,10 +361,11 @@ $output = output(
    stderr => 1,
 );
 
+# Before 2.2 the exit status was 0, but bug 1087804 changed this to 1.
 is(
    $exit_status,
-   0,
+   1,
-   "No host in DSN, zero exit status"
+   "No host in DSN, non-zero exit status"
 );
 
 is(
@@ -490,6 +491,55 @@ is(
    "Bug 821675 (dot): 0 errors"
 );
 
+# #############################################################################
+# Bug 1087804: pt-table-checksum doesn't warn if no slaves are found
+# #############################################################################
+$sb->load_file('master', "$sample/dsn-table.sql");
+$master_dbh->do('TRUNCATE TABLE dsns.dsns');
+$sb->wait_for_slaves;
+
+my $slave1_dsn = $sb->dsn_for('slave1');
+
+$output = output(
+   sub { $exit_status = pt_table_checksum::main(@args,
+      qw(-t sakila.country),
+      "--recursion-method", "dsn=$slave1_dsn,t=dsns.dsns")
+   },
+   stderr => 1,
+);
+
+like(
+   $output,
+   qr/no slaves were found/,
+   "Warns if no slaves are found"
+);
+
+is(
+   $exit_status,
+   1,
+   '...exit status 1'
+);
+
+$output = output(
+   sub { $exit_status = pt_table_checksum::main(@args,
+      qw(-t sakila.country),
+      "--recursion-method", "none")
+   },
+   stderr => 1,
+);
+
+unlike(
+   $output,
+   qr/no slaves were found/,
+   "No warning if no slaves and --recursion-method=none"
+);
+
+is(
+   $exit_status,
+   0,
+   '...exit status 0'
+);
+
 # #############################################################################
 # Done.
 # #############################################################################