Add 'DROP PRIMARY KEY' to --check-alter.

bin/pt-online-schema-change

@@ -8986,6 +8986,28 @@ sub check_alter {
 
    my $ok = 1;
 
+   # ########################################################################
+   # Check for DROP PRIMARY KEY.
+   # ########################################################################
+   if ( $alter =~ m/DROP\s+PRIMARY\s+KEY/i ) {
+      my $msg = "--alter contains 'DROP PRIMARY KEY'.  Dropping and "
+              . "altering the primary key can be very dangerous, "
+              . "especially if the original table does not have other "
+              . "unique indexes.\n";
+      if ( $dry_run ) {
+         print $msg;
+      }
+      else {
+         $ok = 0;
+         warn $msg
+            . "The tool should handle this correctly, but you should "
+            . "test it first and carefully examine the triggers which "
+            . "rely on the PRIMARY KEY or a unique index.  Specify "
+            . "--no-check-alter to disable this check and perform the "
+            . "--alter.\n";
+      }
+   }
+
    # ########################################################################
    # Check for renamed columns.
    # https://bugs.launchpad.net/percona-toolkit/+bug/1068562
@@ -9032,6 +9054,7 @@ sub check_alter {
    }
 
    if ( !$ok ) {
+      # check_alter.t relies on this output.
       die "--check-alter failed.\n";
    }
 
@@ -10286,6 +10309,15 @@ code that does this is not a full-blown SQL parser, so we recommend that users
 run the tool with L<"--dry-run"> and check if it's detecting the renames
 correctly.
 
+=item DROP PRIMARY KEY
+
+If L<"--alter"> contain C<DROP PRIMARY KEY> (case- and space-insensitive),
+a warning is printed and the tool exits unless L<"--dry-run"> is specified.
+Altering the primary key can be very dangerous, but the tool can handle it.
+The tool's triggers, particularly the DELETE trigger, are most affected by
+altering the primary key because the tool prefers to use the primary key
+for its triggers.
+
 =back
 
 =item --[no]check-plan

t/pt-online-schema-change/bugs.t

@@ -210,7 +210,7 @@ $sb->load_file('master', "$sample/del-trg-bug-1062324.sql");
       sub { pt_online_schema_change::main(@args,
          "$master_dsn,D=test,t=t1",
          "--alter", "drop key 2bpk, drop key c3, drop primary key, drop c1, add primary key (c2, c3(4)), add key (c3(4))",
-         qw(--execute --no-drop-new-table --no-swap-tables)) },
+         qw(--no-check-alter --execute --no-drop-new-table --no-swap-tables)) },
    );
 
    # Since _t1_new no longer has the c1 column, the bug caused this
@@ -242,7 +242,7 @@ $sb->load_file('master', "$sample/del-trg-bug-1062324.sql");
       sub { pt_online_schema_change::main(@args,
          "$master_dsn,D=test,t=t1",
          "--alter", "drop primary key, add column _id int unsigned not null primary key auto_increment FIRST",
-         qw(--execute --no-drop-new-table --no-swap-tables)) },
+         qw(--no-check-alter --execute --no-drop-new-table --no-swap-tables)) },
    );
 
    eval {

t/pt-online-schema-change/check_alter.t

@@ -0,0 +1,65 @@
+#!/usr/bin/env perl
+
+BEGIN {
+   die "The PERCONA_TOOLKIT_BRANCH environment variable is not set.\n"
+      unless $ENV{PERCONA_TOOLKIT_BRANCH} && -d $ENV{PERCONA_TOOLKIT_BRANCH};
+   unshift @INC, "$ENV{PERCONA_TOOLKIT_BRANCH}/lib";
+};
+
+use strict;
+use warnings FATAL => 'all';
+use English qw(-no_match_vars);
+use Test::More;
+
+use Data::Dumper;
+use PerconaTest;
+use Sandbox;
+
+require "$trunk/bin/pt-online-schema-change";
+
+my $dp = new DSNParser(opts=>$dsn_opts);
+my $sb = new Sandbox(basedir => '/tmp', DSNParser => $dp);
+my $master_dbh = $sb->get_dbh_for('master');
+my $slave_dbh  = $sb->get_dbh_for('slave1');
+
+if ( !$master_dbh ) {
+   plan skip_all => 'Cannot connect to sandbox master';
+}
+elsif ( !$slave_dbh ) {
+   plan skip_all => 'Cannot connect to sandbox slave1';
+}
+
+# The sandbox servers run with lock_wait_timeout=3 and it's not dynamic
+# so we need to specify --lock-wait-timeout=3 else the tool will die.
+my $master_dsn = 'h=127.1,P=12345,u=msandbox,p=msandbox';
+my @args       = (qw(--lock-wait-timeout 3));
+my $output;
+my $exit_status;
+my $sample  = "t/pt-online-schema-change/samples/";
+
+# #############################################################################
+# DROP PRIMARY KEY
+# #############################################################################
+
+$sb->load_file('master', "$sample/del-trg-bug-1103672.sql");
+
+($output, $exit_status) = full_output(
+   sub { pt_online_schema_change::main(@args,
+      "$master_dsn,D=test,t=t1",
+      "--alter", "drop primary key, add column _id int unsigned not null primary key auto_increment FIRST",
+      qw(--execute)),
+   },
+);
+
+like(
+   $output,
+   qr/--check-alter failed/,
+   "DROP PRIMARY KEY"
+);
+
+# #############################################################################
+# Done.
+# #############################################################################
+$sb->wipe_clean($master_dbh);
+ok($sb->ok(), "Sandbox servers") or BAIL_OUT(__FILE__ . " broke the sandbox");
+done_testing;