MySQL clients return bogus errno for host-not-found errors on Ubuntu 13.04

An upstream bug, which is a CI bug for us:

[2 Jun 8:00] Laurynas Biveinis

Description:
lp:mysql-server/5.1 revno 3997, compiled with BUILD/compile-amd64-debug-max-no-ndb fails mysql and mysql_upgrade testcases with what appears to be wrong errno value -1 in the client.

A quick analysis suggests that it's caused by a gethostbyname_r call that returns -1 (NETDB_INTERNAL) in the errno arg, which then means that the real errno should be read from the global errno var. If that's true, then still no idea why it's returning NETDB_INTERNAL in the first place.

How to repeat:
5.1$ bzr revno
3997
5.1$ BUILD/compile-amd64-debug-max-no-ndb
...
5.1$ cd mysql-test
5.1/mysql-test$ ./mysql-test-run --mem mysql
...
@@ -162,14 +162,14 @@
 ERROR 1049 (42000) at line 1: Unknown database 'invalid'
 Test connect with dbname + hostname
 Test connect with dbname + _invalid_ hostname
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'invalid_hostname' (errno)
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'invalid_hostname' (errno)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'invalid_hostname' (-1)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'invalid_hostname' (-1)
 The commands reported in the bug report
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyril has found a bug :)XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (errno)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyril has found a bug :)XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (-1)
 Too long dbname
 ERROR 1102 (42000) at line 1: Incorrect database name 'test_really_long_dbnamexxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
 Too long hostname
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyrils_superlonghostnameXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (errno)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyrils_superlonghostnameXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (-1)
 1
 1
 ERROR at line 1: DELIMITER cannot contain a backslash character
@@ -198,7 +198,7 @@
 1
 COUNT (*)
 1
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'invalid_hostname' (errno)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'invalid_hostname' (-1)
 End of 5.0 tests
 WARNING: --server-arg option not supported in this configuration.
 Warning (Code 1286): Unknown table engine 'nonexistent'

mysqltest: Result content mismatch

5.1/mysql-test$ ./mysql-test-run --mem mysql_upgrade

@@ -94,7 +94,7 @@
 mysql.user OK
 DROP USER mysqltest1@'%';
 Run mysql_upgrade with a non existing server socket
-mysqlcheck: Got error: 2005: Unknown MySQL server host 'not_existing_host' (errno) when trying to connect
+mysqlcheck: Got error: 2005: Unknown MySQL server host 'not_existing_host' (-1) when trying to connect
 FATAL ERROR: Upgrade failed
 set GLOBAL sql_mode='STRICT_ALL_TABLES,ANSI_QUOTES,NO_ZERO_DATE';
 mtr.global_suppressions OK

Suggested fix:
Will be provided below if my analysis works out.

[2 Jun 8:54] Laurynas Biveinis

It appears that there are two issues.

1) gethostbyname_r returning -1 in h_errno, as described above

If this is fixed to copy errno to h_errno in my_gethostbyname_r(), then the following testcase failure still remains:

 The commands reported in the bug report
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyril has found a bug :)XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (errno)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyril has found a bug :)XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (-1)
 Too long dbname
 ERROR 1102 (42000) at line 1: Incorrect database name 'test_really_long_dbnamexxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'
 Too long hostname
-ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyrils_superlonghostnameXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (errno)
+ERROR 2005 (HY000) at line 1: Unknown MySQL server host 'cyrils_superlonghostnameXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX' (-1)
 1

Thus,
2) this appears to be caused by gethostbyname_r returning 0 (success), but setting hostent *result to NULL.

[2 Jun 9:00] Laurynas Biveinis

The following fixes the testsuite failures for me. I have no idea whether that's really correct, as I was not find info under what circumstances gethostbyname_r returns such values. Also the *h_errno = errno copying makes the my_gethostbyname_r() interface deviate from the underlying gethostbyname_r() interface, which is also suboptimal I guess.

This appears to be a 5.1-only issue BTW.

=== modified file 'mysys/my_net.c'
--- mysys/my_net.c 2013-03-19 12:29:12 +0000
+++ mysys/my_net.c 2013-06-02 08:56:24 +0000
@@ -60,9 +60,15 @@
        int buflen, int *h_errnop)
 {
   struct hostent *hp;
+ int result_hostent;
   DBUG_ASSERT((size_t) buflen >= sizeof(*result));
- if (gethostbyname_r(name,result, buffer, (size_t) buflen, &hp, h_errnop))
+ result_hostent= gethostbyname_r(name,result, buffer, (size_t) buflen, &hp, h_errnop);
+ if (unlikely(!result_hostent || !hp)) {
+ if (*h_errnop == NETDB_INTERNAL) {
+ *h_errnop= errno;
+ }
     return 0;
+ }
   return hp;
 }