MySQL Bugs: #93047: Timeout after a read error returns HA_ERR_INTERNAL

Bug #93047	Timeout after a read error returns HA_ERR_INTERNAL_ERROR (122)
Submitted:	1 Nov 2018 17:30	Modified:	4 Nov 2018 11:33
Reporter:	Olivier Bertrand	Email Updates:
Status:	Unsupported	Impact on me:	None
Category:	Connector / NET	Severity:	S1 (Critical)
Version:	8.0.13	OS:	Windows (10)
Assigned to:		CPU Architecture:	Any

Description:
When called from a C# GUI client, after a read next logical error returning the 122 error code, the GUI is frozen until a timeout occurs.

The problem occurs when using a C# GUI that tries to retrieve a query result via a MySqlDataAdapter.

The code:

dqry = qry;
ln.Clear();
data = new DataTable();
da = new MySqlDataAdapter(qry, conn);
da.FillError += new FillErrorEventHandler(FillError);
 
try {
    da.Fill(data);
} catch (ArgumentNullException) {
    return;
} catch (Exception) {
    throw;
} // end try/catch
 
da.FillError -= new FillErrorEventHandler(FillError);

What happens, in case of read error, is that the Fill function does not return until a timeout occurs. The FillError function is not called and the program resumes at the exception catch saying that a timeout exception was raised.

How to repeat:
For instance, connected to MariaDB, using the CONNECT storage engine:

create table tmout (
n int,
w char(16), /* too small */
f char(3))
engine=connect table_type=csv file_name='foo.csv' lrecl=50;

The file foo.csv being:

26,Hello world!,foo
45,abcdefghijklmnopqrstuvwxyz,bar

Executing:

select * from tmout;

makes the connector not to respond until timeout.

Note that a statement such as

insert into tmout values(45,'abcdefghijklmnopqrstuvwxyz','bar');
correctly returns with an error message.

Suggested fix:
It is very likely that the connector does not expect error packet in the middle of result.

Thank you for the bug report. Server MariaDB isn't supported by MySQL connectors.

Please don't sweep it so quickly under the rug. 

I didn't know how to generate a read error using the MySQL server but finally I found this way:

create table tmout (
n int not null,
w char(16) not null, /* will be too small */
f char(3) not null)
engine=CSV;
insert into tmout values
(26,'Hello world!','foo'),
(45,'abcdefghijklmnop','bar');

Then stop the MySQL server and using an editor increase the second line of tmout.CSV to become:

45,"abcdefghijklmnopqrstuvwxyz"

Restart the MySQL server. Using the standard mysql client, when doing:

select * from tmout;

It replies:

ERROR 1194 (HY000): Table 'tmout' is marked as crashed and should be repaired

Now, using the C# GUI, doing the same thing reproduce the bug: no answer until a timeout occurs.

What do you mean by "works as expected"? That a connector can hide an error, wait for an eventually long time before returning in case of error and do not display the normal error message but just say that a timeout occured?

I tried with another connector and this bug does not occur.