FreeTDS Development Group

[Chris Kings-Lynne <Chris.Kings-Lynne AT navitas.com>]

Hi James,

I am aware of what you say below (having studied the Tenderfoor as well as 
the tsql.c source.  I too wish to suppress the SYBEMSG error, and instead 
print the preceeding message.  In fact, that's what the PHP library is 
attempting to do, however the msgno being sent to the handler is _never_ 
SYBEMSG?

Chris

-----Original Message-----
From: freetds-bounces AT lists.ibiblio.org 
[mailto:freetds-bounces AT lists.ibiblio.org] On Behalf Of James K. Lowden
Sent: Saturday, 14 September 2013 9:58 AM
To: freetds AT lists.ibiblio.org
Subject: Re: [freetds] Problem with message handl

On Fri, 13 Sep 2013 06:51:53 +0000
Chris Kings-Lynne <Chris.Kings-Lynne AT navitas.com> wrote:

> I'm trying to fix the PHP pdo_dblib driver.  The issue is it never 
> returns the actual database error message.  It always says "Check SQL 
> server messages".  I've identified the problem in the message handler 
> code here:
> 
> https://github.com/php/php-src/blob/master/ext/pdo_dblib/pdo_dblib.c#L
> 140

Hmm, they did leave room for improvement, eh?  The handler is called with 
msgno, severity, text, server name, procedure name, and line number, but the 
function keeps only the text.  

It's also possible to get more than one message, but the function 
unilaterally discards any it finds.  

Disregarding messages with serverity 0 isn't very general.  People have been 
known to use the T-SQL PRINT statement, but they're uniformly suppressed.  

I know, there are messages no one really wants to see.  I've handled that in 
libraries I've written by maintaining a do-not-print list of message numbers. 
 That way the application programmer has control over which messages the 
handler handles.  

> The problem is that the msgno always comes through as the correct 
> underlying sql error number (eg. 208), instead of the magic one that's 
> being expected (20018).

I think there may be some confusion on this point.  What you're calling the 
"correct" msgno is attached to a *message* from the server.  It's not 
necessarily an error.  It may be helpful to think of them as runtime errors, 
because in principle they can be caused by things under the user's control.  
Usually the connection remains viable even if the query fails.  Messages are 
handled by the message handler.  

The numbers your referred to in dblib.c are db-lib *errors*.  These are 
caused by using db-lib incorrectly, and originate from the library, not the 
server.  They are logic errors made by the programmer or environmental issues 
(such as network failures) that the user has no control over.  Depending on 
how badly things went awry, the connection may be broken and/or the data 
corrupted.  Errors are handled by the error handler.  

There's a special class of server message that represents an error condition, 
where the server is experiencing heartburn.  Table corruption, disk failure, 
or maybe the DBA killed the spid that was executing the user's query.  These 
kinds of errors have high serverity (where "high" is over 10 IIRC).  In such 
cases, the server will send its message per usual, and db-lib will call the 
message handler, per usual.  Then, just to make sure you got the message (as 
it were) it calls the error handler too, with your friend and mine SYBEMSG.  

I ususally suppress SYBEMSG, because I make very sure to have printed the 
message that preceded it.  

It's like the secretary leaving a "while you were out" message slip on your 
desk.  We used to have people whose job it was to answer phones and take 
messages.  The phone was attached to the wall by a wire, so you couldn't just 
carry it around.  Oh, never mind.  We had offices, too.  

--jkl
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