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From Emmanuel L├ęcharny <elecha...@apache.org>
Subject Re: Exception interceptor pb
Date Thu, 06 Jan 2011 20:11:41 GMT
On 1/6/11 7:02 PM, Alex Karasulu wrote:
> On Thu, Jan 6, 2011 at 6:26 PM, Emmanuel Lecharny<elecharny@gmail.com>wrote:
>
>> Hi,
>>
>> today, while tracking some issue in the server, I jumped into some code in
>> the ExceptionInterceptor that I find questionable. Let me explain something
>> we already discussed on IRC.
>>
>>
> You know I thought about this to a degree after our conversations. I think
> the Cursor is setup to be automatically positioned on the first element if
> one exists. This is why we are checking if an element is available. So
> calling available() might be the right thing to do and makes sense.
Yep. And t's the only way to go (see the mail I just sent).
Although I have no idea how to have the available() method return the 
correct info.

> So even inside a block of code inside an interceptor after Cursor creation
> it might check if the underlying candidate has elements available before
> wrapping it or even adding its own filter.
This part of the code is like the dark side of the moon for me...
>
>> When we go up the chain, and process the ExceptionInterceptor, the
>> following code is executed :
>>     public EntryFilteringCursor search( NextInterceptor nextInterceptor,
>> SearchOperationContext searchContext )
>>     {
>>         EntryFilteringCursor cursor = nextInterceptor.search( searchContext
>> );
>>
>>         // Check that if the cursor is empty, it's not because the DN is
>> invalid.
>>         if ( !cursor.available()&&  !searchContext.getDn().isEmpty()&&
>> !subschemSubentryDn.equals( searchContext.getDn()) )
>>         {
>>             // We just check that the entry exists only if we didn't found
>> any entry
>>             assertHasEntry( searchContext, "Attempt to search under
>> non-existant entry:", searchContext.getDn() );
>>         }
>>
>>         return cursor;
>>     }
>>
>> The assertHasEntry method is :
>>
>>     private void assertHasEntry( OperationContext opContext, String msg, DN
>> dn ) throws LdapException
>>     {
>>         if ( !opContext.hasEntry( dn, ByPassConstants.HAS_ENTRY_BYPASS ) )
>>         {
>>             throw new LdapNoSuchObjectException( msg + dn.getName() );
>>         }
>>     }
>>
>> And now, the problem : the cursor.available() call *always* return false,
>> thus we most of the case do a lookup, which is costly. Every single search
>> we do will always fetch N+1 entry : The N entry to return, plus the first
>> entry a second time.
>>
>>
> Hmmm why does it return the first entry a second time?

It does not return the first entry a second time. What it does is that 
it read the first entry to see if it exists, and if so, the process 
continues and later, while transmitting the SearchResult, we read the 
first entry again. So the first entry is read twice.
>
>> This is useless.
>>
>>
> Yes it should not do this.
>
>
>> The reason why this check is being done is that we need to send a
>> LDAP_NO_SUCH_OBJECT result if there is no entry returned.
>>
>> We discussed some options we have. Currently, we have 2 :
>> - make the cursor return something different from false if there is an
>> entry, even if we don't prefetch the entry.
>> - postpone this test and handle the case in the SearchHandler
>>
>> Option #1 implies a huge refactoring of the cursor API. Not sure we want to
>> do this
>> Option #2 is probably better. We just have to check that it's valid to
>> check for the absence of an entry if the candidate have been discarded by
>> some upper filter (for instance, if we rejected some entry because the ACIs
>> mandated it). In this case, should we send back a LDAP_NO_SUCH_OBJECT?
>>
>> I will experiment with Option #2, re-read the RFC, and update you.
>>
> What worries me is that this worked just fine for years. It should not be
> failing us now: there has to be something wrong here. I can take a look at
> the code and maybe we can discuss this more together on IRC.
It works. It's just that it's not efficient. I removed the cursor.next() 
and replaced it by a cursor.available(), expecting the server to avoid 
fetching the first entry twice, but sadly, as available() returns false 
until the entry is fetched, we still fetch the entry twice...

Btw, see my second mail for the reason why we have to handle this a the 
cursor level, not on the SearchHandler level.


-- 
Regards,
Cordialement,
Emmanuel L├ęcharny
www.iktek.com


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