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Re: (ITS#5487) syncprov_findbase must search the backend from the syncrepl search

On Wed, 30 Apr 2008, Howard Chu wrote:

> rein@OpenLDAP.org wrote:

>> syncprov_findbase() must search the backend saved with the syncrepl
>> operation,
>> not the one from the operation passed as argument.  The backend in the op
>> argument can be a subordinate database, in which case the search for the
>> base in
>> the superior database will fail, and syncrepl consumers will be force to do
>> a an
>> unneccessary full refresh of the database.
> OK.
>>  The patch at the end should fix
>> this.  Note that both fop.o_bd and fop.o_bd->bd_info can be changed by the
>> overlay_op_walk() call, which is the reason for the long pointer traversal
>> to
>> find the correct bd_info to save and restore.

> But the overlay_op_walk call is only appropriate when the DB to be searched
> is the current database, and the current DB is an overlay DB structure.

Ah, the changing of the BackendDB->bd_info that takes place when
overlays are called feels like an open pit I manage to fall into every
time I get close to it...  I wish it could be replaced in a future

> Your patch causes fc->fss->s_op->o_bd's bd_info pointer to change, which is
> not allowed. That's in the original backendDB, which must be treated as
> read-only since multiple threads may be accessing it. The correct approach
> here is to use a new local backendDB variable, copy the s_op->o_bd into it,
> and then just do a regular be_search invocation instead of using
> overlay_op_walk.
> But, this patch must not take effect on the first call to syncprov_findbase
> (which occurred in syncprov_op_search) - in that case, the current code is
> correct. So, you need to tweak things based on whether (s_flags &
> PS_IS_REFRESHING) is true or not - if true, this is the first search, and it
> should use the original code. Else, it must use be_search.

A new patch that I hope should fix this is at the end.  It always use
be_search, after putting back the original bd_info if needed.  I feel
that using the generic be_search is better than interfering directly
with the overlay code as overlay_op_walk does.  I also tested for
SLAP_ISOVERLAY rather than PS_IS_REFRESHING, as that appeared more
generic to me.  But again, I may be totally wrong here.  Does this patch
look better?


Index: OpenLDAP/servers/slapd/overlays/syncprov.c
RCS file: /f/CVSROOT/drift/OpenLDAP/servers/slapd/overlays/syncprov.c,v
retrieving revision
diff -u -u -r1.1.1.18 syncprov.c
--- OpenLDAP/servers/slapd/overlays/syncprov.c	30 Apr 2008 11:17:58 -0000
+++ OpenLDAP/servers/slapd/overlays/syncprov.c	2 May 2008 11:19:46 -0000
@@ -404,7 +404,7 @@
  		slap_callback cb = {0};
  		Operation fop;
  		SlapReply frs = { REP_RESULT };
-		BackendInfo *bi;
+		BackendDB be;
  		int rc;

  		fc->fss->s_flags ^= PS_FIND_BASE;
@@ -413,10 +413,15 @@
  		fop = *fc->fss->s_op;

  		fop.o_hdr = op->o_hdr;
-		fop.o_bd = op->o_bd;
  		fop.o_time = op->o_time;
  		fop.o_tincr = op->o_tincr;
-		bi = op->o_bd->bd_info;
+		if ( SLAP_ISOVERLAY( fop.o_bd )) {
+			slap_overinst *on = (slap_overinst *)fop.o_bd->bd_info;
+			be = *fop.o_bd;
+			be.bd_info = (BackendInfo *)on->on_info;
+			fop.o_bd = &be;
+		}

  		cb.sc_response = findbase_cb;
  		cb.sc_private = fc;
@@ -434,8 +439,7 @@
  		fop.ors_filter = &generic_filter;
  		fop.ors_filterstr = generic_filterstr;

-		rc = overlay_op_walk( &fop, &frs, op_search, on->on_info, on );
-		op->o_bd->bd_info = bi;
+		rc = fop.o_bd->be_search( &fop, &frs );
  	} else {
  		ldap_pvt_thread_mutex_unlock( &fc->fss->s_mutex );
  		fc->fbase = 1;