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Commit4f91af8

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Fix dangling smgr_owner pointer when a fake relcache entry is freed.
A fake relcache entry can "own" a SmgrRelation object, like a regularrelcache entry. But when it was free'd, the owner field in SmgrRelationwas not cleared, so it was left pointing to free'd memory.Amazingly this apparently hasn't caused crashes in practice, or we would'veheard about it earlier. Andres found this with Valgrind.Report and fix by Andres Freund, with minor modifications by me. Backpatchto all supported versions.
1 parent8332fe7 commit4f91af8

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3 files changed

+42
-4
lines changed

3 files changed

+42
-4
lines changed

‎src/backend/access/transam/xlogutils.c

Lines changed: 3 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -445,6 +445,9 @@ CreateFakeRelcacheEntry(RelFileNode rnode)
445445
void
446446
FreeFakeRelcacheEntry(Relationfakerel)
447447
{
448+
/* make sure the fakerel is not referenced by the SmgrRelation anymore */
449+
if (fakerel->rd_smgr!=NULL)
450+
smgrclearowner(&fakerel->rd_smgr,fakerel->rd_smgr);
448451
pfree(fakerel);
449452
}
450453

‎src/backend/storage/smgr/smgr.c

Lines changed: 38 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -84,6 +84,7 @@ static SMgrRelation first_unowned_reln = NULL;
8484

8585
/* local function prototypes */
8686
staticvoidsmgrshutdown(intcode,Datumarg);
87+
staticvoidadd_to_unowned_list(SMgrRelationreln);
8788
staticvoidremove_from_unowned_list(SMgrRelationreln);
8889

8990

@@ -174,9 +175,8 @@ smgropen(RelFileNode rnode, BackendId backend)
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for (forknum=0;forknum <=MAX_FORKNUM;forknum++)
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reln->md_fd[forknum]=NULL;
176177

177-
/* place it at head of unowned list (to make smgrsetowner cheap) */
178-
reln->next_unowned_reln=first_unowned_reln;
179-
first_unowned_reln=reln;
178+
/* it has no owner yet */
179+
add_to_unowned_list(reln);
180180
}
181181

182182
returnreln;
@@ -191,7 +191,7 @@ smgropen(RelFileNode rnode, BackendId backend)
191191
void
192192
smgrsetowner(SMgrRelation*owner,SMgrRelationreln)
193193
{
194-
/* We don'tcurrentlysupport "disowning" an SMgrRelation here */
194+
/* We don't support "disowning" an SMgrRelation here, use smgrclearowner */
195195
Assert(owner!=NULL);
196196

197197
/*
@@ -213,6 +213,40 @@ smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
213213
*owner=reln;
214214
}
215215

216+
/*
217+
* smgrclearowner() -- Remove long-lived reference to an SMgrRelation object
218+
* if one exists
219+
*/
220+
void
221+
smgrclearowner(SMgrRelation*owner,SMgrRelationreln)
222+
{
223+
/* Do nothing if the SMgrRelation object is not owned by the owner */
224+
if (reln->smgr_owner!=owner)
225+
return;
226+
227+
/* unset the owner's reference */
228+
*owner=NULL;
229+
230+
/* unset our reference to the owner */
231+
reln->smgr_owner=NULL;
232+
233+
add_to_unowned_list(reln);
234+
}
235+
236+
/*
237+
* add_to_unowned_list -- link an SMgrRelation onto the unowned list
238+
*
239+
* Check remove_from_unowned_list()'s comments for performance
240+
* considerations.
241+
*/
242+
staticvoid
243+
add_to_unowned_list(SMgrRelationreln)
244+
{
245+
/* place it at head of the list (to make smgrsetowner cheap) */
246+
reln->next_unowned_reln=first_unowned_reln;
247+
first_unowned_reln=reln;
248+
}
249+
216250
/*
217251
* remove_from_unowned_list -- unlink an SMgrRelation from the unowned list
218252
*

‎src/include/storage/smgr.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -80,6 +80,7 @@ extern void smgrinit(void);
8080
externSMgrRelationsmgropen(RelFileNodernode,BackendIdbackend);
8181
externboolsmgrexists(SMgrRelationreln,ForkNumberforknum);
8282
externvoidsmgrsetowner(SMgrRelation*owner,SMgrRelationreln);
83+
externvoidsmgrclearowner(SMgrRelation*owner,SMgrRelationreln);
8384
externvoidsmgrclose(SMgrRelationreln);
8485
externvoidsmgrcloseall(void);
8586
externvoidsmgrclosenode(RelFileNodeBackendrnode);

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