Reduce overhead of cache-clobber testing in LookupOpclassInfo().

Commit 03ffc4d6d added logic to bypass all caching behavior in
LookupOpclassInfo when CLOBBER_CACHE_ALWAYS is enabled.  It doesn't
look like I stopped to think much about what that would cost, but
recent investigation shows that the cost is enormous: it roughly
doubles the time needed for cache-clobber test runs.

There does seem to be value in this behavior when trying to test
the opclass-cache loading logic itself, but for other purposes the
cost is excessive.  Hence, let's back off to doing this only when
debug_invalidate_system_caches_always is at least 3; or in older
branches, when CLOBBER_CACHE_RECURSIVELY is defined.

While here, clean up some other minor issues in LookupOpclassInfo.
Re-order the code so we aren't left with broken cache entries (leading
to later core dumps) in the unlikely case that we suffer OOM while
trying to allocate space for a new entry.  (That seems to be my
oversight in 03ffc4d6d.)  Also, in >= v13, stop allocating one array
entry too many.  That's evidently left over from sloppy reversion in
851b14b0c.

Back-patch to all supported branches, mainly to reduce the runtime
of cache-clobbering buildfarm animals.

Discussion: https://postgr.es/m/1370856.1625428625@sss.pgh.pa.us

diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index be50de462ed1ee37aefc9c683957db9a8f2f98bc..3680e071b1c8b2b377b2b8c083a9534a079a6396 100644 (file)
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -1612,15 +1612,15 @@ LookupOpclassInfo(Oid operatorClassOid,
        /* First time through: initialize the opclass cache */
        HASHCTL     ctl;
 
+       /* Also make sure CacheMemoryContext exists */
+       if (!CacheMemoryContext)
+           CreateCacheMemoryContext();
+
        MemSet(&ctl, 0, sizeof(ctl));
        ctl.keysize = sizeof(Oid);
        ctl.entrysize = sizeof(OpClassCacheEnt);
        OpClassCache = hash_create("Operator class cache", 64,
                                   &ctl, HASH_ELEM | HASH_BLOBS);
-
-       /* Also make sure CacheMemoryContext exists */
-       if (!CacheMemoryContext)
-           CreateCacheMemoryContext();
    }
 
    opcentry = (OpClassCacheEnt *) hash_search(OpClassCache,
@@ -1629,16 +1629,10 @@ LookupOpclassInfo(Oid operatorClassOid,
 
    if (!found)
    {
-       /* Need to allocate memory for new entry */
+       /* Initialize new entry */
        opcentry->valid = false;    /* until known OK */
        opcentry->numSupport = numSupport;
-
-       if (numSupport > 0)
-           opcentry->supportProcs = (RegProcedure *)
-               MemoryContextAllocZero(CacheMemoryContext,
-                                      numSupport * sizeof(RegProcedure));
-       else
-           opcentry->supportProcs = NULL;
+       opcentry->supportProcs = NULL;  /* filled below */
    }
    else
    {
@@ -1646,13 +1640,15 @@ LookupOpclassInfo(Oid operatorClassOid,
    }
 
    /*
-    * When testing for cache-flush hazards, we intentionally disable the
-    * operator class cache and force reloading of the info on each call. This
-    * is helpful because we want to test the case where a cache flush occurs
-    * while we are loading the info, and it's very hard to provoke that if
-    * this happens only once per opclass per backend.
+    * When aggressively testing cache-flush hazards, we disable the operator
+    * class cache and force reloading of the info on each call.  This models
+    * no real-world behavior, since the cache entries are never invalidated
+    * otherwise.  However it can be helpful for detecting bugs in the cache
+    * loading logic itself, such as reliance on a non-nailed index.  Given
+    * the limited use-case and the fact that this adds a great deal of
+    * expense, we enable it only in CLOBBER_CACHE_RECURSIVELY mode.
     */
-#if defined(CLOBBER_CACHE_ALWAYS)
+#if defined(CLOBBER_CACHE_RECURSIVELY)
    opcentry->valid = false;
 #endif
 
@@ -1660,8 +1656,15 @@ LookupOpclassInfo(Oid operatorClassOid,
        return opcentry;
 
    /*
-    * Need to fill in new entry.
-    *
+    * Need to fill in new entry.  First allocate space, unless we already did
+    * so in some previous attempt.
+    */
+   if (opcentry->supportProcs == NULL && numSupport > 0)
+       opcentry->supportProcs = (RegProcedure *)
+           MemoryContextAllocZero(CacheMemoryContext,
+                                  numSupport * sizeof(RegProcedure));
+
+   /*
     * To avoid infinite recursion during startup, force heap scans if we're
     * looking up info for the opclasses used by the indexes we would like to
     * reference here.