Fix cache flush hazard in event trigger cache.

Bug spotted by Jeff Davis using -DCLOBBER_CACHE_ALWAYS.

diff --git a/src/backend/utils/cache/evtcache.c b/src/backend/utils/cache/evtcache.c
index fc9f694e5a78999e1b5be96a3b824ec2488b6cf0..258b5001ee3b115c5f1e4d166e507b7e51973370 100644 (file)
--- a/src/backend/utils/cache/evtcache.c
+++ b/src/backend/utils/cache/evtcache.c
@@ -29,6 +29,13 @@
 #include "utils/snapmgr.h"
 #include "utils/syscache.h"
 
+typedef enum
+{
+   ETCS_NEEDS_REBUILD,
+   ETCS_REBUILD_STARTED,
+   ETCS_VALID
+} EventTriggerCacheStateType;
+
 typedef struct
 {
    EventTriggerEvent   event;
@@ -37,6 +44,7 @@ typedef struct
 
 static HTAB *EventTriggerCache;
 static MemoryContext EventTriggerCacheContext;
+static EventTriggerCacheStateType EventTriggerCacheState = ETCS_NEEDS_REBUILD;
 
 static void BuildEventTriggerCache(void);
 static void InvalidateEventCacheCallback(Datum arg,
@@ -55,7 +63,7 @@ EventCacheLookup(EventTriggerEvent event)
 {
    EventTriggerCacheEntry *entry;
 
-   if (EventTriggerCache == NULL)
+   if (EventTriggerCacheState != ETCS_VALID)
        BuildEventTriggerCache();
    entry = hash_search(EventTriggerCache, &event, HASH_FIND, NULL);
    return entry != NULL ? entry->triggerlist : NULL;
@@ -77,12 +85,9 @@ BuildEventTriggerCache(void)
    if (EventTriggerCacheContext != NULL)
    {
        /*
-        * The cache has been previously built, and subsequently invalidated,
-        * and now we're trying to rebuild it.  Normally, there won't be
-        * anything in the context at this point, because the invalidation
-        * will have already reset it.  But if the previous attempt to rebuild
-        * the cache failed, then there might be some junk lying around
-        * that we want to reclaim.
+        * Free up any memory already allocated in EventTriggerCacheContext.
+        * This can happen either because a previous rebuild failed, or
+        * because an invalidation happened before the rebuild was complete.
         */
        MemoryContextResetAndDeleteChildren(EventTriggerCacheContext);
    }
@@ -109,12 +114,10 @@ BuildEventTriggerCache(void)
    /* Switch to correct memory context. */
    oldcontext = MemoryContextSwitchTo(EventTriggerCacheContext);
 
-   /*
-    * Create a new hash table, but don't assign it to the global variable
-    * until it accurately represents the state of the catalogs, so that
-    * if we fail midway through this we won't end up with incorrect cache
-    * contents.
-    */
+   /* Prevent the memory context from being nuked while we're rebuilding. */
+   EventTriggerCacheState = ETCS_REBUILD_STARTED;
+
+   /* Create new hash table. */
    MemSet(&ctl, 0, sizeof(ctl));
    ctl.keysize = sizeof(EventTriggerEvent);
    ctl.entrysize = sizeof(EventTriggerCacheEntry);
@@ -195,8 +198,17 @@ BuildEventTriggerCache(void)
    /* Restore previous memory context. */
    MemoryContextSwitchTo(oldcontext);
 
-   /* Cache is now valid. */
+   /* Install new cache. */
    EventTriggerCache = cache;
+
+   /*
+    * If the cache has been invalidated since we entered this routine, we
+    * still use and return the cache we just finished constructing, to avoid
+    * infinite loops, but we leave the cache marked stale so that we'll
+    * rebuild it again on next access.  Otherwise, we mark the cache valid.
+    */
+   if (EventTriggerCacheState == ETCS_REBUILD_STARTED)
+       EventTriggerCacheState = ETCS_VALID;
 }
 
 /*
@@ -238,6 +250,17 @@ DecodeTextArrayToCString(Datum array, char ***cstringp)
 static void
 InvalidateEventCacheCallback(Datum arg, int cacheid, uint32 hashvalue)
 {
-   MemoryContextResetAndDeleteChildren(EventTriggerCacheContext);
-   EventTriggerCache = NULL;
+   /*
+    * If the cache isn't valid, then there might be a rebuild in progress,
+    * so we can't immediately blow it away.  But it's advantageous to do
+    * this when possible, so as to immediately free memory.
+    */
+   if (EventTriggerCacheState == ETCS_VALID)
+   {
+       MemoryContextResetAndDeleteChildren(EventTriggerCacheContext);
+       EventTriggerCache = NULL;
+   }
+
+   /* Mark cache for rebuild. */
+   EventTriggerCacheState = ETCS_NEEDS_REBUILD;
 }