1 files changed, 41 insertions, 18 deletions

diff --git a/src/pcache1.c b/src/pcache1.c
index 4f84a20b5..a47087fa1 100644
--- a/src/pcache1.c
+++ b/src/pcache1.c
@@ -106,19 +106,24 @@ typedef struct PGroup PGroup;
 ** corrupt, code at the b-tree layer may overread the page buffer and 
 ** read part of this structure before the corruption is detected. This
 ** can cause a valgrind error if the unitialized gap is accessed. Using u16
-** ensures there is no such gap, and therefore no bytes of unitialized memory
-** in the structure.
+** ensures there is no such gap, and therefore no bytes of uninitialized
+** memory in the structure.
+**
+** The pLruNext and pLruPrev pointers form a double-linked circular list
+** of all pages that are unpinned.  The PGroup.lru element (which should be
+** the only element on the list with PgHdr1.isAnchor set to 1) forms the
+** beginning and the end of the list.
 */
 struct PgHdr1 {
-  sqlite3_pcache_page page;      /* Base class. Must be first. pBuf & pExtra */
-  unsigned int iKey;             /* Key value (page number) */
-  u16 isBulkLocal;               /* This page from bulk local storage */
-  u16 isAnchor;                  /* This is the PGroup.lru element */
-  PgHdr1 *pNext;                 /* Next in hash table chain */
-  PCache1 *pCache;               /* Cache that currently owns this page */
-  PgHdr1 *pLruNext;              /* Next in LRU list of unpinned pages */
-  PgHdr1 *pLruPrev;              /* Previous in LRU list of unpinned pages */
-                                 /* NB: pLruPrev is only valid if pLruNext!=0 */
+  sqlite3_pcache_page page; /* Base class. Must be first. pBuf & pExtra */
+  unsigned int iKey;        /* Key value (page number) */
+  u16 isBulkLocal;          /* This page from bulk local storage */
+  u16 isAnchor;             /* This is the PGroup.lru element */
+  PgHdr1 *pNext;            /* Next in hash table chain */
+  PCache1 *pCache;          /* Cache that currently owns this page */
+  PgHdr1 *pLruNext;         /* Next in circular LRU list of unpinned pages */
+  PgHdr1 *pLruPrev;         /* Previous in LRU list of unpinned pages */
+                            /* NB: pLruPrev is only valid if pLruNext!=0 */
 };
 
 /*
@@ -1114,23 +1119,41 @@ static void pcache1Rekey(
   PCache1 *pCache = (PCache1 *)p;
   PgHdr1 *pPage = (PgHdr1 *)pPg;
   PgHdr1 **pp;
-  unsigned int h; 
+  unsigned int hOld, hNew; 
   assert( pPage->iKey==iOld );
   assert( pPage->pCache==pCache );
+  assert( iOld!=iNew );               /* The page number really is changing */
 
   pcache1EnterMutex(pCache->pGroup);
-
-  h = iOld%pCache->nHash;
-  pp = &pCache->apHash[h];
+  
+  assert( pcache1FetchNoMutex(p, iOld, 0)==pPage ); /* pPg really is iOld */
+  hOld = iOld%pCache->nHash;
+  pp = &pCache->apHash[hOld];
   while( (*pp)!=pPage ){
     pp = &(*pp)->pNext;
   }
   *pp = pPage->pNext;
 
-  h = iNew%pCache->nHash;
+  hNew = iNew%pCache->nHash;
+  pp = &pCache->apHash[hNew];
+  while( *pp ){
+    if( (*pp)->iKey==iNew ){
+      /* If there is already another pcache entry at iNew, change it to iOld,
+      ** thus swapping the positions of iNew and iOld */
+      PgHdr1 *pOld = *pp;
+      *pp = pOld->pNext;
+      pOld->pNext = pCache->apHash[hOld];
+      pCache->apHash[hOld] = pOld;
+      pOld->iKey = iOld;
+      break;
+    }else{
+      pp = &(*pp)->pNext;
+    }
+  }
+
   pPage->iKey = iNew;
-  pPage->pNext = pCache->apHash[h];
-  pCache->apHash[h] = pPage;
+  pPage->pNext = pCache->apHash[hNew];
+  pCache->apHash[hNew] = pPage;
   if( iNew>pCache->iMaxKey ){
     pCache->iMaxKey = iNew;
   }