Keep track of the optimal number of reserved bytes (by looking at reserve

byte requests in calls to sqlite3BtreeSetPageSize()) and then change the
reserve byte count to the optimal when doing a VACUUM or when using the
backup API.

FossilOrigin-Name: 28c2b726285ea88b334acfd6390a057d2d244838

7 files changed, 32 insertions, 15 deletions

diff --git a/src/attach.c b/src/attach.c
index de8742938..7e35fa67c 100644
--- a/src/attach.c
+++ b/src/attach.c
@@ -191,7 +191,7 @@ static void attachFunc(
       case SQLITE_NULL:
         /* No key specified.  Use the key from the main database */
         sqlite3CodecGetKey(db, 0, (void**)&zKey, &nKey);
-        if( nKey>0 || sqlite3BtreeGetReserve(db->aDb[0].pBt)>0 ){
+        if( nKey>0 || sqlite3BtreeGetOptimalReserve(db->aDb[0].pBt)>0 ){
           rc = sqlite3CodecAttach(db, db->nDb-1, zKey, nKey);
         }
         break;
diff --git a/src/backup.c b/src/backup.c
index e3f869035..81c8b5c5f 100644
--- a/src/backup.c
+++ b/src/backup.c
@@ -247,7 +247,7 @@ static int backupOnePage(
   ** guaranteed that the shared-mutex is held by this thread, handle
   ** p->pSrc may not actually be the owner.  */
   int nSrcReserve = sqlite3BtreeGetReserveNoMutex(p->pSrc);
-  int nDestReserve = sqlite3BtreeGetReserve(p->pDest);
+  int nDestReserve = sqlite3BtreeGetOptimalReserve(p->pDest);
 #endif
   int rc = SQLITE_OK;
   i64 iOff;
diff --git a/src/btree.c b/src/btree.c
index eb5151351..8957b74c1 100644
--- a/src/btree.c
+++ b/src/btree.c
@@ -2405,6 +2405,9 @@ int sqlite3BtreeSetPageSize(Btree *p, int pageSize, int nReserve, int iFix){
   BtShared *pBt = p->pBt;
   assert( nReserve>=-1 && nReserve<=255 );
   sqlite3BtreeEnter(p);
+#if SQLITE_HAS_CODEC
+  if( nReserve>pBt->optimalReserve ) pBt->optimalReserve = (u8)nReserve;
+#endif
   if( pBt->btsFlags & BTS_PAGESIZE_FIXED ){
     sqlite3BtreeLeave(p);
     return SQLITE_READONLY;
@@ -2434,7 +2437,6 @@ int sqlite3BtreeGetPageSize(Btree *p){
   return p->pBt->pageSize;
 }
 
-#if defined(SQLITE_HAS_CODEC) || defined(SQLITE_DEBUG)
 /*
 ** This function is similar to sqlite3BtreeGetReserve(), except that it
 ** may only be called if it is guaranteed that the b-tree mutex is already
@@ -2447,25 +2449,33 @@ int sqlite3BtreeGetPageSize(Btree *p){
 ** database handle that owns *p, causing undefined behavior.
 */
 int sqlite3BtreeGetReserveNoMutex(Btree *p){
+  int n;
   assert( sqlite3_mutex_held(p->pBt->mutex) );
-  return p->pBt->pageSize - p->pBt->usableSize;
+  n = p->pBt->pageSize - p->pBt->usableSize;
+  return n;
 }
-#endif /* SQLITE_HAS_CODEC || SQLITE_DEBUG */
 
-#if !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM)
 /*
 ** Return the number of bytes of space at the end of every page that
 ** are intentually left unused.  This is the "reserved" space that is
 ** sometimes used by extensions.
+**
+** If SQLITE_HAS_MUTEX is defined then the number returned is the
+** greater of the current reserved space and the maximum requested
+** reserve space.
 */
-int sqlite3BtreeGetReserve(Btree *p){
+int sqlite3BtreeGetOptimalReserve(Btree *p){
   int n;
   sqlite3BtreeEnter(p);
-  n = p->pBt->pageSize - p->pBt->usableSize;
+  n = sqlite3BtreeGetReserveNoMutex(p);
+#ifdef SQLITE_HAS_CODEC
+  if( n<p->pBt->optimalReserve ) n = p->pBt->optimalReserve;
+#endif
   sqlite3BtreeLeave(p);
   return n;
 }
 
+
 /*
 ** Set the maximum page count for a database if mxPage is positive.
 ** No changes are made if mxPage is 0 or negative.
@@ -2496,7 +2506,6 @@ int sqlite3BtreeSecureDelete(Btree *p, int newFlag){
   sqlite3BtreeLeave(p);
   return b;
 }
-#endif /* !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM) */
 
 /*
 ** Change the 'auto-vacuum' property of the database. If the 'autoVacuum'
diff --git a/src/btree.h b/src/btree.h
index b57d500c7..77d12f78d 100644
--- a/src/btree.h
+++ b/src/btree.h
@@ -73,10 +73,8 @@ int sqlite3BtreeGetPageSize(Btree*);
 int sqlite3BtreeMaxPageCount(Btree*,int);
 u32 sqlite3BtreeLastPage(Btree*);
 int sqlite3BtreeSecureDelete(Btree*,int);
-int sqlite3BtreeGetReserve(Btree*);
-#if defined(SQLITE_HAS_CODEC) || defined(SQLITE_DEBUG)
+int sqlite3BtreeGetOptimalReserve(Btree*);
 int sqlite3BtreeGetReserveNoMutex(Btree *p);
-#endif
 int sqlite3BtreeSetAutoVacuum(Btree *, int);
 int sqlite3BtreeGetAutoVacuum(Btree *);
 int sqlite3BtreeBeginTrans(Btree*,int);
diff --git a/src/btreeInt.h b/src/btreeInt.h
index ed4d75ee9..87d0ef1bb 100644
--- a/src/btreeInt.h
+++ b/src/btreeInt.h
@@ -418,6 +418,9 @@ struct BtShared {
 #endif
   u8 inTransaction;     /* Transaction state */
   u8 max1bytePayload;   /* Maximum first byte of cell for a 1-byte payload */
+#ifdef SQLITE_HAS_CODEC
+  u8 optimalReserve;    /* Desired amount of reserved space per page */
+#endif
   u16 btsFlags;         /* Boolean parameters.  See BTS_* macros below */
   u16 maxLocal;         /* Maximum local payload in non-LEAFDATA tables */
   u16 minLocal;         /* Minimum local payload in non-LEAFDATA tables */
diff --git a/src/test_stat.c b/src/test_stat.c
index 615df3d80..daa84de2c 100644
--- a/src/test_stat.c
+++ b/src/test_stat.c
@@ -301,8 +301,11 @@ static int statDecodePage(Btree *pBt, StatPage *p){
 
   if( p->nCell ){
     int i;                        /* Used to iterate through cells */
-    int nUsable = szPage - sqlite3BtreeGetReserve(pBt);
+    int nUsable;                  /* Usable bytes per page */
 
+    sqlite3BtreeEnter(pBt);
+    nUsable = szPage - sqlite3BtreeGetReserveNoMutex(pBt);
+    sqlite3BtreeLeave(pBt);
     p->aCell = sqlite3_malloc((p->nCell+1) * sizeof(StatCell));
     memset(p->aCell, 0, (p->nCell+1) * sizeof(StatCell));
 
@@ -425,7 +428,11 @@ statNextRestart:
     while( p->iCell<p->nCell ){
       StatCell *pCell = &p->aCell[p->iCell];
       if( pCell->iOvfl<pCell->nOvfl ){
-        int nUsable = sqlite3BtreeGetPageSize(pBt)-sqlite3BtreeGetReserve(pBt);
+        int nUsable;
+        sqlite3BtreeEnter(pBt);
+        nUsable = sqlite3BtreeGetPageSize(pBt) - 
+                        sqlite3BtreeGetReserveNoMutex(pBt);
+        sqlite3BtreeLeave(pBt);
         pCsr->zName = (char *)sqlite3_column_text(pCsr->pStmt, 0);
         pCsr->iPageno = pCell->aOvfl[pCell->iOvfl];
         pCsr->zPagetype = "overflow";
diff --git a/src/vacuum.c b/src/vacuum.c
index 9df8e08b2..dca43e217 100644
--- a/src/vacuum.c
+++ b/src/vacuum.c
@@ -184,7 +184,7 @@ int sqlite3RunVacuum(char **pzErrMsg, sqlite3 *db){
   ** cause problems for the call to BtreeSetPageSize() below.  */
   sqlite3BtreeCommit(pTemp);
 
-  nRes = sqlite3BtreeGetReserve(pMain);
+  nRes = sqlite3BtreeGetOptimalReserve(pMain);
 
   /* A VACUUM cannot change the pagesize of an encrypted database. */
 #ifdef SQLITE_HAS_CODEC