Change the definition of SQLITE_CONFIG_SCRATCH so that at most one scratch

buffer is used per thread.  Use the generic heap memory allocator for the
WalIterator object when running a checkpoint.

FossilOrigin-Name: 391c9b85abcb5ba300fb2e116384639310c69ed2

src/btree.c

@@ -6650,12 +6650,9 @@ static int balance_nonroot(
      + nMaxCells*sizeof(u16)                       /* szCell */
      + pBt->pageSize;                              /* aSpace1 */
 
-  /* EVIDENCE-OF: R-37926-08392 SQLite will never request a scratch buffer
-  ** that is more than 6 times the database page size, except when
-  ** performing a checkpoint in WAL mode when the scratch buffer request
-  ** size is a small fraction of the size of the WAL file. */
+  /* EVIDENCE-OF: R-28375-38319 SQLite will never request a scratch buffer
+  ** that is more than 6 times the database page size. */
   assert( szScratch<=6*pBt->pageSize );
-
   apCell = sqlite3ScratchMalloc( szScratch ); 
   if( apCell==0 ){
     rc = SQLITE_NOMEM;

src/malloc.c

@@ -377,11 +377,12 @@ void *sqlite3ScratchMalloc(int n){
 
 
 #if SQLITE_THREADSAFE==0 && !defined(NDEBUG)
-  /* Verify that no more than two scratch allocations per thread
-  ** are outstanding at one time.  (This is only checked in the
-  ** single-threaded case since checking in the multi-threaded case
-  ** would be much more complicated.) */
-  assert( scratchAllocOut<=1 );
+  /* EVIDENCE-OF: R-12970-05880 SQLite will not use more than one scratch
+  ** buffers per thread.
+  **
+  ** This can only be checked in single-threaded mode.
+  */
+  assert( scratchAllocOut==0 );
   if( p ) scratchAllocOut++;
 #endif
 

src/sqlite.h.in

@@ -1541,13 +1541,9 @@ struct sqlite3_mem_methods {
 ** and the maximum number of scratch allocations (N).)^
 ** The first argument must be a pointer to an 8-byte aligned buffer
 ** of at least sz*N bytes of memory.
-** ^SQLite will not use more than two scratch buffers per thread and not
-** more than one scratch buffer per thread when not performing
-** a [checkpoint] in [WAL mode].
+** ^SQLite will not use more than one scratch buffers per thread.
 ** ^SQLite will never request a scratch buffer that is more than 6
-** times the database page size, except when performing a [checkpoint]
-** in [WAL mode] when the scratch buffer request size is a small fraction
-** of the size of the WAL file.
+** times the database page size.
 ** ^If SQLite needs needs additional
 ** scratch memory beyond what is provided by this configuration option, then 
 ** [sqlite3_malloc()] will be used to obtain the memory needed.<p>

src/wal.c

@@ -1504,7 +1504,7 @@ static void walMergesort(
 ** Free an iterator allocated by walIteratorInit().
 */
 static void walIteratorFree(WalIterator *p){
-  sqlite3ScratchFree(p);
+  sqlite3_free(p);
 }
 
 /*
@@ -1539,7 +1539,7 @@ static int walIteratorInit(Wal *pWal, WalIterator **pp){
   nByte = sizeof(WalIterator) 
         + (nSegment-1)*sizeof(struct WalSegment)
         + iLast*sizeof(ht_slot);
-  p = (WalIterator *)sqlite3ScratchMalloc(nByte);
+  p = (WalIterator *)sqlite3_malloc(nByte);
   if( !p ){
     return SQLITE_NOMEM;
   }
@@ -1549,7 +1549,7 @@ static int walIteratorInit(Wal *pWal, WalIterator **pp){
   /* Allocate temporary space used by the merge-sort routine. This block
   ** of memory will be freed before this function returns.
   */
-  aTmp = (ht_slot *)sqlite3ScratchMalloc(
+  aTmp = (ht_slot *)sqlite3_malloc(
       sizeof(ht_slot) * (iLast>HASHTABLE_NPAGE?HASHTABLE_NPAGE:iLast)
   );
   if( !aTmp ){
@@ -1586,7 +1586,7 @@ static int walIteratorInit(Wal *pWal, WalIterator **pp){
       p->aSegment[i].aPgno = (u32 *)aPgno;
     }
   }
-  sqlite3ScratchFree(aTmp);
+  sqlite3_free(aTmp);
 
   if( rc!=SQLITE_OK ){
     walIteratorFree(p);