The sqlite3BtreeInsert() routine tries to overwrite an existing cell with

modified content if the new content is the same size.  Pages are only dirtied
if they change.  This prototype works some, but still has issues.

FossilOrigin-Name: 489451b378819621537231c1c8a07704437e11c1f5384fd53b09f3977d2213a4

manifest

@@ -1,5 +1,5 @@
-C Prevent\sVACUUM\sfrom\srunning\sany\scommands\sin\ssqlite_master.sql\sother\sthan\nCREATE\sstatements.\s\sThat\sis\sall\sthat\sshould\sbe\sthere\sanyhow.\s\sThis\sfixes\na\sproblem\sdiscovered\sby\sOSSFuzz.\s\sTest\scases\sin\sTH3.
-D 2018-05-02T15:00:26.033
+C The\ssqlite3BtreeInsert()\sroutine\stries\sto\soverwrite\san\sexisting\scell\swith\nmodified\scontent\sif\sthe\snew\scontent\sis\sthe\ssame\ssize.\s\sPages\sare\sonly\sdirtied\nif\sthey\schange.\s\sThis\sprototype\sworks\ssome,\sbut\sstill\shas\sissues.
+D 2018-05-03T03:59:02.523
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F Makefile.in 5ce9343cba9c189046f1afe6d2bcc1f68079439febc05267b98aec6ecc752439
@@ -434,7 +434,7 @@ F src/auth.c 6277d63837357549fe14e723490d6dc1a38768d71c795c5eb5c0f8a99f918f73
 F src/backup.c faf17e60b43233c214aae6a8179d24503a61e83b
 F src/bitvec.c 17ea48eff8ba979f1f5b04cc484c7bb2be632f33
 F src/btmutex.c 8acc2f464ee76324bf13310df5692a262b801808984c1b79defb2503bbafadb6
-F src/btree.c fa7da0a5584e5182b92536bc4b7622a154a468997a075d5901345efb79a05ffe
+F src/btree.c d1b1bd0602381cd668fefdc21bb376083d34a202c7f978ddd6cdfe8b576880fa
 F src/btree.h 0866c0a08255142ea0e754aabd211c843cab32045c978a592a43152405ed0c84
 F src/btreeInt.h 620ab4c7235f43572cf3ac2ac8723cbdf68073be4d29da24897c7b77dda5fd96
 F src/build.c 0c2be5839f22aa2938f217c6c6c2120d9fc96872a546a37541a8271541cb355e
@@ -1727,7 +1727,10 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 0c67150749cb3d067e14b2dcac9c3489e0f14bd18c0387f1d9bc93d21fc96fe5
-R 05ae73916277803fcebbb023b759986c
+P ab0d99d0b5edece4c639baa47ce1ca2c02774cb2515e5b7f36d9bd312ccd3310
+R d19653c30e874527a8180a99ea6a4aad
+T *branch * cell-overwrite-prototype
+T *sym-cell-overwrite-prototype *
+T -sym-trunk *
 U drh
-Z 0a57dfa0969cac36c6f9349555316be2
+Z 4db008fe8cce4e65f98c2fd1a101b30c

manifest.uuid

@@ -1 +1 @@
-ab0d99d0b5edece4c639baa47ce1ca2c02774cb2515e5b7f36d9bd312ccd3310
+489451b378819621537231c1c8a07704437e11c1f5384fd53b09f3977d2213a4

src/btree.c

@@ -8152,6 +8152,83 @@ static int balance(BtCursor *pCur){
   return rc;
 }
 
+/* Overwrite content from pX into pDest.  Only do the write if the
+** content is different from what is already there.
+*/
+static int btreeOverwriteContent(
+  MemPage *pPage,           /* MemPage on which writing will occur */
+  u8 *pDest,                /* Pointer to the place to start writing */
+  const BtreePayload *pX,   /* Source of data to write */
+  int iOffset,              /* Offset of first byte to write */
+  int iAmt                  /* Number of bytes to be written */
+){
+  int nData = pX->nData - iOffset;
+  if( nData<=0 ){
+    /* Overwritting with zeros */
+    int i;
+    for(i=0; i<iAmt && pDest[i]==0; i++){}
+    if( i<iAmt ){
+      int rc = sqlite3PagerWrite(pPage->pDbPage);
+      if( rc ) return rc;
+      memset(pDest + i, 0, iAmt - i);
+    }
+  }else{
+    if( nData<iAmt ){
+      /* Mixed read data and zeros at the end.  Make a recursive call
+      ** to write the zeros then fall through to write the real data */
+      btreeOverwriteContent(pPage, pDest+nData, pX, iOffset+nData, iAmt-nData);
+      iAmt = nData;
+    }
+    if( memcmp(pDest, ((u8*)pX->pData) + iOffset, iAmt)!=0 ){
+      int rc = sqlite3PagerWrite(pPage->pDbPage);
+      if( rc ) return rc;
+      memcpy(pDest, ((u8*)pX->pData) + iOffset, iAmt);
+    }
+  }
+  return SQLITE_OK;
+}
+
+/*
+** Overwrite the cell that cursor pCur is pointing to with fresh content
+** contained in pX.
+*/
+static int btreeOverwriteCell(BtCursor *pCur, const BtreePayload *pX){
+  int iOffset;                        /* Next byte of pX->pData to write */
+  int nTotal = pX->nData + pX->nZero; /* Total bytes of to write */
+  int rc;                             /* Return code */
+  MemPage *pPage = pCur->pPage;       /* Page being written */
+  BtShared *pBt;                      /* Btree */
+  Pgno ovflPgno;                      /* Next overflow page to write */
+  u32 ovflPageSize;                   /* Size to write on overflow page */
+
+  /* Overwrite the local portion first */
+  rc = btreeOverwriteContent(pPage, pCur->info.pPayload, pX,
+                             0, pCur->info.nLocal);
+  if( rc ) return rc;
+  if( pCur->info.nLocal==nTotal ) return SQLITE_OK;
+
+  /* Now overwrite the overflow pages */
+  iOffset = pCur->info.nLocal;
+  ovflPgno = get4byte(pCur->info.pPayload + iOffset);
+  pBt = pPage->pBt;
+  ovflPageSize = pBt->usableSize - 4;
+  do{
+    rc = btreeGetPage(pBt, ovflPgno, &pPage, 0);
+    if( rc ) return rc;
+    if( iOffset+ovflPageSize>nTotal ){
+      ovflPgno = get4byte(pPage->aData + ovflPageSize);
+    }else{
+      ovflPageSize = nTotal - iOffset;
+    }
+    rc = btreeOverwriteContent(pPage, pPage->aData, pX,
+                               iOffset, ovflPageSize);
+    if( rc ) return rc;
+    iOffset += ovflPageSize;
+    sqlite3PagerUnref(pPage->pDbPage);
+  }while( iOffset<nTotal );
+  return SQLITE_OK;    
+}
+
 
 /*
 ** Insert a new record into the BTree.  The content of the new record
@@ -8250,6 +8327,11 @@ int sqlite3BtreeInsert(
     ** new row onto the end, set the "loc" to avoid an unnecessary
     ** btreeMoveto() call */
     if( (pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey ){
+      /* The current is currently pointing to the entry that is to be
+      ** overwritten */
+      if( pCur->info.nPayload==pX->nData+pX->nZero ){
+        return btreeOverwriteCell(pCur, pX);
+      }
       loc = 0;
     }else if( loc==0 ){
       rc = sqlite3BtreeMovetoUnpacked(pCur, 0, pX->nKey, flags!=0, &loc);