SQLite

** This is an eponymous virtual table so it does not need to be created before
** use.  The optional argument to the sqlite_dbpage() table name is the
** schema for the database file that is to be read.  The default schema is
** "main".
**
** The data field of sqlite_dbpage table can be updated.  The new
** value must be a BLOB which is the correct page size, otherwise the
** update fails.  INSERT operations also work, and operate as if they
** where REPLACE.  The size of the database can be extended by INSERT-ing
** new pages on the end.
**
** Rows may not be deleted.  However, doing an INSERT to page number N
** with NULL page data causes the N-th page and all subsequent pages to be
** deleted and the database to be truncated.
*/

#include "sqliteInt.h"   /* Requires access to internal data structures */
#if (defined(SQLITE_ENABLE_DBPAGE_VTAB) || defined(SQLITE_TEST)) \
    && !defined(SQLITE_OMIT_VIRTUALTABLE)

typedef struct DbpageTable DbpageTable;

  int iDb;                        /* Index of database to analyze */
  int szPage;                     /* Size of each page in bytes */
};

struct DbpageTable {
  sqlite3_vtab base;              /* Base class.  Must be first */
  sqlite3 *db;                    /* The database */
  int nTrunc;                     /* Entries in aTrunc[] */
  Pgno *aTrunc;                   /* Truncation size for each database */
};

/* Columns */
#define DBPAGE_COLUMN_PGNO    0
#define DBPAGE_COLUMN_DATA    1
#define DBPAGE_COLUMN_SCHEMA  2



/*
** Connect to or create a dbpagevfs virtual table.
*/
static int dbpageConnect(
  sqlite3 *db,

  return rc;
}

/*
** Disconnect from or destroy a dbpagevfs virtual table.
*/
static int dbpageDisconnect(sqlite3_vtab *pVtab){
  DbpageTable *pTab = (DbpageTable *)pVtab;
  sqlite3_free(pTab->aTrunc);
  sqlite3_free(pVtab);
  return SQLITE_OK;
}

/*
** idxNum:
**

  DbPage *pDbPage = 0;
  int rc = SQLITE_OK;
  char *zErr = 0;
  int iDb;
  Btree *pBt;
  Pager *pPager;
  int szPage;
  int isInsert;

  (void)pRowid;
  if( pTab->db->flags & SQLITE_Defensive ){
    zErr = "read-only";
    goto update_fail;
  }
  if( argc==1 ){
    zErr = "cannot delete";
    goto update_fail;
  }
  if( sqlite3_value_type(argv[0])==SQLITE_NULL ){
    pgno = (Pgno)sqlite3_value_int(argv[2]);
    isInsert = 1;
  }else{
    pgno = sqlite3_value_int(argv[0]);
    if( (Pgno)sqlite3_value_int(argv[1])!=pgno ){
      zErr = "cannot insert";
      goto update_fail;
    }
    isInsert = 0;
  }
  if( sqlite3_value_type(argv[4])==SQLITE_NULL ){
    iDb = 0;
  }else{
    const char *zSchema = (const char*)sqlite3_value_text(argv[4]);
    iDb = sqlite3FindDbName(pTab->db, zSchema);
    if( iDb<0 ){
      zErr = "no such schema";
      goto update_fail;
    }
  }
  pBt = pTab->db->aDb[iDb].pBt;
  if( pgno<1 || NEVER(pBt==0) ){
    zErr = "bad page number";
    goto update_fail;
  }
  szPage = sqlite3BtreeGetPageSize(pBt);
  if( sqlite3_value_type(argv[3])!=SQLITE_BLOB 
   || sqlite3_value_bytes(argv[3])!=szPage
  ){
    if( sqlite3_value_type(argv[3])==SQLITE_NULL && isInsert ){
      if( iDb>=pTab->nTrunc ){
        testcase( pTab->aTrunc!=0 );
        pTab->aTrunc = sqlite3_realloc(pTab->aTrunc, (iDb+1)*sizeof(Pgno));
        if( pTab->aTrunc ){
          int j;
          for(j=pTab->nTrunc; j<iDb; j++) pTab->aTrunc[j] = 0;
          pTab->nTrunc = iDb+1;
        }else{
          return SQLITE_NOMEM;
        }
      }
      pTab->aTrunc[iDb] = pgno;
    }else{
      zErr = "bad page value";
      goto update_fail;
    }
  }
  pPager = sqlite3BtreePager(pBt);
  rc = sqlite3PagerGet(pPager, pgno, (DbPage**)&pDbPage, 0);
  if( rc==SQLITE_OK ){
    const void *pData = sqlite3_value_blob(argv[3]);


    if( (rc = sqlite3PagerWrite(pDbPage))==SQLITE_OK && pData ){

      unsigned char *aPage = sqlite3PagerGetData(pDbPage);
      memcpy(aPage, pData, szPage);
    }
  }
  sqlite3PagerUnref(pDbPage);
  return rc;

update_fail:
  sqlite3_free(pVtab->zErrMsg);

static int dbpageBegin(sqlite3_vtab *pVtab){
  DbpageTable *pTab = (DbpageTable *)pVtab;
  sqlite3 *db = pTab->db;
  int i;
  for(i=0; i<db->nDb; i++){
    Btree *pBt = db->aDb[i].pBt;
    if( pBt ) (void)sqlite3BtreeBeginTrans(pBt, 1, 0);
  }
  if( pTab->nTrunc>0 ){
    memset(pTab->aTrunc, 0, sizeof(pTab->aTrunc[0])*pTab->nTrunc);
  }
  return SQLITE_OK;
}

/* Invoke sqlite3PagerTruncate() as necessary, just prior to COMMIT
*/
static int dbpageSync(sqlite3_vtab *pVtab){
  int iDb;
  DbpageTable *pTab = (DbpageTable *)pVtab;

  for(iDb=0; iDb<pTab->nTrunc; iDb++){
    if( pTab->aTrunc[iDb]>0 ){
      Btree *pBt = pTab->db->aDb[iDb].pBt;
      Pager *pPager = sqlite3BtreePager(pBt);
      sqlite3PagerTruncateImage(pPager, pTab->aTrunc[iDb]);
      pTab->aTrunc[iDb] = 0;
    }
  }
  return SQLITE_OK;
}


/*
** Invoke this routine to register the "dbpage" virtual table module

    dbpageFilter,                 /* xFilter - configure scan constraints */
    dbpageNext,                   /* xNext - advance a cursor */
    dbpageEof,                    /* xEof - check for end of scan */
    dbpageColumn,                 /* xColumn - read data */
    dbpageRowid,                  /* xRowid - read data */
    dbpageUpdate,                 /* xUpdate */
    dbpageBegin,                  /* xBegin */
    dbpageSync,                   /* xSync */
    0,                            /* xCommit */
    0,                            /* xRollback */
    0,                            /* xFindMethod */
    0,                            /* xRename */
    0,                            /* xSavepoint */
    0,                            /* xRelease */
    0,                            /* xRollbackTo */