SQLite

int sqlite3_snapshot_get(
  sqlite3 *db, 
  const char *zDb,
  sqlite3_snapshot **ppSnapshot
){
  int rc = SQLITE_ERROR;
#ifndef SQLITE_OMIT_WAL


#ifdef SQLITE_ENABLE_API_ARMOR
  if( !sqlite3SafetyCheckOk(db) ){
    return SQLITE_MISUSE_BKPT;
  }
#endif
  sqlite3_mutex_enter(db->mutex);

  if( db->autoCommit==0 ){
    int iDb = sqlite3FindDbName(db, zDb);
    if( iDb==0 || iDb>1 ){
      Btree *pBt = db->aDb[iDb].pBt;
      if( 0==sqlite3BtreeIsInTrans(pBt) ){
        rc = sqlite3BtreeBeginTrans(pBt, 0);
        if( rc==SQLITE_OK ){
          rc = sqlite3PagerSnapshotGet(sqlite3BtreePager(pBt), ppSnapshot);
        }
      }
    }
  }

  sqlite3_mutex_leave(db->mutex);
#endif   /* SQLITE_OMIT_WAL */
  return rc;

** EXPERIMENTAL
**
** ^The [sqlite3_snapshot_get(D,S,P)] interface attempts to make a
** new [sqlite3_snapshot] object that records the current state of
** schema S in database connection D.  ^On success, the
** [sqlite3_snapshot_get(D,S,P)] interface writes a pointer to the newly
** created [sqlite3_snapshot] object into *P and returns SQLITE_OK.
** If there is not already a read-transaction open on schema S when
** this function is called, one is opened automatically. 
**
** The following must be true for this function to succeed. If any of
** the following statements are false when sqlite3_snapshot_get() is
** called, SQLITE_ERROR is returned. The final value of *P is undefined
** in this case. 
**
** <ul>
**   <li> The database handle must be in [autocommit mode].
**
**   <li> Schema S of [database connection] D must be a [WAL mode] database.
**
**   <li> There must not be a write transaction open on schema S of database
**        connection D.
**
**   <li> One or more transactions must have been written to the current wal
**        file since it was created on disk (by any connection). This means
**        that a snapshot cannot be taken on a wal mode database with no wal 
**        file immediately after it is first opened. At least one transaction
**        must be written to it first.
** </ul>
**
** This function may also return SQLITE_NOMEM.  If it is called with the
** database handle in autocommit mode but fails for some other reason, 
** whether or not a read transaction is opened on schema S is undefined.

**
** The [sqlite3_snapshot] object returned from a successful call to
** [sqlite3_snapshot_get()] must be freed using [sqlite3_snapshot_free()]
** to avoid a memory leak.
**
** The [sqlite3_snapshot_get()] interface is only available when the
** SQLITE_ENABLE_SNAPSHOT compile-time option is used.

  eval $tcl

  #-------------------------------------------------------------------------
  # Check some error conditions in snapshot_get(). It is an error if:
  #
  #  1) snapshot_get() is called on a non-WAL database, or
  #  2) there is an open write transaction on the database.
  #  3) the database handle is in auto-commit mode
  #
  do_execsql_test $tn.1.0 {
    CREATE TABLE t1(a, b);
    INSERT INTO t1 VALUES(1, 2);
    INSERT INTO t1 VALUES(3, 4);
  }

  do_test $tn.1.1.1 {
    execsql { BEGIN; SELECT * FROM t1; }
    list [catch { snapshot_get db main } msg] $msg
  } {1 SQLITE_ERROR}
  do_execsql_test $tn.1.1.2 COMMIT

  do_test $tn.1.2.1 {
    execsql {
      PRAGMA journal_mode = WAL;
      BEGIN;
        INSERT INTO t1 VALUES(5, 6);
        INSERT INTO t1 VALUES(7, 8);
    }
    list [catch { snapshot_get db main } msg] $msg
  } {1 SQLITE_ERROR}
  do_execsql_test $tn.1.2.2 COMMIT

  do_test $tn.1.3.1 {
    list [catch { snapshot_get db main } msg] $msg
  } {1 SQLITE_ERROR}
  do_test $tn.1.3.2 {
    db trans { set snap [snapshot_get db main] }
    snapshot_free $snap
  } {}

  #-------------------------------------------------------------------------
  # Check that a simple case works. Reuse the database created by the
  # block of tests above.
  #
  do_execsql_test $tn.2.1.0 {
    BEGIN;

    execsql COMMIT
    execsql COMMIT db2
    db2 close
  } {}

  do_test $tn.2.3.1 {
    execsql { DELETE FROM t1 WHERE a>6 }
    db trans { set snapshot [snapshot_get db main] }
    execsql {
      INSERT INTO t1 VALUES('a', 'b');
      INSERT INTO t1 VALUES('c', 'd');
      SELECT * FROM t1;
    }
  } {1 2 3 4 5 6 a b c d}
  do_test $tn.2.3.2 {

    execsql {
      COMMIT;
      BEGIN;
        INSERT INTO t2 VALUES('g', 'h');
    }
    list [catch {snapshot_open db main $snapshot } msg] $msg
  } {1 SQLITE_ERROR}
  do_execsql_test $tn.3.2.4 COMMIT

  do_test $tn.3.3.1 {
    execsql { PRAGMA journal_mode = DELETE }
    execsql { BEGIN }
    list [catch {snapshot_open db main $snapshot } msg] $msg
  } {1 SQLITE_ERROR}