Fossil

    "CREATE TEMP TABLE undo_vmerge_2 AS SELECT * FROM vmerge;"
    "DELETE FROM vmerge;"
    "INSERT INTO vmerge SELECT * FROM undo_vmerge;"
    "DELETE FROM undo_vmerge;"
    "INSERT INTO undo_vmerge SELECT * FROM undo_vmerge_2;"
    "DROP TABLE undo_vmerge_2;"
  );
  if(db_exists("SELECT 1 FROM \"%w\".sqlite_master"
               " WHERE name='undo_stash'", zDb) ){
    if( redoFlag ){
      db_multi_exec(
        "DELETE FROM stash WHERE stashid IN (SELECT stashid FROM undo_stash);"
        "DELETE FROM stashfile"
        " WHERE stashid NOT IN (SELECT stashid FROM stash);"
      );
    }else{

  static const char zSql[] =
    @ DROP TABLE IF EXISTS undo;
    @ DROP TABLE IF EXISTS undo_vfile;
    @ DROP TABLE IF EXISTS undo_vmerge;
    @ DROP TABLE IF EXISTS undo_stash;
    @ DROP TABLE IF EXISTS undo_stashfile;
    ;
  db_multi_exec(zSql /*works-like:""*/);
  db_lset_int("undo_available", 0);
  db_lset_int("undo_checkout", 0);
}

/*
** The following variable stores the original command-line of the
** command that is a candidate to be undone.

/*
** Begin capturing a snapshot that can be undone.
*/
void undo_begin(void){
  int cid;
  const char *zDb = db_name("localdb");
  static const char zSql[] = 
    @ CREATE TABLE "%w".undo(
    @   pathname TEXT UNIQUE,             -- Name of the file
    @   redoflag BOOLEAN,                 -- 0 for undoable.  1 for redoable
    @   existsflag BOOLEAN,               -- True if the file exists
    @   isExe BOOLEAN,                    -- True if the file is executable
    @   isLink BOOLEAN,                   -- True if the file is symlink
    @   content BLOB                      -- Saved content
    @ );
    @ CREATE TABLE "%w".undo_vfile AS SELECT * FROM vfile;
    @ CREATE TABLE "%w".undo_vmerge AS SELECT * FROM vmerge;
  ;
  if( undoDisable ) return;
  undo_reset();
  db_multi_exec(zSql/*works-like:"%w,%w,%w"*/, zDb, zDb, zDb);
  cid = db_lget_int("checkout", 0);
  db_lset_int("undo_checkout", cid);
  db_lset_int("undo_available", 1);
  db_lset("undo_cmdline", undoCmd);
  undoActive = 1;
}

/*
** Make the current state of stashid undoable.
*/
void undo_save_stash(int stashid){
  const char *zDb = db_name("localdb");
  db_multi_exec(
    "CREATE TABLE IF NOT EXISTS \"%w\".undo_stash"
    "  AS SELECT * FROM stash WHERE 0;"
    "INSERT INTO undo_stash"
    " SELECT * FROM stash WHERE stashid=%d;",
    zDb, stashid
  );
  db_multi_exec(
    "CREATE TABLE IF NOT EXISTS \"%w\".undo_stashfile"
    "  AS SELECT * FROM stashfile WHERE 0;"
    "INSERT INTO undo_stashfile"
    " SELECT * FROM stashfile WHERE stashid=%d;",
    zDb, stashid
  );
}