[tcl] sqlite db eval locked fail 解决方法

yjh777

操作一个sqlite db，有读进程 有写进程，时常会出现 db eval {sql} 时出先数据库已加锁的问题
如果，显式的做错误处理，代码太复杂了。搜索官方文档终于找到答案:

可以设定一个 timeout时间，如果不设定的话不等待锁释放，直接退出。


详情: https://www.sqlite.org/tclsqlite.html

The "timeout" method

The "timeout" method is used to control how long the SQLite library will wait for locks to clear before giving up on a database transaction. The default timeout is 0 millisecond. (In other words, the default behavior is not to wait at all.)

The SQLite database allows multiple simultaneous readers or a single writer but not both. If any process is writing to the database no other process is allows to read or write. If any process is reading the database other processes are allowed to read but not write. The entire database shared a single lock.

When SQLite tries to open a database and finds that it is locked, it can optionally delay for a short while and try to open the file again. This process repeats until the query times out and SQLite returns a failure. The timeout is adjustable. It is set to 0 by default so that if the database is locked, the SQL statement fails immediately. But you can use the "timeout" method to change the timeout value to a positive number. For example:

    db1 timeout 2000

The argument to the timeout method is the maximum number of milliseconds to wait for the lock to clear. So in the example above, the maximum delay would be 2 seconds.
The "busy" method

The "busy" method, like "timeout", only comes into play when the database is locked. But the "busy" method gives the programmer much more control over what action to take. The "busy" method specifies a callback Tcl procedure that is invoked whenever SQLite tries to open a locked database. This callback can do whatever is desired. Presumably, the callback will do some other useful work for a short while (such as service GUI events) then return so that the lock can be tried again. The callback procedure should return "0" if it wants SQLite to try again to open the database and should return "1" if it wants SQLite to abandon the current operation.