I tried to find a REST API to allow to reset community user's password, and send emai to user. 
I tried DELETE sobjects/User/<user id>/password from https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_sobject_user_password.htm. It changed the password and didn't send emai. 

Not sure if there is a way to do that. Thanks in advance. 
 

We are using REST API to authenticate user. It works pretty well, until we changed setting "Permitted Users" in the connected Apps to 
"Admin approved users are pre-authorized"

Our app still works for "approved users", but for un-approved users, OAuth calls back to the "callback URL" with error:

https://our_web_app.com/loginsuccessful?error=OAUTH_APP_ACCESS_DENIED&error_description=user+is+not+admin+approved+to+access+this+app&state=%2F

Since this is an error, our app redirects to the salesforce login page, but saleforce redirect to the above callback url immediately, it becomes an infinte loop.

The question here is how to properly display salesforce login again to correct username/password
 

Is it possiblle to have set up trigger using REST API, when when condition meets, salesforce will callback to my service?
We are creating a service that will notify user when certain data in salesforce changed, the other option, not ideal, is we can have a worker that periodically poll the data to check the changes.

Is it possiblle to have set up trigger using REST API, when when condition meets, salesforce will callback to my service?
We are creating a service that will notify user when certain data in salesforce changed, the other option, not ideal, is we can have a worker that periodically poll the data to check the changes.