Elasticsearch version (bin/elasticsearch --version): master,7.5.1,6.8.2

Plugins installed: []

JVM version (java -version): 14

OS version (uname -a if on a Unix-like system): macos 10.13.6

Description of the problem including expected versus actual behavior:

When executing Get Snapshots API, if the target snapshot does not exist, and ignore_unavailable is set to true, then the API returns all the snapshots which are in progress.

Steps to reproduce:

1. Create a snapshot which will take a long time to execute.

PUT _snapshot/my_repository/snap1
{
 "indices":["y"]
}

2. Get an unknown snapshot, and set ignore_unavailable to true

GET _snapshot/my_repository/xxx?ignore_unavailable=true

then you can see the snap1 existed in the response.