Get Snapshot API: Listing all snapshots across all repositories does not work

[spinscale]

Elasticsearch version (bin/elasticsearch --version): 7.13.4

Description of the problem including expected versus actual behavior:

The documentation for the GET Snapshot API states for its path parameters:

GET /_snapshot/<repository>/<snapshot>

<repository>
(Required, string) Snapshot repository name used to limit the request.

To get information about all snapshot repositories registered in the cluster, omit this parameter or use * or _all.

<snapshot>
(Required, string) Comma-separated list of snapshot names to retrieve. Also accepts wildcards (*).

To get information about all snapshots in a registered repository, use a wildcard (*) or _all.

However trying to retrieve all snapshots from all repositories does not work.

Steps to reproduce:

GET _snapshot/_all/_all

returns

{
  "error" : {
    "root_cause" : [
      {
        "type" : "repository_missing_exception",
        "reason" : "[_all] missing"
      }
    ],
    "type" : "repository_missing_exception",
    "reason" : "[_all] missing"
  },
  "status" : 404
}

Either this is a bug, or maybe the documentation needs to state that this is not supported.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DaveCTurner]

TLDR this is a docs bug, you can only get from multiple repositories in ≥7.14.0, we shouldn't have backported this info to the docs for earlier versions.

Bit of a tricky one to disentangle however, we first introduced multi-repo support in master (#42090) but it was a breaking change to the response format so couldn't be backported to 7.x. But then we changed our minds about the new response format and adjusted master to keep using the old format (with slight tweaks). And then we backported a bunch of stuff in #74676 which included the multi-repo support.

The docs backport was #59238, looks like this went all the way back to 7.9.