feat(list_snap): zfs listsnap CLI

[vishnuitta]

This PR adds the zfs listsnap <dataset> CLI.
For this command, output will be in json format as:

vitta@vitta-laptop:~/openebs/lzfs$ ./cmd/zfs/zfs listsnap pool1/ds0 | jq
{
  "name": "pool1/ds0",
  "snaplist": {
    "istgt1": null,
    "snap4": null,
    "istgt2": null,
    "snap1": null,
    "snap2": null,
    "istgt3": null,
    "snap3": null
  }
}

This will NOT list the snapshots created through 'zfs snapshot' until the zrepl is restarted. However, it lists the snapshots created through istgt iscsi target.

Even though existing zfs list -t snapshot gives the list of snapshots, difference with this CLI is just NOT about printing in json format. Main difference is in caching the response. This command implementation will NOT read the contents from disk for every trigger of the command. It does read from disk one time, and keeps updating it with the new snapshots name whenever istgt sends SNAP_CREATE command.

Working on testcases.

Signed-off-by: Vitta vitta@mayadata.io

[richardelling]

It isn't clear to me why we're adding a new command, when zfs list -t snapshot already exists

[pawanpraka1]

looks good. one minor comment.

[mynktl]

looks good.

[vishnuitta]

incorporated review comments

[pawanpraka1]

@vishnuitta can you update the PR description with the modified output?

[vishnuitta]

@vishnuitta can you update the PR description with the modified output?

PR description already have modified output

[vishnuitta]

It isn't clear to me why we're adding a new command, when zfs list -t snapshot already exists

This command caches the response. As the control plane triggers the request very frequently (say every 30 seconds), reading from disk doesn't make sense. So, this command is introduced to cache the response and keep the in-memory data up-to-date.

[pawanpraka1]

@vishnuitta can you update the PR description with the modified output?

PR description already have modified output

I see in the PR you have change the o/p to fprintf(stdout, "%s\n", str_val); from fprintf(stdout, "%s: %s\n", nvpair_name(elem),str_val)?

[vishnuitta]

@vishnuitta can you update the PR description with the modified output?

PR description already have modified output

I see in the PR you have change the o/p to fprintf(stdout, "%s\n", str_val); from fprintf(stdout, "%s: %s\n", nvpair_name(elem),str_val)?

yes.. I updated the output after changing this print in the code

[mittachaitu]

changes are good