`influxd-ctl backup` now validates `-from` node exists

[devanbenz]

ETA: 1.12.3

influxd-ctl backup now validates that the data node specified with -from exists in the cluster before starting the backup. Previously, specifying a non-existent node could lead to confusing errors. Now the command fails immediately with a clear error message: data node "<addr>" does not exist.

InfluxDB Enterprise 1.12.3+

[jstirnaman]

@devanbenz — Source analysis for v1.12.3 docs found additional behavior beyond what this issue describes:

Beyond -from validation, a later PR added smarter node selection.

plutonium#4406 (cherry-picked as #4412, merged 2026-02-17) adds fallback logic: when the -from node doesn't own the shard being backed up, the command now falls back to other shard owners sorted by recency instead of failing.

Should the docs mention this fallback behavior, or just the validation that the node exists in the cluster?

[devanbenz]

@devanbenz — Source analysis for v1.12.3 docs found additional behavior beyond what this issue describes:

Beyond -from validation, a later PR added smarter node selection.

plutonium#4406 (cherry-picked as #4412, merged 2026-02-17) adds fallback logic: when the -from node doesn't own the shard being backed up, the command now falls back to other shard owners sorted by recency instead of failing.

Should the docs mention this fallback behavior, or just the validation that the node exists in the cluster?

Yes, add info about the fallback behavior.