lib/logstorage: querying existing duplicate stream fields can panic

[func25]

Describe the bug

VictoriaLogs v1.51.0 and earlier can store logs with duplicate stream fields. After upgrading to a version containing #1504, querying this existing data causes a fatal panic because the new validation rejects duplicate stream tag names.

To Reproduce

1. Start VictoriaLogs v1.51.0 or earlier.
2. Ingest a log with duplicate stream fields:

curl -H 'Content-Type: application/json' \
  -H 'VL-Stream-Fields: service' \
  --data-binary '{"_msg":"test","service":"foo","service":"bar"}' \
  http://localhost:9428/insert/jsonline

3. Stop VictoriaLogs and open the same storage with current master.
4. Run query=*.

VictoriaLogs panics with FATAL: cannot unmarshal StreamTags

Version

Reproduced after upgrading from v1.50.0 to current master. The query panic was introduced by #1504.

Logs

No response

Screenshots

No response

Used command-line flags

No response

Additional information

No response

[func25]

Starting from the commit 664c6c1, VictoriaLogs accepts duplicate stream tag names when reading stream metadata. This prevents queries from panicking on data containing duplicate stream fields written by earlier VictoriaLogs versions. Duplicate stream tags are currently preserved. A follow-up change will deduplicate all fields with duplicate names, including stream fields, by keeping only one field per name. See #1286.

This commit will be included in the next release of VictoriaLogs. In the mean time it is possible to verify the fix by building VictoriaLogs from the commit according to these instructions.