Fix return values of Matrix and Vector during query range in QueryShardingMiddleware

[kavirajk]

What this PR does / why we need it:
During querysharding, when result type is either Matrix or Vector and when the results are empty, instead of returning empty responses, it return nil which breaks the final JSON encoding.

Which issue(s) this PR fixes:
Fixes: #4420

Special notes for your reviewer:
NA

Checklist

• Documentation added
• Tests updated

[cyriltovena]

Nice let's verify with a test

[kavirajk]

Tried writing quick test yesterda, but realized its bit tricky. As these handlers are setup inside initQueryWorker(which needs lots of dependency) plus these handlers are available inside Process (interface) under service.Service (again an interface).

I will split setting up this handlers to separate function. So easy to test only the middleware chaining.

[cyriltovena]

LGTM

[cyriltovena]

Can you test after and before in dev please too ?

[kavirajk]

http "http://localhost:3100/loki/api/v1/query_range?query=count_over_time({foo=%22bar%22}[1m])"

Before fix

{
    "data": {
        "result": null,
        "resultType": "matrix",
        "stats": {
            "ingester": {
                "compressedBytes": 0,
                "decompressedBytes": 0,
                "decompressedLines": 0,
                "headChunkBytes": 0,
                "headChunkLines": 0,
                "totalBatches": 0,
                "totalChunksMatched": 0,
                "totalDuplicates": 0,
                "totalLinesSent": 0,
                "totalReached": 1
            },
            "store": {
                "chunksDownloadTime": 0,
                "compressedBytes": 0,
                "decompressedBytes": 0,
                "decompressedLines": 0,
                "headChunkBytes": 0,
                "headChunkLines": 0,
                "totalChunksDownloaded": 0,
                "totalChunksRef": 0,
                "totalDuplicates": 0
            },
            "summary": {
                "bytesProcessedPerSecond": 0,
                "execTime": 0.002188372,
                "linesProcessedPerSecond": 0,
                "totalBytesProcessed": 0,
                "totalLinesProcessed": 0
            }
        }
    },
    "status": "success"
}

After fix

{
    "data": {
        "result": [],
        "resultType": "matrix",
        "stats": {
            "ingester": {
                "compressedBytes": 0,
                "decompressedBytes": 0,
                "decompressedLines": 0,
                "headChunkBytes": 0,
                "headChunkLines": 0,
                "totalBatches": 0,
                "totalChunksMatched": 0,
                "totalDuplicates": 0,
                "totalLinesSent": 0,
                "totalReached": 28
            },
            "store": {
                "chunksDownloadTime": 0,
                "compressedBytes": 0,
                "decompressedBytes": 0,
                "decompressedLines": 0,
                "headChunkBytes": 0,
                "headChunkLines": 0,
                "totalChunksDownloaded": 0,
                "totalChunksRef": 0,
                "totalDuplicates": 0
            },
            "summary": {
                "bytesProcessedPerSecond": 0,
                "execTime": 0.005709232,
                "linesProcessedPerSecond": 0,
                "totalBytesProcessed": 0,
                "totalLinesProcessed": 0
            }
        }
    },
    "status": "success"
}

NOTE: response result key
Looks like it happens only on single-binary mode.

[cyriltovena]

Nice work kavi ❤️

[cyriltovena]

Can you sync back on the issue to make sure it was runing on single binary just to make sure we didn;'t miss anything please.

[BitProcessor]

Can you sync back on the issue to make sure it was runing on single binary just to make sure we didn;'t miss anything please.

@cyriltovena
I am the author of #4420 and encountered the issue in distributed mode.

Retested today, with Grafana 8.2.1 and Loki in distributed mode, using image docker.io/grafana/loki:2.3.0 for the various components.
Source:

  chart:
    repository: https://grafana.github.io/helm-charts
    name: loki-distributed
    version: 0.38.0

Error in distributed mode