Package plugin: populate ValueList.DSNames when calling write callbacks.

[pleimer]

Plugin writes do not include new dsnames but instead default to value. To confirm this issue, I used the following code from the docs but changed the dsname. I listened for the messages using the amqp1.

package main

import (
        "context"
        "fmt"
        "time"

        "collectd.org/api"
        "collectd.org/plugin"
)

type examplePlugin struct{}

func (examplePlugin) Read(ctx context.Context) error {
        vl := &api.ValueList{
                Identifier: api.Identifier{
                        Host:   "example.com",
                        Plugin: "goplug",
                        Type:   "gauge",
                },
                Time:     time.Now(),
                Interval: 10 * time.Second,
                Values:   []api.Value{api.Gauge(42)},
                DSNames:  []string{"read"},
        }
        if err := plugin.Write(ctx, vl); err != nil {
                return fmt.Errorf("plugin.Write: %w", err)
        }

        return nil
}

func init() {
	plugin.RegisterRead("example", examplePlugin{})
}

func main() {}

Received Message

[
  {
    "values": [
      42
    ],
    "dstypes": [
      "gauge"
    ],
    "dsnames": [
      "value"
    ],
    "time": 1591894480.918,
    "interval": 10,
    "host": "example.com",
    "plugin": "goplug",
    "plugin_instance": "",
    "type": "gauge",
    "type_instance": ""
  }
]

Expected Message

[
  {
    "values": [
      42
    ],
    "dstypes": [
      "gauge"
    ],
    "dsnames": [
      "read"
    ],
    "time": 1591894480.918,
    "interval": 10,
    "host": "example.com",
    "plugin": "goplug",
    "plugin_instance": "",
    "type": "gauge",
    "type_instance": ""
  }
]

The dsnames field of the message does not container read, but rather the default string value

[octo]

Hi @pleimer, thank you for bringing this up!

This is a limitation of collectd's C API – the "DS names" are not included in the value_list_t struct. This data structure will change significantly with the next major version of collectd, which we're planning to release later this year. So it's more than unlikely that we'll patch the "DS names" through the entire daemon – it's significant work and soon to be obsolete.

We should, however, populate the ValueList.DSNames field when calling write callbacks. You would not (necessarily) get the names passed to plugin.Write, but the names configured in the types.db(5) file. Ideally those two would be the same.

Hope this helps!
—octo

[octo]

I just now saw that you were using the AMQP plugin to verify: that is not going to get fixed, sorry. You'll have to add a entry to the types.db file using the expected data source names.

[pleimer]

This is a limitation of collectd's C API – the "DS names" are not included in the value_list_t struct. This data structure will change significantly with the next major version of collectd, which we're planning to release later this year. So it's more than unlikely that we'll patch the "DS names" through the entire daemon – it's significant work and soon to be obsolete.

I see thanks for pointing that out, I did not notice that in my original research.

We should, however, populate the ValueList.DSNames field when calling write callbacks. You would not (necessarily) get the names passed to plugin.Write, but the names configured in the types.db(5) file. Ideally those two would be the same.

I did some additional experimentation by including the extra types in a custom types.db file and now I do see the new data source names. It seems that anything specified in ValueList.DSNames is simply ignored, however. Is the purpose of the field purely a convenient way to track the names when writing a plugin?