Make JSON format compatible with nulls

[jouir]

Telegraf expects either an object or an array of objects but nulls are valid JSON values. This commit detects if query returns a null or the document is a null to return an empty metric instead of an error.

For example, with this configuration:

test.json:

{"error":null,"result":{"data":null,"items_per_page":10,"total_items":0,"total_pages":0}}

test.conf:

[[inputs.file]]
  files = ["test.json"]
  data_format = "json"
  json_query = "result.data"

The query returns null:

$ cat test.json | jq ".result.data"
null

Latest version returns an error:

$ docker run -it -v $(pwd):/mnt/ -w "/mnt" --rm -e telegraf:latest telegraf -test -debug -config test.conf
2021-04-09T09:31:08Z I! Starting Telegraf 1.18.1
2021-04-09T09:31:08Z D! [agent] Initializing plugins
2021-04-09T09:31:08Z D! [agent] Starting service inputs
2021-04-09T09:31:08Z E! [inputs.file] Error in plugin: E! Query path must lead to a JSON object or array of objects, but lead to: Null
2021-04-09T09:31:08Z D! [agent] Stopping service inputs
2021-04-09T09:31:08Z D! [agent] Input channel closed
2021-04-09T09:31:08Z D! [agent] Stopped Successfully
2021-04-09T09:31:08Z E! [telegraf] Error running agent: input plugins recorded 1 errors

This commits skip the error:

$ ./telegraf -test -debug -config test.conf
2021-04-09T09:34:02Z I! Starting Telegraf 
2021-04-09T09:34:02Z D! [agent] Initializing plugins
2021-04-09T09:34:02Z D! [agent] Starting service inputs
2021-04-09T09:34:02Z D! [agent] Stopping service inputs
2021-04-09T09:34:02Z D! [agent] Input channel closed
2021-04-09T09:34:02Z D! [agent] Stopped Successfully

In the end, in both cases, no metric is written to the outputs.

Same tests for a null document:

test.json:

null

Document is valid:

$ cat test.json | jq
null

Unpatched:

$ docker run -it -v $(pwd):/mnt/ -w "/mnt" --rm -e telegraf:latest telegraf -test -debug -config test.conf
2021-04-09T09:42:17Z I! Starting Telegraf 1.18.1
2021-04-09T09:42:17Z D! [agent] Initializing plugins
2021-04-09T09:42:17Z D! [agent] Starting service inputs
2021-04-09T09:42:17Z E! [inputs.file] Error in plugin: E! Query path must lead to a JSON object or array of objects, but lead to: Null
2021-04-09T09:42:17Z D! [agent] Stopping service inputs
2021-04-09T09:42:17Z D! [agent] Input channel closed
2021-04-09T09:42:17Z D! [agent] Stopped Successfully
2021-04-09T09:42:17Z E! [telegraf] Error running agent: input plugins recorded 1 errors

Patched:

$ ./telegraf -test -debug -config test.conf
2021-04-09T09:42:33Z I! Starting Telegraf 
2021-04-09T09:42:33Z D! [agent] Initializing plugins
2021-04-09T09:42:33Z D! [agent] Starting service inputs
2021-04-09T09:42:33Z D! [agent] Stopping service inputs
2021-04-09T09:42:33Z D! [agent] Input channel closed
2021-04-09T09:42:33Z D! [agent] Stopped Successfully

Required for all PRs:

• Updated associated README.md.
• Wrote appropriate unit tests.

Return an empty list of metrics when:

• result type is gjson.Null if a query is detected
• type of parsed JSON document is nil

[telegraf-tiger]

🤝 ✅ CLA has been signed. Thank you!

[telegraf-tiger]

Looks like new artifacts were built from this PR. Get them here!

Artifact URLs

• aarch64.rpm

• armel.rpm

• armv6hl.rpm

• i386.rpm

• ppc64le.rpm

• s390x.rpm

• x86_64.rpm

• darwin_amd64.tar.gz

• freebsd_amd64.tar.gz

• freebsd_i386.tar.gz

• linux_amd64.tar.gz

• linux_arm64.tar.gz

• linux_armel.tar.gz

• linux_armhf.tar.gz

• linux_i386.tar.gz

• linux_mips.tar.gz

• linux_mipsel.tar.gz

• linux_ppc64le.tar.gz

• linux_s390x.tar.gz

• static_linux_amd64.tar.gz

• windows_amd64.zip

• windows_i386.zip

• amd64.deb

• arm64.deb

• armel.deb

• armhf.deb

• i386.deb

• mips.deb

• mipsel.deb

• ppc64el.deb

• s390x.deb

[ivorybilled]

This seems to make sense to me.

[ssoroka]

looks good, makes sense. I prefer []telegraf.Metric{} to calling make, but returning nil should be acceptable

[telegraf-tiger]

Looks like new artifacts were built from this PR. Get them here!

Artifact URLs

• aarch64.rpm

• armel.rpm

• armv6hl.rpm

• i386.rpm

• ppc64le.rpm

• s390x.rpm

• x86_64.rpm

• darwin_amd64.tar.gz

• freebsd_amd64.tar.gz

• freebsd_i386.tar.gz

• linux_amd64.tar.gz

• linux_arm64.tar.gz

• linux_armel.tar.gz

• linux_armhf.tar.gz

• linux_i386.tar.gz

• linux_mips.tar.gz

• linux_mipsel.tar.gz

• linux_ppc64le.tar.gz

• linux_s390x.tar.gz

• static_linux_amd64.tar.gz

• windows_amd64.zip

• windows_i386.zip

• amd64.deb

• arm64.deb

• armel.deb

• armhf.deb

• i386.deb

• mips.deb

• mipsel.deb

• ppc64el.deb

• s390x.deb