This was discovered when using Discords Slack compatible webhook.
Slack webhooks works without the escape, however Discord rightly refuses
the POST as it contains invalid JSON.
https://discord.com/developers/docs/resources/webhook#execute-slackcompatible-webhook
Valid (while escaping the newline:
```
+ msg_json='{"text": "*ZFS scrub_finish error for test on quartz*\nZFS has detected a data error:\n\n eid: 124\n class: scrub_finish\n host: quartz\n time: \n error: \n objid: :\n pool: test\n"}'
```
Invalid (no escape):
```
+ msg_json='{"text": "*ZFS scrub_finish error for test on quartz*
ZFS has detected a data error:\n\n eid: 124\n class: scrub_finish\n host: quartz\n time: \n error: \n objid: :\n pool: test\n"}'
```
The new line gets rendered and not sent inside the JSON as intended.
```
++ curl -X POST https://discord.com/api/webhooks/{webhook.id}/{webhook.token}/slack --header 'Content-Type: application/json' --data-binary '{"text": "*ZFS scrub_finish error for test on quartz*
ZFS has detected a data error:\n\n eid: 124\n class: scrub_finish\n host: quartz\n time: \n error: \n objid: :\n pool: test\n"}'
+ msg_out='{"message": "Cannot send an empty message", "code": 50006}'
```
Test method:
`root@quartz:/etc/zfs/zed.d# export ZED_ZEDLET_DIR=/etc/zfs/zed.d; export ZEVENT_EID=124; export ZEVENT_SUBCLASS=scrub_finish; export ZEVENT_POOL=test; export ZED_NOTIFY_DATA=1; bash -x ./data-notify.sh`
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Philip Kauffman <philip@kauffman.me>
Closes#13049
aa9905d89b