zed-functions.sh: escape newline to produce valid json.

[papamoose]

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}'

Debug method:

1. Get the relevant information from zed to use in the below command.
2. Set ZED_NOTIFY_INTERVAL_SECS=1 # only so we can reuse the same event repeatedly for testing.
3. 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

Test method:

1. Run a scrub on a test zpool:

cd /tmp
dd if=/dev/zero of=sparse_file bs=1 count=0 seek=512M
zpool create test /tmp/sparse_file
zpool scrub test

2. Use both Slack and Discord (slack compatible) webhooks.

Motivation and Context

Description

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Good find!

[behlendorf]

@papamoose can you added your signed-off-by to the commit message of this PR and force update it.

git commit --amend -s