Issue Bug report: Watchdog disk threshold alert email contains literal placeholders instead of values

[Zenon]

Server operating system version

Debian 12.15 x86_64

Plesk version and microupdate number

Obsidian 18.0.79.6 (build 2026/08/10, revision e72b2a625b7155e90dde0af0efc1c2d0d38b7df0)

Environment​

• Plesk: Obsidian 18.0.79.6 (build 2026/08/10, revision e72b2a625b7155e90dde0af0efc1c2d0d38b7df0)
• OS: Debian 12.15 x86_64
• Watchdog extension: 3.1.10-15879
• Monit: 5.33.0 (distribution package, dpkg -l monit → 1:5.33.0-1)

Problem description​

The email alert sent by Watchdog when a disk usage threshold is reachedcontains unresolved placeholders <fs_size> and <used_size> instead ofthe actual numeric values. The other template fields (Device name, Mountpoint) are populated correctly.

Email received:​

Disk space usage has reached the specified threshold on host [SERVER_HOSTNAME]:
Device name: /dev/vda1
Mount point: /
Capacity: <fs_size> kilobytes
Used disk space: <used_size> kilobytes

Expected content (matching the historical Watchdog format, confirmed in​

Plesk forum threads dating back to 2008):



Disk space usage has reached the specified threshold on host [SERVER_HOSTNAME]:
Device name: /dev/vda1
Mount point: /
Capacity: 123679096 kilobytes
Used disk space: 50388404 kilobytes

Diagnostics performed​

1. Confirmed that Postfix/SMTP is working correctly (the email is delivered;the issue is only with the message content).
2. Confirmed that Monit has correct, complete data for the partition:

$ monit status | grep -A 15 "vda1"
Filesystem 'dev/vda1'
status OK
monitoring status Monitored
monitoring mode active
on reboot start
filesystem type ext4
filesystem flags rw,relatime,discard,errors=remount-ro
permission 660
uid 0
gid 6
block size 4 kB
space total 117.9 GB (of which 4.1% is reserved for root user)
space free for non superuser 65.1 GB [55.2%]
space free total 69.9 GB [59.3%]
inodes total 7856128
inodes free 7601248 [96.8%]


$ df -T /
Filesystem Type 1K-blocks Used Available Use% Mounted on
/dev/vda1 ext4 123679096 50388404 68226904 43%

Additional test: full uninstall and reinstall of the extension​

To rule out the hypothesis of a leftover inconsistent state from a previousextension update (cf. EXTPLESK-13679), the Watchdog extension was fullyuninstalled via Extensions > My Extensions > Uninstall, then reinstalledfrom the Extensions Catalog, and disk monitoring was reconfigured fromscratch for the /dev/vda1 partition with the same threshold.


Result: the issue occurred identically on the freshly installedextension. The email still contains unresolved <fs_size> and<used_size> placeholders. This rules out a corrupted/leftover local stateas the cause and points to a systematic bug in the Watchdog 3.1.10-15879code itself (or in how it communicates with Monit 5.33.0 on Debian 12),rather than an artifact of this particular installation's update history.

Possible connection to Plesk Obsidian 18.0.79​

The issue appeared on a server updated to version 18.0.79.6. The changelogfor this version (and neighboring microupdates) includes fixes in a relatedarea — disk usage display/calculation and Watchdog's internal state afteran update:

• PPPM-15422 — "Fixed the issue where the mail account disk-usage bar wasmissing after the update to Plesk version 18.0.79."
• EXTPLESK-13679 — "Fixed an issue where, after updating the Watchdogextension to the latest version, obsolete cron entries and scriptsrelated to the extension's earlier version were not properly removedfrom the server."

It is not confirmed that this is the same underlying mechanism as thefs_size/used_size issue described here, but the timing and thematicoverlap (disk usage + Watchdog state after update) suggest a possibleconnection worth investigating.

Conclusion​

Since Monit correctly reports the partition size data, but Watchdog stillfails to substitute it into two specific template fields (fs_size,used_size), the issue lies in the layer of Watchdog responsible forfetching/mapping size data from Monit into the notification email content —most likely a mismatch between the data format returned by this specificMonit version/build and what the Watchdog code expects.

Steps to reproduce​

1. Install Watchdog 3.1.10-15879 on Debian with Monit 1:5.33.0-1(distribution package).
2. Enable disk monitoring for any partition.
3. Temporarily lower the usage threshold below the current usage.
4. Wait for the email alert (default polling interval: 5 minutes).
5. Observe the unresolved placeholders in the email content.