node_filesystem_readonly seems to be reporting _everything_ as read-write

[io7m]

Hello!

Possibly related to #3484 although the symptom in my case seems a little more extreme. The node_filesystem_readonly always reports 0 indicating that all filesystems are read-write.

In my case, most read-only filesystems are archival ZFS filesystems set as read-only. For example:

$ grep vanilla /proc/1/mounts
storage/backup/archived/vanilla /backup/archived/vanilla zfs ro,seclabel,relatime,xattr,noacl,casesensitive 0 0

$ mount | grep vanilla
storage/backup/archived/vanilla on /backup/archived/vanilla type zfs (ro,relatime,seclabel,xattr,noacl,casesensitive)

node_filesystem_readonly{device="storage/backup/archived/vanilla"}  => 0

This has caused a lot of alerting to start complaining, because it thinks those filesystems are full (because they're resized with a quota down to 100% usage before being made read-only, and read-only filesystems are normally excluded from alerting here).

I'm on:

$ /usr/bin/prometheus-node-exporter --version
node_exporter, version 1.10.2 (branch: tarball, revision: 3.el9)
  build user:       
  build date:       20260130
  go version:       go1.25.5 (Red Hat 1.25.5-2.el9_7)
  platform:         linux/amd64
  tags:             rpm_crashtraceback,libtrust_openssl,netgo,osusergo,static_build

$ uname -a
Linux backup02 5.14.0-611.36.1.el9_7.x86_64 #1 SMP PREEMPT_DYNAMIC Tue Mar 3 17:30:12 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux

[Vedant-Mhatre]

I rechecked this locally on current master (March 13, 2026) and could reproduce it with a targeted Linux collector test.

This appears to be the same underlying bug as #3484 / PR #3507.

Root cause:

• mountOptionsString() currently concatenates mount options without commas
• isFilesystemReadOnly() later splits on commas and looks for "ro"

So a mount like ro,seclabel,relatime,xattr,noacl,casesensitive is turned into a single string, which causes readonly detection to fail.

I also verified locally that applying the same comma-separated join logic from #3507 fixes the repro.