False disk space warning on immutable/composefs root filesystems (Directory.GetDirectoryRoot always returns / on Linux)

Description

Self-hosted runners on immutable Linux systems (bootc, Fedora CoreOS, ostree-based) report a false disk space warning every job:

Warning: You are running out of disk space. The runner will stop working when the machine runs out of disk space. Free space left: 0 MB

There is plenty of real disk space available. The warning fires because of a platform-specific bug in CheckDiskSpaceAsync.

Root Cause

In src/Runner.Worker/JobExtension.cs, the disk space check does:

var workDirRoot = Directory.GetDirectoryRoot(HostContext.GetDirectory(WellKnownDirectory.Work));
var driveInfo = new DriveInfo(workDirRoot);
var freeSpaceInMB = driveInfo.AvailableFreeSpace / 1024 / 1024;

On Linux, Directory.GetDirectoryRoot() always returns / regardless of which filesystem the work directory is actually mounted on. This means DriveInfo always inspects / — not the filesystem backing the work directory.

On immutable systems, / is a composefs mount (the read-only image layer). composefs correctly reports 0 bytes available because it is a read-only filesystem. The runner interprets this as the disk being full.

Environment

• Runner version: 2.334.0
• OS: Fedora CoreOS / MethidOS (bootc-managed image)
• Root filesystem: composefs (immutable, read-only — this is by design)
• Actual writable partitions: hundreds of GB free (verified with df -h)

Example df -h output from an affected machine:

Filesystem      Size  Used Avail Use% Mounted on
composefs        47M   47M     0 100% /              ← always 0, read-only by design
/dev/nvme0n1p5  141G   24G  110G  18% /etc
/dev/nvme0n1p3  619G  106G  514G  17% /var/lib/docker
/dev/nvme0n1p4  186G    2G  175G   2% /var/home

The runner work directory (/var/lib/github-runner/_work) is on /dev/nvme0n1p5 with 110 GB free. GetDirectoryRoot returns / for this path, causing DriveInfo to check composefs instead.

Expected Behavior

The disk space check should inspect the filesystem that actually backs the work directory, not always /.

Suggested Fix

Replace Directory.GetDirectoryRoot() + DriveInfo with a direct statvfs(2) call (via P/Invoke or DriveInfo on the full work path) on Linux, or use the df-equivalent .NET API that resolves the actual mount point for a given path:

// Instead of:
var workDirRoot = Directory.GetDirectoryRoot(HostContext.GetDirectory(WellKnownDirectory.Work));
var driveInfo = new DriveInfo(workDirRoot);

// Use the work path directly — DriveInfo accepts a directory path, not just a root:
var driveInfo = new DriveInfo(HostContext.GetDirectory(WellKnownDirectory.Work));

new DriveInfo(path) on .NET resolves the actual volume for the given path on Windows, but on Linux it still resolves to /. The correct Linux fix is to call statvfs directly on the work directory path so the kernel returns stats for the actual mount.

Additional Context

This affects any self-hosted runner on an ostree/bootc/composefs-based OS, including Fedora CoreOS (41+), Universal Blue images, and any custom bootc image. These systems are increasingly common for edge/embedded CI workloads. The warning is purely cosmetic today but causes user confusion and will become a hard failure if the runner ever starts refusing jobs based on this check.

There is no runner-side configuration knob to suppress or redirect this check — system.runner.lowdiskspacethreshold is pushed by GitHub's service in the job payload and cannot be overridden from the runner's .env file.

Read src/Runner.Worker/JobExtension.cs and trace CheckDiskSpaceAsync, starting with how it derives the work-directory filesystem and calculates available space. Verify the change against a composefs root with a writable work-directory mount; done means the warning reports the work filesystem's available space instead of the read-only root filesystem.