fix(files): skip directory symlinks that loop back onto the scanned path

[nikaiw]

• resolves [Bug]: files:scan follows symlink -> endless loop #41563

Summary

A directory symlink whose target is one of its own ancestors, e.g. device -> .. in a copied /sys tree or docker-ce's bundles/latest -> ., makes the scanner re-enter the same tree until the 4000 character path limit:

.../4:0:0:0/bsg/4:0:0:0/device/bsg/4:0:0:0/device/... (~140 levels)

Each level adds a filecache row, so scanning such a storage grows oc_filecache table without bound and the background scan never finishes.

In effect this is a DoS: anything able to write symlinks to the backing storage (a build job, rsync, sftp) can grow the database until the disk is full. On one instance two stale build trees produced 30M bogus rows and a 185 GB database, and at their peak filled the node's disk, taking the server down.

The bug has been open since 2017 (#6395, see also #20197 and #23022). The previous attempt, #21723, compared the logical path to the resolved one, which cannot match when an ancestor is itself a symlink, and was closed unmerged.

Skip a child symlink in Local::getDirectoryContent() when its resolved target equals the listed directory or one of its ancestors, comparing resolved paths on both sides.

This also catches indirect cycles (a -> b, b -> a). The loop above now stops where it closes, with a single log line instead of a runaway descent:

Skipping looping directory symlink '.../4:0:0:0/bsg/4:0:0:0/device'
-> '.../4:0:0:0'

Only listing is affected; files behind such links stay readable and writable (see testDisallowSymlinksInsideDatadir).

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

Also #41563

[susnux]

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