[19] Cross-Filesystem Temp Files Broke an ‘Atomic’ Config Write on Linux

The trick: a rename is only atomic within one filesystem. Write your temp file into the same directory as the final target, never into the system default temp directory.

Issue

A config-persistence routine used the standard temp-file-then-rename pattern for an atomic write. On Linux specifically, it reliably failed.

Root Cause

The temp file was created in the system’s default temp directory, commonly its own filesystem — often an in-memory one. Renaming across filesystems isn’t atomic; it isn’t even a rename. It fails outright with a cross-device error, since a rename can only relink within one filesystem.

Solution

Create the temp file in the same directory as the target. Same directory guarantees same filesystem, which guarantees the rename is genuinely atomic.

💡 Takeaway

• Temp-file-then-rename is only atomic when both files share a filesystem — the system temp directory is a common way to violate that silently.
• A cross-filesystem rename fails outright rather than degrading gracefully; expect it on Linux specifically.
• Check whether the failure path of an “atomic” operation can itself hold a lock open.