fix: null-terminate readlink buffer in GetSelfPath

[MorningLightMountain713]

Summary

• Null-terminate the buffer returned by readlink() before passing it to dirname() in GetSelfPath()

Problem

Observed fluxd failing to start intermittently with:

2026-03-13 11:27:52 Path: /usr/local/bin/fluxd
2026-03-13 11:27:52 Error: Failed to find benchmark application
2026-03-13 11:27:52 Shutdown: In progress...

After systemd restarted the service, it worked fine:

2026-03-13 11:28:37 Path: /usr/local/bin
2026-03-13 11:28:37 Found fluxbenchd in /usr/local/bin

On the failed start, GetSelfPath() returned /usr/local/bin/fluxd (full binary path) instead of /usr/local/bin (directory only). This caused FindBenchmarkPath("fluxbenchd", "/usr/local/bin/fluxd") to look for /usr/local/bin/fluxd/fluxbenchd which doesn't exist.

Root cause

readlink() does not append a null byte to the buffer. GetSelfPath() passes the un-terminated buffer directly to dirname(), which reads past the valid bytes into whatever garbage is on the stack. If that garbage happens to contain a / character, dirname() finds it and returns the wrong directory. Whether the stack contains a / depends on prior call history, ASLR, environment size, etc. — hence the intermittent nature.

Test plan

Built a test binary that reproduces the issue by simulating dirty stack memory:

broken raw: /usr/local/bin/fluxdaaaaa/aaaa
broken dirname: /usr/local/bin/fluxdaaaaa    <-- wrong

fixed  raw: /usr/local/bin/fluxd
fixed  dirname: /usr/local/bin               <-- correct

🤖 Generated with Claude Code