SSH config writes are not idempotent, and tests write fixture hosts into the user's real ~/.ssh/config (164 duplicate blocks observed)

定位 SSH 配置的写入路径，以及创建 my_cluster fixture 的测试；首先确认哪些测试可以访问 ~/.ssh/config。完成的标准是：重复写入不会创建重复的主机块，生成的条目能够安全地隔离或替换，并且测试使用临时配置，同时保留真实的用户条目。

Summary

clustrix appends entries to the user's real ~/.ssh/config without deduplicating, and it writes test-fixture hosts into that personal config. On my machine this accumulated to 164 identical placeholder blocks.

Evidence

Measured on a real user config (macOS, ~/.ssh/config):

total lines:        1202
total Host entries:  172

Frequency by host (real hostnames redacted):

164  Host my_cluster          <- test fixture, repeated 164x
  1  Host <gpu-host-alias>
  1  Host <slurm-host-alias>
  1  Host test_cli
  1  Host test_cleanup
  1  Host <slurm-host-alias-2>
  1  Host <slurm-host>
  1  Host <slurm-host-alias-3>
  1  Host <institution-domain>

Every duplicate block is byte-identical and self-labelled:

# Clustrix auto-generated entry for my_cluster
Host my_cluster
    HostName cluster.example.com
    User testuser
    IdentityFile ~/.ssh/id_ed25519_clustrix_testuser_my_cluster
    IdentitiesOnly yes

cluster.example.com / testuser is a test fixture, not a real host. After removing only the my_cluster blocks, the file went from 1202 lines / 172 Host entries to 52 lines / 8 Host entries, with every genuine entry preserved.

Two distinct bugs

1. No deduplication on write. Each run appends a new block for a host that already has one. Nothing detects or replaces the existing entry, so the file grows without bound across runs.
2. Tests write to the real user config. my_cluster → cluster.example.com / testuser is fixture data. A test suite should never mutate ~/.ssh/config; it should write to a temp file and point SSH at it via -F / ssh_config path injection.

Impact

• The user's personal SSH config becomes unmanageable (1202 lines of which ~96% is generated noise).
• Real entries get buried among fixtures, making the file hard to audit.
• Functionally the duplicates are inert — OpenSSH takes the first match for a given Host pattern — so this is a hygiene and trust problem rather than a broken-connection problem. But it means clustrix silently rewrites a security-relevant file in the user's home directory.

Suggested fixes

• Make config writes idempotent: look for an existing block for the same Host alias and replace it in place, rather than appending.
• Delimit generated regions with explicit markers (e.g. # >>> clustrix managed >>> / # <<< clustrix managed <<<) so the tool can rewrite only its own section and users can see what it owns.
• Consider writing to a dedicated ~/.ssh/clustrix_config and having users add a single Include clustrix_config line, so clustrix never touches the main file.
• In the test suite, redirect all SSH-config writes to a tmp_path fixture. No test should be able to modify ~/.ssh/config.
• Optionally ship a clustrix ssh-config --prune command to clean up configs already affected.

Environment

• clustrix installed from source (repo master)
• macOS, OpenSSH client