🐋🔧：write out the one key, not everything offered

[DerekNonGeneric]

Found while reviewing the port of this file into the portal, where the fix landed as 86ecc82.

The count and the thing written were not the same list.

count is how many of ssh-add -L's lines look like a key type this accepts. The file was then written from identities, which is every line the agent offered. An agent forwarding a certificate alongside the key it belongs to prints two lines and matches one — the certificate is ssh-ed25519-cert-v01@openssh.com, deliberately not an accepted type — so the guard passed and wrote both.

Two keys in that file breaks both readers of it. ssh-keygen -Y sign -f wants one public key, and allowed_signers takes its second field from awk '{print $1, $2}' over the whole file, which becomes a second line in a grammar that has room for none.

The matching lines are kept now and the count taken from those, so what is counted is what is written.

How it was checked

agent holds	count	lines written
one ed25519	1	1
ed25519 + its certificate	1	1 (was 2)
two real keys	2	refused
ECDSA nistp256	1	1
The agent has no identities.	0	refused

bash -n and sh -n clean, mode 755 preserved, verify.all shows no new failures. With this the file is byte-identical to the portal's again.

Summary by CodeRabbit

• Bug Fixes
  • Improved SSH signing-key detection in development environments.
  • Only supported public keys are now considered, while certificate entries are excluded.
  • When exactly one valid key is available, the correct key is selected consistently for signing operations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Team

Run ID: 717b94b7-ee16-4830-980a-925c7e15346b

📥 Commits

Reviewing files that changed from the base of the PR and between 09ad280 and cac3f3d.

📒 Files selected for processing (1)

• .devcontainer/post-start.sh

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📝 Walkthrough

Walkthrough

The container startup script now filters SSH agent identities before counting and writing the signing-key file. It excludes unsupported keys and certificates from both operations.

Changes

SSH signing-key filtering

Layer / File(s)	Summary
Filter identities before writing .devcontainer/post-start.sh	The script stores supported, non-certificate public keys in matched, counts that list, and writes the filtered identity when exactly one match exists.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to cac3f

The startup script now writes only one accepted SSH key and excludes accompanying certificates or unsupported identities. The stated validation covers the intended key and failure cases, with no current merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: writing only the single matching SSH key instead of all offered identities. The wording is concise and specific.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch infra/one-key-not-all

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