MCP 2b-3: GitHub App credential backend#31
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Third slice of #17's [2b] items: - src/app_token.rs: AppTokenProvider — RS256 JWT (60s backdate, 9min validity) signed with the App private key, exchanged for an installation token; cached and lazily refreshed 5 min before expiry. Installation id explicit or discovered from owner (org, then user) on first mint. - [mcp.github_app] config: app_id + private_key (secret-ref resolved, \n-escape normalized) + installation_id/owner. When present, the MCP path injects installation tokens; the PAT pool remains for REST/GraphQL and as the MCP fallback. - Session pins now carry the exact credential (PinnedCred::Pat by identity / ::App by token value + expiry). Sessions cannot outlive their credential: an expired App-token pin terminates the session (404 → client re-initializes) — completing the session-credential lifetime half of the [2b] binding gate. Provider refresh does not disturb in-flight sessions (pinned token stays valid upstream until its own expiry). - e2e: optional App-backend section (runs when APP_ID/APP_PRIVATE_KEY are present — CI passes the repo secrets): boots a second ghpool in App mode, asserts mint + pin-to-github-app + issue_read. - New dep: jsonwebtoken 9.3 (RS256). 67 tests (8 new: JWT shape/PEM validation/caching+refresh via mock GitHub API; App-mode pinning, pinned-token reuse, expiry termination via mock MCP upstream).
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Summary
Third slice of #17's
[2b]gate items: the GitHub App credential backend — the Revision 2 architecture's recommended credential for the MCP path, resolving the PAT-pooling compliance concern. Closes gate items [2b] GitHub App credential backend and the session-credential-lifetime half of [2b] Session-to-agent binding.Design
src/app_token.rs(new):AppTokenProvider— RS256 JWT (60s clock-skew backdate, 9-minute validity) signed with the App private key →POST /app/installations/{id}/access_tokens. Tokens cached and lazily refreshed 5 minutes before expiry. Installation id set explicitly or discovered fromowner(org endpoint, then user) on first mint.[mcp.github_app]config:app_id,private_key(secret-ref resolved;\n-escapes normalized for env/JSON sources),installation_id/owner. When present, the MCP path injects installation tokens; the PAT pool is untouched for REST/GraphQL and remains the MCP fallback when the section is absent.PinnedCred::Patby identity /::Appby token value + expiry). An expired App-token pin terminates the session (404 → client re-initializes per MCP spec). A provider refresh does not disturb in-flight sessions — the pinned token remains valid upstream until its own expiry, so sessions drain naturally across rotations (conservative choice given MCP proxy Phase 0: compliance & compatibility spike (hosted endpoint contract, App tokens) #22's untested token-switch-mid-session behavior).[via github-app]([agent=… via github-app]in agent mode).jsonwebtoken 9.3(pinned minor; RS256). Test PEM intestdata/is a throwaway generated for JWT-shape tests — not a credential.Tested
Bearer ghs_mock_token, pins by value, reuses the pinned token on follow-ups, and terminates expired-credential sessions (404, pin removed, upstream never called). Clippy-D warningsclean.[mcp.github_app](repo'sAPP_ID/APP_PRIVATE_KEYsecrets), and ghpool signed a real JWT, discovered the openabdev installation, minted a real installation token, and servedissue_readvia the hosted MCP server — 18/18 checks (13 PAT-mode + 5 App-mode). Section skips cleanly when App credentials are absent (verified locally).Remaining 2b
2b-4 durable fail-closed audit → 2b-5 enable writes (+ pin required App permission set docs).