fix(ci): add retry logic to sccache --start-server in setup-sccache action

[mendral-app]

Summary

• Wrap sccache --start-server in a 3-attempt retry loop (5s delay between attempts) to handle transient WebDAV 500 errors that have been causing ~1-2 CI failures per week for 4+ months
• On persistent failure, gracefully degrade by unsetting RUSTC_WRAPPER so builds complete (slower, without cache) rather than failing the entire job

Context

Related insight: sccache WebDAV backend 500 errors causing recurring Rust CI failures

The WebDAV cache backend intermittently returns HTTP 500 on the startup health check (GET .sccache_check). Since the setup-sccache action had no retry logic, every transient error immediately failed the step and the entire job. 22+ occurrences observed across 14+ distinct backend IPs over 4 months.

---

Note

Created by Mendral. Tag @mendral-app with feedback or questions.

• View implementation session
• Addresses this insight

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
.github/actions/setup-sccache/action.yml:71-87
Setting `RUSTC_WRAPPER=sccache` before the retry loop and then attempting to clear it with `RUSTC_WRAPPER=` (empty string) is risky. GitHub Actions sets the env var to an empty string rather than unsetting it, and cargo's handling of `RUSTC_WRAPPER=""` varies by version — older cargo releases may attempt to exec an empty string as the wrapper instead of treating it as "no wrapper", causing builds to fail anyway. The intent of graceful degradation is better served by only writing `RUSTC_WRAPPER=sccache` on success, eliminating the need for the override entirely.

```suggestion
              echo "$RUNNER_TEMP/sccache" >> "$GITHUB_PATH"
              for i in 1 2 3; do
                if "$RUNNER_TEMP/sccache/sccache" --start-server; then
                  echo "sccache server started successfully"
                  echo "RUSTC_WRAPPER=sccache" >> "$GITHUB_ENV"
                  break
                fi
                if [ "$i" -eq 3 ]; then
                  echo "::warning::sccache failed to start after 3 attempts, continuing without cache"
                  exit 0
                fi
                echo "sccache startup attempt $i failed, retrying in 5 seconds..."
                "$RUNNER_TEMP/sccache/sccache" --stop-server 2>/dev/null || true
                sleep 5
              done
              "$RUNNER_TEMP/sccache/sccache" --version
```

_{Reviews (1): Last reviewed commit: "fix(ci): add retry logic to sccache --st..." | Re-trigger Greptile}

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-05-29 13:41 UTC	Run
prod-us	✅ Deployed	2026-05-29 14:09 UTC	Run
prod-eu	✅ Deployed	2026-05-29 14:12 UTC	Run