Recoding — Code Runner

A lightweight Go HTTP service that compiles and executes participant code for the Recoding Exercise Platform.

Live at: https://code-runner-cpqt.onrender.com

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What it does

When a participant clicks Run in the coding editor, the platform sends their Go code to this service. The runner:

1. Validates the API key
2. Writes the code to a temporary directory
3. Runs go run with a 10-second timeout
4. Captures stdout, stderr, and exit code
5. Returns the result as JSON
6. Cleans up the temp directory

The banner file (standard.txt) is automatically injected as stdin by the main app for ASCII art exercises — participants read it via os.Stdin without needing file I/O.

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API

POST /run

Execute Go code.

Headers:

Authorization: Bearer <RUNNER_API_KEY>
Content-Type: application/json

Request:

{
  "code": "package main\nimport \"fmt\"\nfunc main(){fmt.Println(\"hello\")}",
  "language": "go",
  "stdin": ""
}

Response:

{
  "stdout": "hello\n",
  "stderr": "",
  "exit_code": 0,
  "compile_output": ""
}

GET /health

Returns {"status":"ok"}. Used by Render for health checks.

GET /

Status page showing the server is online with endpoint documentation.

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Limits

Limit	Value
Code size	64 KB
Output size	64 KB
Execution timeout	10 seconds
Supported languages	Go only

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Security

All requests to /run require a Bearer token:

Authorization: Bearer <RUNNER_API_KEY>

Set RUNNER_API_KEY as an environment variable on Render. The main app reads the same key from its RUNNER_API_KEY env var and sends it automatically.

Generate a key:

openssl rand -base64 32

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Deploy to Render

1. Go to render.com → New → Web Service
2. Connect github.com/jvcByte/code-runner
3. Runtime: Docker
4. Add environment variable: RUNNER_API_KEY=<your-secret>
5. Deploy

After deploy, set in the main app:

RUNNER_URL=https://your-service.onrender.com
RUNNER_API_KEY=<same-secret>

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Local development

# Build and run
go build -o runner . && ./runner
# Server starts on :3001

# Test
curl -X POST http://localhost:3001/run \
  -H "Content-Type: application/json" \
  -d '{"code":"package main\nimport \"fmt\"\nfunc main(){fmt.Println(\"hello\")}","language":"go","stdin":""}'

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Project structure

main.go                      — Entry point, server setup, route registration
internal/
  executor/
    executor.go              — Runs go code in temp dir with timeout and output limits
  handlers/
    run.go                   — POST /run — validates request, calls executor, returns JSON
    health.go                — GET /health — returns {"status":"ok"}
    index.go                 — GET / — HTML status page with server rack icon
  middleware/
    auth.go                  — Bearer token authentication (skipped if RUNNER_API_KEY unset)
    logger.go                — Structured JSON request logging (IP, agent, status, duration)

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Logging

All requests are logged as structured JSON to stdout (visible in Render's log viewer):

{"time":"2026-04-14T10:00:00Z","level":"INFO","msg":"request","method":"POST","path":"/run","status":200,"ip":"1.2.3.4","agent":"...","duration":"523ms"}

Log levels:

• INFO — successful requests (2xx)
• WARN — client errors (4xx) and execution timeouts
• ERROR — server errors (5xx) and execution failures

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Notes

• Render free tier sleeps after 15 minutes of inactivity — first request after idle will be slow (~30s cold start)
• No sandboxing beyond the 10s timeout and output limits — code runs as the server process user
• Only Go is supported; other languages return an error response