Gin Example: Restrict File Upload Size with http.MaxBytesReader

This example demonstrates how to use Gin and Go's http.MaxBytesReader to strictly limit uploaded file size and return custom error messages when the size limit is exceeded.

Features

• REST API - POST /upload endpoint for file upload.
• Server-side enforcement of maximum file size (default: 1MB).
• Immediate error response (413 Request Entity Too Large with custom JSON) when file is too large.
• Well-commented code for clarity.
• Unit + integration tests verifying successful uploads, oversized file rejection, and edge cases.

How It Works

• The core implementation wraps the incoming request body using Go's http.MaxBytesReader, which limits how many bytes the server will read.
• If a client uploads a file exceeding this limit, parsing fails and Gin responds with a custom error message.
• See main.go for details and comments.

API Usage

Start the server

go run main.go

Server listens on port 8080.

Upload a file

cURL valid file (within 1MB):

curl -F "[email protected]" http://localhost:8080/upload
# Response: {"message":"upload successful"}

cURL oversized file (e.g., 2MB):

curl -F "[email protected]" http://localhost:8080/upload
# Response: {"error":"file too large (max: 1048576 bytes)"}

Error cases

• Missing file: {"error":"file form required"} (status 400)
• File too large: {"error":"file too large (max: 1048576 bytes)"} (status 413)

Testing

This example includes unit/integration tests in main_test.go:

go test

• TestUploadWithinLimit: uploads a file under the limit, expects success.
• TestUploadOverLimit: uploads a file over the limit, expects a custom error.
• TestUploadMissingFile: no file uploaded, expects validation error.

Code Reference

• main.go: Gin server setup, upload handler with file size enforcement.
• main_test.go: Contains all unit and integration tests.

Modify the Limit

Change MaxUploadSize constant in main.go and main_test.go to test other limits.

License: MIT
Author: Gin Example Contributors