Implementing ETag Handling in a Go HTTP Server

ETags (Entity Tags) are a mechanism for web servers and browsers to validate cached resources. This challenge asks you to build a simple Go HTTP server that generates and validates ETags for a given resource, allowing clients to efficiently revalidate the resource without re-downloading it if it hasn't changed. This is crucial for improving website performance and reducing bandwidth usage.

Problem Description

You need to create a Go program that implements a basic HTTP server. This server should serve a single resource (a simple text file) and generate an ETag for it based on its content's hash. The server should:

1. Generate ETag: Calculate an ETag for the resource based on its content. A simple MD5 hash is sufficient for this exercise.
2. Serve Resource with ETag: Include the generated ETag in the ETag header of the HTTP response.
3. Handle If-None-Match Header: Accept the If-None-Match header from the client. If the client's If-None-Match value matches the server's ETag, respond with a 304 Not Modified status code and an empty body.
4. Serve Resource if Not Matching: If the client's If-None-Match value does not match the server's ETag, serve the resource with the 200 OK status code and the resource content in the response body.

Key Requirements:

• The server must handle both GET requests.
• The server must correctly generate and validate ETags.
• The server must return the appropriate HTTP status codes (200 OK, 304 Not Modified).
• The server should handle the case where the resource doesn't exist gracefully (return a 404 Not Found).

Expected Behavior:

• The first request for the resource should return the resource content and an ETag header.
• Subsequent requests with a matching If-None-Match header should return a 304 Not Modified response.
• Subsequent requests with a non-matching If-None-Match header should return the resource content and the updated ETag.
• If the resource is modified (simulated by changing the file content), subsequent requests with the old If-None-Match header should return 200 OK with the new content and ETag.
• If the resource is not found, return a 404 Not Found.

Edge Cases to Consider:

• Resource not found.
• Empty resource content.
• Client sends an If-None-Match header with a value that is not a valid ETag (e.g., an empty string). Treat this as a non-match.
• Resource modification (simulated by changing the file content).

Examples

Example 1:

Input: GET request for "/resource" (first request)
Output: HTTP 200 OK
        ETag: "e5d8e5a2a799a9999999999999999999"
        Body: "This is the resource content."
Explanation: The server generates an ETag for the resource content and returns it along with the content.

Example 2:

Input: GET request for "/resource" with If-None-Match: "e5d8e5a2a799a9999999999999999999"
Output: HTTP 304 Not Modified
        ETag: "e5d8e5a2a799a9999999999999999999"
Explanation: The client's `If-None-Match` matches the server's ETag, so a 304 response is returned.

Example 3:

Input: GET request for "/resource" with If-None-Match: "wrong_etag"
Output: HTTP 200 OK
        ETag: "f1a2b3c4d5e6f7a8b9c0d1e2f3a4b5c6" (new ETag after modification)
        Body: "This is the modified resource content."
Explanation: The client's `If-None-Match` does not match the server's ETag (because the resource was modified), so the resource and a new ETag are returned.

Constraints

• The resource file path is hardcoded to "resource.txt" for simplicity.
• The ETag generation should use MD5 hashing. You can use the crypto/md5 package.
• The server should listen on port 8080.
• The resource file should be small (less than 1KB) to avoid performance issues.
• The server should be able to handle at least 10 concurrent requests without significant performance degradation.

Notes

• Consider using the net/http package for building the HTTP server.
• You'll need to read the resource file content and calculate its MD5 hash.
• Remember to handle errors appropriately, such as file not found errors.
• The If-None-Match header value is a string.
• Focus on the core ETag handling logic; error handling and advanced features (e.g., wildcard ETags) are not required for this challenge.
• Simulate resource modification by manually changing the content of "resource.txt" between requests.