Implementing ETag Generation and Validation in Go

This challenge focuses on building robust ETag (Entity Tag) handling within a Go web application. ETags are crucial for HTTP caching, enabling clients to avoid re-downloading resources that haven't changed. You will implement functions to generate a strong ETag from resource data and to validate an incoming ETag against the current resource state.

Problem Description

Your task is to create a Go package that provides functionalities for generating and validating HTTP ETags. This is essential for optimizing web server performance and improving client-side user experience by leveraging conditional HTTP requests.

What needs to be achieved:

1. ETag Generation: Implement a function that takes the content (or a representation of the content) of a resource and generates a unique, strong ETag for it. A "strong" ETag is one that is guaranteed to be different if the content is different.
2. ETag Validation: Implement a function that compares an incoming ETag (from an If-None-Match or If-Match HTTP header) against a newly generated ETag for the current resource.

Key requirements:

• Generation: The ETag should be a string. A common and recommended approach is to use a cryptographic hash (like MD5 or SHA-1) of the resource's content, encoded in hexadecimal. For this challenge, you can use MD5 for simplicity, though SHA-256 would be more secure in production. The ETag should be enclosed in double quotes (e.g., "abcdef123456").
• Validation: The validation function should return true if the incoming ETag matches the generated ETag, and false otherwise. It should handle the double quotes correctly (i.e., compare the content inside the quotes).
• Efficiency: While not a strict performance bottleneck for small resources, consider that ETag generation might be performed frequently.

Expected behavior:

• Generation: Given the same input data, the generation function must always produce the same ETag. Given different input data, it's highly probable (though not strictly guaranteed for MD5 in theory) that different ETags will be produced.
• Validation:
  • If If-None-Match header value matches the current ETag, the resource has not changed, and the server should respond with 304 Not Modified.
  • If If-Match header value matches the current ETag, the condition for modification is met, and the server can proceed with the update.
  • If the ETags do not match for either header, appropriate responses (e.g., 200 OK for If-None-Match, 412 Precondition Failed for If-Match) should be sent.

Edge cases to consider:

• Empty resource content.
• ETags with incorrect formatting (e.g., missing quotes, invalid characters). While the focus is on generation and basic validation, a robust system would handle malformed incoming ETags gracefully. For this challenge, assume well-formed incoming ETags that may or may not match.

Examples

Example 1:

Input Data (byte slice representing resource content): []byte("Hello, World!")

Output ETag: "65a8e27d8879283ada094a3111b3968a" (assuming MD5 hash)

Explanation: The MD5 hash of "Hello, World!" is calculated, and the hexadecimal representation is enclosed in double quotes.

Example 2:

Input Data: []byte("Another piece of content")

Output ETag: "c2e9161b8e1915810073f956a0223255" (assuming MD5 hash)

Explanation: A different MD5 hash is generated for the new content and enclosed in double quotes.

Example 3: Validation

Scenario:

• Current resource content: []byte("Some data")
• Generated ETag for current content: "8a1b1c0d4e5f6a7b8c9d0e1f2a3b4c5d" (hypothetical)
• Incoming If-None-Match header: "8a1b1c0d4e5f6a7b8c9d0e1f2a3b4c5d"

Validation Result: true

Explanation: The incoming If-None-Match ETag exactly matches the current resource's ETag.

Example 4: Validation (Mismatch)

Scenario:

• Current resource content: []byte("Some data")
• Generated ETag for current content: "8a1b1c0d4e5f6a7b8c9d0e1f2a3b4c5d" (hypothetical)
• Incoming If-Match header: "different1234567890abcdef"

Validation Result: false

Explanation: The incoming If-Match ETag does not match the current resource's ETag.

Constraints

• The ETag generation function should accept []byte as input.
• The ETag validation function should accept two string arguments: the incoming ETag and the generated ETag.
• The generated ETag must be a string of the format "<hex_hash>".
• Consider that the input data for generation can be of variable length, from 0 bytes up to several megabytes.
• The validation logic should be case-insensitive for hexadecimal characters in the ETag.

Notes

• For generating the hash, you can use the crypto/md5 package in Go.
• Remember to handle the conversion from the hash sum to a hexadecimal string using encoding/hex.
• When comparing ETags, be mindful of stripping potential leading/trailing whitespace and handling the double quotes.
• Think about how you would incorporate this into a typical HTTP handler in Go's net/http package. While you don't need to build a full server, understanding the context is helpful.
• For production systems, consider using a more collision-resistant hash like SHA-256 and explore mechanisms for more efficient ETag generation for large, frequently changing files (e.g., using file modification times or checksums combined with other metadata). However, for this challenge, a simple hash of the content is sufficient.