ngit-grasp/docs/explanation/decisions.md

Architecture Decision Summary

Question: Pre-receive Hook vs. Inline Authorization?

After investigating the git-http-backend Rust crate and the reference implementation, we have determined that inline authorization is both pragmatic and superior.

Investigation Findings

git-http-backend Crate Analysis

The git-http-backend crate (v0.1.3) provides:

1. Low-level Git protocol handling via actix-web handlers
2. Process spawning of git-receive-pack and git-upload-pack
3. Stream-based I/O between HTTP and Git processes
4. Flexible path rewriting through the GitConfig trait

Key Finding: The crate spawns Git as a subprocess in git_receive_pack.rs. We can intercept before this spawn happens.

Reference Implementation (ngit-relay) Analysis

The Go-based reference uses:

1. nginx as HTTP frontend
2. git-http-backend (C binary) for Git protocol
3. Pre-receive hook (Go binary) for authorization
4. Khatru (Go) for Nostr relay
5. supervisord for process management
6. Docker for packaging

The pre-receive hook:

• Reads ref updates from stdin
• Queries local Nostr relay via WebSocket
• Validates each ref against state events
• Exits with 0 (accept) or 1 (reject)
• Errors printed to stderr appear as remote: messages in git client

Decision: Inline Authorization ✅

Why This Is Pragmatic

1. The crate supports it: We can implement a custom git_receive_pack handler that validates before spawning Git
2. Better error handling: Direct HTTP responses vs. parsing hook stderr
3. Simpler deployment: Single binary, no hook management
4. Easier testing: Pure Rust unit tests, no shell scripts
5. Performance: Avoid spawning Git for invalid pushes
6. Type safety: Share types between Git and Nostr modules

Implementation Approach

// Instead of using git-http-backend's handler as-is:
pub async fn git_receive_pack(
    req: HttpRequest,
    body: web::Payload,
    state: web::Data<AppState>,
) -> Result<HttpResponse> {
    // 1. Parse repository path from URL
    let (npub, identifier) = parse_repo_path(&req)?;
    
    // 2. Buffer enough of the request to parse ref updates
    let ref_updates = parse_ref_updates(&body).await?;
    
    // 3. VALIDATE AGAINST NOSTR STATE
    let validator = PushValidator::new(&state.nostr_client);
    match validator.validate_push(&npub, &identifier, &ref_updates).await {
        Ok(_) => {
            // 4. Valid! Spawn git-receive-pack and stream
            spawn_git_receive_pack(req, body, state).await
        }
        Err(e) => {
            // 5. Invalid! Return HTTP error
            Ok(HttpResponse::Forbidden()
                .body(format!("Push rejected: {}", e)))
        }
    }
}

Advantages Over Hooks

Aspect	Pre-receive Hook	Inline Authorization
Error messages	Via stderr, prefixed with remote:	Direct HTTP response body
Testing	Requires Git repo setup	Pure Rust unit tests
Debugging	Hook logs separate from server	Unified logging
Deployment	Symlinks, permissions, hook scripts	Single binary
Performance	Always spawn Git	Skip Git for invalid pushes
State sharing	IPC or network	Direct memory access
Type safety	Separate binaries	Shared Rust types

Potential Concerns & Mitigations

Concern: "What if we need to validate the actual pack data, not just refs?"

Mitigation: We can still do this inline! Parse the pack stream before forwarding to Git. The git-http-backend crate already buffers the request body.

Concern: "Doesn't Git expect hooks for certain operations?"

Mitigation: We're not eliminating hooks entirely. Post-receive hooks might still be useful for notifications. We're just moving authorization out of hooks.

Concern: "What about compatibility with standard Git setups?"

Mitigation: The Git Smart HTTP protocol is standardized. Our inline validation is transparent to clients. We're still using real Git repositories and spawning real git-receive-pack.

Comparison with Reference Implementation

Reference (ngit-relay)

Client → nginx → git-http-backend → Git → pre-receive hook → validate → accept/reject
                                              ↓
                                    Query Nostr relay (WebSocket)

Our Approach (ngit-grasp)

Client → actix-web → validate → Git → accept
                        ↓
                Query Nostr relay (in-process)
                        ↓
                     reject ← return HTTP error

Implementation Complexity

Hook-based (if we went that route)

• ✅ Simpler: Follow reference implementation
• ❌ More components: Hook binaries, symlinks
• ❌ More complex testing: Need Git repos, shell scripts
• ❌ More complex deployment: Hook installation, permissions

Inline (our choice)

• ❌ More complex: Custom Git protocol handling
• ✅ Fewer components: Single binary
• ✅ Simpler testing: Pure Rust
• ✅ Simpler deployment: Just run the binary

Verdict: Slightly more complex initially, but much simpler long-term.

Code Reuse from Reference

We can still reuse the logic from the reference implementation:

• Maintainer recursion algorithm
• State validation logic
• Event filtering policies
• Repository provisioning workflow

We're just implementing it in Rust within our HTTP handlers rather than in Git hooks.

Conclusion

Inline authorization is both pragmatic and superior for a Rust implementation.

The git-http-backend crate provides sufficient flexibility through its handler architecture. By intercepting at the HTTP layer, we gain:

1. Better error handling and user experience
2. Simpler deployment and operations
3. Easier testing and debugging
4. Better performance characteristics
5. Tighter integration between components

The additional complexity of parsing the Git protocol is minimal compared to the benefits, and we're still using the standard Git binaries for the actual repository operations.

Next Steps

1. ✅ Document architecture (this file + ARCHITECTURE.md)
2. ⏭️ Set up project structure with Cargo workspace
3. ⏭️ Implement core types (RefUpdate, RepositoryState, etc.)
4. ⏭️ Implement Git protocol parsing
5. ⏭️ Implement Nostr relay with policies
6. ⏭️ Implement push validation logic
7. ⏭️ Integration tests
8. ⏭️ GRASP-01 compliance testing

Purgatory Implementation (2025-12-23)

Implemented according to design specification in purgatory-design.md. No significant deviations from original design.

Implementation approach:

• Phases 1-7 completed sequentially as planned
• All data structures match design specifications
• Integration points implemented as designed

Key technical choices:

1. Optional Purgatory in Git Handlers: Used Option<Arc<Purgatory>> in git handler signatures for backward compatibility. This allows git handlers to function even when no purgatory is provided (e.g., in minimal test setups).

2. Cleanup Interval: Background cleanup task runs every 60 seconds as designed, removing expired entries from both state and PR stores.

3. Thread-Safe Storage: Used Arc<DashMap> for lock-free concurrent access, enabling safe sharing between HTTP handlers, WebSocket handlers, and background tasks.

4. Late Binding Implementation: Ref extraction logic in helpers.rs extracts refs at git push time, not event arrival time, as specified in the design.

Test integration:

• Existing test code in grasp-audit was uncommented (Phases 7)
• No new integration tests added (as instructed)
• Test verification enabled in grasp-audit/src/client.rs and grasp-audit/src/specs/grasp01/push_authorization.rs

Related Documentation:

• Design: purgatory-design.md
• Architecture: architecture.md
• Implementation Plan: ../purgatory-implementation-plan.md