huskies/server/src/agents/pool/auto_assign/merge.rs

//! Merge stage dispatch: trigger server-side merges and auto-spawn mergemaster for content conflicts.

use std::path::Path;

use crate::config::ProjectConfig;
use crate::slog;
use crate::slog_error;
use crate::slog_warn;
use crate::worktree;

use super::super::super::PipelineStage;
use super::super::AgentPool;
use super::scan::{find_free_agent_for_stage, is_story_assigned_for_stage, scan_stage_items};
use super::story_checks::{
    has_content_conflict_failure, has_merge_failure, has_mergemaster_attempted, has_review_hold,
    has_unmet_dependencies, is_story_blocked, is_story_frozen,
};

impl AgentPool {
    /// Process stories in `4_merge/`: trigger server-side squash-merges and auto-spawn
    /// a mergemaster agent when a content-conflict failure is detected.
    ///
    /// Stories with a recorded merge failure may be eligible for automatic mergemaster
    /// dispatch when the failure is a content conflict — otherwise they need human
    /// intervention.  Each eligible story without an active merge job triggers
    /// `trigger_server_side_merge`.
    pub(super) async fn assign_merge_stage(&self, project_root: &Path, config: &ProjectConfig) {
        // ── 4_merge: deterministic server-side merge (no LLM agent) ──────────
        //
        // Stories in 4_merge/ are handled directly by the server rather than by
        // a mergemaster agent.  For each eligible story, trigger start_merge_agent_work
        // which runs the squash-merge pipeline in a background task.  On success
        // the story advances to 5_done automatically.  On failure merge_failure is
        // written to the CRDT and a notification is emitted; the story stays in
        // 4_merge/ until a human intervenes or an explicit `start_agent mergemaster`
        // call invokes the LLM-driven recovery path.
        let merge_items = scan_stage_items(project_root, "4_merge");
        for story_id in &merge_items {
            // Stories with a recorded merge failure may be eligible for
            // automatic mergemaster dispatch when the failure is a content
            // conflict — otherwise they need human intervention.
            if has_merge_failure(project_root, "4_merge", story_id) {
                // Auto-spawn mergemaster for content conflicts, but only once.
                if has_content_conflict_failure(project_root, "4_merge", story_id)
                    && !has_mergemaster_attempted(project_root, "4_merge", story_id)
                    && !is_story_blocked(project_root, "4_merge", story_id)
                {
                    // Find the mergemaster agent.
                    let mergemaster_agent = {
                        let agents = match self.agents.lock() {
                            Ok(a) => a,
                            Err(e) => {
                                slog_error!(
                                    "[auto-assign] Failed to lock agents for mergemaster check: {e}"
                                );
                                continue;
                            }
                        };
                        if is_story_assigned_for_stage(
                            config,
                            &agents,
                            story_id,
                            &PipelineStage::Mergemaster,
                        ) {
                            // Already running — don't spawn again.
                            None
                        } else {
                            find_free_agent_for_stage(config, &agents, &PipelineStage::Mergemaster)
                                .map(str::to_string)
                        }
                    };

                    if let Some(agent_name) = mergemaster_agent {
                        slog!(
                            "[auto-assign] Content conflict on '{story_id}'; \
                             auto-spawning mergemaster '{agent_name}'."
                        );
                        // Record mergemaster_attempted before spawning so a
                        // crash/restart doesn't re-trigger an infinite loop.
                        if let Some(contents) = crate::db::read_content(story_id) {
                            let updated =
                                crate::io::story_metadata::write_mergemaster_attempted_in_content(
                                    &contents,
                                );
                            crate::db::write_content(story_id, &updated);
                            crate::db::write_item_with_content(story_id, "4_merge", &updated);
                        }
                        crate::crdt_state::set_mergemaster_attempted(story_id, true);
                        if let Err(e) = self
                            .start_agent(project_root, story_id, Some(&agent_name), None, None)
                            .await
                        {
                            slog!(
                                "[auto-assign] Failed to start mergemaster '{agent_name}' \
                                 for '{story_id}': {e}"
                            );
                        }
                    }
                }
                continue;
            }

            if has_review_hold(project_root, "4_merge", story_id) {
                continue;
            }

            if is_story_frozen(project_root, "4_merge", story_id) {
                slog!("[auto-assign] Story '{story_id}' in 4_merge/ is frozen; skipping.");
                continue;
            }

            if is_story_blocked(project_root, "4_merge", story_id) {
                continue;
            }

            if has_unmet_dependencies(project_root, "4_merge", story_id) {
                slog!("[auto-assign] Story '{story_id}' in 4_merge/ has unmet deps; skipping.");
                continue;
            }

            // AC6: Detect empty-diff stories before starting the merge pipeline.
            // If the worktree has no commits on the feature branch, block the
            // story immediately via the state machine — no merge job needed.
            if let Some(wt_path) = worktree::find_worktree_path(project_root, story_id)
                && !crate::agents::gates::worktree_has_committed_work(&wt_path)
            {
                let empty_diff_reason = "Feature branch has no code changes — the coder agent \
                     did not produce any commits.";
                slog_warn!(
                    "[auto-assign] Story '{story_id}' in 4_merge/ has no commits \
                     on feature branch. Blocking via state machine."
                );
                if let Err(e) =
                    crate::agents::lifecycle::transition_to_blocked(story_id, empty_diff_reason)
                {
                    slog_error!("[auto-assign] Failed to transition '{story_id}' to Blocked: {e}");
                }
                let _ = self
                    .watcher_tx
                    .send(crate::io::watcher::WatcherEvent::StoryBlocked {
                        story_id: story_id.to_string(),
                        reason: empty_diff_reason.to_string(),
                    });
                continue;
            }

            // Skip if a merge job is already running for this story (e.g. triggered
            // by a previous auto-assign pass or by pipeline advancement).
            let already_running = crate::crdt_state::read_merge_job(story_id.as_str())
                .is_some_and(|job| job.status == "running");
            if already_running {
                continue;
            }

            // Skip if an explicit mergemaster LLM agent is already running
            // (operator-driven failure recovery path).
            let has_mergemaster = {
                let agents = match self.agents.lock() {
                    Ok(a) => a,
                    Err(e) => {
                        slog_error!("[auto-assign] Failed to lock agents: {e}");
                        break;
                    }
                };
                is_story_assigned_for_stage(config, &agents, story_id, &PipelineStage::Mergemaster)
            };
            if has_mergemaster {
                continue;
            }

            slog!("[auto-assign] Triggering server-side merge for '{story_id}' in 4_merge/");
            self.trigger_server_side_merge(project_root, story_id);
        }
    }
}