fusion/packages/engine/src/plan-artifact-writeback.ts

import { readFile } from "node:fs/promises";
import { join } from "node:path";

import type { TaskStore } from "@fusion/core";

/*
FNXC:PlanArtifactPersistence 2026-07-26-03:55:
Planning sessions run in the TASK's own worktree (see FNXC:NodeWorktreeIsolation in triage.ts), and they
carry the full coding tool surface. The system prompt tells the planner to persist through
`fn_task_prompt_write`, but that is a soft instruction: a planner that reaches for the generic write tool
writes the relative spec path (`.fusion/tasks/<id>/PROMPT.md`) against its own cwd, so the spec lands
INSIDE the worktree. Triage then finalizes by reading `<rootDir>/<promptPath>`, sees nothing, and fails
deterministic validation — and the worktree copy is destroyed with the worktree.

Two durability requirements follow, and this module owns both:
1. A plan written inside a worktree is copied back into the main project `.fusion/` folder. The copy goes
   through `store.updateTask({ prompt })`, which is the single validated persistence path (File Scope
   validation, root PROMPT.md write, and task.json sync stay together and stay atomic).
2. The authoritative plan is mirrored into the project database. `project.tasks` has no `prompt` column —
   PROMPT.md is filesystem-only and is hydrated on read — so losing the project checkout loses every spec.
   The mirror uses the existing `task_documents` store under the `plan` key, which triage already reads as
   a planning-draft fallback (`readNonEmptyPlanningDraft`), so recovery has a DB-backed source of truth.
*/

/** Relative, cwd-anchored path handed to the planning agent for a task's spec. */
export function relativePromptPath(taskId: string): string {
  return `.fusion/tasks/${taskId}/PROMPT.md`;
}

/** The `task_documents` key used to mirror the authoritative plan into the project DB. */
export const PLAN_DOCUMENT_KEY = "plan";

export interface PlanWritebackLogger {
  log?: (message: string) => void;
  warn?: (message: string) => void;
}

export interface ReconcileWorktreePlanArtifactOptions {
  store: TaskStore;
  taskId: string;
  /** Project root checkout — the authoritative `.fusion/` location. */
  rootDir: string;
  /** cwd the planning session ran in. Equal to `rootDir` when planning did not get a worktree. */
  planningCwd: string;
  logger?: PlanWritebackLogger;
}

export type PlanWritebackOutcome =
  /** Planning ran in the project root; there is no separate copy to reconcile. */
  | "not-worktree"
  /** No spec file inside the worktree — the planner used the durable writer, as instructed. */
  | "no-worktree-artifact"
  /** Worktree copy is empty/whitespace; nothing worth rescuing. */
  | "worktree-artifact-empty"
  /** Worktree copy matches the authoritative root copy already. */
  | "already-authoritative"
  /** Worktree copy was copied back into the project `.fusion/` folder. */
  | "recovered"
  /** A worktree copy existed but persisting it failed; the root copy is untouched. */
  | "recovery-failed";

export interface ReconcileWorktreePlanArtifactResult {
  outcome: PlanWritebackOutcome;
  /** Authoritative plan content after reconciliation, when one could be resolved. */
  content?: string;
}

async function readIfPresent(path: string): Promise<string | null> {
  try {
    return await readFile(path, "utf-8");
  } catch {
    return null;
  }
}

/**
 * FNXC:PlanArtifactPersistence 2026-07-26-03:55:
 * Copy a worktree-local PROMPT.md back into the main project `.fusion/` folder.
 *
 * Only rescues a STRANDED plan: when the root copy already matches, or the worktree holds nothing, this
 * is a no-op. The root copy is never overwritten with an empty or whitespace-only worktree file, so a
 * planner that used `fn_task_prompt_write` correctly cannot have its spec clobbered by a stale stub the
 * worktree happened to inherit.
 *
 * Failure is non-fatal: triage's own deterministic validation still owns the "no usable spec" verdict, and
 * a failed rescue must not convert a recoverable planning pass into a hard error.
 */
export async function reconcileWorktreePlanArtifact(
  options: ReconcileWorktreePlanArtifactOptions,
): Promise<ReconcileWorktreePlanArtifactResult> {
  const { store, taskId, rootDir, planningCwd, logger } = options;
  const relPath = relativePromptPath(taskId);

  const rootContent = await readIfPresent(join(rootDir, relPath));
  if (planningCwd === rootDir) {
    return { outcome: "not-worktree", content: rootContent ?? undefined };
  }

  const worktreeContent = await readIfPresent(join(planningCwd, relPath));
  if (worktreeContent === null) {
    return { outcome: "no-worktree-artifact", content: rootContent ?? undefined };
  }
  if (worktreeContent.trim().length === 0) {
    return { outcome: "worktree-artifact-empty", content: rootContent ?? undefined };
  }
  if (rootContent === worktreeContent) {
    return { outcome: "already-authoritative", content: rootContent };
  }

  try {
    // The single validated persistence path: File Scope validation + root PROMPT.md write + task.json sync.
    await store.updateTask(taskId, { prompt: worktreeContent });
    logger?.log?.(
      `${taskId}: recovered a worktree-local PROMPT.md into the project .fusion folder (${planningCwd})`,
    );
    return { outcome: "recovered", content: worktreeContent };
  } catch (error) {
    const message = error instanceof Error ? error.message : String(error);
    logger?.warn?.(
      `${taskId}: failed to copy the worktree-local PROMPT.md back into the project .fusion folder: ${message}`,
    );
    return { outcome: "recovery-failed", content: rootContent ?? undefined };
  }
}

/**
 * FNXC:PlanArtifactPersistence 2026-07-26-03:55:
 * Mirror the authoritative plan into the project database under the `plan` task document.
 *
 * `project.tasks` carries no `prompt` column, so without this the spec exists only as a file in the
 * project checkout. The `plan` document is already the draft surface triage falls back to when PROMPT.md
 * is absent, so mirroring here makes that recovery path DB-backed instead of filesystem-only.
 *
 * Best-effort by design: a mirror failure must never fail the planning pass that just produced a good spec.
 * Re-mirroring identical content is skipped so repeated prompt writes do not churn document revisions.
 */
export async function mirrorPlanToProjectDb(
  store: TaskStore,
  taskId: string,
  content: string,
  options: { author?: string; logger?: PlanWritebackLogger } = {},
): Promise<boolean> {
  if (content.trim().length === 0) return false;
  if (typeof store.upsertTaskDocument !== "function") return false;

  try {
    if (typeof store.getTaskDocument === "function") {
      const existing = await store.getTaskDocument(taskId, PLAN_DOCUMENT_KEY);
      if (typeof existing?.content === "string" && existing.content === content) return false;
    }
    await store.upsertTaskDocument(taskId, {
      key: PLAN_DOCUMENT_KEY,
      content,
      author: options.author ?? "agent",
    });
    return true;
  } catch (error) {
    const message = error instanceof Error ? error.message : String(error);
    options.logger?.warn?.(`${taskId}: failed to mirror the plan into the project database: ${message}`);
    return false;
  }
}

/**
 * FNXC:PlanArtifactPersistence 2026-07-26-03:55:
 * Combined post-planning durability pass: rescue a worktree-stranded spec into the project `.fusion/`
 * folder, then mirror whatever is authoritative afterwards into the project database. Callers run this
 * BEFORE reading the finalized spec so the read observes the recovered content.
 */
export async function persistPlanArtifact(
  options: ReconcileWorktreePlanArtifactOptions & { author?: string },
): Promise<ReconcileWorktreePlanArtifactResult & { mirrored: boolean }> {
  const result = await reconcileWorktreePlanArtifact(options);
  const mirrored = result.content
    ? await mirrorPlanToProjectDb(options.store, options.taskId, result.content, {
      author: options.author,
      logger: options.logger,
    })
    : false;
  return { ...result, mirrored };
}