feat(tools): guarded write CAS core with per-path FIFO chain (S4a, #1375)

src/core/task/Task.ts

@@ -103,6 +103,7 @@ import { ToolRepetitionDetector } from "../tools/ToolRepetitionDetector"
 import { restoreTodoListForTask } from "../tools/UpdateTodoListTool"
 import { FileContextTracker } from "../context-tracking/FileContextTracker"
 import { RooIgnoreController } from "../ignore/RooIgnoreController"
+import { ObservationRegistry } from "./observationRegistry"
 import { RooProtectedController } from "../protect/RooProtectedController"
 import { type AssistantMessageContent, presentAssistantMessage } from "../assistant-message"
 import { NativeToolCallParser } from "../assistant-message/NativeToolCallParser"
@@ -181,6 +182,7 @@ export class Task extends EventEmitter<TaskEvents> implements TaskLike {
 	readonly parentTask: Task | undefined = undefined
 	readonly taskNumber: number
 	readonly workspacePath: string
+	readonly observationRegistry = new ObservationRegistry()
 
 	/**
 	 * The mode associated with this task. Persisted across sessions

src/core/task/__tests__/observationRegistry.spec.ts

@@ -0,0 +1,72 @@
+import { describe, it, expect, vi } from "vitest"
+
+import { ObservationRegistry } from "../observationRegistry"
+
+describe("ObservationRegistry", () => {
+	it("observe → get returns the recorded version and observedAt", () => {
+		const reg = new ObservationRegistry()
+		reg.observe("/a/b/c.ts", "1:2:300:4000000000:5000000000")
+
+		const obs = reg.get("/a/b/c.ts")
+		expect(obs).toBeDefined()
+		expect(obs!.version).toBe("1:2:300:4000000000:5000000000")
+		expect(typeof obs!.observedAt).toBe("number")
+	})
+
+	it("re-observe replaces the entry with a fresh observedAt", () => {
+		vi.useFakeTimers()
+		const reg = new ObservationRegistry()
+		reg.observe("/a/b/c.ts", "v1")
+		const first = reg.get("/a/b/c.ts")!
+		expect(first.version).toBe("v1")
+
+		vi.advanceTimersByTime(50)
+		reg.observe("/a/b/c.ts", "v2")
+		const second = reg.get("/a/b/c.ts")!
+		expect(second.version).toBe("v2")
+		expect(second.observedAt).toBeGreaterThan(first.observedAt)
+
+		vi.useRealTimers()
+	})
+
+	it("has returns true for observed paths, false otherwise", () => {
+		const reg = new ObservationRegistry()
+		reg.observe("/x.ts", "t1")
+		expect(reg.has("/x.ts")).toBe(true)
+		expect(reg.has("/y.ts")).toBe(false)
+	})
+
+	it("size reflects the number of observed entries", () => {
+		const reg = new ObservationRegistry()
+		expect(reg.size).toBe(0)
+		reg.observe("/a.ts", "t1")
+		reg.observe("/b.ts", "t2")
+		expect(reg.size).toBe(2)
+	})
+
+	it("clear removes all entries and resets size to 0", () => {
+		const reg = new ObservationRegistry()
+		reg.observe("/a.ts", "t1")
+		reg.observe("/b.ts", "t2")
+		reg.clear()
+		expect(reg.size).toBe(0)
+		expect(reg.get("/a.ts")).toBeUndefined()
+		expect(reg.has("/b.ts")).toBe(false)
+	})
+
+	it("get on empty registry returns undefined", () => {
+		const reg = new ObservationRegistry()
+		expect(reg.get("/any.ts")).toBeUndefined()
+	})
+
+	it("separate instances are independent — observing in one does not appear in the other", () => {
+		const regA = new ObservationRegistry()
+		const regB = new ObservationRegistry()
+		regA.observe("/shared.ts", "v1")
+		expect(regA.get("/shared.ts")).toBeDefined()
+		expect(regB.get("/shared.ts")).toBeUndefined()
+		regB.observe("/shared.ts", "v2")
+		expect(regA.get("/shared.ts")!.version).toBe("v1")
+		expect(regB.get("/shared.ts")!.version).toBe("v2")
+	})
+})

src/core/task/observationRegistry.ts

@@ -0,0 +1,47 @@
+/**
+ * Per-task file observation registry (upstream epic #1375, phase A2).
+ *
+ * Each Task owns its own instance so parent and subtask observations are
+ * independent. The S4 guarded-write will compare these versions against the
+ * token recomputed pre-write to detect stale reads or file replacement.
+ *
+ * Pure in-memory — zero I/O, no dependencies. No behavior change in this PR:
+ * observations are recorded but not consulted.
+ */
+
+export interface FileObservation {
+	/** Version token derived from on-disk fs.stat (bigint mode). */
+	version: string
+	/** Millisecond timestamp when the observation was recorded. */
+	observedAt: number
+}
+
+export class ObservationRegistry {
+	private readonly entries = new Map<string, FileObservation>()
+
+	/**
+	 * Record an observation for a file at its absolute path.
+	 *
+	 * Re-observing replaces the entry with a fresh observedAt timestamp and
+	 * the new version token.
+	 */
+	observe(absolutePath: string, version: string): void {
+		this.entries.set(absolutePath, { version, observedAt: Date.now() })
+	}
+
+	get(absolutePath: string): FileObservation | undefined {
+		return this.entries.get(absolutePath)
+	}
+
+	has(absolutePath: string): boolean {
+		return this.entries.has(absolutePath)
+	}
+
+	clear(): void {
+		this.entries.clear()
+	}
+
+	get size(): number {
+		return this.entries.size
+	}
+}

src/core/tools/ReadFileTool.ts

@@ -16,6 +16,7 @@ import type { ReadFileParams, ReadFileMode, ReadFileToolParams, FileEntry, LineR
 import { isLegacyReadFileParams, type ClineSayTool } from "@roo-code/types"
 
 import { Task } from "../task/Task"
+import { versionTokenOfStat } from "../../utils/versionToken"
 import { formatResponse } from "../prompts/responses"
 import { RecordSource } from "../context-tracking/FileContextTrackerTypes"
 import { isPathOutsideWorkspace } from "../../utils/pathUtils"
@@ -214,12 +215,29 @@ export class ReadFileTool extends BaseTool<"read_file"> {
 					// Read text file content with lossy UTF-8 conversion
 					// Reading as Buffer first allows graceful handling of non-UTF8 bytes
 					// (they become U+FFFD replacement characters instead of throwing)
+					// A2 (epic #1375): capture the on-disk token before the read so a mutation
+					// landing mid-read is detected by the post-read stat below.
+					const preReadStats = await fs.stat(fullPath, { bigint: true }).catch(() => undefined)
 					const buffer = await fs.readFile(fullPath)
 					const fileContent = buffer.toString("utf-8")
 					const result = this.processTextFile(fileContent, entry)
 
 					await task.fileContextTracker.trackFileContext(relPath, "read_tool" as RecordSource)
 
+					// A2 (plan #33 / epic #1375): record the observed on-disk version for the future write guard.
+					// The token is captured before AND after the read; the target is observed only
+					// when both match — a mutation between the two stats means the content the model
+					// received is not the on-disk state, and observing it would let a later write
+					// match a token the model never saw. A stat failure leaves the target
+					// unobserved and never fails the read.
+					const postReadStats = await fs.stat(fullPath, { bigint: true }).catch(() => undefined)
+					if (preReadStats && postReadStats) {
+						const preReadToken = versionTokenOfStat(preReadStats)
+						if (preReadToken === versionTokenOfStat(postReadStats)) {
+							task.observationRegistry.observe(fullPath, preReadToken)
+						}
+					}
+
 					updateFileResult(relPath, {
 						nativeContent: `File: ${relPath}\n${result}`,
 					})
@@ -768,6 +786,9 @@ export class ReadFileTool extends BaseTool<"read_file"> {
 				}
 
 				// Read text file
+				// A2 (epic #1375): capture the on-disk token before the read so a mutation
+				// landing mid-read is detected by the post-read stat below.
+				const preReadStats = await fs.stat(fullPath, { bigint: true }).catch(() => undefined)
 				const rawContent = await fs.readFile(fullPath, "utf8")
 
 				// Handle line ranges if specified
@@ -799,6 +820,19 @@ export class ReadFileTool extends BaseTool<"read_file"> {
 
 				// Track file in context
 				await task.fileContextTracker.trackFileContext(relPath, "read_tool")
+
+				// A2 (plan #33 / epic #1375): mirror the native path — record the observed
+				// on-disk version so legacy-format reads also feed the future write guard.
+				// Observe only when the pre-read and post-read tokens match (a mutation between
+				// them means the returned content is not the on-disk state). A stat failure
+				// leaves the target unobserved and never fails the read.
+				const postReadStats = await fs.stat(fullPath, { bigint: true }).catch(() => undefined)
+				if (preReadStats && postReadStats) {
+					const preReadToken = versionTokenOfStat(preReadStats)
+					if (preReadToken === versionTokenOfStat(postReadStats)) {
+						task.observationRegistry.observe(fullPath, preReadToken)
+					}
+				}
 			} catch (error) {
 				const errorMsg = error instanceof Error ? error.message : String(error)
 				results.push(`File: ${relPath}\nError: ${errorMsg}`)