fix(tmux): drain kill-pane stdout to prevent pipe backpressure hang

closeTmuxPane spawned kill-pane with stdout: "pipe" but never drained the
stream, which could leave the subprocess hanging indefinitely when tmux
wrote anything to stdout (for example under --force-close race conditions).

- send-keys now uses stdout: "ignore" so there is no pipe to drain
- kill-pane keeps the pipe but drains stdout/stderr alongside proc.exited
- switch imports to the new spawn-process helper so the behavior is
  covered by hermetic tests that mock the spawn boundary

src/shared/tmux/tmux-utils/pane-close.test.ts

@@ -0,0 +1,197 @@
+import { beforeEach, describe, expect, it, mock } from "bun:test"
+
+type CloseTmuxPane = typeof import("./pane-close").closeTmuxPane
+
+type SpawnCall = {
+	command: string[]
+	options: {
+		stdout?: string
+		stderr?: string
+	}
+}
+
+type FakeSubprocess = {
+	exited: Promise<number>
+	stdout: ReadableStream<Uint8Array>
+	stderr: ReadableStream<Uint8Array>
+}
+
+const TIMEOUT = Symbol("timeout")
+const spawnCalls: SpawnCall[] = []
+const queuedProcesses: FakeSubprocess[] = []
+
+function createClosedStream(): ReadableStream<Uint8Array> {
+	return new ReadableStream<Uint8Array>({
+		start(controller) {
+			controller.close()
+		},
+	})
+}
+
+type DrainSignal = { onPull: () => void }
+
+function createDrainSensitiveStream(byteLength: number, signal: DrainSignal): ReadableStream<Uint8Array> {
+	let remainingBytes = byteLength
+	const chunk = new TextEncoder().encode("x".repeat(16 * 1024))
+
+	return new ReadableStream<Uint8Array>({
+		pull(controller) {
+			signal.onPull()
+
+			if (remainingBytes <= 0) {
+				controller.close()
+				return
+			}
+
+			const nextChunkSize = Math.min(remainingBytes, chunk.byteLength)
+			controller.enqueue(chunk.subarray(0, nextChunkSize))
+			remainingBytes -= nextChunkSize
+		},
+	})
+}
+
+function createProcess(exitCode: number): FakeSubprocess {
+	return {
+		exited: Promise.resolve(exitCode),
+		stdout: createClosedStream(),
+		stderr: createClosedStream(),
+	}
+}
+
+function createStdoutSensitiveProcess(exitCode: number, stdoutBytes: number): FakeSubprocess {
+	let resolveDrained: () => void = () => undefined
+	const drained = new Promise<void>((resolve) => {
+		resolveDrained = resolve
+	})
+	const stdout = createDrainSensitiveStream(stdoutBytes, { onPull: () => resolveDrained() })
+
+	return {
+		exited: drained.then(() => exitCode),
+		stdout,
+		stderr: createClosedStream(),
+	}
+}
+
+const spawnMock = mock((command: string[], options: { stdout?: string; stderr?: string } = {}): FakeSubprocess => {
+	spawnCalls.push({ command, options })
+
+	const process = queuedProcesses.shift()
+	if (!process) {
+		throw new Error(`No fake subprocess configured for ${command.join(" ")}`)
+	}
+
+	return process
+})
+
+const isInsideTmuxMock = mock((): boolean => true)
+const getTmuxPathMock = mock(async (): Promise<string | undefined> => "tmux")
+const logMock = mock(() => undefined)
+
+const paneCloseSpecifier = import.meta.resolve("./pane-close")
+const environmentSpecifier = import.meta.resolve("./environment")
+const loggerSpecifier = import.meta.resolve("../../logger")
+const spawnProcessSpecifier = import.meta.resolve("./spawn-process")
+const tmuxPathResolverSpecifier = import.meta.resolve("../../../tools/interactive-bash/tmux-path-resolver")
+
+async function loadCloseTmuxPane(): Promise<CloseTmuxPane> {
+	const module = await import(`${paneCloseSpecifier}?test=${crypto.randomUUID()}`)
+	return module.closeTmuxPane
+}
+
+function registerModuleMocks(): void {
+	mock.module(spawnProcessSpecifier, () => ({ spawn: spawnMock }))
+	mock.module(environmentSpecifier, () => ({ isInsideTmux: isInsideTmuxMock }))
+	mock.module(tmuxPathResolverSpecifier, () => ({ getTmuxPath: getTmuxPathMock }))
+	mock.module(loggerSpecifier, () => ({ log: logMock }))
+}
+
+function resolveWithin<TResult>(promise: Promise<TResult>, milliseconds: number): Promise<TResult | typeof TIMEOUT> {
+	return Promise.race([
+		promise,
+		new Promise<typeof TIMEOUT>((resolve) => {
+			setTimeout(() => resolve(TIMEOUT), milliseconds)
+		}),
+	])
+}
+
+describe("closeTmuxPane", () => {
+	beforeEach(() => {
+		registerModuleMocks()
+		spawnCalls.length = 0
+		queuedProcesses.length = 0
+		spawnMock.mockClear()
+		isInsideTmuxMock.mockClear()
+		getTmuxPathMock.mockClear()
+		logMock.mockClear()
+
+		isInsideTmuxMock.mockImplementation((): boolean => true)
+		getTmuxPathMock.mockImplementation(async (): Promise<string | undefined> => "tmux")
+	})
+
+	it("#given pane exists #when closeTmuxPane called #then returns true and invokes send-keys + kill-pane in order", async () => {
+		// given
+		const closeTmuxPane = await loadCloseTmuxPane()
+		queuedProcesses.push(createProcess(0), createProcess(0))
+
+		// when
+		const result = await closeTmuxPane("%42")
+
+		// then
+		expect(result).toBe(true)
+		expect(spawnCalls).toEqual([
+			{ command: ["tmux", "send-keys", "-t", "%42", "C-c"], options: { stdout: "ignore", stderr: "ignore" } },
+			{ command: ["tmux", "kill-pane", "-t", "%42"], options: { stdout: "pipe", stderr: "pipe" } },
+		])
+	})
+
+	it("#given not inside tmux #when closeTmuxPane called #then returns false without spawn", async () => {
+		// given
+		const closeTmuxPane = await loadCloseTmuxPane()
+		isInsideTmuxMock.mockImplementation((): boolean => false)
+
+		// when
+		const result = await closeTmuxPane("%42")
+
+		// then
+		expect(result).toBe(false)
+		expect(spawnCalls).toHaveLength(0)
+	})
+
+	it("#given tmux not found #when closeTmuxPane called #then returns false without spawn", async () => {
+		// given
+		const closeTmuxPane = await loadCloseTmuxPane()
+		getTmuxPathMock.mockImplementation(async (): Promise<string | undefined> => undefined)
+
+		// when
+		const result = await closeTmuxPane("%42")
+
+		// then
+		expect(result).toBe(false)
+		expect(spawnCalls).toHaveLength(0)
+	})
+
+	it("#given kill-pane fails #when closeTmuxPane called #then returns false", async () => {
+		// given
+		const closeTmuxPane = await loadCloseTmuxPane()
+		queuedProcesses.push(createProcess(0), createProcess(1))
+
+		// when
+		const result = await closeTmuxPane("%42")
+
+		// then
+		expect(result).toBe(false)
+	})
+
+	it("#given kill-pane stdout stream waits for drain #when closeTmuxPane called #then returns true once drainer consumes stdout", async () => {
+		// given
+		const closeTmuxPane = await loadCloseTmuxPane()
+		queuedProcesses.push(createProcess(0), createStdoutSensitiveProcess(0, 16 * 1024))
+
+		// when
+		const result = await resolveWithin(closeTmuxPane("%42"), 2000)
+
+		// then
+		expect(result).not.toBe(TIMEOUT)
+		expect(result).toBe(true)
+	})
+})

src/shared/tmux/tmux-utils/pane-close.ts

@@ -1,13 +1,18 @@
-import { spawn } from "bun"
-import { getTmuxPath } from "../../../tools/interactive-bash/tmux-path-resolver"
-import { isInsideTmux } from "./environment"
-
 function delay(milliseconds: number): Promise<void> {
 	return new Promise((resolve) => setTimeout(resolve, milliseconds))
 }
 
+async function readStream(stream: ReadableStream<Uint8Array> | null | undefined): Promise<string> {
+	return stream ? new Response(stream).text() : ""
+}
+
 export async function closeTmuxPane(paneId: string): Promise<boolean> {
-	const { log } = await import("../../logger")
+	const [{ log }, { isInsideTmux }, { getTmuxPath }, { spawn }] = await Promise.all([
+		import("../../logger"),
+		import("./environment"),
+		import("../../../tools/interactive-bash/tmux-path-resolver"),
+		import("./spawn-process"),
+	])
 
 	if (!isInsideTmux()) {
 		log("[closeTmuxPane] SKIP: not inside tmux")
@@ -22,8 +27,8 @@ export async function closeTmuxPane(paneId: string): Promise<boolean> {
 
 	log("[closeTmuxPane] sending Ctrl+C for graceful shutdown", { paneId })
 	const ctrlCProc = spawn([tmux, "send-keys", "-t", paneId, "C-c"], {
-		stdout: "pipe",
+		stdout: "ignore",
-		stderr: "pipe",
+		stderr: "ignore",
 	})
 	await ctrlCProc.exited
 
@@ -31,12 +36,15 @@ export async function closeTmuxPane(paneId: string): Promise<boolean> {
 
 	log("[closeTmuxPane] killing pane", { paneId })
 
-	const proc = spawn([tmux, "kill-pane", "-t", paneId], {
+	const killPaneProc = spawn([tmux, "kill-pane", "-t", paneId], {
 		stdout: "pipe",
 		stderr: "pipe",
 	})
-	const exitCode = await proc.exited
+	const [, stderr, exitCode] = await Promise.all([
-	const stderr = await new Response(proc.stderr).text()
+		readStream(killPaneProc.stdout),
+		readStream(killPaneProc.stderr),
+		killPaneProc.exited,
+	])
 
 	if (exitCode !== 0) {
 		log("[closeTmuxPane] FAILED", { paneId, exitCode, stderr: stderr.trim() })

src/shared/tmux/tmux-utils/spawn-process.ts

@@ -0,0 +1 @@
+export { spawn } from "bun"