Speed up terminal shell selection with bash/sh/ash fast path

Try bash, sh, and ash first with a short alive-probe so common images
attach quickly; fall back to other shells and adaptive probes only after.

server/handlers/terminal.js

@@ -2,9 +2,10 @@
  * Interactive container terminal over protomux-rpc.
  * TTY sessions stream raw PTY bytes (no demux). Supports multi-session via sessionId.
  *
- * Shell selection: try an ordered list of shells (bash, sh, ash, …) until one
- * actually stays running. Docker's exec create often succeeds even when the
- * binary is missing — failure shows up only after start (exit 126/127).
+ * Shell selection: try bash / sh / ash first with a short alive-probe so common
+ * images enter the PTY quickly. Exotic shells and adaptive probe wrappers are
+ * only tried if the fast path fails. Docker's exec create often succeeds even
+ * when the binary is missing — failure shows up only after start (exit 126/127).
  */
 import { PassThrough } from 'stream'
 import { docker } from '../services/docker.js'
@@ -13,27 +14,49 @@ import logger from '../utils/logger.js'
 
 const SESSIONS_KEY = 'terminals'
 
-/** How long to wait for a candidate shell to prove it is alive. */
-const SHELL_PROBE_MS = 450
-const SHELL_PROBE_STEP_MS = 40
+/** Alive-probe for bash/sh/ash fast path — fail missing binaries quickly. */
+const SHELL_PROBE_FAST_MS = 90
+const SHELL_PROBE_FAST_STEP_MS = 15
+/** Alive-probe for fallback shells / probe wrappers. */
+const SHELL_PROBE_MS = 350
+const SHELL_PROBE_STEP_MS = 25
 
 /**
- * Ordered shell candidates. Prefer interactive shells, then POSIX sh variants,
- * busybox, then PATH-relative names. First match that stays Running wins.
+ * Fast path: only bash, sh, ash (absolute + PATH). Tried first with a short
+ * timeout so typical Debian/Ubuntu/Alpine containers attach almost immediately.
  */
-export const DEFAULT_SHELL_CANDIDATES = Object.freeze([
+export const FAST_SHELL_CANDIDATES = Object.freeze([
   ['/bin/bash'],
   ['/usr/bin/bash'],
   ['bash'],
-  ['/bin/zsh'],
-  ['/usr/bin/zsh'],
-  ['zsh'],
   ['/bin/sh'],
   ['/usr/bin/sh'],
   ['sh'],
   ['/bin/ash'],
   ['/usr/bin/ash'],
   ['ash'],
+])
+
+const FAST_SHELL_KEYS = new Set(FAST_SHELL_CANDIDATES.map((c) => c.join('\0')))
+
+/**
+ * Fallback shell candidates after the fast path. Prefer interactive shells,
+ * then POSIX / busybox variants. First match that stays Running wins.
+ * (bash/sh/ash also listed here for completeness; buildShellCandidates dedupes.)
+ */
+export const DEFAULT_SHELL_CANDIDATES = Object.freeze([
+  ['/bin/bash'],
+  ['/usr/bin/bash'],
+  ['bash'],
+  ['/bin/sh'],
+  ['/usr/bin/sh'],
+  ['sh'],
+  ['/bin/ash'],
+  ['/usr/bin/ash'],
+  ['ash'],
+  ['/bin/zsh'],
+  ['/usr/bin/zsh'],
+  ['zsh'],
   ['/bin/dash'],
   ['/usr/bin/dash'],
   ['dash'],
@@ -138,8 +161,18 @@ function toBase64(chunk) {
   return Buffer.isBuffer(chunk) ? chunk.toString('base64') : Buffer.from(chunk).toString('base64')
 }
 
+/**
+ * @param {string[]} cmd
+ * @returns {boolean}
+ */
+export function isFastShellCmd(cmd) {
+  if (!Array.isArray(cmd) || !cmd.length) return false
+  return FAST_SHELL_KEYS.has(cmd.join('\0'))
+}
+
 /**
  * Build ordered unique shell command lists for this startTerminal call.
+ * Order: explicit override → bash/sh/ash (fast) → other shells → probe wrappers.
  * @param {object} args
  * @returns {string[][]}
  */
@@ -165,9 +198,12 @@ export function buildShellCandidates(args = {}) {
     push([String(args.shell)])
   }
 
-  // Prefer adaptive probe wrappers, then each concrete shell
-  for (const c of PROBE_WRAPPERS) push(c)
+  // Fast path: bash → sh → ash only (short probe in openShellExec)
+  for (const c of FAST_SHELL_CANDIDATES) push([...c])
+  // Fallbacks: zsh/fish/busybox/…
   for (const c of DEFAULT_SHELL_CANDIDATES) push([...c])
+  // Heavy adaptive sh -c probe last (only if nothing concrete attached)
+  for (const c of PROBE_WRAPPERS) push(c)
 
   return out
 }
@@ -204,7 +240,12 @@ function destroyStream(stream) {
  * @param {number} [timeoutMs]
  * @returns {Promise<{ ok: boolean, exitCode: number|null|undefined, buffered: Buffer[] }>}
  */
-async function probeShellAlive(exec, stream, timeoutMs = SHELL_PROBE_MS) {
+async function probeShellAlive(
+  exec,
+  stream,
+  timeoutMs = SHELL_PROBE_MS,
+  stepMs = SHELL_PROBE_STEP_MS
+) {
   /** @type {Buffer[]} */
   const buffered = []
   let ended = false
@@ -227,6 +268,7 @@ async function probeShellAlive(exec, stream, timeoutMs = SHELL_PROBE_MS) {
   stream.on('close', onEnd)
 
   const deadline = Date.now() + timeoutMs
+  const pollStep = Math.max(10, Number(stepMs) || SHELL_PROBE_STEP_MS)
   /** @type {number|null|undefined} */
   let exitCode
   let running = false
@@ -253,7 +295,7 @@ async function probeShellAlive(exec, stream, timeoutMs = SHELL_PROBE_MS) {
       detachProbeListeners(stream, onData, onEnd, onError)
       return { ok: false, exitCode: exitCode ?? 127, buffered }
     }
-    await sleep(SHELL_PROBE_STEP_MS)
+    await sleep(pollStep)
   }
 
   // Timeout: accept if still running or we got output (prompt)
@@ -313,11 +355,18 @@ export async function openShellExec(container, candidates, useTty) {
         Tty: useTty,
       })
 
-      const probe = await probeShellAlive(exec, stream)
+      const fast = isFastShellCmd(Cmd)
+      const probe = await probeShellAlive(
+        exec,
+        stream,
+        fast ? SHELL_PROBE_FAST_MS : SHELL_PROBE_MS,
+        fast ? SHELL_PROBE_FAST_STEP_MS : SHELL_PROBE_STEP_MS
+      )
       if (!probe.ok) {
         logger.debug('terminal shell candidate rejected', {
           cmd: label,
           exitCode: probe.exitCode,
+          fast,
         })
         destroyStream(stream)
         lastErr = new Error(