fix(mux): require configured herdr anchor (#12)

Author: leduckhc

docs/specs/SPEC-04-multiplexer-adapter-layer.md

@@ -86,8 +86,9 @@ export function getMultiplexer(
 - Keep it pure mechanism: no pino-session knowledge here.
 
 ## Acceptance criteria
-- [x] `HerdrAdapter.isAvailable()` returns true inside a herdr environment,
-      false when `herdr` is missing.
+- [x] `HerdrAdapter.isAvailable()` returns true when herdr is reachable and the
+      configured anchor pane exists, false when `herdr` is missing or the anchor
+      is not usable.
 - [x] `spawnPane({cwd, command:'echo hi; sleep 30', label:'pino: test'})` creates
       a **new, unfocused** pane running the command; `paneExists` is true;
       `setLabel` shows the label; `closePane` removes it and is safe to call twice.
@@ -100,9 +101,11 @@ export function getMultiplexer(
       clean.
 
 ## Decisions (formerly open questions)
-- **Anchor pane / workspace:** `PINO_MUX_ANCHOR` env or `config.json` key
-  `mux.anchor` (default `"pino"`). New panes split into that anchor so sessions
-  don't fragment the user's active layout. Callers can override per-host.
+- **Anchor pane:** `PINO_MUX_ANCHOR` env or `config.json` key `mux.anchor`
+  (default `"pino"` as a setup label). New panes split from that anchor pane id
+  so sessions don't fragment the user's active layout. `HerdrAdapter.isAvailable()`
+  returns false until the configured anchor appears in `herdr pane list`; callers
+  can then fall back instead of failing on `pane_not_found`.
 - **Exec injection:** yes — `HerdrAdapter` accepts an optional `exec` fn in its
   constructor (defaults to `execFile`). Keeps the adapter testable without a real
   herdr binary.

server/src/mux/config.ts

@@ -11,7 +11,7 @@ import { log } from "../log.js";
 export interface MuxConfig {
   /** Active multiplexer name, e.g. "herdr". Env PINO_MUX wins. */
   mux: string;
-  /** Anchor pane id or workspace for new splits. Env PINO_MUX_ANCHOR wins. */
+  /** Anchor pane id for new splits. Env PINO_MUX_ANCHOR wins. */
   anchor: string;
 }
 
@@ -39,7 +39,7 @@ function readConfigFile(file: string): ConfigFileShape {
   }
 }
 
-/** Default anchor: dedicated "pino" workspace so sessions don't fragment the active layout. */
+/** Default anchor label; herdr is unavailable until this resolves to an actual pane id. */
 export const DEFAULT_MUX_ANCHOR = "pino";
 
 /** Load mux config. Env vars override file values. */

server/src/mux/herdr.test.ts

@@ -68,12 +68,20 @@ test("parsePaneList returns [] on garbage", () => {
 
 test("isAvailable true when pane list succeeds", async () => {
   const exec = recordingExec({
-    "herdr pane list": paneListJson([]),
+    "herdr pane list": paneListJson(["w1:p1"]),
   });
-  const adapter = new HerdrAdapter({ exec, anchor: "pino" });
+  const adapter = new HerdrAdapter({ exec, anchor: "w1:p1" });
   assert.equal(await adapter.isAvailable(), true);
 });
 
+test("isAvailable false when configured anchor is missing", async () => {
+  const exec = recordingExec({
+    "herdr pane list": paneListJson(["w1:p1"]),
+  });
+  const adapter = new HerdrAdapter({ exec, anchor: "pino" });
+  assert.equal(await adapter.isAvailable(), false);
+});
+
 test("isAvailable false when herdr missing", async () => {
   const failing: ExecFn = async () => {
     throw Object.assign(new Error("ENOENT"), { code: "ENOENT" });

server/src/mux/herdr.ts

@@ -70,8 +70,8 @@ export class HerdrAdapter implements MultiplexerAdapter {
 
   async isAvailable(): Promise<boolean> {
     try {
-      await this.exec(MUX, ["pane", "list"]);
-      return true;
+      const { stdout } = await this.exec(MUX, ["pane", "list"]);
+      return parsePaneList(stdout).includes(this.anchor);
     } catch {
       return false;
     }