fix(mcp): serialize shared panel key repair and credit report

CHANGELOG.md

@@ -59,13 +59,13 @@
 
 ## [Unreleased]
 
-- Keep the globe panel working when an MCP client starts more than one
-  server process. Each stdio server made its own panel key, so a client that
-  read the panel page from one process and sent its requests to another
-  (seen in Claude Desktop) got "Only the God's Eye View panel may make this
-  request". Stdio servers of one install now share a key, created once in
-  `.gev-cache/`; a server that cannot store it keeps a key of its own, as
-  before (#927).
+- Stdio servers from one install share a panel key so a page read from one
+  process can make requests through another. Concurrent malformed-key repairs
+  now select one winner; unavailable storage or a busy repair retains the
+  logged per-process fallback. Thanks to [Jibran Tahir](https://github.com/jibraaan)
+  for #958, [MarvinNL046](https://github.com/MarvinNL046) for the report and
+  proposed diagnosis in #927, and [kvnloo](https://github.com/kvnloo) for identifying
+  the repair race. Windows/Claude Desktop Cowork confirmation remains outstanding.
 
 - Street Level: a street-level imagery layer modelled on the iD editor's photo
   overlay, with Mapillary as its first provider (free client token, CC BY-SA 4.0

docs/CURRENT-STATE.md

@@ -14,6 +14,19 @@ new views from the page that frames it; framing is off unless
 `GEV_EMBED_FRAME_ANCESTORS` allows the framing page. MCP leads with the tools that find
 what to show. See [tools and the MCP server](TOOLS.md).
 
+Stdio processes from one install share the panel key in
+`.gev-cache/mcp-panel-key`, resolved from the checkout rather than the client's
+working directory. First creation is atomic; malformed-file repairs select one
+winner and concurrent processes re-read it. A busy repair waits at most two
+seconds before falling back. If the cache cannot be read/written or repair remains
+busy, stderr reports the fallback to a per-process key; cross-process panels can
+still fail in that mode. An abandoned `.repair-lock` is left intact: after
+confirming no repair process is running, remove it to allow repair on restart.
+HTTP MCP servers retain per-server keys. This key is a panel routing guard, not
+authentication. Large-response continuations remain process-local and require
+consistent tool-call routing. Two-process reproduction is covered locally;
+Windows/Claude Desktop Cowork confirmation remains outstanding (#927).
+
 ## Tools and local MCP server — October 1, 2026
 
 `gods-eye-view/tools` defines queries that answer questions from the app's data,

server/mcp/panelKey.js

@@ -20,17 +20,52 @@ function readKey(file) {
   return KEY_PATTERN.test(text) ? text : null;
 }
 
+/** Serialize malformed-file repair; ordinary first creation still uses link(). */
+function repairKey(file, tmp, key) {
+  const lock = `${file}.repair-lock`;
+  const deadline = Date.now() + 2000;
+  const pause = new Int32Array(new SharedArrayBuffer(4));
+  let fd;
+  for (;;) {
+    try {
+      fd = fs.openSync(lock, 'wx', 0o600);
+      break;
+    } catch (error) {
+      if (error.code !== 'EEXIST') throw error;
+      // Another repair may already have published a usable key.
+      const winner = readKey(file);
+      if (winner) return winner;
+      if (Date.now() >= deadline) {
+        throw Object.assign(new Error('panel key repair is busy'), {
+          code: 'ETIMEDOUT',
+        });
+      }
+      Atomics.wait(pause, 0, 0, 10);
+    }
+  }
+  try {
+    // A repair completed between our earlier read and acquiring the lock.
+    const winner = readKey(file);
+    if (winner) return winner;
+    fs.renameSync(tmp, file);
+    return key;
+  } finally {
+    fs.closeSync(fd);
+    fs.rmSync(lock, { force: true });
+  }
+}
+
 /**
  * The panel key every stdio server of this install shares.
  *
  * A client can start more than one server process for one connection: Claude
- * Desktop has been seen serving the globe panel's page from one and sending
- * the page's `panel_request` calls to the other. A key made per process then
- * never matches, and the panel reports that only it may make the request
- * (#927). The first process to run creates the key, without replacing one
+ * Desktop was reported to start two processes; serving the panel's page from
+ * one and sending its `panel_request` calls to the other reproduces #927.
+ * Per-process keys then mismatch, and the panel refuses the request.
+ * The first process to run creates the key, without replacing one
  * another process created at the same moment; the rest read it. When the file
- * cannot be read or written, the process falls back to a key of its own,
- * which is what every process used before.
+ * cannot be read or written, or repair stays busy for two seconds, the process
+ * falls back to its own key, as every process did before.
  *
  * As before, the key keeps `panel_request` from clients that only list it; it
  * is not access control.
@@ -71,10 +106,7 @@ export function sharedPanelKey({
       if (error.code !== 'EEXIST') throw error;
       const winner = readKey(file);
       if (winner) return winner;
-      // A malformed file left by something else: replace it.
-      fs.renameSync(tmp, file);
-      tmp = null;
-      return key;
+      return repairKey(file, tmp, key);
     }
   } catch (error) {
     log(notShared(error));

src/tools/mcpPanelKey.test.mjs

@@ -122,3 +122,79 @@ test('processes starting at the same moment agree on one key', async (t) => {
   assert.equal(new Set(keys).size, 1);
   assert.equal(fs.readFileSync(file, 'utf8').trim(), keys[0]);
 });
+
+test('concurrent malformed-file repairs return the same persisted key', async (t) => {
+  const file = tempKeyFile(t);
+  fs.mkdirSync(path.dirname(file), { recursive: true });
+  fs.writeFileSync(file, 'not a key\n');
+  const script = `
+    import fs from 'node:fs';
+    const { sharedPanelKey } = await import(${JSON.stringify(PANEL_KEY_MODULE)});
+    const [file, marker, other] = process.argv.slice(1);
+    const original = fs.readFileSync;
+    let reads = 0;
+    fs.readFileSync = function (target, ...args) {
+      const text = original.call(this, target, ...args);
+      if (target === file && ++reads === 2) {
+        // Both processes capture the actual malformed contents before either
+        // can replace them. The old unchecked rename returns two different keys.
+        fs.writeFileSync(marker, 'ready');
+        const deadline = Date.now() + 5000;
+        while (!fs.existsSync(other)) {
+          if (Date.now() >= deadline) throw new Error('repair barrier timed out');
+          Atomics.wait(new Int32Array(new SharedArrayBuffer(4)), 0, 0, 5);
+        }
+      }
+      return text;
+    };
+    process.stdout.write(sharedPanelKey({ file }));
+  `;
+  const run = (index) =>
+    new Promise((resolve, reject) => {
+      const marker = (id) => path.join(path.dirname(file), `ready-${id}`);
+      const child = spawn(
+        process.execPath,
+        [
+          '--input-type=module',
+          '-e',
+          script,
+          file,
+          marker(index),
+          marker(1 - index),
+        ],
+        { stdio: ['ignore', 'pipe', 'inherit'] },
+      );
+      t.after(() => child.kill());
+      let out = '';
+      child.stdout.on('data', (chunk) => (out += chunk));
+      child.on('error', reject);
+      child.on('close', (code) =>
+        code === 0 ? resolve(out) : reject(new Error(`exit ${code}`)),
+      );
+    });
+  const keys = await Promise.all([run(0), run(1)]);
+  assert.equal(new Set(keys).size, 1);
+  assert.equal(fs.readFileSync(file, 'utf8').trim(), keys[0]);
+  assert.equal(fs.existsSync(`${file}.repair-lock`), false);
+  assert.equal(
+    fs.readdirSync(path.dirname(file)).some((name) => name.endsWith('.tmp')),
+    false,
+  );
+});
+
+test('an unavailable repair lock falls back without hanging or removing its owner’s lock', (t) => {
+  const file = tempKeyFile(t);
+  fs.mkdirSync(path.dirname(file), { recursive: true });
+  fs.writeFileSync(file, 'not a key\n');
+  fs.writeFileSync(`${file}.repair-lock`, 'held');
+  const logged = [];
+  const key = sharedPanelKey({ file, log: (line) => logged.push(line) });
+  assert.match(key, /^[A-Za-z0-9_-]{43}$/);
+  assert.match(logged[0], /ETIMEDOUT/);
+  assert.equal(fs.readFileSync(file, 'utf8'), 'not a key\n');
+  assert.equal(fs.readFileSync(`${file}.repair-lock`, 'utf8'), 'held');
+  assert.deepEqual(fs.readdirSync(path.dirname(file)).sort(), [
+    'mcp-panel-key',
+    'mcp-panel-key.repair-lock',
+  ]);
+});