fix(scripts): stop unconfigured siblings from falsely claiming the sync port base

Independent review caught a real logic defect: readSyncPort defaulted to
SYNC_PORT_BASE whenever a sibling worktree had no sync.port set at all,
so every worktree that hasn't run sync yet (the common case — sync: stays
absent until mongo.uri is present) looked like it was actively occupying
port 3010 to every other worktree's conflict check. That's the exact
opposite of the comment above findNextSyncPort, which assumes the base is
usually free since most worktrees have never had one assigned.

readSyncPort now returns null when sync.port isn't actually configured,
and readSiblingSyncPorts filters those out, so only a worktree that has
genuinely claimed a sync port counts as a claim.

Author: tyler-dane

packages/scripts/src/commands/ensure-sync-config.test.ts

@@ -35,12 +35,14 @@ describe("readSyncPort", () => {
     expect(readSyncPort("sync:\n  port: 3011\n")).toBe(3011);
   });
 
-  it("falls back to the base when unset", () => {
-    expect(readSyncPort("web:\n  port: 9080\n")).toBe(3010);
+  it("returns null when no sync.port is configured, not a default guess", () => {
+    // A worktree with no sync: block yet must not read as claiming
+    // SYNC_PORT_BASE to every other worktree's conflict check.
+    expect(readSyncPort("web:\n  port: 9080\n")).toBeNull();
   });
 
-  it("falls back to the base for malformed yaml", () => {
-    expect(readSyncPort("{{ not yaml")).toBe(3010);
+  it("returns null for malformed yaml", () => {
+    expect(readSyncPort("{{ not yaml")).toBeNull();
   });
 });
 

packages/scripts/src/commands/ensure-sync-config.ts

@@ -31,12 +31,17 @@ import { readFileSync, writeFileSync } from "node:fs";
 export const SYNC_PORT_BASE = 3010;
 const SYNC_DATABASE_NAME = "compass_sync";
 
-export function readSyncPort(yamlText: string): number {
+// null means no sync.port is actually configured — distinct from "configured
+// to SYNC_PORT_BASE", so an unconfigured sibling (the common case; sync: is
+// nullish until mongo.uri is present) doesn't falsely read as claiming the
+// default port to every other worktree's conflict check.
+export function readSyncPort(yamlText: string): number | null {
   try {
     const config = parse(yamlText) as { sync?: { port?: string | number } };
-    return Number(config?.sync?.port) || SYNC_PORT_BASE;
+    const port = Number(config?.sync?.port);
+    return port > 0 ? port : null;
   } catch {
-    return SYNC_PORT_BASE;
+    return null;
   }
 }
 
@@ -93,9 +98,9 @@ export function ensureSyncConfig(
 }
 
 export function readSiblingSyncPorts(root: string): number[] {
-  return siblingConfigPaths(root).map((file) =>
-    readSyncPort(readFileSync(file, "utf8")),
-  );
+  return siblingConfigPaths(root)
+    .map((file) => readSyncPort(readFileSync(file, "utf8")))
+    .filter((port): port is number => port !== null);
 }
 
 // Unlike dev-ports.ts's findNextPorts, this searches from the base itself