From 2c5cd845a2ec3512cf983f62f10bd88eea61ee25 Mon Sep 17 00:00:00 2001
From: OrcaWin <alpha-eng@stably.ai>
Date: Tue, 8 Sep 2026 19:40:10 -0700
Subject: [PATCH] perf: append to owned project source lists without repeated
 copying (#19465)

* perf: append to owned project source lists without repeated copying

* docs(projects): record draft-ownership rule for in-place project merge

---------

Co-authored-by: m4air <m4air@m4airs-MacBook-Air.local>
Co-authored-by: Neil <4138956+nwparker@users.noreply.github.com>
---
 .../project-host-setup-projection.test.ts     | 30 ++++++++++++++
 src/shared/project-host-setup-projection.ts   | 40 ++++++++++---------
 2 files changed, 52 insertions(+), 18 deletions(-)

diff --git a/src/shared/project-host-setup-projection.test.ts b/src/shared/project-host-setup-projection.test.ts
index 67fca1ad6b5..f6eeb9a0bdb 100644
--- a/src/shared/project-host-setup-projection.test.ts
+++ b/src/shared/project-host-setup-projection.test.ts
@@ -684,3 +684,33 @@ describe('getProjectHostSetupWorktreeMeta host selection', () => {
     expect(getProjectHostSetupWorktreeMeta(setups, sshRepo).hostId).toBe('ssh:build-box')
   })
 })
+
+it('appends project source IDs without repeatedly copying an accumulating array', () => {
+  const repos = Array.from({ length: 2000 }, (_, i) =>
+    repo({
+      id: `source-${i}`,
+      path: `/repos/${i}`,
+      displayName: `Repo ${i}`,
+      upstream: { owner: 'acme', repo: 'project' }
+    })
+  )
+  let copied = 0
+  const iterator = Array.prototype[Symbol.iterator]
+  Array.prototype[Symbol.iterator] = function (this: unknown[]) {
+    if (this[0] === 'source-0') {
+      copied += this.length
+    }
+    return iterator.call(this)
+  }
+  let result: ReturnType<typeof projectHostSetupProjectionFromRepos>
+  try {
+    result = projectHostSetupProjectionFromRepos([...repos, repos[0]])
+  } finally {
+    Array.prototype[Symbol.iterator] = iterator
+  }
+  expect(copied).toBeLessThan(10_000)
+  expect(result.projects).toHaveLength(1)
+  expect(result.projects[0].sourceRepoIds).toEqual(repos.map((repo) => repo.id))
+  expect(result.setups).toHaveLength(2001)
+  expect(repos[0].displayName).toBe('Repo 0')
+})
diff --git a/src/shared/project-host-setup-projection.ts b/src/shared/project-host-setup-projection.ts
index 954208ee27d..794fd625b12 100644
--- a/src/shared/project-host-setup-projection.ts
+++ b/src/shared/project-host-setup-projection.ts
@@ -8,8 +8,15 @@ import { githubRepoIdentityKey, isDefaultGitHubHost } from './github/repository-
 import type { Project, ProjectHostSetup, ProjectProviderIdentity } from './project-types'
 import type { Repo } from './repo-types'
 
+/**
+ * A projection-local draft. `project` is always freshly built by `createProjectFromRepo`, never a
+ * caller's or persisted row, because `mergeProjectRepo` writes to it in place; seeding it from an
+ * existing `Project` would leak those writes to whoever else holds that row. `sourceRepoIds`
+ * mirrors `project.sourceRepoIds` so membership is a lookup rather than a rescan.
+ */
 type ProjectAccumulator = {
   project: Project
+  sourceRepoIds: Set<string>
 }
 
 export type ProjectHostSetupProjection = {
@@ -262,19 +269,16 @@ function createProjectFromRepo(repo: Repo): Project {
   }
 }
 
-function mergeProjectRepo(project: Project, repo: Repo): Project {
-  const sourceRepoIds = project.sourceRepoIds.includes(repo.id)
-    ? project.sourceRepoIds
-    : [...project.sourceRepoIds, repo.id]
-  // Why unknown-aware on both sides: the accumulator itself carries 0 when the first repo of the
-  // project had no addedAt, so a plain min() would let repo order decide the project's createdAt.
-  const addedAt = catalogTimestampFromAddedAt(repo.addedAt)
-  return {
-    ...project,
-    sourceRepoIds,
-    createdAt: mergeCatalogCreatedAt(project.createdAt, addedAt),
-    updatedAt: mergeCatalogUpdatedAt(project.updatedAt, addedAt)
+/** Mutates the draft in place — only ever call this on an accumulator this projection owns. */
+function mergeProjectRepo(accumulator: ProjectAccumulator, repo: Repo): void {
+  const { project, sourceRepoIds } = accumulator
+  if (!sourceRepoIds.has(repo.id)) {
+    sourceRepoIds.add(repo.id)
+    project.sourceRepoIds.push(repo.id)
   }
+  const addedAt = catalogTimestampFromAddedAt(repo.addedAt)
+  project.createdAt = mergeCatalogCreatedAt(project.createdAt, addedAt)
+  project.updatedAt = mergeCatalogUpdatedAt(project.updatedAt, addedAt)
 }
 
 function createSetupFromRepo(repo: Repo, projectId: string): ProjectHostSetup {
@@ -312,15 +316,15 @@ export function projectHostSetupProjectionFromRepos(
   for (const repo of repos) {
     const projectId = getProjectId(repo)
     const existing = projectById.get(projectId)
-    const project = existing
-      ? mergeProjectRepo(existing.project, repo)
-      : createProjectFromRepo(repo)
+    if (existing) {
+      mergeProjectRepo(existing, repo)
+    } else {
+      const project = createProjectFromRepo(repo)
+      projectById.set(projectId, { project, sourceRepoIds: new Set(project.sourceRepoIds) })
+    }
     // Why normalize here: a repo row is untrusted persisted/wire data too, and these
     // constructors copy repo.path / repo.id straight onto fields consumers call .trim() on.
     const setup = normalizeProjectHostSetupRow(createSetupFromRepo(repo, projectId))
-    projectById.set(projectId, {
-      project
-    })
     setups.push(setup)
   }