src/workspace/affected.ts — git-relative project selection

Purpose

Power --affected[=<base>] and the [<since>] filter form. Resolves the set of project names whose files changed between <since> and the current working tree.

Public surface

export interface AffectedArgs {
  workspaceRoot: string
  since: string // required: ref / commit / branch
  projects: readonly ProjectMeta[]
}

export function affectedProjects(args: AffectedArgs): Promise<Set<string>>

/** Default base when `--affected` has no value. */
export function defaultAffectedBase(workspaceRoot: string): Promise<string>

Algorithm

1. verifyRef(workspaceRoot, since) — git rev-parse --verify --quiet <ref>. Throws UserError if the ref doesn’t resolve locally.
2. git diff --name-only <since> — emits the union of committed + staged + unstaged changes. Matches Turbo’s [<since>] semantics.
3. Each changed path is mapped to its owning project. Projects are sorted by directory length descending so a nested project wins over its parent when a file is a descendant of both (consistent with the project-boundary rule).
4. Returns the deduped set of project names.

defaultAffectedBase:

• Try git symbolic-ref --short -q refs/remotes/origin/HEAD (e.g. origin/main).
• Fall back to HEAD~1 if the symbolic-ref isn’t set (common in CI shallow clones).

Filter integration

cli/run.ts:resolveFilters invokes affectedProjects once per [<since>] filter, stuffs the result in affectedByFilter: Map<ParsedFilter, Set<string>>, then calls applyFilters({ filters, projects, graph, affectedByFilter }).

This separation keeps workspace/filter.ts pure (no FS / no git spawns) — easy to test against in-memory fixtures.

What this does NOT do

• Doesn’t run git fetch. If the local clone is stale, affectedProjects operates on stale refs. CI scripts should fetch first.
• Doesn’t honor .gitattributes or --diff-filter. A whitespace-only commit still marks projects as affected.
• Doesn’t intersect with cache.inputs.files. A project is affected if any file changed under its dir — even if no cached task lists that file as an input. (We default to “be permissive”; Turbo behaves the same.)

Tests

tests/affected.test.ts:

• single file change → owning project selected.
• file in nested project → nested project wins over parent.
• file outside any project → no project selected.
• bad git ref → UserError with the ref name.
• defaultAffectedBase returns origin/HEAD symref then HEAD~1.