feat(runtime/go): smart ShouldRebuild via go list -deps

[alessandrolattao]

Why

The Go runtime today rebuilds every Lambda in the same go.mod tree whenever any file changes. That's fine with the documented layout (one go.mod per handler), but as soon as a project chooses a single-module monorepo with many handlers — share a services/shared package across N Lambdas, keep one set of dependencies, one go.mod tidy, etc. — sst dev becomes unusable: editing one handler triggers a rebuild of every other handler in the module.

Side effect of one-go.mod-per-Lambda also pays a recurring cost on every CI lint and CI test pass that scales linearly with the Lambda count, since each handler is its own golangci-lint + go test invocation.

This is the same problem the Node runtime already solved: it uses esbuild's metafile to know exactly which files compile into each bundle and rebuilds only the Lambdas whose actual deps changed.

What

Capture each handler's transitive import graph at Build time via go list -deps -json and use it as the rebuild scope. ShouldRebuild fires only when the changed file is in that handler's graph. Works in every layout (per-Lambda, per-domain, single-module monorepo).

Side fix: go build arguments now use a ./ prefix on the source path so layouts where the source is a sub-path don't fail with package <path> is not in std. With the legacy one-go.mod-per-Lambda layout src was always ., so the bug never surfaced.

The previous directory-scope behavior was deliberately removed rather than kept as a fallback. In a single-module repo it would silently fan out to every Lambda when captureDeps failed, hiding a real problem behind a worse one. A captureDeps failure now logs a warning with full context (goflags, gomodcache) and disables ShouldRebuild for that function until the next successful Build.

Tests

`pkg/runtime/golang/golang_test.go` — pure unit tests for `Properties` JSON contract, the goarch mapping, `ShouldRebuild` edge cases (non-go file, missing graph, file hit/miss, relative path resolution), the `GOMODCACHE`/stdlib filter via a fixture JSON stream, a 50×50 race-detector concurrency probe, and a cross-OS `isUnderDir` helper.

`pkg/runtime/golang/build_test.go` — hermetic integration tests against the real `go list` toolchain (env scrubbed: `GOPROXY=off`, `GOTOOLCHAIN=local`, redirected `GOMODCACHE`/`GOCACHE`/`HOME`). Skipped under `-short` and when `go` is not in `PATH`.

`go test ./pkg/runtime/golang/... -race`: 1.6s full, 1.0s under `-short`. `go vet` clean.

Tested manually

Validated end-to-end on a real repo with ~60 Go Lambdas consolidated into one root `services/go.mod`. With this patch:

• editing a single Lambda's `handler.go` rebuilds only that Lambda;
• editing a file in the shared package rebuilds only the Lambdas that import it (cross-Lambda fan-out is correct);
• editing a `_test.go` triggers no rebuild (test files don't compile into the binary);
• `bun run go-lint` time on the same repo dropped from ~10s to <1s warm cache (one `golangci-lint` invocation over the whole module instead of 60).

[Copilot]

Pull request overview

This PR improves the Go runtime’s incremental rebuild behavior in sst dev by capturing each handler’s transitive dependency file set via go list -deps -json during Build, and using that set in ShouldRebuild to rebuild only the functions actually affected by a file change. It also fixes go build invocation for sub-path handlers by forcing ./-prefixed relative package paths, and adds unit + integration tests for the new dependency-capture and rebuild-scope logic.

Changes:

• Capture per-function dependency graphs at build time (go list -deps -json) and use them to scope ShouldRebuild.
• Fix go build argument to use ./-prefixed relative handler paths to avoid stdlib-package misresolution.
• Add unit tests for graph parsing / path helpers / concurrency and hermetic integration tests against the real Go toolchain.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
pkg/runtime/golang/golang.go	Implements dependency capture, new rebuild decision logic, GOMODCACHE filtering, and path fix for go build.
pkg/runtime/golang/golang_test.go	Adds unit tests for Properties contract, arch mapping, ShouldRebuild, isUnderDir, and go-list parsing filters.
pkg/runtime/golang/build_test.go	Adds integration tests exercising captureDeps with real go list in a scrubbed environment.

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[alessandrolattao]

I've just realized that the current version fails to trigger a rebuild for Lambdas when code shared via workspaces/replaces is modified, even if the Lambda depends on it. This update also fixes that issue.