nodejs_npm --build-in-source silently produces a dependency-less artifact for an npm workspaces monorepo #933

Description

For an npm workspaces monorepo built with --build-in-source, the NodejsNpmBuilder workflow (the plain one, not NodejsNpmEsbuildBuilder) produces an artifact with no dependencies at all, and reports success.

npm hoists the workspace package's dependencies to the monorepo root, so packages/<fn>/node_modules is never created. NodejsNpmWorkflow._actions_for_linking_source_dependencies_to_artifacts then links source_dir/node_modules into the artifacts directory, and LinkSinglePathAction.execute treats a missing source as nothing to do:

if not source_path.exists():
    # Source path doesn't exist, nothing to symlink
    LOG.debug("Source path %s does not exist, skipping generating symlink", source_path)
    return

The result is a Lambda package that fails at invoke time with Cannot find module, with only a debug-level log at build time.

Steps to reproduce

An npm workspaces monorepo, one function per workspace package:

package.json            { "workspaces": ["packages/*"] }
package-lock.json
packages/fn/package.json  { "dependencies": { "minimal-request-promise": "^1.3.0" } }
packages/fn/included.js   require("minimal-request-promise")

Build packages/fn in source:

LambdaBuilder(language="nodejs", dependency_manager="npm", application_framework=None).build(
    source_dir,          # <monorepo>/packages/fn
    artifacts_dir,
    scratch_dir,
    os.path.join(source_dir, "package.json"),
    runtime="nodejs22.x",
    build_in_source=True,
)

Observed result

artifacts dir:                  ['included.js', 'package.json']
artifacts/node_modules:         ABSENT
source packages/fn/node_modules: None            <- npm hoisted, nothing to link
monorepo root node_modules:     ['.package-lock.json', '@workspaces-monorepo', 'minimal-request-promise']
require.resolve from artifacts dir: UNRESOLVABLE

The build exits successfully.

Expected result

Either the hoisted dependencies reach the artifact, or the build says out loud that it could not find any — a warning rather than a debug log, so the failure surfaces at build time instead of at invoke time.

NodejsNpmEsbuildBuilder is unaffected: esbuild bundles the dependency into the output file, resolving it through the root node_modules.

Additional environment details

• aws-lambda-builders develop (measured at 587257c) and unchanged by fix: honour the lockfile when building nodejs dependencies in source #931
• npm 11.19.0, node 22, Linux; the hoisting behaviour is the same on npm 8/9/10

Raised out of review on #931, which changes which npm command installs those dependencies but not where npm puts them, so the gap predates it and is not made worse by it. Filing it so the workspaces coverage added there does not imply the plain workflow's artifact is covered.