fix(module-loader): resolve a barrel's relative imports against its own directory (depends on #2999)

src/transforms/mdx/esm-module-loader/module-fetcher/index.ts

@@ -21,7 +21,7 @@ import { getModulePathCache } from "../cache/index.ts";
 import { hashString } from "../utils/hash.ts";
 import { resolveModuleFile } from "../resolution/file-finder.ts";
 import { getTransformCacheKey, getVersionedPathCacheKey } from "./cache-keys.ts";
-import { resolveNestedModuleImports } from "./nested-imports.ts";
+import { resolveNestedImportBase, resolveNestedModuleImports } from "./nested-imports.ts";
 import { readDistributedCache } from "./distributed-cache.ts";
 import { resolveUnresolvedModuleViaHttpFallback } from "./http-fallback.ts";
 import { normalizePath } from "./module-cache.ts";
@@ -285,6 +285,7 @@ async function doFetchAndCacheModule(
       moduleCode,
       esmCacheDir,
       normalizedPath,
+      parentBasePath: resolveNestedImportBase(normalizedPath, actualFilePath),
       strictMissingModules: context.strictMissingModules ?? true,
       projectSlug,
       fetchAndCacheModule: fetchAndCacheModuleFn,

src/transforms/mdx/esm-module-loader/module-fetcher/nested-imports.test.ts

@@ -4,6 +4,7 @@ import { describe, it } from "#veryfront/testing/bdd.ts";
 import {
   findNestedImports,
   hasUnresolvedImports,
+  resolveNestedImportBase,
   resolveNestedModuleImports,
 } from "./nested-imports.ts";
 
@@ -159,3 +160,56 @@ import { bar } from "./local.js";
     });
   });
 });
+
+describe("resolveNestedImportBase", () => {
+  // A barrel lives at lib/index.ts but is addressed as _vf_modules/lib.
+  // Resolving its children against "_vf_modules/lib.js" drops the "lib"
+  // segment, so ./constants.js resolved to _vf_modules/constants.js — one
+  // directory too high. The file was then stubbed and the barrel silently
+  // stopped re-exporting: "does not provide an export named 'COLORS'".
+  it("keeps the directory segment for an index module", () => {
+    assertEquals(
+      resolveNestedImportBase("_vf_modules/lib.js", "/project/lib/index.ts"),
+      "_vf_modules/lib/index.js",
+    );
+    assertEquals(
+      resolveNestedImportBase("_vf_modules/components.js", "/project/components/index.tsx"),
+      "_vf_modules/components/index.js",
+    );
+  });
+
+  it("leaves a plain module untouched", () => {
+    assertEquals(
+      resolveNestedImportBase("_vf_modules/lib/constants.js", "/project/lib/constants.ts"),
+      "_vf_modules/lib/constants.js",
+    );
+  });
+
+  it("does not double up when the path already names index", () => {
+    assertEquals(
+      resolveNestedImportBase("_vf_modules/lib/index.js", "/project/lib/index.ts"),
+      "_vf_modules/lib/index.js",
+    );
+  });
+
+  it("is a no-op without a resolved file path", () => {
+    assertEquals(resolveNestedImportBase("_vf_modules/lib.js"), "_vf_modules/lib.js");
+  });
+
+  it("does not treat a file merely named index-something as an index module", () => {
+    assertEquals(
+      resolveNestedImportBase("_vf_modules/lib.js", "/project/lib/indexer.ts"),
+      "_vf_modules/lib.js",
+    );
+  });
+
+  it("recognises every index extension the resolver accepts", () => {
+    for (const ext of ["ts", "tsx", "js", "jsx", "mdx", "md"]) {
+      assertEquals(
+        resolveNestedImportBase("_vf_modules/lib.js", `/project/lib/index.${ext}`),
+        "_vf_modules/lib/index.js",
+        ext,
+      );
+    }
+  });
+});

src/transforms/mdx/esm-module-loader/module-fetcher/nested-imports.ts

@@ -132,6 +132,38 @@ export interface ResolveNestedModuleImportsInput {
   strictMissingModules: boolean;
   fetchAndCacheModule: (path: string, parent?: string) => Promise<string | null>;
   log?: Logger;
+  /**
+   * Path this module's relative imports resolve against. Defaults to
+   * `normalizedPath`; see {@link resolveNestedImportBase}.
+   */
+  parentBasePath?: string;
+}
+
+/**
+ * The path a module's own relative imports should resolve against.
+ *
+ * A directory barrel lives at `lib/index.ts` but is addressed as
+ * `_vf_modules/lib`. Resolving its children against `_vf_modules/lib.js` drops
+ * the trailing segment as if it were a filename, so `./constants.js` becomes
+ * `_vf_modules/constants.js` — one directory too high. The file is then not
+ * found and gets replaced by a stub, and the barrel silently stops re-exporting
+ * anything: `does not provide an export named 'COLORS'`.
+ *
+ * When the module actually resolved to an index file, keep the directory
+ * segment by addressing it as `<dir>/index.js`.
+ */
+export function resolveNestedImportBase(
+  normalizedPath: string,
+  actualFilePath?: string,
+): string {
+  if (!actualFilePath || !/(?:^|\/)index\.(?:tsx?|jsx?|mdx|md)$/.test(actualFilePath)) {
+    return normalizedPath;
+  }
+
+  const withoutExt = normalizedPath.replace(/\.(?:js|mjs)$/, "");
+  if (withoutExt.endsWith("/index")) return normalizedPath;
+
+  return `${withoutExt}/index.js`;
 }
 
 /**
@@ -167,7 +199,10 @@ export async function resolveNestedModuleImports(
       original,
       start,
       end,
-      nestedFilePath: await input.fetchAndCacheModule(path, input.normalizedPath),
+      nestedFilePath: await input.fetchAndCacheModule(
+        path,
+        input.parentBasePath ?? input.normalizedPath,
+      ),
       [key]: path,
     })),
   );