Export PATH and ERRNO_CODES from Emscripten (#2582)

BrowserFS can mount custom filesystems into Emscripten. 
However it requires the PATH and ERRNO_CODES exports from
Emscripten in addition to FS.

This exports `PATH` and `ERRNO_CODES` from `Module` into the `pyodide`
Javascript API so they can be used with BrowserFS.

Makefile.envs

@@ -113,7 +113,8 @@ export MAIN_MODULE_LDFLAGS= $(LDFLAGS_BASE) \
 	--exclude-file "*/tests/*" \
 	--exclude-file "*/distutils/*" \
 	--pre-js src/core/pre.js \
-	--pre-js src/js/_pyodide.out.js
+	--pre-js src/js/_pyodide.out.js \
+	--post-js src/core/post.js
 
 
 export SIDE_MODULE_CXXFLAGS = $(CXXFLAGS_BASE)

docs/project/changelog.md

@@ -86,6 +86,9 @@ substitutions:
   `clear_interval`, `add_event_listener` and `remove_event_listener` for the corresponding JavaScript functions.
   {pr}`2456`
 
+- {{ Enhancement }} Pyodide now directly exposes the Emscripten `PATH` and `ERRNO_CODES` APIs.
+  {pr}`2582`
+
 ### micropip
 
 - {{ Fix }} micropip now correctly handles package names that include dashes

src/core/main.c

@@ -90,13 +90,6 @@ static struct PyModuleDef core_module_def = {
 int
 main(int argc, char** argv)
 {
-  EM_ASM({
-    // For some reason emscripten doesn't make UTF8ToString available on Module
-    // by default...
-    Module.UTF8ToString = UTF8ToString;
-    Module.wasmTable = wasmTable;
-  });
-
   // This exits and prints a message to stderr on failure,
   // no status code to check.
   initialize_python();

src/core/post.js

@@ -0,0 +1,5 @@
+// Emscripten doesn't make UTF8ToString or wasmTable available on Module by default...
+Module.UTF8ToString = UTF8ToString;
+Module.wasmTable = wasmTable;
+// Emscripten has a bug where it accidentally exposes an empty object as Module.ERRNO_CODES
+Module.ERRNO_CODES = ERRNO_CODES;

src/js/api.ts

@@ -449,6 +449,8 @@ export function checkInterrupt() {
 export type PyodideInterface = {
   globals: typeof globals;
   FS: typeof FS;
+  PATH: typeof PATH;
+  ERRNO_CODES: typeof ERRNO_CODES;
   pyodide_py: typeof pyodide_py;
   version: typeof version;
   loadPackage: typeof loadPackage;
@@ -486,14 +488,32 @@ export type PyodideInterface = {
  */
 export let FS: any;
 
+/**
+ * An alias to the `Emscripten Path API
+ * <https://github.com/emscripten-core/emscripten/blob/main/src/library_path.js>`_.
+ *
+ * This provides a variety of operations for working with file system paths, such as
+ * ``dirname``, ``normalize``, and ``splitPath``.
+ */
+export let PATH: any;
+
+/**
+ * An alias to the Emscripten ERRNO_CODES map of standard error codes.
+ */
+export let ERRNO_CODES: any;
+
 /**
  * @private
  */
 API.makePublicAPI = function (): PyodideInterface {
   FS = Module.FS;
+  PATH = Module.PATH;
+  ERRNO_CODES = Module.ERRNO_CODES;
   let namespace = {
     globals,
     FS,
+    PATH,
+    ERRNO_CODES,
     pyodide_py,
     version,
     loadPackage,

src/js/test/integration/filesystem.test.js

@@ -25,3 +25,24 @@ describe("FS", () => {
     chai.assert.isTrue(result);
   });
 });
+
+describe("PATH", () => {
+  it("exists", async () => {
+    chai.assert.exists(pyodide.PATH);
+  });
+  it("has expected keys", async () => {
+    chai.assert.exists(pyodide.PATH.dirname);
+    chai.assert.exists(pyodide.PATH.normalize);
+  });
+});
+
+describe("ERRNO_CODES", () => {
+  it("exists", async () => {
+    chai.assert.exists(pyodide.ERRNO_CODES);
+  });
+  it("has expected keys", async () => {
+    chai.assert.exists(pyodide.ERRNO_CODES.ENOENT);
+    chai.assert.exists(pyodide.ERRNO_CODES.EPERM);
+    chai.assert.exists(pyodide.ERRNO_CODES.EEXIST);
+  });
+});