Bazel: Pass nodejs binary path as an environment variable. (#1518)

The current way to derive the binary path relies on a specific name for
the nodejs repository. This blocks migrating to bzlmod, as bzlmod
prefixes repository directories with the module name that created them.
By asking bazel for the path instead we always get the correct path, so
we can work with both bzlmod and WORKSPACE based dependencies at the
same time.

The repository @nodejs, used in the build label, refers to nodejs for
the host platform and is generated by the following macro (wasn't too
obvious to me):

d19d695275/nodejs/repositories.bzl (L452)

This is some work towards solving #1509.

bazel/emscripten_toolchain/BUILD.bazel

@@ -62,6 +62,7 @@ emscripten_cc_toolchain_config_rule(
     cpu = "wasm",
     em_config = "@emscripten_cache//:emscripten_config",
     emscripten_binaries = "@emsdk//:compiler_files",
+    nodejs_bin = "@nodejs//:node",
     script_extension = select({
         "@bazel_tools//src/conditions:host_windows": "bat",
         "//conditions:default": "sh",

bazel/emscripten_toolchain/default_config

@@ -5,19 +5,12 @@ ROOT_DIR = os.environ["ROOT_DIR"]
 EMSCRIPTEN_ROOT = os.environ["EMSCRIPTEN"]
 BINARYEN_ROOT = os.path.join(ROOT_DIR, os.environ["EM_BIN_PATH"])
 LLVM_ROOT = os.path.join(BINARYEN_ROOT, "bin")
+NODE_JS = os.path.join(ROOT_DIR, os.environ["NODE_JS_PATH"])
 FROZEN_CACHE = True
 
-system = platform.system()
-
-machine = "arm64" if platform.machine() in ('arm64', 'aarch64') else "amd64"
-nodejs_binary = "bin/nodejs/node.exe" if(system =="Windows") else "bin/node"
-NODE_JS = ROOT_DIR + "/external/nodejs_{}_{}/{}".format(system.lower(), machine, nodejs_binary)
-
-
 # This works around an issue with Bazel RBE where the symlinks in node_modules/.bin
 # are uploaded as the linked files, which means the cli.js cannot load its
 # dependencies from the expected locations.
 # See https://github.com/emscripten-core/emscripten/pull/16640 for more
-if system != "Windows":
-    CLOSURE_COMPILER = [NODE_JS, os.path.join(EMSCRIPTEN_ROOT, "node_modules",
-                                              "google-closure-compiler", "cli.js")]
+CLOSURE_COMPILER = [NODE_JS, os.path.join(EMSCRIPTEN_ROOT, "node_modules",
+                                          "google-closure-compiler", "cli.js")]

bazel/emscripten_toolchain/toolchain.bzl

@@ -71,6 +71,7 @@ def _impl(ctx):
     cc_target_os = "emscripten"
 
     emscripten_dir = ctx.attr.emscripten_binaries.label.workspace_root
+    nodejs_path = ctx.file.nodejs_bin.path
 
     builtin_sysroot = emscripten_dir + "/emscripten/cache/sysroot"
 
@@ -1060,6 +1061,10 @@ def _impl(ctx):
                     key = "EM_CONFIG_PATH",
                     value = ctx.file.em_config.path,
                 ),
+                env_entry(
+                    key = "NODE_JS_PATH",
+                    value = nodejs_path,
+                ),
             ],
         ),
         # Use llvm backend.  Off by default, enabled via --features=llvm_backend
@@ -1134,6 +1139,7 @@ emscripten_cc_toolchain_config_rule = rule(
         "cpu": attr.string(mandatory = True, values = ["asmjs", "wasm"]),
         "em_config": attr.label(mandatory = True, allow_single_file = True),
         "emscripten_binaries": attr.label(mandatory = True, cfg = "exec"),
+        "nodejs_bin": attr.label(mandatory = True, allow_single_file = True),
         "script_extension": attr.string(mandatory = True, values = ["sh", "bat"]),
     },
     provides = [CcToolchainConfigInfo],