Better URL resolution for url-file

- Expose the URL() constructor's resolution behavior via url.js
- Add slashes '/' between the base and path to give what we think is
  the least surprising behavior for punning filesystem and URL – so

  url-file("https://a.com/b/c", "../d.arr")

  resolves to "https://a.com/b/d.arr", not "https://a.com/d.arr"

  And this:

  url-file("https://a.com/b/c", "d.arr")

  resolves to "https://a.com/b/c/d.arr"

  Without inserting the trailing slashes, URL and path resolution would be
  pretty different: URL resolution would always "chop off" the part of the path
  after the last slash

- Write a test for this behavior getting the correct module identity when
  ".." is used in some url-file paths and not others

Author: jpolitz

src/arr/compiler/cli-module-loader.arr

@@ -13,6 +13,7 @@ import filesystem as Filesystem
 import file as F
 import error as ERR
 import system as SYS
+import url as URL
 import file("js-ast.arr") as J
 import file("concat-lists.arr") as C
 import file("compile-lib.arr") as CL
@@ -308,6 +309,13 @@ fun get-real-path(current-load-path :: String, this-path :: String):
   end
 end
 
+fun maybe-add-slash(s):
+  last-index = string-length(s) - 1
+  if string-char-at(s, last-index) == "/": s
+  else: s + "/"
+  end
+end
+
 fun locate-file(ctxt :: CLIContext, rel-path :: String):
   clp = ctxt.current-load-path
   real-path = get-real-path(clp, rel-path)
@@ -329,7 +337,8 @@ fun module-finder(ctxt :: CLIContext, dep :: CS.Dependency):
       else if protocol == "url":
         CL.located(UL.url-locator(dep.arguments.get(0), CS.standard-globals), ctxt)
       else if protocol == "url-file":
-        full-url = args.get(0) + "/" + args.get(1)
+        base = maybe-add-slash(args.get(0))
+        full-url = URL.resolve(args.get(1), base)
         cases(CS.UrlFileMode) ctxt.url-file-mode:
           | all-remote =>
             CL.located(UL.url-locator(full-url, CS.standard-globals), ctxt)

src/js/trove/url.js

@@ -0,0 +1,24 @@
+({
+    provides: {
+        values: {
+          resolve: ["arrow", ["String", "String"], "String"]
+        },
+        types: {},
+    },
+    requires: [ ],
+    nativeRequires: [],
+    theModule: function(runtime, _, uri) {
+      function resolve(path, base) {
+        runtime.checkArgsInternal2("url", "resolve", path, runtime.String, base, runtime.String);
+        try {
+          return new URL(path, base).href;
+        }
+        catch(e) {
+          runtime.ffi.throwMessageException("url.resolve() failed: " + String(e));
+        }
+      }
+      return runtime.makeModuleReturn({
+        resolve: runtime.makeFunction(resolve)
+      }, {});
+    }
+})

tests/io-tests/tests/library-code.arr

@@ -2,4 +2,6 @@
 
 provide *
 
-x = "same dir library code"
+x = "same dir library code"
+
+data D: d() end

tests/io-tests/tests/nested/imports-library-with-dotdot.arr

@@ -0,0 +1,5 @@
+import url-file("http://0.0.0.0:7999/nested/", "../library-code.arr") as LC
+
+provide from LC:
+  data D
+end

tests/io-tests/tests/test-imports-library-and-nested.arr

@@ -0,0 +1,9 @@
+###> Looks shipshape
+
+import url-file("http://0.0.0.0:7999/", "library-code.arr") as LC
+import url-file("http://0.0.0.0:7999/", "nested/imports-library-with-dotdot.arr") as N
+
+check:
+  N.d() is LC.d()
+end
+