Compute File.writeable based on FileWrite instead of FileRead. (ponyl…

…ang#2698)

* Compute File.writeable based on FileWrite

`File._descriptor(...)` was computing the `writeable` flag by checking
if the `FileRead` capability was set, rather than the `FileWrite`
capability.

* Catch "inner" errors in the writeable test.

`try ... then ... end` can mask errors and give a false positive to
tests.

* Tidy code style to comply with stdlib guidelines.

Space after : before type.

packages/files/_test.pony

@@ -26,6 +26,7 @@ actor Main is TestList
     test(_TestFileCreateExistsNotWriteable)
   ifdef not windows then
     test(_TestFileCreateDirNotWriteable)
+    test(_TestFileOpenInDirNotWriteable)
     test(_TestFileOpenPermissionDenied)
   end
     test(_TestFileCreateMissingCaps)
@@ -433,6 +434,39 @@ class iso _TestFileCreateDirNotWriteable is _NonRootTest
       end
     end
 
+class iso _TestFileOpenInDirNotWriteable is UnitTest
+  fun name(): String => "files/File.open-dir-not-writeable"
+  fun apply(h: TestHelper) =>
+    ifdef not windows then
+      try
+        // make a temporary directory
+        let dir_path = FilePath.mkdtemp(
+          h.env.root as AmbientAuth,
+          "tmp.open-dir-not-writeable")?
+        try
+          let dir = Directory(dir_path)?
+
+          // create a file (rw)
+          let created: File = dir.create_file("created")?
+          h.assert_true(created.valid())
+          h.assert_true(created.writeable)
+          created.dispose()
+
+          // open a file (ro)
+          let readonly: File = dir.open_file("created")?
+          h.assert_true(readonly.valid())
+          h.assert_false(readonly.writeable)
+          readonly.dispose()
+        else
+          h.fail("Unhandled inner error!")
+        then
+          dir_path.remove()
+        end
+      else
+        h.fail("Unhandled error!")
+      end
+    end
+
 class iso _TestFileCreateMissingCaps is UnitTest
   fun name(): String => "files/File.create-missing-caps"
   fun apply(h: TestHelper) =>

packages/files/file.pony

@@ -185,7 +185,7 @@ class File
     end
 
     path = from
-    writeable = from.caps(FileRead)
+    writeable = from.caps(FileWrite)
     _fd = fd
 
     _FileDes.set_rights(_fd, path, writeable)?