Add support for file URIs and extracting the file path from file URIs

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 73ce45b906d466733aa000276c1a4b5fa424c034..88d4763df54524853cf49504b95a27ac64236a28 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -263,6 +263,134 @@ class Uri {
   }
 
   /**
+   * Creates a new `file` URI from an absolute or relative file path.
+   *
+   * The file path is passed in [path].
+   *
+   * This path is interpreted using either Windows or non-Windows
+   * semantics. For Windows semantics the backslash separator is used

[ahe, 2013/07/29 15:30:16]

For Windows semantics, one can use either backslash or slash.

[Bill Hesse, 2013/07/30 11:29:32]

Forward slash must be supported as a Windows path separator - plenty of code is
ending up with a mixture of forward and backward slashes in Windows file paths,
and we can't stop people from doing it, because it works in everything
(including package path).

[Søren Gjesse, 2013/07/30 13:41:26]

Done.

[Søren Gjesse, 2013/07/30 13:41:26]

Done.

+   * to separate path segments. For non-Windows semantics the slash is
+   * used to separate path segments. If the path starts with a path
+   * separator an absolute URI is created. Otherwise a relative URI is
+   * created. One exception from this rule is that with Windows
+   * semantics and a path which starts with a drive letter then an
+   * absolute URI is created.
+   *
+   * The default for whether to use Windows or non-Windows semantics
+   * determined from the platform Dart is running on. When running in
+   * the standalone VM this is detected by the VM based on the
+   * operating system. When running in the browser non-Windows
+   * semantics is always used.
+   *
+   * To override the automatic detection of which semantics to use pass
+   * a value for [windowsPath]. Passing `true` will use Windows
+   * semantics and passing `false` will use non-Windows semantics.
+   *
+   * If the path passed is not a legal file path [ArgumentError] is thrown.
+   */

[floitsch, 2013/07/29 15:16:54]

Give some examples.

[Søren Gjesse, 2013/07/30 13:41:26]

Done.

+  factory Uri.file(String path, {bool windowsPath}) {

[floitsch, 2013/07/29 15:16:54]

doesn't sound like a boolean name, but so far haven't found a better one.

[Søren Gjesse, 2013/07/30 13:41:26]

Agree that this is a pretty bad name - sounds like an alternative path value for
use on Windows.

How about

  windowsSemantics (my favorite)
  useWindowsSemantics

Or add an enum

class FilePathSemantics {
  static const DEFAULT = const FilePathSemantics._internal(0);
  static const NON_WINDOWS = const FilePathSemantics._internal(1);
  static const WINDOWS = const FilePathSemantics._internal(2);
  const FilePathSemantics._internal(int this._semantics);
  final int _semantics;
}

factory Uri.file(String path, {FilePathSemantics windowsPath:
FilePathSemantics.DEFAULT})

[Bill Hesse, 2013/07/30 15:03:15]

I think that just "windows:" would be a great name for this parameter. 
Uri.file("C:/My Docu~1/timeshee.xls", windows: true)
for the win!

+    windowsPath = windowsPath == null ? Uri._isWindows : windowsPath;
+    return windowsPath ? _makeWindowsFileUrl(path) : _makeFileUri(path);
+  }
+
+  external static bool get _isWindows;

[Bill Hesse, 2013/07/30 11:29:32]

Could you call this something else (like _isWindowsRuntime), and
declare

static bool _isWindows = _isWindowsRuntime;

and let lazy static initialization take care of caching the value?

[Søren Gjesse, 2013/07/30 13:41:26]

Good point. Done.

+
+  // Characters which are not allowed in file names on Windows.
+  static final _windowsPathReservedCharacters = [
+    _DOUBLE_QUOTE,
+    _ASTERISK,
+    _SLASH,
+    _COLON,  // Colon is only allowed after drive letter.
+    _LESS,
+    _GREATER,
+    _QUESTION,
+    _BACKSLASH,  // Backslash can only be separator.
+    _BAR,
+  ];
+
+  static _checkWindowsPathReservedCharacters(List<String> segments,
+                                             bool argumentError,
+                                             [int firstSegment = 0]) {
+    for (int i = firstSegment; i < segments.length; i++) {
+      var segment = segments[i];
+      for (int j = 0; j < segment.length; j++) {
+        if (_windowsPathReservedCharacters.contains(segment.codeUnitAt(j))) {
+          if (argumentError) {
+            throw new ArgumentError("Illegal path character ${segment[j]}");

[floitsch, 2013/07/29 15:16:54]

You could maybe use FormatException if you think that this is something that
users could provide, and that is something that could happen without a
programming error.

[Søren Gjesse, 2013/07/30 13:41:26]

Shouldn't we stick to argument checking throwing ArgumentError?

[Bill Hesse, 2013/07/30 15:03:15]

I think that a badly formatted file path should be an exception, rather than an
error, so I like the idea of using format exception.
It is not an input of the wrong type, for example.

+          } else {
+            throw new UnsupportedError("Illegal path character ${segment[j]}");

[ahe, 2013/07/29 15:30:16]

StateError?

[Søren Gjesse, 2013/07/30 13:41:26]

I did also think about this. However as the Uri object is immutable this somehow
sounds wrong, but I am fine with changing it, as UnsupportedError is also
somewhat strange.

+          }
+        }
+      }
+    }
+  }
+
+  static _checkWindowsDriveLetter(int charCode, bool argumentError) {
+    if ((_UPPER_CASE_A <= charCode && charCode <= _UPPER_CASE_Z) ||
+        (_LOWER_CASE_A <= charCode && charCode <= _LOWER_CASE_Z)) {
+      return;
+    }
+    if (argumentError) {
+      throw new ArgumentError("Illegal drive letter " +

[floitsch, 2013/07/29 15:16:54]

ditto.

[Søren Gjesse, 2013/07/30 13:41:26]

See above.

+                              new String.fromCharCode(charCode));
+    } else {
+      throw new UnsupportedError("Illegal drive letter " +

[ahe, 2013/07/29 15:30:16]

StateError?

[Søren Gjesse, 2013/07/30 13:41:26]

See above.

+                              new String.fromCharCode(charCode));
+    }
+  }
+
+  static _makeFileUri(String path) {
+    String sep = "/";
+    if (path.length > 0 && path[0] == sep) {
+      // Absolute file:// URI.
+      return new Uri(scheme: "file", pathSegments: path.split(sep));
+    } else {
+      // Relative URI.
+      return new Uri(pathSegments: path.split(sep));

[ahe, 2013/07/29 15:30:16]

Check that this works as you expect for the empty string.

[Søren Gjesse, 2013/07/30 13:41:26]

Added tests with empty path.

+    }
+  }
+
+  static _makeWindowsFileUrl(String path) {

[ahe, 2013/07/29 15:30:16]

I'm not sure this works if the path is using / instead of \.

[Søren Gjesse, 2013/07/30 13:41:26]

It did not, but changed it so it now does. Also updated tests.

+    String sep = "\\";
+    if (path.length > 1 && path[1] == ":") {
+      _checkWindowsDriveLetter(path.codeUnitAt(0), true);
+      if (path.length == 2 || path.codeUnitAt(2) != _BACKSLASH) {

[Bill Hesse, 2013/07/30 11:29:32]

Why not path.length >= 2?

[Søren Gjesse, 2013/07/30 13:41:26]

Not sure I understand. If path.length is == 2 I cannot look at
path.codeUnitAt(2).

[Bill Hesse, 2013/07/30 15:03:15]

Nevermind - I read || as && and 2 as 3.

+        throw new ArgumentError(
+            "Windows paths with drive letter must be absolute");
+      }

[Bill Hesse, 2013/07/30 11:29:32]

The windows paths "\foo\bar" and "c:" (and even "c:foo") are both weird,
semi-absolute paths in their semantics, but you accept the first and not the
second.

I guess this is OK, but why accept the first?  The semantics depend on the
user's current drive.

[Søren Gjesse, 2013/07/30 13:41:26]

I think we need to accept the first, as it is widely used. And if you run
everything on the same drive it all works.

The second is strange, and cannot be captured in the file: URI syntax. The code
currently turns URIs like file://c:d into the Windows path of c:/d, but maybe we
should just make that an error instead.

+      // Absolute file://C:/ URI.
+      var pathSegments = path.split(sep);
+      _checkWindowsPathReservedCharacters(pathSegments, true, 1);
+      return new Uri(scheme: "file", pathSegments: pathSegments);
+    }
+
+    if (path.length > 0 && path[0] == sep) {
+      if (path.length > 1 && path[1] == sep) {
+        // Absolute file:// URI with host.
+        int pathStart = path.indexOf("\\", 2);
+        String hostPart =
+            pathStart == -1 ? path.substring(2) : path.substring(2, pathStart);
+        String pathPart =
+            pathStart == -1 ? "" : path.substring(pathStart + 1);
+        var pathSegments = pathPart.split(sep);
+        _checkWindowsPathReservedCharacters(pathSegments, true);
+        return new Uri(
+            scheme: "file", host: hostPart, pathSegments: pathSegments);
+      } else {
+        // Absolute file:// URI.
+        var pathSegments = path.split(sep);
+        _checkWindowsPathReservedCharacters(pathSegments, true);
+        return new Uri(scheme: "file", pathSegments: pathSegments);
+      }
+    } else {
+      // Relative URI.
+      var pathSegments = path.split(sep);
+      _checkWindowsPathReservedCharacters(pathSegments, true);
+      return new Uri(pathSegments: pathSegments);
+    }
+  }
+
+  /**
    * Returns the URI path split into its segments. Each of the
    * segments in the returned list have been decoded. If the path is
    * empty the empty list will be returned. A leading slash `/` does
@@ -651,6 +779,86 @@ class Uri {
     return "$scheme://$host:$port";
   }
 
+  /**
+   * Returns the file path from a file URI.
+   *
+   * The returned path has either Windows or non-Windows
+   * semantics. For Windows semantics the backslash separator is used
+   * to separate path segments. For non Windows semantics the slash is
+   * used to separate path segments. If the URI is absolute the path

[ahe, 2013/07/29 15:30:16]

An absolute URI means that the URI has a scheme. Not that the path is absolute.

What happens if I do:

Uri.parse('file:fisk.txt').toFilePath()

[Søren Gjesse, 2013/07/30 13:41:26]

As

  print(Uri.parse('file:fisk.txt'))

prints

  file:///fisk.txt

This returns /test.txt or \test.txt. Which I think is the right thing. A URI
with a scheme is absolute and then the path is absolute.

On Windows I also made paths like C:a (relative to the current directory on a
drive) absolute, that is

Uri.parse('file:C:a').toFilePath()

Returns either /C:a (non-Windows) or C:\a (Windows).

Added tests for that.

Maybe we should make this an error instead. See below.

+   * statrs with a path separator unless Windows semantics is used and

[ahe, 2013/07/29 15:30:16]

statrs -> starts

[Søren Gjesse, 2013/07/30 13:41:26]

Done.

+   * the first path segment is a drive letter. When Windows semantics
+   * is used a host component in the uri in interpreted as a file
+   * server and a UNC path is returned.
+   *
+   * The default for whether to use Windows or non-Windows semantics
+   * determined from the platform Dart is running on. When running in
+   * the standalone VM this is detected by the VM based on the
+   * operating system. When running in the browser non-Windows
+   * semantics is always used.
+   *
+   * To override the automatic detection of which semantics to use pass
+   * a value for [windowsPath]. Passing `true` will use Windows
+   * semantics and passing `false` will use non-Windows semantics.
+   *
+   * If the URI is not a file URI calling this throws
+   * [UnsupportedError].

[ahe, 2013/07/29 15:30:16]

StateError?

[Søren Gjesse, 2013/07/30 13:41:26]

See above.

+   *
+   * If the URI cannot be converted to a file path calling this throws
+   * [UnsupportedError].

[ahe, 2013/07/29 15:30:16]

StateError?

[Søren Gjesse, 2013/07/30 13:41:26]

See above.

+   */

[floitsch, 2013/07/29 15:16:54]

add examples.

[Søren Gjesse, 2013/07/30 13:41:26]

Done.

+  String toFilePath({bool windowsPath}) {
+    if (scheme != "" && scheme != "file") {
+      throw new UnsupportedError(
+          "Cannot extract a file path from a $scheme URI");
+    }
+    if (windowsPath == null) windowsPath = _isWindows;
+    return windowsPath ? _toWindowsFilePath() : _toFilePath();
+  }
+
+  String _toFilePath() {
+    if (host != "") {
+      throw new UnsupportedError(

[ahe, 2013/07/29 15:30:16]

StateError?

[Søren Gjesse, 2013/07/30 13:41:26]

See above.

+          "Cannot extract a non-Windows file path from a file URI "
+          "with an authority");
+    }
+    var result = new StringBuffer();
+    if (isAbsolute) result.write("/");

[ahe, 2013/07/29 15:30:16]

isAbsolute doesn't mean what you think it means.

[Søren Gjesse, 2013/07/30 13:41:26]

I think this is the right thing to do. Parsing file:xxx and file:///xxx results
in exactly the same URI object which is absolute with one path component.

+    result.writeAll(pathSegments, "/");
+    return result.toString();
+  }
+
+  String _toWindowsFilePath() {
+    bool hasDriveLetter = false;
+    var segments = pathSegments;
+    if (segments.length > 0 &&
+        segments[0].length >= 2 &&
+        segments[0].codeUnitAt(1) == _COLON) {
+      _checkWindowsDriveLetter(segments[0].codeUnitAt(0), false);

[ahe, 2013/07/29 15:30:16]

Seems like you accept stuff like:

c:b

[Bill Hesse, 2013/07/30 11:29:32]

These cases have really weird semantics, and I think they should throw
exceptions.  At least they should have the same exceptions when encoding and
when decoding.

[Søren Gjesse, 2013/07/30 13:41:26]

Yes, file:///c:d is turned into path c:\d on Windows. My argument is that it is
absolute and starts with a drive letter.

There is now a test for it and it is in the examples.

However we could also make it an error as the first path segment "c:d" contains
an illegal character in a windows path. What I am currently doing is
artificially splitting the first path segment into two segments - which is also
unexpected.

Btw. Chrome turns file:c:d into file:///c:/d when typed in the omnibox.

[Søren Gjesse, 2013/07/30 13:41:26]

I am OK with making it an error.

[Bill Hesse, 2013/07/30 15:03:15]

I am now more convinced by your argument that URI semantics imply that anything
that specifies the drive or share (i.e., the "root") must be an absolute path,
so that any conversions to or from URI to Windows path should convert weird
c:relative_path to c:\relative_path.  I think an error would be fine too.

+      if (segments[0].length > 2) {

[ahe, 2013/07/29 15:30:16]

And then you just add another segment for 'b'?

[Søren Gjesse, 2013/07/30 13:41:26]

See above.

+        var originalSegments = segments;
+        segments = new List<String>(originalSegments.length + 1);
+        segments[0] = originalSegments[0].substring(0, 2);
+        segments[1] = originalSegments[0].substring(2);
+        segments.setRange(2, segments.length, originalSegments, 1);
+      }
+      _checkWindowsPathReservedCharacters(segments, false, 1);
+      hasDriveLetter = true;
+    } else {
+      _checkWindowsPathReservedCharacters(segments, false);
+    }
+    var result = new StringBuffer();
+    if (isAbsolute && !hasDriveLetter) result.write("\\");
+    if (host != "") {
+      result.write("\\");
+      result.write(host);
+      result.write("\\");
+    }
+    result.writeAll(segments, "\\");
+    if (hasDriveLetter && segments.length == 1) result.write("\\");
+    return result.toString();
+  }
+
   void _writeAuthority(StringSink ss) {
     _addIfNonEmpty(ss, userInfo, userInfo, "@");
     ss.write(host == null ? "null" :
@@ -840,17 +1048,26 @@ class Uri {
   }
 
   // Frequently used character codes.
+  static const int _DOUBLE_QUOTE = 0x22;
   static const int _PERCENT = 0x25;
+  static const int _ASTERISK = 0x2A;
   static const int _PLUS = 0x2B;
   static const int _SLASH = 0x2F;
   static const int _ZERO = 0x30;
   static const int _NINE = 0x39;
   static const int _COLON = 0x3A;
+  static const int _LESS = 0x3C;
+  static const int _GREATER = 0x3E;
+  static const int _QUESTION = 0x3F;
   static const int _AT_SIGN = 0x40;
   static const int _UPPER_CASE_A = 0x41;
   static const int _UPPER_CASE_F = 0x46;
+  static const int _UPPER_CASE_Z = 0x5A;
+  static const int _BACKSLASH = 0x5C;
   static const int _LOWER_CASE_A = 0x61;
   static const int _LOWER_CASE_F = 0x66;
+  static const int _LOWER_CASE_Z = 0x7A;
+  static const int _BAR = 0x7C;
 
   /**
    * This is the internal implementation of JavaScript's encodeURI function.