lua-uri-file(3) File URI support for Lua URI library

## Description

The class "uri.file" is used for URIs with the "file" scheme. It inherits from the uri class.

A file URI without an authority doesn't have a well defined meaning. This library considers such URIs to be invalid when they have a path which does not start with '/' (for example "file:foo/bar"). It is likely that any such URI should really be a relative URI reference. If the path does start with a slash then this library will attempt to 'repair' the URI by adding an empty authority part, so that "file:/foo/bar" will be changed automatically to "file:///foo/bar".

A host value of "localhost" is normalized to an empty host, so that "file://localhost/foo" will become "file:///foo". An empty path is normalized to '/'.

The path part is always considered to be case sensitive, so no case folding is done even when converting to a filesystem path for Windows.

Query parts and fragments are left alone by this library, but are not used in converting URIs to filesystem paths.

## Converting between URIs and filesystem paths

A "uri.file" object can be converted into an absolute path suitable for use on a particular operating system by calling the "filesystem_path" method:

    local uri = assert(URI:new("file:///foo/bar"))
print(uri:filesystem_path("unix"))  -- /foo/bar
print(uri:filesystem_path("win32")) -- \foo\bar


This method will throw an exception if the path cannot be converted. For example, a file URI containing a host name cannot be represented on a Unix filesystem, but on a Win32 system it will be converted to a UNC path:

    local uri = assert(URI:new("file://server/path"))
print(uri:filesystem_path("unix"))  -- error
print(uri:filesystem_path("win32")) -- \\server\path


To convert a filesystem path into a URI, call the class method "make_file_uri":

    local FileURI = require "uri.file"
local uri = FileURI.make_file_uri("/foo/bar", "unix")
print(uri)      -- file:///foo/bar
uri = FileURI.make_file_uri("C:\foo\bar", "win32")
print(uri)      -- file:///C:/foo/bar


To convert a relative URI reference (a uri._relative object) into a filesystem path you should first resolve it against an appropriate "file" URI, and then call the "filesystem_path" method on that.

## Methods

All the methods defined in lua-uri(3) are supported. The "userinfo", and "port" methods will always return nil, and will throw an exception when passed anything other than nil. The "host" method will normalize "localhost" to an empty host name, and will throw an exception if given a new value of nil. The "path" method will normalize an empty path or nil value to '/'.

In addition to the standard methods, file URIs support the "filesystem_path" method, and the "uri.file" class contains the "make_file_uri" function, both of which are described above.

## Operating systems supported

The conversion between a file URI and a path suitable for use on a particular operating system are defined in additional classes, which are loaded automatically based on the operating system name supplied to the two conversion functions. For example, passing the string "win32" to the functions will invoke the implementation in the class "uri.file.win32". An exception will be thrown if no class exists to support a given operating system. The following operating system classes are provided:
"uri.file.unix"
A URI containing a host name will cause an exception to be thrown, as there is no obvious way for these to be represented in Unix paths. If the path contains an encoded null byte (%00) or encoded slash (%2F) then an exception will be thrown.

Attempting to convert a relative path to a URI will cause an exception.

"uri.file.win32"
Forward slashes ('/') in URIs will be converted to backslashes ('\') in paths, and vice versa.

URIs containing host names will be converted to UNC paths, starting with a '\\' followed by the hostname and then the path part. If the path part of a URI appears to begin with a drive letter, then the first slash will be removed so that the resulting path starts with the letter. Encoded pipe characters ('%7C') will be recognized as equivalent to colons for the purpose of identifying drive letters, since they have been historically used in that way, but I believe they are not allowed to occur in the path unencoded in a URI nowadays.

The operating system names are case insensitive, and are folded to lowercase before being converted into a Lua module name.

Currently there is no way for this library to recognise the operating system it is running on, since Lua has no built-in way of providing that information.

## References

The most up to date IETF standard for the "file" URI scheme is still RFC 1738 section 3.10'', but this does not specify exactly how to convert between URIs and filesystem paths on particular platforms. It does however specify the equivalence between 'localhost' and an empty host.

The correct form of file URI to represent a Windows filesystem path is described in a blog article: <http://blogs.msdn.com/ie/archive/2006/12/06/file-uris-in-windows.aspx>

There is a standard of sorts describing the conversion between Unix paths and file URIs: <http://equinox-project.org/spec/file-uri-spec.txt>