FoundationEssentials: simplify path normalization

We would previously conditionally call `GetFullPathNameW` and do string
manipulations for normalising the path. Instead, always call
`GetFullPathNameW` to normalise the path as per Windows' rules. This
more importantly will collapse runs of the arc separator, which is
crucial when using extended paths as the NT path form must always use
the normalised paths or the access will fail.

Author: compnerd

Sources/FoundationEssentials/String/String+Internals.swift

@@ -28,51 +28,23 @@ extension String {
             throw CocoaError.errorWithFilePath(.fileReadInvalidFileName, "")
         }
 
-        // 1. Normalize the path first.
-
-        var path = self
+        var iter = self.makeIterator()
+        let bLeadingSlash = if iter.next() == "/", iter.next()?.isLetter ?? false, iter.next() == ":" { true } else { false }
 
         // Strip the leading `/` on a RFC8089 path (`/[drive-letter]:/...` ).  A
         // leading slash indicates a rooted path on the drive for the current
         // working directory.
-        var iter = path.makeIterator()
-        if iter.next() == "/", iter.next()?.isLetter ?? false, iter.next() == ":" {
-            path.removeFirst()
-        }
-
-        // Win32 APIs can support `/` for the arc separator. However,
-        // symlinks created with `/` do not resolve properly, so normalize
-        // the path.
-        path.replace("/", with: "\\")
-
-        // Drop trailing slashes unless it follows a drive specification.  The
-        // trailing arc separator after a drive specifier indicates the root as
-        // opposed to a drive relative path.
-        while path.count > 1, path.last == "\\" {
-            let first = path.startIndex
-            let second = path.index(after: path.startIndex)
-            if path.count == 3, path[first].isLetter, path[second] == ":" {
-                break;
-            }
-            path.removeLast()
-        }
-
-        // 2. Perform the operation on the normalized path.
-
-        return try path.withCString(encodedAs: UTF16.self) { pwszPath in
-            guard !path.hasPrefix(#"\\"#) else { return try body(pwszPath) }
-
-            let dwLength = GetFullPathNameW(pwszPath, 0, nil, nil)
-            let path = try withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwLength)) {
-                guard GetFullPathNameW(pwszPath, DWORD($0.count), $0.baseAddress, nil) == dwLength - 1 else {
-                    throw CocoaError.errorWithFilePath(path, win32: GetLastError(), reading: true)
+        return try self.dropFirst(bLeadingSlash ? 1 : 0).withCString(encodedAs: UTF16.self) { pwszPath in
+            // 1. Normalize the path first.
+            let dwLength: DWORD = GetFullPathNameW(pwszPath, 0, nil, nil)
+            return try withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwLength)) {
+                guard GetFullPathNameW(pwszPath, DWORD($0.count), $0.baseAddress, nil) > 0 else {
+                    throw CocoaError.errorWithFilePath(self, win32: GetLastError(), reading: true)
                 }
-                return String(decodingCString: $0.baseAddress!, as: UTF16.self)
-            }
-            guard !path.hasPrefix(#"\\"#) else {
-                return try path.withCString(encodedAs: UTF16.self, body)
+
+                // 2. Perform the operation on the normalized path.
+                return try body($0.baseAddress!)
             }
-            return try #"\\?\\#(path)"#.withCString(encodedAs: UTF16.self, body)
         }
     }
 }