Refactor virtual path management of StaticFilesModule (fixes #272 et…

… al.) (#275)

* Avoid creating a temporary Dictionary.

* Remove excess parenthesis.

* Refactor virtual path management of StaticFilesModule (fixes #272 et al.)

* Determinism:
  - always evaluate virtual path in reverse ordinal order;
  - normalize URL paths to simplify code and ensure consistent mapping.

* Separation of concerns:
  - have methods that deal with mapping URL paths to local paths,
    and other methods that deal with the existence of local paths.

* Caching:
  - keep track of _how_ and _why_ paths were mapped;
  - discard cached mapped paths whose generating data (virtual path,
    default extension, default documkent name) has changed.

* Support for dynamic file systems:
  - allow the user to deactivate path caching in situations where the
    contents of served directories may change over time.

* Compatibility:
  - keep existing constructors, methods, and properties of
    StaticFilesModule;
  - keep existing exception semantics (e.g. throw
    InvalidOperationException instead of ArgumentException for invalid
    paths passed to RegisterVirtualPath).

* Behavior changes:
  - scenarios where files could be added to a served directory after
    being requested (resulting in error 404) are no longer supported:
    said files will continue to give error 404, unless path caching
    is disabled.
  - scenarios where files are continuously added and/or deleted,
    which previously resulted in spurious error 404s (#272), can now be
    supported by disabling path caching.

* Modify test that would not throw on machines where drive E: actually exists.

src/Unosquare.Labs.EmbedIO/Core/PathHelper.cs

@@ -0,0 +1,95 @@
+namespace Unosquare.Labs.EmbedIO.Core
+{
+    using System;
+    using System.IO;
+    using System.Text.RegularExpressions;
+
+    internal static class PathHelper
+    {
+        private static readonly Regex _multipleSlashRegex = new Regex("//+", RegexOptions.Compiled | RegexOptions.CultureInvariant);
+
+        static readonly char[] _invalidLocalPathChars = GetInvalidLocalPathChars();
+
+        public static bool IsValidUrlPath(string urlPath) => !string.IsNullOrEmpty(urlPath) && urlPath[0] == '/';
+
+        // urlPath must be a valid URL path
+        // (not null, not empty, starting with a slash.)
+        public static string NormalizeUrlPath(string urlPath, bool isBasePath)
+        {
+            // Replace each run of multiple slashes with a single slash
+            urlPath = _multipleSlashRegex.Replace(urlPath, "/");
+
+            // The root path needs no further checking.
+            var length = urlPath.Length;
+            if (length == 1)
+                return urlPath;
+
+            // Base URL paths must end with a slash;
+            // non-base URL paths must NOT end with a slash.
+            // The final slash is irrelevant for the URL itself
+            // (it has to map the same way with or without it)
+            // but makes comparing and mapping URls a lot simpler.
+            var finalPosition = length - 1;
+            var endsWithSlash = urlPath[finalPosition] == '/';
+            return isBasePath
+                ? (endsWithSlash ? urlPath : urlPath + "/")
+                : (endsWithSlash ? urlPath.Substring(0, finalPosition) : urlPath);
+        }
+
+        public static string EnsureValidUrlPath(string urlPath, bool isBasePath)
+        {
+            if (urlPath == null)
+            {
+                throw new InvalidOperationException("URL path is null,");
+            }
+
+            if (urlPath.Length == 0)
+            {
+                throw new InvalidOperationException("URL path is empty.");
+            }
+
+            if (urlPath[0] != '/')
+            {
+                throw new InvalidOperationException($"URL path \"{urlPath}\"does not start with a slash.");
+            }
+
+            return NormalizeUrlPath(urlPath, isBasePath);
+        }
+
+        public static string EnsureValidLocalPath(string localPath)
+        {
+            if (localPath == null)
+            {
+                throw new InvalidOperationException("Local path is null.");
+            }
+
+            if (localPath.Length == 0)
+            {
+                throw new InvalidOperationException("Local path is empty.");
+            }
+
+            if (string.IsNullOrWhiteSpace(localPath))
+            {
+                throw new InvalidOperationException("Local path contains only white space.");
+            }
+
+            if (localPath.IndexOfAny(_invalidLocalPathChars) >= 0)
+            {
+                throw new InvalidOperationException($"Local path \"{localPath}\"contains one or more invalid characters.");
+            }
+
+            return localPath;
+        }
+
+        private static char[] GetInvalidLocalPathChars()
+        {
+            var systemChars = Path.GetInvalidPathChars();
+            var p = systemChars.Length;
+            var result = new char[p + 2];
+            Array.Copy(systemChars, result, p);
+            result[p++] = '*';
+            result[p] = '?';
+            return result;
+        }
+    }
+}

src/Unosquare.Labs.EmbedIO/Core/PathMappingResult.cs

@@ -0,0 +1,38 @@
+namespace Unosquare.Labs.EmbedIO.Core
+{
+    using System;
+
+    [Flags]
+    internal enum PathMappingResult
+    {
+        /// <summary>
+        /// The mask used to extract the mapping result.
+        /// </summary>
+        MappingMask = 0xF,
+
+        /// <summary>
+        /// The path was not found.
+        /// </summary>
+        NotFound = 0,
+
+        /// <summary>
+        /// The path was mapped to a file.
+        /// </summary>
+        IsFile = 0x1,
+
+        /// <summary>
+        /// The path was mapped to a directory.
+        /// </summary>
+        IsDirectory = 0x2,
+
+        /// <summary>
+        /// The default extension has been appended to the path.
+        /// </summary>
+        DefaultExtensionUsed = 0x1000,
+
+        /// <summary>
+        /// The default document name has been appended to the path.
+        /// </summary>
+        DefaultDocumentUsed = 0x2000,
+    }
+}

src/Unosquare.Labs.EmbedIO/Core/ReverseOrdinalStringComparer.cs

@@ -0,0 +1,19 @@
+namespace Unosquare.Labs.EmbedIO.Core
+{
+    using System;
+    using System.Collections.Generic;
+
+    // Sorts strings in reverse order to obtain the evaluation order of virtual paths
+    internal sealed class ReverseOrdinalStringComparer : IComparer<string>
+    {
+        private static readonly IComparer<string> _directComparer = StringComparer.Ordinal;
+
+        private ReverseOrdinalStringComparer()
+        {
+        }
+
+        public static IComparer<string> Instance { get; } = new ReverseOrdinalStringComparer();
+
+        public int Compare(string x, string y) => _directComparer.Compare(y, x);
+    }
+}

src/Unosquare.Labs.EmbedIO/Core/VirtualPath.cs

@@ -0,0 +1,42 @@
+namespace Unosquare.Labs.EmbedIO.Core
+{
+    using System;
+    using System.IO;
+
+    internal sealed class VirtualPath
+    {
+        public VirtualPath(string baseUrlPath, string baseLocalPath)
+        {
+            BaseUrlPath = PathHelper.EnsureValidUrlPath(baseUrlPath, true);
+            try
+            {
+                BaseLocalPath = Path.GetFullPath(PathHelper.EnsureValidLocalPath(baseLocalPath));
+            }
+#pragma warning disable CA1031
+            catch (Exception e)
+            {
+                throw new InvalidOperationException($"Cannot determine the full local path for \"{baseLocalPath}\".", e);
+            }
+#pragma warning restore CA1031
+        }
+
+        public string BaseUrlPath { get; }
+
+        public string BaseLocalPath { get; }
+
+        internal bool CanMapUrlPath(string urlPath) => urlPath.StartsWith(BaseUrlPath, StringComparison.Ordinal);
+
+        internal bool TryMapUrlPathLoLocalPath(string urlPath, out string localPath)
+        {
+            if (!CanMapUrlPath(urlPath))
+            {
+                localPath = null;
+                return false;
+            }
+
+            var relativeUrlPath = urlPath.Substring(BaseUrlPath.Length);
+            localPath = Path.Combine(BaseLocalPath, relativeUrlPath.Replace('/', Path.DirectorySeparatorChar));
+            return true;
+        }
+    }
+}