[mono] Implement Environment.GetFolderPath on iOS (#34022)

* Implement Environment.GetFolderPath on iOS

* Address feedback

* Move GetFolderPathCore to Environment.Unix.GetFolderPathCore.cs

* Fix build issue

* Address feedback

* cache all special directories

* Fix build issue

* remove a whitespace

* Fix UserProfile issue

* undo changes in GetEnvironmentVariableCore

* Update Environment.Unix.Mono.cs

* Extract to InternalGetEnvironmentVariable

* Fix build issue

* Return emtpy string if underlying native function returns null

* Add nullability

src/libraries/Common/src/Interop/OSX/System.Native/Interop.SearchPath.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#nullable enable
+using System;
+using System.Runtime.InteropServices;
+
+internal static partial class Interop
+{
+    internal static partial class Sys
+    {
+        [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_SearchPath")]
+        internal static extern string? SearchPath(NSSearchPathDirectory folderId);
+
+        internal enum NSSearchPathDirectory
+        {
+            NSApplicationDirectory = 1,
+            NSLibraryDirectory = 5,
+            NSUserDirectory = 7,
+            NSDocumentDirectory = 9,
+            NSDesktopDirectory = 12,
+            NSCachesDirectory = 13,
+            NSMoviesDirectory = 17,
+            NSMusicDirectory = 18,
+            NSPicturesDirectory = 19
+        }
+    }
+}

src/libraries/Native/Unix/System.Native/CMakeLists.txt

@@ -23,7 +23,9 @@ set(NATIVE_SOURCES
 )
 
 if (CLR_CMAKE_TARGET_IOS)
-    set(NATIVE_SOURCES ${NATIVE_SOURCES} pal_log.m)
+    set(NATIVE_SOURCES ${NATIVE_SOURCES}
+        pal_log.m
+        pal_searchpath.m)
 else ()
     set(NATIVE_SOURCES ${NATIVE_SOURCES} pal_console.c)
 endif ()

src/libraries/Native/Unix/System.Native/pal_searchpath.h

@@ -0,0 +1,10 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#pragma once
+
+#include "pal_compiler.h"
+#include "pal_types.h"
+
+PALEXPORT const char* SystemNative_SearchPath(int32_t folderId);

src/libraries/Native/Unix/System.Native/pal_searchpath.m

@@ -0,0 +1,14 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+#include "pal_searchpath.h"
+#import <Foundation/Foundation.h>
+
+const char* SystemNative_SearchPath(int32_t folderId)
+{
+    NSSearchPathDirectory spd = (NSSearchPathDirectory) folderId;
+    NSURL* url = [[[NSFileManager defaultManager] URLsForDirectory:spd inDomains:NSUserDomainMask] lastObject];
+    const char* path = [[url path] UTF8String];
+    return path == NULL ? NULL : strdup (path);
+}

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -1723,6 +1723,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Diagnostics\Tracing\RuntimeEventSourceHelper.Unix.cs" Condition="'$(FeaturePerfTracing)' == 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Environment.NoRegistry.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Environment.Unix.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Environment.Unix.GetFolderPathCore.cs" Condition="'$(TargetsiOS)' != 'true'" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\CalendarData.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\CompareInfo.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\CultureData.Unix.cs" />

src/libraries/System.Private.CoreLib/src/System/Environment.Unix.GetFolderPathCore.cs

@@ -0,0 +1,250 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading;
+
+namespace System
+{
+    public static partial class Environment
+    {
+        private static Func<string, object>? s_directoryCreateDirectory;
+
+        private static string GetFolderPathCore(SpecialFolder folder, SpecialFolderOption option)
+        {
+            // Get the path for the SpecialFolder
+            string path = GetFolderPathCoreWithoutValidation(folder);
+            Debug.Assert(path != null);
+
+            // If we didn't get one, or if we got one but we're not supposed to verify it,
+            // or if we're supposed to verify it and it passes verification, return the path.
+            if (path.Length == 0 ||
+                option == SpecialFolderOption.DoNotVerify ||
+                Interop.Sys.Access(path, Interop.Sys.AccessMode.R_OK) == 0)
+            {
+                return path;
+            }
+
+            // Failed verification.  If None, then we're supposed to return an empty string.
+            // If Create, we're supposed to create it and then return the path.
+            if (option == SpecialFolderOption.None)
+            {
+                return string.Empty;
+            }
+            else
+            {
+                Debug.Assert(option == SpecialFolderOption.Create);
+
+                Func<string, object> createDirectory = LazyInitializer.EnsureInitialized(ref s_directoryCreateDirectory, () =>
+                {
+                    Type dirType = Type.GetType("System.IO.Directory, System.IO.FileSystem, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", throwOnError: true)!;
+                    MethodInfo mi = dirType.GetTypeInfo().GetDeclaredMethod("CreateDirectory")!;
+                    return (Func<string, object>)mi.CreateDelegate(typeof(Func<string, object>));
+                });
+                createDirectory(path);
+
+                return path;
+            }
+        }
+
+        private static string GetFolderPathCoreWithoutValidation(SpecialFolder folder)
+        {
+            // First handle any paths that involve only static paths, avoiding the overheads of getting user-local paths.
+            // https://www.freedesktop.org/software/systemd/man/file-hierarchy.html
+            switch (folder)
+            {
+                case SpecialFolder.CommonApplicationData: return "/usr/share";
+                case SpecialFolder.CommonTemplates: return "/usr/share/templates";
+#if TARGET_OSX
+                case SpecialFolder.ProgramFiles: return "/Applications";
+                case SpecialFolder.System: return "/System";
+#endif
+            }
+
+            // All other paths are based on the XDG Base Directory Specification:
+            // https://specifications.freedesktop.org/basedir-spec/latest/
+            string? home = null;
+            try
+            {
+                home = PersistedFiles.GetHomeDirectory();
+            }
+            catch (Exception exc)
+            {
+                Debug.Fail($"Unable to get home directory: {exc}");
+            }
+
+            // Fall back to '/' when we can't determine the home directory.
+            // This location isn't writable by non-root users which provides some safeguard
+            // that the application doesn't write data which is meant to be private.
+            if (string.IsNullOrEmpty(home))
+            {
+                home = "/";
+            }
+
+            // TODO: Consider caching (or precomputing and caching) all subsequent results.
+            // This would significantly improve performance for repeated access, at the expense
+            // of not being responsive to changes in the underlying environment variables,
+            // configuration files, etc.
+
+            switch (folder)
+            {
+                case SpecialFolder.UserProfile:
+                case SpecialFolder.MyDocuments: // same value as Personal
+                    return home;
+                case SpecialFolder.ApplicationData:
+                    return GetXdgConfig(home);
+                case SpecialFolder.LocalApplicationData:
+                    // "$XDG_DATA_HOME defines the base directory relative to which user specific data files should be stored."
+                    // "If $XDG_DATA_HOME is either not set or empty, a default equal to $HOME/.local/share should be used."
+                    string? data = GetEnvironmentVariable("XDG_DATA_HOME");
+                    if (string.IsNullOrEmpty(data) || data[0] != '/')
+                    {
+                        data = Path.Combine(home, ".local", "share");
+                    }
+                    return data;
+
+                case SpecialFolder.Desktop:
+                case SpecialFolder.DesktopDirectory:
+                    return ReadXdgDirectory(home, "XDG_DESKTOP_DIR", "Desktop");
+                case SpecialFolder.Templates:
+                    return ReadXdgDirectory(home, "XDG_TEMPLATES_DIR", "Templates");
+                case SpecialFolder.MyVideos:
+                    return ReadXdgDirectory(home, "XDG_VIDEOS_DIR", "Videos");
+
+#if TARGET_OSX
+                case SpecialFolder.MyMusic:
+                    return Path.Combine(home, "Music");
+                case SpecialFolder.MyPictures:
+                    return Path.Combine(home, "Pictures");
+                case SpecialFolder.Fonts:
+                    return Path.Combine(home, "Library", "Fonts");
+                case SpecialFolder.Favorites:
+                    return Path.Combine(home, "Library", "Favorites");
+                case SpecialFolder.InternetCache:
+                    return Path.Combine(home, "Library", "Caches");
+#else
+                case SpecialFolder.MyMusic:
+                    return ReadXdgDirectory(home, "XDG_MUSIC_DIR", "Music");
+                case SpecialFolder.MyPictures:
+                    return ReadXdgDirectory(home, "XDG_PICTURES_DIR", "Pictures");
+                case SpecialFolder.Fonts:
+                    return Path.Combine(home, ".fonts");
+#endif
+            }
+
+            // No known path for the SpecialFolder
+            return string.Empty;
+        }
+
+        private static string GetXdgConfig(string home)
+        {
+            // "$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored."
+            // "If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used."
+            string? config = GetEnvironmentVariable("XDG_CONFIG_HOME");
+            if (string.IsNullOrEmpty(config) || config[0] != '/')
+            {
+                config = Path.Combine(home, ".config");
+            }
+            return config;
+        }
+
+        private static string ReadXdgDirectory(string homeDir, string key, string fallback)
+        {
+            Debug.Assert(!string.IsNullOrEmpty(homeDir), $"Expected non-empty homeDir");
+            Debug.Assert(!string.IsNullOrEmpty(key), $"Expected non-empty key");
+            Debug.Assert(!string.IsNullOrEmpty(fallback), $"Expected non-empty fallback");
+
+            string? envPath = GetEnvironmentVariable(key);
+            if (!string.IsNullOrEmpty(envPath) && envPath[0] == '/')
+            {
+                return envPath;
+            }
+
+            // Use the user-dirs.dirs file to look up the right config.
+            // Note that the docs also highlight a list of directories in which to look for this file:
+            // "$XDG_CONFIG_DIRS defines the preference-ordered set of base directories to search for configuration files in addition
+            //  to the $XDG_CONFIG_HOME base directory. The directories in $XDG_CONFIG_DIRS should be separated with a colon ':'. If
+            //  $XDG_CONFIG_DIRS is either not set or empty, a value equal to / etc / xdg should be used."
+            // For simplicity, we don't currently do that.  We can add it if/when necessary.
+
+            string userDirsPath = Path.Combine(GetXdgConfig(homeDir), "user-dirs.dirs");
+            if (Interop.Sys.Access(userDirsPath, Interop.Sys.AccessMode.R_OK) == 0)
+            {
+                try
+                {
+                    using (var reader = new StreamReader(userDirsPath))
+                    {
+                        string? line;
+                        while ((line = reader.ReadLine()) != null)
+                        {
+                            // Example lines:
+                            // XDG_DESKTOP_DIR="$HOME/Desktop"
+                            // XDG_PICTURES_DIR = "/absolute/path"
+
+                            // Skip past whitespace at beginning of line
+                            int pos = 0;
+                            SkipWhitespace(line, ref pos);
+                            if (pos >= line.Length) continue;
+
+                            // Skip past requested key name
+                            if (string.CompareOrdinal(line, pos, key, 0, key.Length) != 0) continue;
+                            pos += key.Length;
+
+                            // Skip past whitespace and past '='
+                            SkipWhitespace(line, ref pos);
+                            if (pos >= line.Length - 4 || line[pos] != '=') continue; // 4 for ="" and at least one char between quotes
+                            pos++; // skip past '='
+
+                            // Skip past whitespace and past first quote
+                            SkipWhitespace(line, ref pos);
+                            if (pos >= line.Length - 3 || line[pos] != '"') continue; // 3 for "" and at least one char between quotes
+                            pos++; // skip past opening '"'
+
+                            // Skip past relative prefix if one exists
+                            bool relativeToHome = false;
+                            const string RelativeToHomePrefix = "$HOME/";
+                            if (string.CompareOrdinal(line, pos, RelativeToHomePrefix, 0, RelativeToHomePrefix.Length) == 0)
+                            {
+                                relativeToHome = true;
+                                pos += RelativeToHomePrefix.Length;
+                            }
+                            else if (line[pos] != '/') // if not relative to home, must be absolute path
+                            {
+                                continue;
+                            }
+
+                            // Find end of path
+                            int endPos = line.IndexOf('"', pos);
+                            if (endPos <= pos) continue;
+
+                            // Got we need.  Now extract it.
+                            string path = line.Substring(pos, endPos - pos);
+                            return relativeToHome ?
+                                Path.Combine(homeDir, path) :
+                                path;
+                        }
+                    }
+                }
+                catch (Exception exc)
+                {
+                    // assembly not found, file not found, errors reading file, etc. Just eat everything.
+                    Debug.Fail($"Failed reading {userDirsPath}: {exc}");
+                }
+            }
+
+            return Path.Combine(homeDir, fallback);
+        }
+
+        private static void SkipWhitespace(string line, ref int pos)
+        {
+            while (pos < line.Length && char.IsWhiteSpace(line[pos])) pos++;
+        }
+    }
+}