Implement AppContext.BaseDirectory for Linux (#3588)

4 files changed, 70 insertions, 21 deletions

diff --git a/src/System.Private.CoreLib/src/System.Private.CoreLib.csproj b/src/System.Private.CoreLib/src/System.Private.CoreLib.csproj
index 1b992a98e..c080ce9d6 100644
--- a/src/System.Private.CoreLib/src/System.Private.CoreLib.csproj
+++ b/src/System.Private.CoreLib/src/System.Private.CoreLib.csproj
@@ -658,6 +658,9 @@
     <Compile Include="..\..\Common\src\Interop\Unix\System.Private.CoreLib.Native\Interop.GetEnv.cs">
       <Link>Interop\Unix\System.Private.CoreLib.Native\Interop.GetEnv.cs</Link>
     </Compile>
+    <Compile Include="..\..\Common\src\Interop\Unix\System.Private.CoreLib.Native\Interop.GetExecutableAbsolutePath.cs">
+      <Link>Interop\Unix\System.Private.CoreLib.Native\Interop.GetExecutableAbsolutePath.cs</Link>
+    </Compile>
     <Compile Include="..\..\Common\src\Interop\Unix\System.Private.CoreLib.Native\Interop.MemAllocFree.cs">
       <Link>Interop\Unix\System.Private.CoreLib.Native\Interop.MemAllocFree.cs</Link>
     </Compile>
diff --git a/src/System.Private.CoreLib/src/System/AppContext.Unix.cs b/src/System.Private.CoreLib/src/System/AppContext.Unix.cs
index 9a59c759e..6067804e8 100644
--- a/src/System.Private.CoreLib/src/System/AppContext.Unix.cs
+++ b/src/System.Private.CoreLib/src/System/AppContext.Unix.cs
@@ -2,8 +2,10 @@
 // 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.Buffers;
 using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Runtime.ExceptionServices;
 using System.Runtime.InteropServices;
@@ -13,11 +15,41 @@ namespace System
 {
     public static partial class AppContext
     {
-        public static string BaseDirectory
+        /// <summary>
+        /// Return the directory of the executable image for the current process
+        /// as the default value for AppContext.BaseDirectory
+        /// </summary>
+        private static string GetBaseDirectoryCore()
         {
-            get
+            // Start with a relatively small buffer
+            int currentSize = 256;
+            for (;;)
             {
-                throw new NotImplementedException();
+                char[] buffer = ArrayPool<char>.Shared.Rent(currentSize);
+
+                // Get full path to the executable image
+                int actualSize = Interop.Sys.GetExecutableAbsolutePath(buffer, buffer.Length);
+
+                if (actualSize < 0)
+                {
+                    // The call to GetExecutableAbsolutePath function failed.
+                    Interop.ErrorInfo error = Interop.Sys.GetLastErrorInfo();
+                    ArrayPool<char>.Shared.Return(buffer);
+                    throw Interop.GetExceptionForIoErrno(error);
+                }
+
+                Debug.Assert(actualSize > 0);
+                if (actualSize <= buffer.Length)
+                {
+                    string fileName = new string(buffer, 0, actualSize);
+                    ArrayPool<char>.Shared.Return(buffer);
+
+                    // Return path to the executable image including the terminating slash
+                    return fileName.Substring(0, fileName.LastIndexOf(Path.DirectorySeparatorChar) + 1);
+                }
+
+                ArrayPool<char>.Shared.Return(buffer);
+                currentSize = actualSize;
             }
         }
     }
diff --git a/src/System.Private.CoreLib/src/System/AppContext.Windows.cs b/src/System.Private.CoreLib/src/System/AppContext.Windows.cs
index d14936129..7d535c5de 100644
--- a/src/System.Private.CoreLib/src/System/AppContext.Windows.cs
+++ b/src/System.Private.CoreLib/src/System/AppContext.Windows.cs
@@ -13,29 +13,31 @@ namespace System
 {
     public static partial class AppContext
     {
-        public static string BaseDirectory
+        /// <summary>
+        /// Return the directory of the executable image for the current process
+        /// as the default value for AppContext.BaseDirectory
+        /// </summary>
+        private static string GetBaseDirectoryCore()
         {
-            get
+            StringBuilder buffer = new StringBuilder(Interop.mincore.MAX_PATH);
+            while (true)
             {
-                StringBuilder buffer = new StringBuilder(Interop.mincore.MAX_PATH);
-                while (true)
+                int size = Interop.mincore.GetModuleFileName(IntPtr.Zero, buffer, buffer.Capacity);
+                if (size == 0)
                 {
-                    int size = Interop.mincore.GetModuleFileName(IntPtr.Zero, buffer, buffer.Capacity);
-                    if (size == 0)
-                    {
-                        throw Win32Marshal.GetExceptionForWin32Error(Marshal.GetLastWin32Error());
-                    }
-
-                    if (Marshal.GetLastWin32Error() == Interop.mincore.ERROR_INSUFFICIENT_BUFFER)
-                    {
-                        // Enlarge the buffer and try again.
-                        buffer.EnsureCapacity(buffer.Capacity * 2);
-                        continue;
-                    }
+                    throw Win32Marshal.GetExceptionForWin32Error(Marshal.GetLastWin32Error());
+                }
 
-                    string fileName = buffer.ToString();
-                    return fileName.Substring(0, fileName.LastIndexOf('\\'));
+                if (Marshal.GetLastWin32Error() == Interop.mincore.ERROR_INSUFFICIENT_BUFFER)
+                {
+                    // Enlarge the buffer and try again.
+                    buffer.EnsureCapacity(buffer.Capacity * 2);
+                    continue;
                 }
+
+                // Return path to the executable image including the terminating slash
+                string fileName = buffer.ToString();
+                return fileName.Substring(0, fileName.LastIndexOf('\\') + 1);
             }
         }
     }
diff --git a/src/System.Private.CoreLib/src/System/AppContext.cs b/src/System.Private.CoreLib/src/System/AppContext.cs
index cd688f2ba..5b593b70c 100644
--- a/src/System.Private.CoreLib/src/System/AppContext.cs
+++ b/src/System.Private.CoreLib/src/System/AppContext.cs
@@ -23,6 +23,7 @@ namespace System
         }
         private static readonly Dictionary<string, SwitchValueState> s_switchMap = new Dictionary<string, SwitchValueState>();
         private static Dictionary<String, Object> s_localStore = new Dictionary<String, Object>();
+        private static string s_defaultBaseDirectory;
 
         static AppContext()
         {
@@ -38,6 +39,17 @@ namespace System
             }
         }
 
+        public static string BaseDirectory
+        {
+            get
+            {
+                // The value of APP_CONTEXT_BASE_DIRECTORY key has to be a string and it is not allowed to be any other type. 
+                // Otherwise the caller will get invalid cast exception
+                return (string)GetData("APP_CONTEXT_BASE_DIRECTORY") ?? 
+                    (s_defaultBaseDirectory ?? (s_defaultBaseDirectory = GetBaseDirectoryCore()));
+            }
+        }
+
         public static object GetData(string name)
         {
             if (name == null)