Fix various problems with install path and app path variables (#968)

Author: Matthew John Cheetham

src/shared/Core/ApplicationBase.cs

@@ -91,7 +91,7 @@ public static string GetEntryApplicationPath()
 
         public static string GetInstallationDirectory()
         {
-            return Path.GetDirectoryName(Environment.GetCommandLineArgs()[0]);
+            return AppContext.BaseDirectory;
         }
 
         /// <summary>

src/shared/Core/Authentication/AuthenticationBase.cs

@@ -140,6 +140,11 @@ protected bool TryFindHelperCommand(string envar, string configName, string defa
 
                 Context.Trace.WriteLine($"UI helper override specified: '{helperName}'.");
             }
+            else if (string.IsNullOrWhiteSpace(defaultValue))
+            {
+                Context.Trace.WriteLine("No default UI supplied.");
+                return false;
+            }
             else
             {
                 Context.Trace.WriteLine($"Using default UI helper: '{defaultValue}'.");

src/shared/Core/Interop/Windows/WindowsEnvironment.cs

@@ -22,7 +22,10 @@ internal WindowsEnvironment(IFileSystem fileSystem, IReadOnlyDictionary<string,
 
         protected override string[] SplitPathVariable(string value)
         {
-            return value.Split(';');
+            // Ensure we don't return empty values here - callers may use this as the base
+            // path for `Path.Combine(..)`, for which an empty value means 'current directory'.
+            // We only ever want to use the current directory for path resolution explicitly.
+            return value.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries);
         }
 
         public override void AddDirectoryToPath(string directoryPath, EnvironmentVariableTarget target)

src/shared/Core/PlatformUtils.cs

@@ -289,7 +289,11 @@ private static string GetWindowsEntryPath()
             IntPtr argv0Ptr = Marshal.ReadIntPtr(argvPtr);
             string argv0 = Marshal.PtrToStringAuto(argv0Ptr);
             Interop.Windows.Native.Kernel32.LocalFree(argvPtr);
-            return argv0;
+
+            // If this isn't absolute then we should return null to prevent any
+            // caller that expect only an absolute path from mis-using this result.
+            // They will have to fall-back to other mechanisms for getting the entry path.
+            return Path.IsPathRooted(argv0) ? argv0 : null;
         }
 
         #endregion

src/shared/Git-Credential-Manager/Program.cs

@@ -43,16 +43,24 @@ public static void Main(string[] args)
                 //
                 // On UNIX systems we do the same check, except instead of a copy we use a symlink.
                 //
-                string oldName = PlatformUtils.IsWindows()
-                    ? "git-credential-manager-core.exe"
-                    : "git-credential-manager-core";
 
-                if (appPath?.EndsWith(oldName, StringComparison.OrdinalIgnoreCase) ?? false)
+                if (!string.IsNullOrWhiteSpace(appPath))
                 {
-                    context.Streams.Error.WriteLine(
-                        "warning: git-credential-manager-core was renamed to git-credential-manager");
-                    context.Streams.Error.WriteLine(
-                        $"warning: see {Constants.HelpUrls.GcmExecRename} for more information");
+                    // Trim any (.exe) file extension if we're on Windows
+                    // Note that in some circumstances (like being called by Git when config is set
+                    // to just `helper = manager-core`) we don't always have ".exe" at the end.
+                    if (PlatformUtils.IsWindows() && appPath.EndsWith(".exe", StringComparison.OrdinalIgnoreCase))
+                    {
+                        appPath = appPath.Substring(0, appPath.Length - 4);
+                    }
+
+                    if (appPath.EndsWith("git-credential-manager-core", StringComparison.OrdinalIgnoreCase))
+                    {
+                        context.Streams.Error.WriteLine(
+                            "warning: git-credential-manager-core was renamed to git-credential-manager");
+                        context.Streams.Error.WriteLine(
+                            $"warning: see {Constants.HelpUrls.GcmExecRename} for more information");
+                    }
                 }
 
                 // Register all supported host providers at the normal priority.