env: introduce TryLocateExecutable method

Introduce a `TryX` method version of the existing `LocateExecutable`
method on `IEnvironment`, for locating executables with `which` and
`where.exe`.

Author: mjcheetham

src/shared/Microsoft.Git.CredentialManager/EnvironmentBase.cs

@@ -41,8 +41,9 @@ public interface IEnvironment
         /// Locate an executable on the current PATH.
         /// </summary>
         /// <param name="program">Executable program name.</param>
-        /// <returns>List of all instances of the found executable program, in order of most specific to least.</returns>
-        string LocateExecutable(string program);
+        /// <param name="path">First instance of the found executable program.</param>
+        /// <returns>True if the executable was found, false otherwise.</returns>
+        bool TryLocateExecutable(string program, out string path);
     }
 
     public abstract class EnvironmentBase : IEnvironment
@@ -75,6 +76,25 @@ public bool IsDirectoryOnPath(string directoryPath)
 
         protected abstract string[] SplitPathVariable(string value);
 
-        public abstract string LocateExecutable(string program);
+        public abstract bool TryLocateExecutable(string program, out string path);
+    }
+
+    public static class EnvironmentExtensions
+    {
+        /// <summary>
+        /// Locate an executable on the current PATH.
+        /// </summary>
+        /// <param name="environment">The <see cref="IEnvironment"/>.</param>
+        /// <param name="program">Executable program name.</param>
+        /// <returns>List of all instances of the found executable program, in order of most specific to least.</returns>
+        public static string LocateExecutable(this IEnvironment environment, string program)
+        {
+            if (environment.TryLocateExecutable(program, out string path))
+            {
+                return path;
+            }
+
+            throw new Exception($"Failed to locate '{program}' executable on the path.");
+        }
     }
 }

src/shared/Microsoft.Git.CredentialManager/Interop/Posix/PosixEnvironment.cs

@@ -31,7 +31,7 @@ protected override string[] SplitPathVariable(string value)
             return value.Split(':');
         }
 
-        public override string LocateExecutable(string program)
+        public override bool TryLocateExecutable(string program, out string path)
         {
             const string whichPath = "/usr/bin/which";
             var psi = new ProcessStartInfo(whichPath, program)
@@ -45,19 +45,21 @@ public override string LocateExecutable(string program)
                 where.Start();
                 where.WaitForExit();
 
-                if (where.ExitCode != 0)
+                switch (where.ExitCode)
                 {
-                    throw new Exception($"Failed to locate '{program}' using {whichPath}. Exit code: {where.ExitCode}.");
-                }
+                    case 0: // found
+                        string stdout = where.StandardOutput.ReadToEnd();
+                        string[] results = stdout.Split(new[] {'\n'}, StringSplitOptions.RemoveEmptyEntries);
+                        path = results.First();
+                        return true;
 
-                string stdout = where.StandardOutput.ReadToEnd();
-                if (string.IsNullOrWhiteSpace(stdout))
-                {
-                    return null;
-                }
+                    case 1: // not found
+                        path = null;
+                        return false;
 
-                string[] results = stdout.Split(new[] {'\n'}, StringSplitOptions.RemoveEmptyEntries);
-                return results.FirstOrDefault();
+                    default:
+                        throw new Exception($"Unknown error locating '{program}' using {whichPath}. Exit code: {where.ExitCode}.");
+                }
             }
         }
 

src/shared/Microsoft.Git.CredentialManager/Interop/Windows/WindowsEnvironment.cs

@@ -65,7 +65,7 @@ public override void RemoveDirectoryFromPath(string directoryPath, EnvironmentVa
             }
         }
 
-        public override string LocateExecutable(string program)
+        public override bool TryLocateExecutable(string program, out string path)
         {
             string wherePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.System), "where.exe");
             var psi = new ProcessStartInfo(wherePath, program)
@@ -79,19 +79,21 @@ public override string LocateExecutable(string program)
                 where.Start();
                 where.WaitForExit();
 
-                if (where.ExitCode != 0)
+                switch (where.ExitCode)
                 {
-                    throw new Exception($"Failed to locate '{program}' using where.exe. Exit code: {where.ExitCode}.");
+                    case 0: // found
+                        string stdout = where.StandardOutput.ReadToEnd();
+                        string[] results = stdout.Split(new[] {'\r', '\n'}, StringSplitOptions.RemoveEmptyEntries);
+                        path = results.First();
+                        return true;
+
+                    case 1: // not found
+                        path = null;
+                        return false;
+
+                    default:
+                        throw new Exception($"Unknown error locating '{program}' using where.exe. Exit code: {where.ExitCode}.");
                 }
-
-                string stdout = where.StandardOutput.ReadToEnd();
-                if (string.IsNullOrWhiteSpace(stdout))
-                {
-                    return null;
-                }
-
-                string[] results = stdout.Split(new[] {'\r', '\n'}, StringSplitOptions.RemoveEmptyEntries);
-                return results.FirstOrDefault();
             }
         }
 

src/shared/TestInfrastructure/Objects/TestEnvironment.cs

@@ -80,21 +80,24 @@ public void RemoveDirectoryFromPath(string directoryPath, EnvironmentVariableTar
             Variables["PATH"] = string.Join(_envPathSeparator, Path);
         }
 
-        public string LocateExecutable(string program)
+        public bool TryLocateExecutable(string program, out string path)
         {
             if (WhichFiles.TryGetValue(program, out ICollection<string> paths))
             {
-                return paths.FirstOrDefault();
+                path = paths.First();
+                return true;
             }
 
             if (!System.IO.Path.HasExtension(program) && PlatformUtils.IsWindows())
             {
                 // If we're testing on a Windows platform, don't have a file extension, and were unable to locate
                 // the executable file.. try appending .exe.
-                return WhichFiles.TryGetValue($"{program}.exe", out paths) ? paths.FirstOrDefault() : null;
+                path = WhichFiles.TryGetValue($"{program}.exe", out paths) ? paths.First() : null;
+                return !(path is null);
             }
 
-            return null;
+            path = null;
+            return false;
         }
 
         #endregion