C#: Improve logic for looking up .NET runtime in standalone mode

Instead of only considering a fixed set of paths for `dotnet` and `mono`,
first attempt to lookup the paths based on the `PATH` environment variable.
This change also fixes a potential `System.IO.DirectoryNotFoundException` exception,
which could be thrown when the `shared/Microsoft.NETCore.App` folder was not
present.

Author: hvitved

csharp/extractor/Semmle.Extraction.CSharp.Standalone/Runtime.cs

@@ -3,6 +3,7 @@
 using System.Runtime.InteropServices;
 using System.IO;
 using System.Linq;
+using Semmle.Util;
 
 namespace Semmle.Extraction.CSharp.Standalone
 {
@@ -20,11 +21,14 @@ public static IEnumerable<string> CoreRuntimes
         {
             get
             {
-                string[] dotnetDirs = { "/usr/share/dotnet", @"C:\Program Files\dotnet" };
+                var dotnetPath = FileUtils.FindExecutableOnPath("dotnet");
+                var dotnetDirs = dotnetPath != null
+                    ? new[] { dotnetPath }
+                    : new[] { "/usr/share/dotnet", @"C:\Program Files\dotnet" };
+                var coreDirs = dotnetDirs.Select(d => Path.Combine(d, "shared", "Microsoft.NETCore.App"));
 
-                foreach (var dir in dotnetDirs.Where(Directory.Exists))
-                    return Directory.EnumerateDirectories(Path.Combine(dir, "shared", "Microsoft.NETCore.App")).
-                        OrderByDescending(d => Path.GetFileName(d));
+                foreach (var dir in coreDirs.Where(Directory.Exists))
+                    return Directory.EnumerateDirectories(dir).OrderByDescending(Path.GetFileName);
                 return Enumerable.Empty<string>();
             }
         }
@@ -37,19 +41,22 @@ public static IEnumerable<string> DesktopRuntimes
         {
             get
             {
-                string[] monoDirs = { "/usr/lib/mono", @"C:\Program Files\Mono\lib\mono" };
+                var monoPath = FileUtils.FindExecutableOnPath("mono");
+                var monoDirs = monoPath != null
+                    ? new[] { monoPath }
+                    : new[] { "/usr/lib/mono", @"C:\Program Files\Mono\lib\mono" };
 
                 if (Directory.Exists(@"C:\Windows\Microsoft.NET\Framework64"))
                 {
-                    return System.IO.Directory.EnumerateDirectories(@"C:\Windows\Microsoft.NET\Framework64", "v*").
-                        OrderByDescending(d => Path.GetFileName(d));
+                    return Directory.EnumerateDirectories(@"C:\Windows\Microsoft.NET\Framework64", "v*").
+                        OrderByDescending(Path.GetFileName);
                 }
 
                 foreach (var dir in monoDirs.Where(Directory.Exists))
                 {
-                    return System.IO.Directory.EnumerateDirectories(dir).
+                    return Directory.EnumerateDirectories(dir).
                         Where(d => Char.IsDigit(Path.GetFileName(d)[0])).
-                        OrderByDescending(d => Path.GetFileName(d));
+                        OrderByDescending(Path.GetFileName);
                 }
 
                 return Enumerable.Empty<string>();

csharp/extractor/Semmle.Util/FileUtils.cs

@@ -1,4 +1,6 @@
+using System;
 using System.IO;
+using System.Linq;
 
 namespace Semmle.Util
 {
@@ -50,5 +52,23 @@ public static void TryDelete(string file)
                 // Ignore
             }
         }
+
+        /// <summary>
+        /// Finds the path for the executable <paramref name="exe"/> based on the
+        /// <code>PATH</code> environment variable, and in the case of Windows the
+        /// <code>PATHEXT</code> environment variable.
+        /// 
+        /// Returns <code>null</code> of no path can be found.
+        /// </summary>
+        public static string FindExecutableOnPath(string exe)
+        {
+            var isWindows = Win32.IsWindows();
+            var paths = Environment.GetEnvironmentVariable("PATH").Split(isWindows ? ';' : ':');
+            var exes = isWindows
+                ? Environment.GetEnvironmentVariable("PATHEXT").Split(';').Select(ext => exe + ext)
+                : new[] { exe };
+            var candidates = paths.Where(path => exes.Any(exe0 => File.Exists(Path.Combine(path, exe0))));
+            return candidates.FirstOrDefault();
+        }
     }
 }