Don't try and use old runtimes (#2)

README.md

@@ -12,3 +12,10 @@ let info = DotnetEnvironmentInfo.Get ()
 // or, if you already know a path to the `dotnet` executable...
 let info = DotnetEnvironmentInfo.Get "/path/to/dotnet"
 ```
+
+## Troubleshooting
+
+If you have a *very* strange setup, we may be unable to locate the `libhostfxr` library we use to find the runtimes.
+In that case, you can supply the environment variable `WOOFWARE_DOTNET_LOCATOR_LIBHOSTFXR`,
+which should be a full path to a `libhostfxr` DLL on your system.
+(Normally this is in `/usr/share/dotnet/host/fxr/{runtime}/libhostfxr.so`; you must make sure your version is from runtime 6 or greater, because the required symbols were not added until then.)

WoofWare.DotnetRuntimeLocator/DotnetEnvironmentInfo.cs

@@ -26,6 +26,16 @@ public record DotnetEnvironmentInfo(
 {
     private static readonly Lazy<FileInfo> HostFxr = new(() =>
     {
+        switch (Environment.GetEnvironmentVariable("WOOFWARE_DOTNET_LOCATOR_LIBHOSTFXR"))
+        {
+            case null:
+                break;
+            case var s:
+            {
+                return new FileInfo(s);
+            }
+        }
+
         // First, we might be self-contained: try and find it next to us.
         var selfContainedAttempt = Directory.GetParent(Assembly.GetExecutingAssembly().Location);
         if (selfContainedAttempt != null)
@@ -42,7 +52,15 @@ public record DotnetEnvironmentInfo(
         var parent3 = parent2.Parent ??
                       throw new Exception("Unable to locate the host/fxr directory in the .NET runtime");
         var fxrDir = new DirectoryInfo(Path.Combine(parent3.FullName, "host", "fxr"));
-        return fxrDir.EnumerateDirectories().First().EnumerateFiles("*hostfxr*").First();
+        Func<DirectoryInfo, bool> isAcceptableName =
+            di =>
+            {
+                // Until net6, libhostfxr did not contain the entrypoint we use, and I can't be bothered to reimplement
+                // it on those runtimes. I'm just going to assume you have no runtimes earlier than 3 installed.
+                return !di.Name.StartsWith("3.", StringComparison.Ordinal) &&
+                       !di.Name.StartsWith("5.", StringComparison.Ordinal);
+            };
+        return fxrDir.EnumerateDirectories().First(isAcceptableName).EnumerateFiles("*hostfxr*").First();
     });
 
     private static FileInfo ResolveAllSymlinks(FileInfo f)