C#: Resolve the hostfxr path using dotnet CLI

Instead of trying to get the location of the dotnet CLI from PATH (which is unavailable in some platforms that don't allow reading environment variables), we execute the dotnet CLI to list the available SDKs and find the hostfxr location that way.

modules/mono/mono_gd/gd_mono.cpp

@@ -40,6 +40,7 @@
 
 #ifdef TOOLS_ENABLED
 #include "../editor/hostfxr_resolver.h"
+#include "../editor/semver.h"
 #endif
 
 #include "core/config/project_settings.h"
@@ -105,6 +106,60 @@ const char_t *get_data(const HostFxrCharString &p_char_str) {
 	return (const char_t *)p_char_str.get_data();
 }
 
+#ifdef TOOLS_ENABLED
+bool try_get_dotnet_root_from_command_line(String &r_dotnet_root) {
+	String pipe;
+	List<String> args;
+	args.push_back("--list-sdks");
+
+	int exitcode;
+	Error err = OS::get_singleton()->execute("dotnet", args, &pipe, &exitcode, true);
+
+	ERR_FAIL_COND_V_MSG(err != OK, false, String(".NET failed to get list of installed SDKs. Error: ") + error_names[err]);
+	ERR_FAIL_COND_V_MSG(exitcode != 0, false, pipe);
+
+	Vector<String> sdks = pipe.strip_edges().replace("\r\n", "\n").split("\n", false);
+
+	godotsharp::SemVerParser sem_ver_parser;
+
+	godotsharp::SemVer latest_sdk_version;
+	String latest_sdk_path;
+
+	for (const String &sdk : sdks) {
+		// The format of the SDK lines is:
+		// 8.0.401 [/usr/share/dotnet/sdk]
+		String version_string = sdk.get_slice(" ", 0);
+		String path = sdk.get_slice(" ", 1);
+		path = path.substr(1, path.length() - 2);
+
+		godotsharp::SemVer version;
+		if (!sem_ver_parser.parse(version_string, version)) {
+			WARN_PRINT("Unable to parse .NET SDK version '" + version_string + "'.");
+			continue;
+		}
+
+		if (!DirAccess::exists(path)) {
+			WARN_PRINT("Found .NET SDK version '" + version_string + "' with invalid path '" + path + "'.");
+			continue;
+		}
+
+		if (version > latest_sdk_version) {
+			latest_sdk_version = version;
+			latest_sdk_path = path;
+		}
+	}
+
+	if (!latest_sdk_path.is_empty()) {
+		print_verbose("Found .NET SDK at " + latest_sdk_path);
+		// The `dotnet_root` is the parent directory.
+		r_dotnet_root = latest_sdk_path.path_join("..").simplify_path();
+		return true;
+	}
+
+	return false;
+}
+#endif
+
 String find_hostfxr() {
 #ifdef TOOLS_ENABLED
 	String dotnet_root;
@@ -113,23 +168,9 @@ String find_hostfxr() {
 		return fxr_path;
 	}
 
-	// hostfxr_resolver doesn't look for dotnet in `PATH`. If it fails, we try to find the dotnet
-	// executable in `PATH` here and pass its location as `dotnet_root` to `get_hostfxr_path`.
-	String dotnet_exe = Path::find_executable("dotnet");
-
-	if (!dotnet_exe.is_empty()) {
-		// The file found in PATH may be a symlink
-		dotnet_exe = Path::abspath(Path::realpath(dotnet_exe));
-
-		// TODO:
-		// Sometimes, the symlink may not point to the dotnet executable in the dotnet root.
-		// That's the case with snaps. The snap install should have been found with the
-		// previous `get_hostfxr_path`, but it would still be better to do this properly
-		// and use something like `dotnet --list-sdks/runtimes` to find the actual location.
-		// This way we could also check if the proper sdk or runtime is installed. This would
-		// allow us to fail gracefully and show some helpful information in the editor.
-
-		dotnet_root = dotnet_exe.get_base_dir();
+	// hostfxr_resolver doesn't look for dotnet in `PATH`. If it fails, we try to use the dotnet
+	// executable in `PATH` to find the `dotnet_root` and get the `hostfxr_path` from there.
+	if (try_get_dotnet_root_from_command_line(dotnet_root)) {
 		if (godotsharp::hostfxr_resolver::try_get_path_from_dotnet_root(dotnet_root, fxr_path)) {
 			return fxr_path;
 		}