fix(launcher): require executable Linux hosts

src/AcDream.Launcher.Core/Orchestration/LauncherExecutableSet.cs

@@ -12,12 +12,14 @@ namespace AcDream.Launcher.Core.Orchestration;
 public sealed class LauncherExecutableSet
 {
     private readonly Func<string, bool> _fileExists;
+    private readonly Func<string, bool> _hasUnixExecutePermission;
 
     public LauncherExecutableSet(
         string graphicalHostPath,
         string headlessHostPath,
         string? workingDirectory = null,
-        Func<string, bool>? fileExists = null)
+        Func<string, bool>? fileExists = null,
+        Func<string, bool>? hasUnixExecutePermission = null)
     {
         ArgumentException.ThrowIfNullOrWhiteSpace(graphicalHostPath);
         ArgumentException.ThrowIfNullOrWhiteSpace(headlessHostPath);
@@ -25,6 +27,8 @@ public sealed class LauncherExecutableSet
         HeadlessHostPath = headlessHostPath;
         WorkingDirectory = workingDirectory;
         _fileExists = fileExists ?? File.Exists;
+        _hasUnixExecutePermission =
+            hasUnixExecutePermission ?? HasUnixExecutePermission;
     }
 
     public string GraphicalHostPath { get; }
@@ -38,17 +42,25 @@ public sealed class LauncherExecutableSet
         string path = mode == LaunchMode.Headless
             ? HeadlessHostPath
             : GraphicalHostPath;
-        if (_fileExists(path))
-        {
-            return LauncherCapability.Available;
-        }
-
         string host = mode == LaunchMode.Headless
             ? "headless host"
             : "graphical client";
-        return LauncherCapability.Unavailable(
-            $"The co-deployed {host} is missing at '{path}'. Reinstall or update "
-            + "the client before launching.");
+        if (!_fileExists(path))
+        {
+            return LauncherCapability.Unavailable(
+                $"The co-deployed {host} is missing at '{path}'. Reinstall or update "
+                + "the client before launching.");
+        }
+
+        if (OperatingSystem.IsLinux() && !_hasUnixExecutePermission(path))
+        {
+            return LauncherCapability.Unavailable(
+                $"The co-deployed {host} at '{path}' exists but is not executable. "
+                + "Restore its executable permission (for example, chmod +x) or "
+                + "reinstall/update the client before launching.");
+        }
+
+        return LauncherCapability.Available;
     }
 
     public LauncherProcessSpec CreatePlaySpec(
@@ -99,4 +111,27 @@ public sealed class LauncherExecutableSet
                 capability.Reason ?? "The selected launcher host is unavailable.");
         }
     }
+
+    private static bool HasUnixExecutePermission(string path)
+    {
+        if (!OperatingSystem.IsLinux())
+        {
+            return true;
+        }
+
+        try
+        {
+            const UnixFileMode executeBits =
+                UnixFileMode.UserExecute
+                | UnixFileMode.GroupExecute
+                | UnixFileMode.OtherExecute;
+            return (File.GetUnixFileMode(path) & executeBits) != 0;
+        }
+        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
+        {
+            // Fail closed if the file vanished or its metadata cannot be read
+            // after the existence check. The next capability refresh retries.
+            return false;
+        }
+    }
 }