Make MCP executable recover on non-FHS Linux

Author: st0012

exe/rubydex_mcp

@@ -1,17 +1,8 @@
 #!/usr/bin/env ruby
 # frozen_string_literal: true
 
-require "rbconfig"
+$LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
 
-host_os = RbConfig::CONFIG.fetch("host_os")
-executable = host_os.match?(/mswin|mingw|cygwin/) ? "rubydex_mcp.exe" : "rubydex_mcp"
-binary = File.expand_path("../lib/rubydex/bin/#{executable}", __dir__)
+require "rubydex/mcp_server_launcher"
 
-unless File.executable?(binary)
-  abort(<<~MESSAGE.chomp)
-    rubydex_mcp is not available at #{binary}.
-    Install a precompiled rubydex gem, or reinstall rubydex with Cargo available so the MCP executable can be built locally.
-  MESSAGE
-end
-
-exec(binary, *ARGV)
+Rubydex::MCPServerLauncher.exec_server(ARGV)

lib/rubydex/mcp_server_launcher.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+require "rbconfig"
+
+module Rubydex
+  # Launcher-only helpers for `exe/rubydex_mcp`.
+  #
+  # Do not require this file from `lib/rubydex.rb`. It exists only so the Ruby executable wrapper can handle
+  # host-specific process launch details before it execs the Rust MCP server.
+  module MCPServerLauncher
+    extend self
+
+    def windows?
+      RbConfig::CONFIG.fetch("host_os").match?(/mswin|mingw|cygwin/)
+    end
+
+    def executable_name
+      windows? ? "rubydex_mcp.exe" : "rubydex_mcp"
+    end
+
+    def binary_path
+      File.expand_path("bin/#{executable_name}", __dir__)
+    end
+
+    def host_loader(maps_path)
+      File.foreach(maps_path) do |line|
+        path = line.split.last
+        next unless path&.start_with?("/")
+        next unless File.basename(path).match?(/\Ald-.*\.so/)
+
+        return path if File.exist?(path)
+      end
+
+      nil
+    rescue Errno::ENOENT
+      nil
+    end
+
+    def exec_server(argv)
+      binary = binary_path
+
+      unless File.executable?(binary)
+        abort(<<~MESSAGE.chomp)
+          rubydex_mcp is not available at #{binary}.
+          Install a precompiled rubydex gem, or reinstall rubydex with Cargo available so the MCP executable can be built locally.
+        MESSAGE
+      end
+
+      exec(binary, *argv)
+    rescue Errno::ENOENT
+      loader = host_loader("/proc/self/maps")
+      raise unless loader
+
+      exec(loader, binary, *argv)
+    end
+  end
+end

test/mcp_server_executable_test.rb

@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require_relative "test_helper"
+
+require "fileutils"
+require "tmpdir"
+require "tempfile"
+require "rubydex/mcp_server_launcher"
+
+class MCPServerExecutableTest < Minitest::Test
+  def test_host_loader_reads_loader_from_maps
+    dir = Dir.mktmpdir
+    loader_path = File.join(dir, "ld-linux-x86-64.so.2")
+    File.write(loader_path, "")
+
+    maps = Tempfile.new("maps")
+    maps.write(<<~MAPS)
+      7f3d10000000-7f3d10022000 r--p 00000000 00:00 0 [heap]
+      7f3d20000000-7f3d20022000 r-xp 00000000 00:00 0 #{loader_path}
+    MAPS
+    maps.close
+
+    assert_equal(loader_path, Rubydex::MCPServerLauncher.host_loader(maps.path))
+  ensure
+    maps&.unlink
+    FileUtils.remove_entry_secure(dir) if dir && File.directory?(dir)
+  end
+
+  def test_host_loader_ignores_missing_loader_paths
+    Dir.mktmpdir do |dir|
+      loader_path = File.join(dir, "ld-linux-x86-64.so.2")
+      File.write(loader_path, "")
+
+      maps = Tempfile.new("maps")
+      maps.write(<<~MAPS)
+        7f3d10000000-7f3d10022000 r-xp 00000000 00:00 0 /missing/ld-linux-x86-64.so.2
+        7f3d20000000-7f3d20022000 r-xp 00000000 00:00 0 #{loader_path}
+      MAPS
+      maps.close
+
+      assert_equal(loader_path, Rubydex::MCPServerLauncher.host_loader(maps.path))
+    ensure
+      maps&.unlink
+    end
+  end
+
+  def test_host_loader_returns_nil_when_maps_file_is_missing
+    assert_nil(Rubydex::MCPServerLauncher.host_loader("/missing/proc/self/maps"))
+  end
+
+  def test_host_loader_returns_existing_linux_loader
+    skip("Linux-only") unless File.file?("/proc/self/maps")
+
+    loader = Rubydex::MCPServerLauncher.host_loader("/proc/self/maps")
+
+    refute_nil(loader)
+    assert_path_exists(loader)
+    assert_match(/\Ald-.*\.so/, File.basename(loader))
+  end
+end