ast/lir: simplify concurrent use of AST, which is globally stateful

Fixes #10415

logstash-core/lib/logstash/config/config_ast.rb

@@ -7,30 +7,62 @@ require "logstash/compiler/treetop_monkeypatches"
 module LogStash; module Config; module AST
   PROCESS_ESCAPE_SEQUENCES = :process_escape_sequences
 
-  def self.deferred_conditionals=(val)
+  class << self
+    # @api private
+    MUTEX = Mutex.new
+
+    # Executes the given block with exclusive access to the AST global variables
+    #
+    # @yieldreturn [Object]: the object that is returned from the block is returned by this method
+    #
+    # @return [Object]
+    def exclusive
+      MUTEX.synchronize { yield }
+    end
+
+    def deferred_conditionals=(val)
+      ensure_exclusive!
       @deferred_conditionals = val
     end
 
-  def self.deferred_conditionals
+    def deferred_conditionals
+      ensure_exclusive!
       @deferred_conditionals
     end
 
-  def self.deferred_conditionals_index
+    def deferred_conditionals_index
+      ensure_exclusive!
       @deferred_conditionals_index
     end
 
-  def self.deferred_conditionals_index=(val)
+    def deferred_conditionals_index=(val)
+      ensure_exclusive!
       @deferred_conditionals_index = val
     end
 
-  def self.plugin_instance_index
+    def plugin_instance_index
+      ensure_exclusive!
       @plugin_instance_index
     end
 
-  def self.plugin_instance_index=(val)
+    def plugin_instance_index=(val)
+      ensure_exclusive!
       @plugin_instance_index = val
     end
 
+    private
+
+    # Raises a descriptive error if the thread in which it is invoked does
+    # not have exclusive access.
+    #
+    # @raise [RuntimeError]
+    def ensure_exclusive!
+      return if MUTEX.owned?
+
+      raise "Illegal access without exclusive lock at `#{caller[1]}`"
+    end
+  end
+
   class Node < Treetop::Runtime::SyntaxNode
     include LogStashCompilerLSCLGrammar::LogStash::Compiler::LSCL::AST::Helpers
 
@@ -46,6 +78,15 @@ module LogStash; module Config; module AST
 
 
     def compile
+      LogStash::Config::AST.exclusive { do_compile }
+    end
+
+    private
+
+    # NON-threadsafe method compiles an AST into executable Ruby code.
+    # @see Config#compile, which is a threadsafe wrapper around this method.
+    # @api private
+    def do_compile
       LogStash::Config::AST.deferred_conditionals = []
       LogStash::Config::AST.deferred_conditionals_index = 0
       LogStash::Config::AST.plugin_instance_index = 0
@@ -491,6 +532,7 @@ module LogStash; module Config; module AST
 end; end; end
 
 
+
 # Monkeypatch Treetop::Runtime::SyntaxNode's inspect method to skip
 # any Whitespace or SyntaxNodes with no children.
 class Treetop::Runtime::SyntaxNode