Use a Fiber attribute for Context

Author: fbogsany

api/lib/opentelemetry/context.rb

@@ -8,11 +8,12 @@
 require 'opentelemetry/context/propagation'
 
 module OpenTelemetry
+  Fiber.attr_accessor :opentelemetry_context
+
   # Manages context on a per-fiber basis
   class Context
     EMPTY_ENTRIES = {}.freeze
-    STACK_KEY = :__opentelemetry_context_storage__
-    private_constant :EMPTY_ENTRIES, :STACK_KEY
+    private_constant :EMPTY_ENTRIES
 
     DetachError = Class.new(OpenTelemetry::Error)
 
@@ -113,11 +114,9 @@ def value(key)
         current.value(key)
       end
 
-      # Clears the fiber-local Context stack. This allocates a new array for the
-      # stack, which is important in some use-cases to avoid sharing the backing
-      # array between fibers.
+      # Clears the fiber-local Context stack.
       def clear
-        Thread.current[STACK_KEY] = []
+        Fiber.current.opentelemetry_context = []
       end
 
       def empty
@@ -127,7 +126,7 @@ def empty
       private
 
       def stack
-        Thread.current[STACK_KEY] ||= []
+        Fiber.current.opentelemetry_context ||= []
       end
     end
 

api/test/opentelemetry/context_test.rb

@@ -289,7 +289,73 @@
     end
   end
 
+  describe 'fibers' do
+    it 'is isolated with respect to Fiber-local variable manipulation' do
+      OpenTelemetry::TestHelpers.with_test_logger do |log_stream|
+        ctx = new_context
+        Context.with_current(ctx) do
+          # This is based on code in ActionController::Live#process:
+          # https://github.com/rails/rails/blob/ad0105c13f61d145a659004efb928a643104e973/actionpack/lib/action_controller/metal/live.rb#L270
+          t1 = Thread.current
+          locals = t1.keys.map { |key| [key, t1[key]] }
+          Fiber.new do
+            t2 = Thread.current
+            locals.each { |k, v| t2[k] = v }
+            # Manipulate _this fiber's_ context stack.
+            Context.attach(new_context)
+          ensure
+            locals.each { |k, _| t2[k] = nil }
+          end.resume
+        end
+        _(log_stream.string).must_be_empty
+      end
+    end
+
+    it 'is isolated with respect to Fiber-local storage manipulation' do
+      OpenTelemetry::TestHelpers.with_test_logger do |log_stream|
+        ctx = new_context
+        Context.with_current(ctx) do
+          # This is based on code in ActionController::Live#process, modified to use Fiber-local storage:
+          # https://github.com/rails/rails/blob/ad0105c13f61d145a659004efb928a643104e973/actionpack/lib/action_controller/metal/live.rb#L270
+          f1 = Fiber.current
+          Fiber[:foo] = :bar
+          locals = f1.storage
+          Fiber.new do
+            f2 = Fiber.current
+            locals.each { |k, v| f2.storage[k] = v }
+            # Manipulate _this fiber's_ context stack.
+            Context.attach(new_context)
+          ensure
+            locals.each { |k, _| f2.storage[k] = nil }
+          end.resume
+        end
+        _(log_stream.string).must_be_empty
+      end
+    end
+  end
+
   describe 'threading' do
+    it 'is isolated with respect to Fiber-local variable manipulation' do
+      OpenTelemetry::TestHelpers.with_test_logger do |log_stream|
+        ctx = new_context
+        Context.with_current(ctx) do
+          # This is based on code in ActionController::Live#process:
+          # https://github.com/rails/rails/blob/ad0105c13f61d145a659004efb928a643104e973/actionpack/lib/action_controller/metal/live.rb#L270
+          t1 = Thread.current
+          locals = t1.keys.map { |key| [key, t1[key]] }
+          Thread.new do
+            t2 = Thread.current
+            locals.each { |k, v| t2[k] = v }
+            # Manipulate _this thread's_ context stack.
+            Context.attach(new_context)
+          ensure
+            locals.each { |k, _| t2[k] = nil }
+          end.join
+        end
+        _(log_stream.string).must_be_empty
+      end
+    end
+
     it 'unwinds the stack on each thread' do
       ctx = new_context
       t1_ctx_before = Context.current