From 53e31a5c562535dca95a24f34fd837298e07e6ee Mon Sep 17 00:00:00 2001
From: Sam Saffron <sam.saffron@gmail.com>
Date: Fri, 13 Sep 2019 17:26:34 +1000
Subject: [PATCH 1/2] FEATURE: add interface compatible constant version of
 thread local var

ThreadLocalVar can be expensive as it spins a thread in finalizers. In some
cases consumers may want to swap in a constant value implementations that
is very cheap.

`Concurrent::ConstantThreadLocalVar` is interface compatible with
AbstractThreadLocalVar, however does not allow any changing of .value
---
 lib/concurrent-ruby/concurrent/atomics.rb     |  1 +
 .../atomic/constant_thread_local_var.rb       | 35 ++++++++++++
 .../atomic/constant_thread_local_var_spec.rb  | 56 +++++++++++++++++++
 3 files changed, 92 insertions(+)
 create mode 100644 lib/concurrent/atomic/constant_thread_local_var.rb
 create mode 100644 spec/concurrent/atomic/constant_thread_local_var_spec.rb

diff --git a/lib/concurrent-ruby/concurrent/atomics.rb b/lib/concurrent-ruby/concurrent/atomics.rb
index 16cbe6610..cb6d06b36 100644
--- a/lib/concurrent-ruby/concurrent/atomics.rb
+++ b/lib/concurrent-ruby/concurrent/atomics.rb
@@ -8,3 +8,4 @@
 require 'concurrent/atomic/reentrant_read_write_lock'
 require 'concurrent/atomic/semaphore'
 require 'concurrent/atomic/thread_local_var'
+require 'concurrent/atomic/constant_thread_local_var'
diff --git a/lib/concurrent/atomic/constant_thread_local_var.rb b/lib/concurrent/atomic/constant_thread_local_var.rb
new file mode 100644
index 000000000..310e865e9
--- /dev/null
+++ b/lib/concurrent/atomic/constant_thread_local_var.rb
@@ -0,0 +1,35 @@
+require 'concurrent/atomic/abstract_thread_local_var'
+
+module Concurrent
+  class ConstantThreadLocalVar < AbstractThreadLocalVar
+    def value
+      default
+    end
+
+    def value=(value)
+      if value != default
+        raise ArgumentError, "Constant thread local vars may not be altered"
+      end
+    end
+
+    def bind(value)
+      self.value = value
+      if block_given?
+        yield
+      end
+    end
+
+    protected
+    def allocate_storage
+      # nothing to do
+    end
+
+    def default
+      if @default_block
+        @default_block.call
+      else
+        @default
+      end
+    end
+  end
+end
diff --git a/spec/concurrent/atomic/constant_thread_local_var_spec.rb b/spec/concurrent/atomic/constant_thread_local_var_spec.rb
new file mode 100644
index 000000000..45ea43776
--- /dev/null
+++ b/spec/concurrent/atomic/constant_thread_local_var_spec.rb
@@ -0,0 +1,56 @@
+require 'rbconfig'
+
+module Concurrent
+
+  require 'concurrent/atomic/constant_thread_local_var'
+
+  RSpec.describe ConstantThreadLocalVar do
+
+    context "#value" do
+      it 'can return default value' do
+        v = described_class.new("apple")
+        expect(v.value).to eq("apple")
+
+        v = described_class.new do
+          "orange"
+        end
+        expect(v.value).to eq("orange")
+      end
+    end
+
+    context "#value=" do
+      it 'can set value to same value' do
+        v = described_class.new("apple")
+        v.value = "apple"
+      end
+
+      it 'will raise an ArgumentError when attempting to change immutable value' do
+        v = described_class.new do
+          "apple"
+        end
+
+        expect do
+          v.value = "orange"
+        end.to raise_error(ArgumentError)
+      end
+    end
+
+    context '#bind' do
+      it 'will raise when attempting to bind to a different value' do
+        v = described_class.new("apple")
+        expect do
+          v.bind("orange") {}
+        end.to raise_error(ArgumentError)
+      end
+
+      it 'works when bind value is the same' do
+
+        v = described_class.new("apple")
+        v.bind("apple") do
+          expect(v.value).to eq("apple")
+        end
+      end
+    end
+  end
+
+end

From 99dc326a0b9c7954bf40ddd574c4d501def1c343 Mon Sep 17 00:00:00 2001
From: Petr Chalupa <git@pitr.ch>
Date: Mon, 10 Feb 2020 21:18:36 +0100
Subject: [PATCH 2/2] Fix placement and add documentation

---
 .../concurrent/atomic/constant_thread_local_var.rb     | 10 ++++++++++
 1 file changed, 10 insertions(+)
 rename lib/{ => concurrent-ruby}/concurrent/atomic/constant_thread_local_var.rb (56%)

diff --git a/lib/concurrent/atomic/constant_thread_local_var.rb b/lib/concurrent-ruby/concurrent/atomic/constant_thread_local_var.rb
similarity index 56%
rename from lib/concurrent/atomic/constant_thread_local_var.rb
rename to lib/concurrent-ruby/concurrent/atomic/constant_thread_local_var.rb
index 310e865e9..dfb02999a 100644
--- a/lib/concurrent/atomic/constant_thread_local_var.rb
+++ b/lib/concurrent-ruby/concurrent/atomic/constant_thread_local_var.rb
@@ -1,17 +1,26 @@
 require 'concurrent/atomic/abstract_thread_local_var'
 
 module Concurrent
+  # Has the same interface as {ThreadLocalVar} but the value can never be changed.
+  # The value is always the default provided to the constructor.
   class ConstantThreadLocalVar < AbstractThreadLocalVar
+
+    # @note the value is always equal to default value
+    # @!macro thread_local_var_method_get
     def value
       default
     end
 
+    # @!macro thread_local_var_method_set
+    # @raise ArgumentError if the value is not equal to the default
     def value=(value)
       if value != default
         raise ArgumentError, "Constant thread local vars may not be altered"
       end
     end
 
+    # @!macro thread_local_var_method_bind
+    # @raise ArgumentError if the value is not equal to the default
     def bind(value)
       self.value = value
       if block_given?
@@ -20,6 +29,7 @@ def bind(value)
     end
 
     protected
+
     def allocate_storage
       # nothing to do
     end