Fix merging race conditions

Ensure we cache and synchronize appropriately so we don't merge more than
once or do unsafe reads of the resultset. This has the added benefit of
reducing the runtime of at_exit hooks by over 66%, since we now do less
than 1/3 the work.

See https://gist.github.com/jenseng/62465f674f8c02de09ef776f23d4dca4
for simple repro script.

Basically there are 3 main problems when using merging:

1. `SimpleCov::ResultMerger.stored_data` doesn't synchronize its read,
   so it can see an empty file if another process is writing it.
2. `SimpleCov::ResultMerger.resultset` calls `.stored_data` twice and
   never caches the result.
3. `SimpleCov.result` doesn't cache the `@result`, so the default `at_exit`
   behavior causes it to store and merge 3 times. Due to 1. and 2.,
   this is extra bad

This can cause the formatter to miss out on coverage data and/or get the
wrong values for covered percentages.

Furthermore, if you use OJ, `JSON.parse("") -> nil`, which means
`.resultset` can be nil, so this race condition causes exceptions as
seen in #406. In addition to fixing the race condition, also add ` || {}`
to make  `.stored_data` a bit more robust and protect against an empty
.resultset.json.

lib/simplecov.rb

@@ -75,23 +75,21 @@ def add_not_loaded_files(result)
     # from cache using SimpleCov::ResultMerger if use_merging is activated (default)
     #
     def result
-      # Ensure the variable is defined to avoid ruby warnings
-      @result = nil unless defined?(@result)
+      return @result if result?
 
       # Collect our coverage result
-      if running && !result?
+      if running
         @result = SimpleCov::Result.new add_not_loaded_files(Coverage.result)
       end
 
       # If we're using merging of results, store the current result
-      # first, then merge the results and return those
+      # first (if there is one), then merge the results and return those
       if use_merging
         SimpleCov::ResultMerger.store_result(@result) if result?
-
-        SimpleCov::ResultMerger.merged_result
-      else
-        @result
+        @result = SimpleCov::ResultMerger.merged_result
       end
+
+      @result
     ensure
       self.running = false
     end
@@ -158,6 +156,13 @@ def usable?
         false
       end
     end
+
+    #
+    # Clear out the previously cached .result. Primarily useful in testing
+    #
+    def clear_result
+      @result = nil
+    end
   end
 end
 

lib/simplecov/result_merger.rb

@@ -17,25 +17,31 @@ def resultset_writelock
         File.join(SimpleCov.coverage_path, ".resultset.json.lock")
       end
 
-      # Loads the cached resultset from JSON and returns it as a Hash
+      # Loads the cached resultset from JSON and returns it as a Hash,
+      # caching it for subsequent accesses.
       def resultset
-        if stored_data
-          begin
-            JSON.parse(stored_data)
-          rescue
+        @resultset ||= begin
+          data = stored_data
+          if data
+            begin
+              JSON.parse(data) || {}
+            rescue
+              {}
+            end
+          else
             {}
           end
-        else
-          {}
         end
       end
 
       # Returns the contents of the resultset cache as a string or if the file is missing or empty nil
       def stored_data
-        return unless File.exist?(resultset_path)
-        data = File.read(resultset_path)
-        return if data.nil? || data.length < 2
-        data
+        synchronize_resultset do
+          return unless File.exist?(resultset_path)
+          data = File.read(resultset_path)
+          return if data.nil? || data.length < 2
+          data
+        end
       end
 
       # Gets the resultset hash and re-creates all included instances
@@ -76,8 +82,9 @@ def merged_result
 
       # Saves the given SimpleCov::Result in the resultset cache
       def store_result(result)
-        File.open(resultset_writelock, "w+") do |f|
-          f.flock(File::LOCK_EX)
+        synchronize_resultset do
+          # Ensure we have the latest, in case it was already cached
+          @resultset = nil
           new_set = resultset
           command_name, data = result.to_hash.first
           new_set[command_name] = data
@@ -87,6 +94,25 @@ def store_result(result)
         end
         true
       end
+
+    private
+
+      # Ensure only one process is reading or writing the resultset at any
+      # given time
+      def synchronize_resultset
+        # make it reentrant
+        return yield if defined?(@resultset_locked) && @resultset_locked
+
+        begin
+          @resultset_locked = true
+          File.open(resultset_writelock, "w+") do |f|
+            f.flock(File::LOCK_EX)
+            yield
+          end
+        ensure
+          @resultset_locked = false
+        end
+      end
     end
   end
 end

spec/result_merger_spec.rb

@@ -4,7 +4,6 @@
   describe SimpleCov::ResultMerger do
     describe "with two faked coverage resultsets" do
       before do
-        SimpleCov.use_merging true
         @resultset1 = {
           source_fixture("sample.rb") => [nil, 1, 1, 1, nil, nil, 1, 1, nil, nil],
           source_fixture("app/models/user.rb") => [nil, 1, 1, 1, nil, nil, 1, 0, nil, nil],
@@ -73,14 +72,23 @@
               expect(SimpleCov::ResultMerger.results.length).to eq(1)
             end
           end
+        end
+      end
 
-          context "with merging disabled" do
-            before { SimpleCov.use_merging false }
+      describe ".store_result" do
+        it "refreshes the resultset" do
+          set = SimpleCov::ResultMerger.resultset
+          SimpleCov::ResultMerger.store_result({})
+          new_set = SimpleCov::ResultMerger.resultset
+          expect(new_set).not_to be(set)
+        end
+      end
 
-            it "returns nil for SimpleCov.result" do
-              expect(SimpleCov.result).to be_nil
-            end
-          end
+      describe ".resultset" do
+        it "caches by default" do
+          set = SimpleCov::ResultMerger.resultset
+          new_set = SimpleCov::ResultMerger.resultset
+          expect(new_set).to be(set)
         end
       end
     end

spec/simplecov_spec.rb

@@ -0,0 +1,109 @@
+require "helper"
+
+if SimpleCov.usable?
+  describe SimpleCov do
+    describe ".result" do
+      before do
+        SimpleCov.clear_result
+        allow(Coverage).to receive(:result).once.and_return({})
+      end
+
+      context "with merging disabled" do
+        before do
+          allow(SimpleCov).to receive(:use_merging).once.and_return(false)
+        end
+
+        context "when not running" do
+          before do
+            allow(SimpleCov).to receive(:running).and_return(false)
+          end
+
+          it "returns nil" do
+            expect(SimpleCov.result).to be_nil
+          end
+        end
+
+        context "when running" do
+          before do
+            allow(SimpleCov).to receive(:running).and_return(true, false)
+          end
+
+          it "uses the result from Coverage" do
+            expect(Coverage).to receive(:result).once.and_return(__FILE__ => [0, 1])
+            expect(SimpleCov.result.filenames).to eq [__FILE__]
+          end
+
+          it "adds not-loaded-files" do
+            expect(SimpleCov).to receive(:add_not_loaded_files).once.and_return({})
+            SimpleCov.result
+          end
+
+          it "doesn't store the current coverage" do
+            expect(SimpleCov::ResultMerger).not_to receive(:store_result)
+            SimpleCov.result
+          end
+
+          it "doesn't merge the result" do
+            expect(SimpleCov::ResultMerger).not_to receive(:merged_result)
+            SimpleCov.result
+          end
+
+          it "caches its result" do
+            result = SimpleCov.result
+            expect(SimpleCov.result).to be(result)
+          end
+        end
+      end
+
+      context "with merging enabled" do
+        let(:the_merged_result) { double }
+
+        before do
+          allow(SimpleCov).to receive(:use_merging).once.and_return(true)
+          allow(SimpleCov::ResultMerger).to receive(:store_result).once
+          allow(SimpleCov::ResultMerger).to receive(:merged_result).once.and_return(the_merged_result)
+        end
+
+        context "when not running" do
+          before do
+            allow(SimpleCov).to receive(:running).and_return(false)
+          end
+
+          it "merges the result" do
+            expect(SimpleCov.result).to be(the_merged_result)
+          end
+        end
+
+        context "when running" do
+          before do
+            allow(SimpleCov).to receive(:running).and_return(true, false)
+          end
+
+          it "uses the result from Coverage" do
+            expect(Coverage).to receive(:result).once.and_return({})
+            SimpleCov.result
+          end
+
+          it "adds not-loaded-files" do
+            expect(SimpleCov).to receive(:add_not_loaded_files).once.and_return({})
+            SimpleCov.result
+          end
+
+          it "stores the current coverage" do
+            expect(SimpleCov::ResultMerger).to receive(:store_result).once
+            SimpleCov.result
+          end
+
+          it "merges the result" do
+            expect(SimpleCov.result).to be(the_merged_result)
+          end
+
+          it "caches its result" do
+            result = SimpleCov.result
+            expect(SimpleCov.result).to be(result)
+          end
+        end
+      end
+    end
+  end
+end