Allow for empty last_run files for parallel tests

CHANGELOG.md

@@ -5,6 +5,8 @@ Unreleased ([changes](https://github.com/colszowka/simplecov/compare/v0.12.0...m
 
 ## Bugfixes
 
+* Fix parallel_tests where one thread ends up running no tests. See [#533](https://github.com/colszowka/simplecov/pull/533) (thanks @cshaffer)
+
 0.12.0 2016-07-02 ([changes](https://github.com/colszowka/simplecov/compare/v0.11.2...v0.12.0))
 =================
 

lib/simplecov/last_run.rb

@@ -9,7 +9,8 @@ def last_run_path
 
       def read
         return nil unless File.exist?(last_run_path)
-        JSON.parse(File.read(last_run_path))
+        json = File.read(last_run_path)
+        JSON.parse(json) unless json.strip.empty?
       end
 
       def write(json)

spec/last_run_spec.rb

@@ -0,0 +1,45 @@
+require "helper"
+
+describe SimpleCov::LastRun do
+  subject { SimpleCov::LastRun }
+
+  it "defines a last_run_path" do
+    expect(subject.last_run_path).to eq(File.join(SimpleCov.coverage_path, ".last_run.json"))
+  end
+
+  it "writes json to its last_run_path" do
+    json = []
+    subject.write(json)
+    expect(File.read(subject.last_run_path).strip).to eq(JSON.pretty_generate(json))
+  end
+
+  context "reading" do
+    context "but the last_run file does not exist" do
+      before { File.delete(subject.last_run_path) if File.exist?(subject.last_run_path) }
+
+      it "returns nil" do
+        expect(subject.read).to be_nil
+      end
+    end
+
+    context "a non empty result" do
+      before { subject.write([]) }
+
+      it "reads json from its last_run_path" do
+        expect(subject.read).to eq(JSON.parse("[]"))
+      end
+    end
+
+    context "an empty result" do
+      before do
+        File.open(subject.last_run_path, "w+") do |f|
+          f.puts ""
+        end
+      end
+
+      it "returns nil" do
+        expect(subject.read).to be_nil
+      end
+    end
+  end
+end if SimpleCov.usable?