Fix thread safety of file reads

Author: maxim

.github/workflows/rspec.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.4', '2.5', '2.6', '2.7' ]
+        ruby: [ '2.5', '2.6', '2.7' ]
 
     name: Ruby ${{ matrix.ruby }}
     steps:

README.md

@@ -12,6 +12,8 @@ Useful in cases where:
 
 ## Installation
 
+Note: Requires at least ruby 2.5 to support `File#pread` function.
+
 Add this line to your application's Gemfile:
 
 ```ruby

lib/wordmap/file_access.rb

@@ -68,8 +68,7 @@ def read_meta(file, spacer)
     def read_at(file, pos, bytes)
       # puts "Seeking in #{file.path.split('.wmap', 2)[1][1..-1]} to #{pos}, " \
       #      "and reading #{bytes} bytes"
-      file.sysseek(pos)
-      file.sysread(bytes)
+      file.pread(bytes, pos)
     end
   end
 end

spec/wordmap_spec.rb

@@ -112,6 +112,19 @@ def reads_breakdown(trace)
         expect(wmap['wrong-isbn', trace: trace]).to eq([])
         expect(reads_breakdown(trace)).to eq([[:bsearch_vec, 3]])
       end
+
+      it 'does thread safe lookups' do
+        wmap['9780385689229'] # Seeking in parent thread.
+        concurrency = 100
+
+        threads = concurrency.times.map {
+          Thread.new { Thread.current[:book] = !wmap['9780385689229'][0].nil? }
+        }
+
+        success_count = threads.map { |t| t.join[:book] }.count(&:itself)
+        expect(success_count).to eq(concurrency),
+          "#{success_count}/#{threads.size} threads succeeded"
+      end
     end
 
     describe '#query' do