race issue in runContainer16's equal() method

[beiwei30]

Run the following code with go test -race:

package roaring

import (
	"github.com/RoaringBitmap/roaring"
	"sync"
	"testing"
)

func TestJoinIfNotEqual(t *testing.T) {
	l := roaring.NewBitmap()
	l.Add(uint32(1))
	l.Add(uint32(2))
	l.Add(uint32(3))
	r := roaring.NewBitmap()
	r.AddRange(1, 4)

	// this is for race condition test
	var wg sync.WaitGroup
	wg.Add(2)
	go func() {
		defer wg.Done()
		l.Equals(r)
	}()

	go func() {
		defer wg.Done()
		l.Equals(r)
	}()
	wg.Wait()
}

Race detector will report the following warning to indicate there's potential data race in runContainer16's equal() method:

=== RUN   TestEqual
==================
WARNING: DATA RACE
Read at 0x00c00015e058 by goroutine 9:
  github.com/RoaringBitmap/roaring.(*runContainer16).cardinality()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/runcontainer.go:984 +0x64
  github.com/RoaringBitmap/roaring.(*runContainer16).getCardinality()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/runcontainer.go:2337 +0x27c
  github.com/RoaringBitmap/roaring.(*runContainer16).equals()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/runcontainer.go:2108 +0x246
  github.com/RoaringBitmap/roaring.(*roaringArray).equals()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/roaringarray.go:434 +0x2ba
  github.com/RoaringBitmap/roaring.(*Bitmap).Equals()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/roaring.go:495 +0xe4
  test-go/roaring.TestEqual.func2()
      /Users/iluo/tmp/test-go/roaring/roaring_map_test.go:27 +0x86

Previous write at 0x00c00015e058 by goroutine 8:
  github.com/RoaringBitmap/roaring.(*runContainer16).cardinality()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/runcontainer.go:992 +0xfc
  github.com/RoaringBitmap/roaring.(*runContainer16).getCardinality()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/runcontainer.go:2337 +0x27c
  github.com/RoaringBitmap/roaring.(*runContainer16).equals()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/runcontainer.go:2108 +0x246
  github.com/RoaringBitmap/roaring.(*roaringArray).equals()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/roaringarray.go:434 +0x2ba
  github.com/RoaringBitmap/roaring.(*Bitmap).Equals()
      /Users/iluo/go/pkg/mod/github.com/!roaring!bitmap/roaring@v0.6.1/roaring.go:495 +0xe4
  test-go/roaring.TestEqual.func1()
      /Users/iluo/tmp/test-go/roaring/roaring_map_test.go:22 +0x86

Goroutine 9 (running) created at:
  test-go/roaring.TestEqual()
      /Users/iluo/tmp/test-go/roaring/roaring_map_test.go:25 +0x21a
  testing.tRunner()
      /usr/local/Cellar/go/1.16.3/libexec/src/testing/testing.go:1193 +0x202

Goroutine 8 (finished) created at:
  test-go/roaring.TestEqual()
      /Users/iluo/tmp/test-go/roaring/roaring_map_test.go:20 +0x1e4
  testing.tRunner()
      /usr/local/Cellar/go/1.16.3/libexec/src/testing/testing.go:1193 +0x202
==================
    testing.go:1092: race detected during execution of test
--- FAIL: TestEqual (0.00s)

=== CONT  
    testing.go:1092: race detected during execution of test
FAIL

[lemire]

The runContainer16 equal() function is single threaded. It does not seem possible that it suffers from a data race.

The code provided in this issue modifies the data structure in two distinct threads, which will indeed introduce a data race if one is not careful...

The core issue in this instance is that roaring will compute the cardinality of run containers and cache it as needed. I do not much like this design to be honest.

[lemire]

@beiwei30 Can you review #306 and tell me what you think?

[lemire]

Release 0.7.1 made data races less likely by removing the cache. Closing.