sdk/log: Fix TestBatchProcessor/DroppedLogs flaky test (#5421)

Fix #5384

Run of `go test  -count=1000000 -run="TestBatchProcessor/DroppedLogs"`
**Before**:
Failed with either `Condition never satisfied` or panic
**After**:
Passed

First, bytes.Buffer is not thread-safe, so writing log and reading
(`bytes.String()`) caused panic. Added `concurrentBuffer`
Second, fixed flaky test with 4 records:
1. Record goes to `testExporter.Export` function and blocks in this
function because of `ExportTrigger`
(https://github.com/open-telemetry/opentelemetry-go/blob/19ee6d4775d578357e251828215213782eafed54/sdk/log/exporter_test.go#L87)
2. Record goes to `bufferExporter.input`
(https://github.com/open-telemetry/opentelemetry-go/blob/7c5e64cccc16710b2e75f3d60e96acc07dacd7a7/sdk/log/exporter.go#L129)
3. Record goes to `BatchProcessor.q` queue and it could not be enqueued
to export, because `bufferExporter.input` is full
4. Record goes to `BatchProcessor.q` and because of overfill, drops
third record

---------

Co-authored-by: Sam Xie <sam@samxie.me>

sdk/log/batch_test.go

@@ -24,6 +24,23 @@ import (
 	"go.opentelemetry.io/otel/log"
 )
 
+type concurrentBuffer struct {
+	b bytes.Buffer
+	m sync.Mutex
+}
+
+func (b *concurrentBuffer) Write(p []byte) (n int, err error) {
+	b.m.Lock()
+	defer b.m.Unlock()
+	return b.b.Write(p)
+}
+
+func (b *concurrentBuffer) String() string {
+	b.m.Lock()
+	defer b.m.Unlock()
+	return b.b.String()
+}
+
 func TestEmptyBatchConfig(t *testing.T) {
 	assert.NotPanics(t, func() {
 		var bp BatchProcessor
@@ -421,7 +438,8 @@ func TestBatchProcessor(t *testing.T) {
 	t.Run("DroppedLogs", func(t *testing.T) {
 		orig := global.GetLogger()
 		t.Cleanup(func() { global.SetLogger(orig) })
-		buf := new(bytes.Buffer)
+		// Use concurrentBuffer for concurrent-safe reading.
+		buf := new(concurrentBuffer)
 		stdr.SetVerbosity(1)
 		global.SetLogger(stdr.New(stdlog.New(buf, "", 0)))
 
@@ -436,15 +454,23 @@ func TestBatchProcessor(t *testing.T) {
 			WithExportTimeout(time.Hour),
 		)
 		var r Record
-		assert.NoError(t, b.OnEmit(ctx, r), "queued")
-		assert.NoError(t, b.OnEmit(ctx, r), "dropped")
-
-		var n int
+		// First record will be blocked by testExporter.Export
+		assert.NoError(t, b.OnEmit(ctx, r), "exported record")
 		require.Eventually(t, func() bool {
-			n = e.ExportN()
-			return n > 0
+			return e.ExportN() > 0
 		}, 2*time.Second, time.Microsecond, "blocked export not attempted")
 
+		// Second record will be written to export queue
+		assert.NoError(t, b.OnEmit(ctx, r), "export queue record")
+		require.Eventually(t, func() bool {
+			return len(b.exporter.input) == cap(b.exporter.input)
+		}, 2*time.Second, time.Microsecond, "blocked queue read not attempted")
+
+		// Third record will be written to BatchProcessor.q
+		assert.NoError(t, b.OnEmit(ctx, r), "first queued")
+		// The previous record will be dropped, as the new one will be written to BatchProcessor.q
+		assert.NoError(t, b.OnEmit(ctx, r), "second queued")
+
 		wantMsg := `"level"=1 "msg"="dropped log records" "dropped"=1`
 		assert.Eventually(t, func() bool {
 			return strings.Contains(buf.String(), wantMsg)