cmd/cli: add internal logging

So in case of no logging enabled, useful data could be sent to ControlD
server for further troubleshooting.

cmd/cli/log_writer_test.go

@@ -0,0 +1,49 @@
+package cli
+
+import (
+	"strings"
+	"sync"
+	"testing"
+)
+
+func Test_logWriter_Write(t *testing.T) {
+	size := 64
+	lw := &logWriter{size: size}
+	lw.buf.Grow(lw.size)
+	data := strings.Repeat("A", size)
+	lw.Write([]byte(data))
+	if lw.buf.String() != data {
+		t.Fatalf("unexpected buf content: %v", lw.buf.String())
+	}
+	newData := "B"
+	halfData := strings.Repeat("A", len(data)/2) + logTruncatedMarker
+	lw.Write([]byte(newData))
+	if lw.buf.String() != halfData+newData {
+		t.Fatalf("unexpected new buf content: %v", lw.buf.String())
+	}
+
+	bigData := strings.Repeat("B", 256)
+	expected := halfData + strings.Repeat("B", 16)
+	lw.Write([]byte(bigData))
+	if lw.buf.String() != expected {
+		t.Fatalf("unexpected big buf content: %v", lw.buf.String())
+	}
+}
+
+func Test_logWriter_ConcurrentWrite(t *testing.T) {
+	size := 64
+	lw := &logWriter{size: size}
+	n := 10
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			lw.Write([]byte(strings.Repeat("A", i)))
+		}()
+	}
+	wg.Wait()
+	if lw.buf.Len() > lw.size {
+		t.Fatalf("unexpected buf size: %v, content: %q", lw.buf.Len(), lw.buf.String())
+	}
+}