refactor: move bytecounting conn in bytecounter pkg (#392)

* refactor: move bytecounting conn in bytecounter pkg

This enables other pieces of code to request bytecounting without
depending on netx or on the perverse using-the-context-to-configure-
byte-counting mechanism.

Also occurred when working on https://github.com/ooni/probe/issues/1687

* fix: add missing docs

internal/bytecounter/conn.go

@@ -0,0 +1,26 @@
+package bytecounter
+
+import "net"
+
+// Conn wraps a network connection and counts bytes.
+type Conn struct {
+	// net.Conn is the underlying net.Conn.
+	net.Conn
+
+	// Counter is the byte counter.
+	Counter *Counter
+}
+
+// Read implements net.Conn.Read.
+func (c *Conn) Read(p []byte) (int, error) {
+	count, err := c.Conn.Read(p)
+	c.Counter.CountBytesReceived(count)
+	return count, err
+}
+
+// Write implements net.Conn.Write.
+func (c *Conn) Write(p []byte) (int, error) {
+	count, err := c.Conn.Write(p)
+	c.Counter.CountBytesSent(count)
+	return count, err
+}

internal/bytecounter/conn_test.go

@@ -0,0 +1,66 @@
+package bytecounter
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/mockablex"
+)
+
+func TestConnWorksOnSuccess(t *testing.T) {
+	counter := New()
+	underlying := &mockablex.Conn{
+		MockRead: func(b []byte) (int, error) {
+			return 10, nil
+		},
+		MockWrite: func(b []byte) (int, error) {
+			return 4, nil
+		},
+	}
+	conn := &Conn{
+		Conn:    underlying,
+		Counter: counter,
+	}
+	if _, err := conn.Read(make([]byte, 128)); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := conn.Write(make([]byte, 1024)); err != nil {
+		t.Fatal(err)
+	}
+	if counter.BytesReceived() != 10 {
+		t.Fatal("unexpected number of bytes received")
+	}
+	if counter.BytesSent() != 4 {
+		t.Fatal("unexpected number of bytes sent")
+	}
+}
+
+func TestConnWorksOnFailure(t *testing.T) {
+	readError := errors.New("read error")
+	writeError := errors.New("write error")
+	counter := New()
+	underlying := &mockablex.Conn{
+		MockRead: func(b []byte) (int, error) {
+			return 0, readError
+		},
+		MockWrite: func(b []byte) (int, error) {
+			return 0, writeError
+		},
+	}
+	conn := &Conn{
+		Conn:    underlying,
+		Counter: counter,
+	}
+	if _, err := conn.Read(make([]byte, 128)); !errors.Is(err, readError) {
+		t.Fatal("not the error we expected", err)
+	}
+	if _, err := conn.Write(make([]byte, 1024)); !errors.Is(err, writeError) {
+		t.Fatal("not the error we expected", err)
+	}
+	if counter.BytesReceived() != 0 {
+		t.Fatal("unexpected number of bytes received")
+	}
+	if counter.BytesSent() != 0 {
+		t.Fatal("unexpected number of bytes sent")
+	}
+}