feat(testingx): add log collector (#1562)

Closes ooni/probe#2713.

We developed this diff in #1560,
which is work in the ooni/probe#2700 context.

internal/testingx/logger.go

@@ -0,0 +1,87 @@
+package testingx
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/logmodel"
+)
+
+// Logger implements [logmodel.Logger] and collects all the log lines.
+//
+// The zero value of this struct is ready to use.
+type Logger struct {
+	// debug contains debug lines.
+	debug []string
+
+	// info contains info lines.
+	info []string
+
+	// mu provides mutual exclusion.
+	mu sync.Mutex
+
+	// warning contains warning lines.
+	warning []string
+}
+
+var _ logmodel.Logger = &Logger{}
+
+// Debug implements logmodel.Logger.
+func (l *Logger) Debug(msg string) {
+	l.mu.Lock()
+	l.debug = append(l.debug, msg)
+	l.mu.Unlock()
+}
+
+// Debugf implements logmodel.Logger.
+func (l *Logger) Debugf(format string, v ...interface{}) {
+	l.Debug(fmt.Sprintf(format, v...))
+}
+
+// Info implements logmodel.Logger.
+func (l *Logger) Info(msg string) {
+	l.mu.Lock()
+	l.info = append(l.info, msg)
+	l.mu.Unlock()
+}
+
+// Infof implements logmodel.Logger.
+func (l *Logger) Infof(format string, v ...interface{}) {
+	l.Info(fmt.Sprintf(format, v...))
+}
+
+// Warn implements logmodel.Logger.
+func (l *Logger) Warn(msg string) {
+	l.mu.Lock()
+	l.warning = append(l.warning, msg)
+	l.mu.Unlock()
+}
+
+// Warnf implements logmodel.Logger.
+func (l *Logger) Warnf(format string, v ...interface{}) {
+	l.Warn(fmt.Sprintf(format, v...))
+}
+
+// DebugLines returns a copy of the observed debug lines.
+func (l *Logger) DebugLines() []string {
+	l.mu.Lock()
+	out := append([]string{}, l.debug...)
+	l.mu.Unlock()
+	return out
+}
+
+// InfoLines returns a copy of the observed info lines.
+func (l *Logger) InfoLines() []string {
+	l.mu.Lock()
+	out := append([]string{}, l.info...)
+	l.mu.Unlock()
+	return out
+}
+
+// WarnLines returns a copy of the observed warn lines.
+func (l *Logger) WarnLines() []string {
+	l.mu.Lock()
+	out := append([]string{}, l.warning...)
+	l.mu.Unlock()
+	return out
+}

internal/testingx/logger_test.go

@@ -0,0 +1,33 @@
+package testingx
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestLogger(t *testing.T) {
+	logger := &Logger{}
+
+	logger.Debug("foobar")
+	logger.Debugf("foo%s", "baz")
+	expectDebug := []string{"foobar", "foobaz"}
+
+	logger.Info("barfoo")
+	logger.Infof("bar%s", "baz")
+	expectInfo := []string{"barfoo", "barbaz"}
+
+	logger.Warn("jarjar")
+	logger.Warnf("jar%s", "baz")
+	expectWarn := []string{"jarjar", "jarbaz"}
+
+	if diff := cmp.Diff(expectDebug, logger.DebugLines()); diff != "" {
+		t.Fatal(diff)
+	}
+	if diff := cmp.Diff(expectInfo, logger.InfoLines()); diff != "" {
+		t.Fatal(diff)
+	}
+	if diff := cmp.Diff(expectWarn, logger.WarnLines()); diff != "" {
+		t.Fatal(diff)
+	}
+}