common: implement a common log package

This change adds two log packages.  One for the external go-ceph
consumers under common/log, and one for the internal use within the
go-ceph code under internal/log.  The external package exports two
functions for consumers, SetWarnf() and SetDebugf(), that set a
logger for warnings and debug messages respectively. They take a
log.Printf compatible function as argument.

The internal package exports these functions as Warnf() and Debugf(),
which can be used by the go-ceph code to log warnings and debug
messages.

Signed-off-by: Sven Anderson <sven@redhat.com>

common/log/log.go

@@ -0,0 +1,22 @@
+//go:build ceph_preview
+// +build ceph_preview
+
+// Package log allows to enable go-ceph logging and integrate it with the
+// logging of the go-ceph consuming code.
+package log
+
+import (
+	intLog "github.com/ceph/go-ceph/internal/log"
+)
+
+// SetWarnf sets the log.Printf compatible receiver for warning logs.
+//  PREVIEW
+func SetWarnf(f func(format string, v ...interface{})) {
+	intLog.Warnf = f
+}
+
+// SetDebugf sets the log.Printf compatible receiver for debug logs.
+//  PREVIEW
+func SetDebugf(f func(format string, v ...interface{})) {
+	intLog.Debugf = f
+}

internal/log/log.go

@@ -0,0 +1,14 @@
+// Package log is the internal package for go-ceph logging. This package is only
+// used from go-ceph code, not from consumers of go-ceph. go-ceph code uses the
+// functions in this package to log information that can't be returned as
+// errors. The functions default to no-ops and can be set with the external log
+// package common/log by the go-ceph consumers.
+package log
+
+func noop(string, ...interface{}) {}
+
+// These variables are set by the common log package.
+var (
+	Warnf  = noop
+	Debugf = noop
+)

internal/log/log_test.go

@@ -0,0 +1,79 @@
+package log_test
+
+import (
+	"io"
+	stdlog "log"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/ceph/go-ceph/common/log"
+	intLog "github.com/ceph/go-ceph/internal/log"
+	"github.com/stretchr/testify/assert"
+)
+
+func testLog() {
+	intLog.Debugf("-%s-", "debug")
+	intLog.Warnf("-%s-", "warn")
+}
+
+var testOut = []string{
+	"<go-ceph>[DBG]log_test.go:16: -debug-",
+	"<go-ceph>[WRN]log_test.go:17: -warn-",
+	"",
+}
+
+func checkLines(t *testing.T, lines []string) {
+	for i := range lines {
+		assert.Equal(t, testOut[len(testOut)-len(lines)+i], lines[i])
+	}
+}
+
+func captureAllOutput(f func()) string {
+	oldout := os.Stdout
+	olderr := os.Stderr
+	oldlog := stdlog.Writer()
+	defer func() {
+		os.Stdout = oldout
+		os.Stderr = olderr
+		stdlog.SetOutput(oldlog)
+	}()
+	r, w, _ := os.Pipe()
+	os.Stdout = w
+	os.Stderr = w
+	stdlog.SetOutput(w)
+	go func() {
+		f()
+		_ = w.Close()
+	}()
+	buf, _ := io.ReadAll(r)
+	return string(buf)
+}
+
+func TestLogOffByDefault(t *testing.T) {
+	out := captureAllOutput(func() { testLog() })
+	assert.Empty(t, out)
+}
+
+func TestLogLevels(t *testing.T) {
+	stdlog.Default()
+	var out strings.Builder
+	warn := stdlog.New(&out, "<go-ceph>[WRN]", stdlog.Lshortfile)
+	log.SetWarnf(warn.Printf)
+	t.Run("Warnf", func(t *testing.T) {
+		out.Reset()
+		testLog()
+		lines := strings.Split(out.String(), "\n")
+		assert.Len(t, lines, 2)
+		checkLines(t, lines)
+	})
+	debug := stdlog.New(&out, "<go-ceph>[DBG]", stdlog.Lshortfile)
+	log.SetDebugf(debug.Printf)
+	t.Run("Debugf", func(t *testing.T) {
+		out.Reset()
+		testLog()
+		lines := strings.Split(out.String(), "\n")
+		assert.Len(t, lines, 3)
+		checkLines(t, lines)
+	})
+}