Allow to configure allowed levels by string value

level/doc.go

@@ -7,6 +7,17 @@
 //    logger = level.NewFilter(logger, level.AllowInfo()) // <--
 //    logger = log.With(logger, "ts", log.DefaultTimestampUTC)
 //
+// It's also possible to configure log level from a string. For instance from
+// a flag, environment variable or configuration file.
+//
+//    fs := flag.NewFlagSet("myprogram")
+//    lvl := fs.String("log", "info", "debug, info, warn, error")
+//
+//    var logger log.Logger
+//    logger = log.NewLogfmtLogger(os.Stderr)
+//    logger = level.NewFilter(logger, level.Allow(level.ParseDefault(*lvl, level.InfoValue()))) // <--
+//    logger = log.With(logger, "ts", log.DefaultTimestampUTC)
+//
 // Then, at the callsites, use one of the level.Debug, Info, Warn, or Error
 // helper methods to emit leveled log events.
 //

level/example_test.go

@@ -2,6 +2,7 @@ package level_test
 
 import (
 	"errors"
+	"flag"
 	"os"
 
 	"github.com/go-kit/log"
@@ -34,6 +35,26 @@ func Example_filtered() {
 	level.Debug(logger).Log("next item", 17) // filtered
 
 	// Output:
-	// level=error caller=example_test.go:32 err="bad data"
-	// level=info caller=example_test.go:33 event="data saved"
+	// level=error caller=example_test.go:33 err="bad data"
+	// level=info caller=example_test.go:34 event="data saved"
+}
+
+func Example_parsed() {
+	fs := flag.NewFlagSet("example", flag.ExitOnError)
+	lvl := fs.String("log-level", "", `"debug", "info", "warn" or "error"`)
+	fs.Parse([]string{"-log-level", "info"})
+
+	// Set up logger with level filter.
+	logger := log.NewLogfmtLogger(os.Stdout)
+	logger = level.NewFilter(logger, level.Allow(level.ParseDefault(*lvl, level.DebugValue())))
+	logger = log.With(logger, "caller", log.DefaultCaller)
+
+	// Use level helpers to log at different levels.
+	level.Error(logger).Log("err", errors.New("bad data"))
+	level.Info(logger).Log("event", "data saved")
+	level.Debug(logger).Log("next item", 17) // filtered
+
+	// Output:
+	// level=error caller=example_test.go:53 err="bad data"
+	// level=info caller=example_test.go:54 event="data saved"
 }

level/level.go

@@ -1,6 +1,14 @@
 package level
 
-import "github.com/go-kit/log"
+import (
+	"errors"
+	"strings"
+
+	"github.com/go-kit/log"
+)
+
+// ErrInvalidLevelString is returned whenever an invalid string is passed to Parse
+var ErrInvalidLevelString = errors.New("invalid level string")
 
 // Error returns a logger that includes a Key/ErrorValue pair.
 func Error(logger log.Logger) log.Logger {
@@ -66,6 +74,22 @@ func (l *logger) Log(keyvals ...interface{}) error {
 // Option sets a parameter for the leveled logger.
 type Option func(*logger)
 
+// Allow the provided log level to pass.
+func Allow(v Value) Option {
+	switch v {
+	case debugValue:
+		return AllowDebug()
+	case infoValue:
+		return AllowInfo()
+	case warnValue:
+		return AllowWarn()
+	case errorValue:
+		return AllowError()
+	default:
+		return AllowNone()
+	}
+}
+
 // AllowAll is an alias for AllowDebug.
 func AllowAll() Option {
 	return AllowDebug()
@@ -100,6 +124,32 @@ func allowed(allowed level) Option {
 	return func(l *logger) { l.allowed = allowed }
 }
 
+// Parse a string to its corresponding level value. Valid strings are "debug",
+// "info", "warn", and "error". Strings are normalized via strings.TrimSpace and
+// strings.ToLower.
+func Parse(level string) (Value, error) {
+	switch strings.TrimSpace(strings.ToLower(level)) {
+	case debugValue.name:
+		return debugValue, nil
+	case infoValue.name:
+		return infoValue, nil
+	case warnValue.name:
+		return warnValue, nil
+	case errorValue.name:
+		return errorValue, nil
+	default:
+		return nil, ErrInvalidLevelString
+	}
+}
+
+// ParseDefault calls Parse and returns the default Value on error.
+func ParseDefault(level string, def Value) Value {
+	if v, err := Parse(level); err == nil {
+		return v
+	}
+	return def
+}
+
 // ErrNotAllowed sets the error to return from Log when it squelches a log
 // event disallowed by the configured Allow[Level] option. By default,
 // ErrNotAllowed is nil; in this case the log event is squelched with no

level/level_test.go

@@ -17,6 +17,45 @@ func TestVariousLevels(t *testing.T) {
 		allowed level.Option
 		want    string
 	}{
+		{
+			"Allow(DebugValue)",
+			level.Allow(level.DebugValue()),
+			strings.Join([]string{
+				`{"level":"debug","this is":"debug log"}`,
+				`{"level":"info","this is":"info log"}`,
+				`{"level":"warn","this is":"warn log"}`,
+				`{"level":"error","this is":"error log"}`,
+			}, "\n"),
+		},
+		{
+			"Allow(InfoValue)",
+			level.Allow(level.InfoValue()),
+			strings.Join([]string{
+				`{"level":"info","this is":"info log"}`,
+				`{"level":"warn","this is":"warn log"}`,
+				`{"level":"error","this is":"error log"}`,
+			}, "\n"),
+		},
+		{
+			"Allow(WarnValue)",
+			level.Allow(level.WarnValue()),
+			strings.Join([]string{
+				`{"level":"warn","this is":"warn log"}`,
+				`{"level":"error","this is":"error log"}`,
+			}, "\n"),
+		},
+		{
+			"Allow(ErrorValue)",
+			level.Allow(level.ErrorValue()),
+			strings.Join([]string{
+				`{"level":"error","this is":"error log"}`,
+			}, "\n"),
+		},
+		{
+			"Allow(nil)",
+			level.Allow(nil),
+			strings.Join([]string{}, "\n"),
+		},
 		{
 			"AllowAll",
 			level.AllowAll(),
@@ -147,7 +186,7 @@ func TestLevelContext(t *testing.T) {
 	logger = log.With(logger, "caller", log.DefaultCaller)
 
 	level.Info(logger).Log("foo", "bar")
-	if want, have := `level=info caller=level_test.go:149 foo=bar`, strings.TrimSpace(buf.String()); want != have {
+	if want, have := `level=info caller=level_test.go:188 foo=bar`, strings.TrimSpace(buf.String()); want != have {
 		t.Errorf("\nwant '%s'\nhave '%s'", want, have)
 	}
 }
@@ -163,7 +202,7 @@ func TestContextLevel(t *testing.T) {
 	logger = level.NewFilter(logger, level.AllowAll())
 
 	level.Info(logger).Log("foo", "bar")
-	if want, have := `caller=level_test.go:165 level=info foo=bar`, strings.TrimSpace(buf.String()); want != have {
+	if want, have := `caller=level_test.go:204 level=info foo=bar`, strings.TrimSpace(buf.String()); want != have {
 		t.Errorf("\nwant '%s'\nhave '%s'", want, have)
 	}
 }
@@ -233,3 +272,122 @@ func TestInjector(t *testing.T) {
 		t.Errorf("wrong level value: got %#v, want %#v", got, want)
 	}
 }
+
+func TestParse(t *testing.T) {
+	testCases := []struct {
+		name    string
+		level   string
+		want    level.Value
+		wantErr error
+	}{
+		{
+			name:    "Debug",
+			level:   "debug",
+			want:    level.DebugValue(),
+			wantErr: nil,
+		},
+		{
+			name:    "Info",
+			level:   "info",
+			want:    level.InfoValue(),
+			wantErr: nil,
+		},
+		{
+			name:    "Warn",
+			level:   "warn",
+			want:    level.WarnValue(),
+			wantErr: nil,
+		},
+		{
+			name:    "Error",
+			level:   "error",
+			want:    level.ErrorValue(),
+			wantErr: nil,
+		},
+		{
+			name:    "Case Insensitive",
+			level:   "ErRoR",
+			want:    level.ErrorValue(),
+			wantErr: nil,
+		},
+		{
+			name:    "Trimmed",
+			level:   " Error ",
+			want:    level.ErrorValue(),
+			wantErr: nil,
+		},
+		{
+			name:    "Invalid",
+			level:   "invalid",
+			want:    nil,
+			wantErr: level.ErrInvalidLevelString,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := level.Parse(tc.level)
+			if err != tc.wantErr {
+				t.Errorf("got unexpected error %#v", err)
+			}
+
+			if got != tc.want {
+				t.Errorf("wrong value: got=%#v, want=%#v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestParseDefault(t *testing.T) {
+	testCases := []struct {
+		name  string
+		level string
+		want  level.Value
+	}{
+		{
+			name:  "Debug",
+			level: "debug",
+			want:  level.DebugValue(),
+		},
+		{
+			name:  "Info",
+			level: "info",
+			want:  level.InfoValue(),
+		},
+		{
+			name:  "Warn",
+			level: "warn",
+			want:  level.WarnValue(),
+		},
+		{
+			name:  "Error",
+			level: "error",
+			want:  level.ErrorValue(),
+		},
+		{
+			name:  "Case Insensitive",
+			level: "ErRoR",
+			want:  level.ErrorValue(),
+		},
+		{
+			name:  "Trimmed",
+			level: " Error ",
+			want:  level.ErrorValue(),
+		},
+		{
+			name:  "Invalid",
+			level: "invalid",
+			want:  level.InfoValue(),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := level.ParseDefault(tc.level, level.InfoValue())
+
+			if got != tc.want {
+				t.Errorf("wrong value: got=%#v, want=%#v", got, tc.want)
+			}
+		})
+	}
+}