zaptest/observer: Support filtering by message or field (#386)

* zaptest/observer: Support filtering by message or field

Add `Filter*` functions to filter a set of ObservedLogs and return
a new set of ObservedLogs. This allows chaining of multiple filters
such as `sink.FilterMessage(..).FilterField(..)` for easier use in
tests.

* Handle duplicate fields

zaptest/observer/observer.go

@@ -80,6 +80,38 @@ func (o *ObservedLogs) AllUntimed() []LoggedEntry {
 	return ret
 }
 
+// FilterMessage filters entries to those that have the specified message.
+func (o *ObservedLogs) FilterMessage(msg string) *ObservedLogs {
+	return o.filter(func(e LoggedEntry) bool {
+		return e.Message == msg
+	})
+}
+
+// FilterField filters entries to those that have the specified field.
+func (o *ObservedLogs) FilterField(field zapcore.Field) *ObservedLogs {
+	return o.filter(func(e LoggedEntry) bool {
+		for _, ctxField := range e.Context {
+			if ctxField == field {
+				return true
+			}
+		}
+		return false
+	})
+}
+
+func (o *ObservedLogs) filter(match func(LoggedEntry) bool) *ObservedLogs {
+	o.mu.RLock()
+	defer o.mu.RUnlock()
+
+	var filtered []LoggedEntry
+	for _, entry := range o.logs {
+		if match(entry) {
+			filtered = append(filtered, entry)
+		}
+	}
+	return &ObservedLogs{logs: filtered}
+}
+
 func (o *ObservedLogs) add(log LoggedEntry) {
 	o.mu.Lock()
 	o.logs = append(o.logs, log)

zaptest/observer/observer_test.go

@@ -118,3 +118,66 @@ func TestObserverWith(t *testing.T) {
 		},
 	}, logs.All(), "expected no field sharing between With siblings")
 }
+
+func TestFilters(t *testing.T) {
+	logs := []LoggedEntry{
+		{
+			Entry:   zapcore.Entry{Level: zap.InfoLevel, Message: "log a"},
+			Context: []zapcore.Field{zap.String("fStr", "1"), zap.Int("a", 1)},
+		},
+		{
+			Entry:   zapcore.Entry{Level: zap.InfoLevel, Message: "log a"},
+			Context: []zapcore.Field{zap.String("fStr", "2"), zap.Int("b", 2)},
+		},
+		{
+			Entry:   zapcore.Entry{Level: zap.InfoLevel, Message: "log b"},
+			Context: []zapcore.Field{zap.Int("a", 1), zap.Int("b", 2)},
+		},
+		{
+			Entry:   zapcore.Entry{Level: zap.InfoLevel, Message: "log c"},
+			Context: []zapcore.Field{zap.Int("a", 1), zap.Namespace("ns"), zap.Int("a", 2)},
+		},
+	}
+
+	logger, sink := New(zap.InfoLevel)
+	for _, log := range logs {
+		logger.Write(log.Entry, log.Context)
+	}
+
+	tests := []struct {
+		msg      string
+		filtered *ObservedLogs
+		want     []LoggedEntry
+	}{
+		{
+			msg:      "filter by message",
+			filtered: sink.FilterMessage("log a"),
+			want:     logs[0:2],
+		},
+		{
+			msg:      "filter by field",
+			filtered: sink.FilterField(zap.String("fStr", "1")),
+			want:     logs[0:1],
+		},
+		{
+			msg:      "filter by message and field",
+			filtered: sink.FilterMessage("log a").FilterField(zap.Int("b", 2)),
+			want:     logs[1:2],
+		},
+		{
+			msg:      "filter by field with duplicate fields",
+			filtered: sink.FilterField(zap.Int("a", 2)),
+			want:     logs[3:4],
+		},
+		{
+			msg:      "filter doesn't match any messages",
+			filtered: sink.FilterMessage("no match"),
+			want:     []LoggedEntry{},
+		},
+	}
+
+	for _, tt := range tests {
+		got := tt.filtered.AllUntimed()
+		assert.Equal(t, tt.want, got, tt.msg)
+	}
+}