Expose zaptest.observer.(*ObservedLogs).Filter

Export method  `observer.(*ObservedLogs).Filter` to allow consumers to
filter logs by arbitrary functions for testing.

Currently consumers can call `(*ObservedLogs).All` and proceed to filter
using `[]LoggedEntry` directly, but the result is then incompatible with
existing methods such as `FilterMessage` because there is now way to
re-construct an `*ObservedLogs` object.

zaptest/observer/observer.go

@@ -80,21 +80,21 @@ func (o *ObservedLogs) AllUntimed() []LoggedEntry {
 
 // 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 o.Filter(func(e LoggedEntry) bool {
 		return e.Message == msg
 	})
 }
 
 // FilterMessageSnippet filters entries to those that have a message containing the specified snippet.
 func (o *ObservedLogs) FilterMessageSnippet(snippet string) *ObservedLogs {
-	return o.filter(func(e LoggedEntry) bool {
+	return o.Filter(func(e LoggedEntry) bool {
 		return strings.Contains(e.Message, snippet)
 	})
 }
 
 // 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 {
+	return o.Filter(func(e LoggedEntry) bool {
 		for _, ctxField := range e.Context {
 			if ctxField.Equals(field) {
 				return true
@@ -106,7 +106,7 @@ func (o *ObservedLogs) FilterField(field zapcore.Field) *ObservedLogs {
 
 // FilterFieldKey filters entries to those that have the specified key.
 func (o *ObservedLogs) FilterFieldKey(key string) *ObservedLogs {
-	return o.filter(func(e LoggedEntry) bool {
+	return o.Filter(func(e LoggedEntry) bool {
 		for _, ctxField := range e.Context {
 			if ctxField.Key == key {
 				return true
@@ -116,13 +116,15 @@ func (o *ObservedLogs) FilterFieldKey(key string) *ObservedLogs {
 	})
 }
 
-func (o *ObservedLogs) filter(match func(LoggedEntry) bool) *ObservedLogs {
+// Filter returns a copy of this ObservedLogs containing only those entries
+// for which the provided function returns true.
+func (o *ObservedLogs) Filter(keep func(LoggedEntry) bool) *ObservedLogs {
 	o.mu.RLock()
 	defer o.mu.RUnlock()
 
 	var filtered []LoggedEntry
 	for _, entry := range o.logs {
-		if match(entry) {
+		if keep(entry) {
 			filtered = append(filtered, entry)
 		}
 	}

zaptest/observer/observer_test.go

@@ -55,7 +55,7 @@ func TestObserver(t *testing.T) {
 	assert.Equal(t, want, logs.AllUntimed(), "Unexpected contents from AllUntimed.")
 
 	all := logs.All()
-	require.Equal(t, 1, len(all), "Unexpected numbed of LoggedEntries returned from All.")
+	require.Equal(t, 1, len(all), "Unexpected number of LoggedEntries returned from All.")
 	assert.NotEqual(t, time.Time{}, all[0].Time, "Expected non-zero time on LoggedEntry.")
 
 	// copy & zero time for stable assertions
@@ -219,6 +219,19 @@ func TestFilters(t *testing.T) {
 			filtered: sink.FilterFieldKey("filterMe"),
 			want:     logs[7:9],
 		},
+		{
+			msg: "filter by arbitrary function",
+			filtered: sink.Filter(func(e LoggedEntry) bool {
+				return len(e.Context) > 1
+			}),
+			want: func() []LoggedEntry {
+				// Do not modify logs slice.
+				w := []LoggedEntry{}
+				w = append(w, logs[0:5]...)
+				w = append(w, logs[7])
+				return w
+			}(),
+		},
 	}
 
 	for _, tt := range tests {