feat: line_format bypass template execution if possible (#15411)

Co-authored-by: Ed Welch <ed@edjusted.com>

Author: cyriltovena

pkg/logql/log/fmt.go

@@ -194,6 +194,7 @@ type LineFormatter struct {
 
 	currentLine []byte
 	currentTs   int64
+	simpleKey   string
 }
 
 // NewFormatter creates a new log line formatter from a given text template.
@@ -213,10 +214,30 @@ func NewFormatter(tmpl string) (*LineFormatter, error) {
 		return nil, fmt.Errorf("invalid line template: %w", err)
 	}
 	lf.Template = t
+
+	// determine if the template is a simple key substitution, e.g. line_format `{{.message}}`
+	// if it is, save the key name and we can use it later to directly copy the string
+	// bytes of the value to avoid copying and allocating a new string.
+	if len(t.Root.Nodes) == 1 && t.Root.Nodes[0].Type() == parse.NodeAction {
+		actionNode := t.Root.Nodes[0].(*parse.ActionNode)
+		if len(actionNode.Pipe.Cmds) == 1 && len(actionNode.Pipe.Cmds[0].Args) == 1 {
+			if fieldNode, ok := actionNode.Pipe.Cmds[0].Args[0].(*parse.FieldNode); ok && len(fieldNode.Ident) == 1 {
+				lf.simpleKey = fieldNode.Ident[0]
+			}
+		}
+	}
+
 	return lf, nil
 }
 
 func (lf *LineFormatter) Process(ts int64, line []byte, lbs *LabelsBuilder) ([]byte, bool) {
+	if lf.simpleKey != "" {
+		if val, ok := lbs.Get(lf.simpleKey); ok {
+			return unsafeGetBytes(val), true
+		}
+		return []byte{}, true
+	}
+
 	lf.buf.Reset()
 	lf.currentLine = line
 	lf.currentTs = ts
@@ -387,7 +408,7 @@ func (lf *LabelsFormatter) Process(ts int64, l []byte, lbs *LabelsBuilder) ([]by
 	lf.currentLine = l
 	lf.currentTs = ts
 
-	var m = smp.Get()
+	m := smp.Get()
 	defer smp.Put(m)
 	for _, f := range lf.formats {
 		if f.Rename {

pkg/logql/log/fmt_test.go

@@ -479,6 +479,33 @@ func Test_lineFormatter_Format(t *testing.T) {
 			labels.FromStrings("foo", "hello"),
 			[]byte("1"),
 		},
+		{
+			"simple key template",
+			newMustLineFormatter("{{.foo}}"),
+			labels.FromStrings("foo", "bar"),
+			0,
+			[]byte("bar"),
+			labels.FromStrings("foo", "bar"),
+			nil,
+		},
+		{
+			"simple key template with space",
+			newMustLineFormatter("{{.foo}}  "),
+			labels.FromStrings("foo", "bar"),
+			0,
+			[]byte("bar  "),
+			labels.FromStrings("foo", "bar"),
+			nil,
+		},
+		{
+			"simple key template with missing key",
+			newMustLineFormatter("{{.missing}}"),
+			labels.FromStrings("foo", "bar"),
+			0,
+			[]byte{},
+			labels.FromStrings("foo", "bar"),
+			nil,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -916,7 +943,7 @@ func TestLabelFormatter_RequiredLabelNames(t *testing.T) {
 }
 
 func TestDecolorizer(t *testing.T) {
-	var decolorizer, _ = NewDecolorizer()
+	decolorizer, _ := NewDecolorizer()
 	tests := []struct {
 		name     string
 		src      []byte
@@ -927,7 +954,7 @@ func TestDecolorizer(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			var result, _ = decolorizer.Process(0, tt.src, nil)
+			result, _ := decolorizer.Process(0, tt.src, nil)
 			require.Equal(t, tt.expected, result)
 		})
 	}
@@ -979,7 +1006,6 @@ func TestMapPoolPanic(_ *testing.T) {
 			}
 			smp.Put(m)
 			wgFinished.Done()
-
 		}()
 	}
 	wg.Done()