From ad2f81ef6ddda642018a271133a68ff7b58ce8fc Mon Sep 17 00:00:00 2001
From: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
Date: Wed, 4 Mar 2026 15:19:17 +0100
Subject: [PATCH] textparse: fix parseLVals to only treat quoted strings as
 metric names

When a label-name position is followed by comma or brace-close, only
treat it as a metric name shorthand if the token was a double-quoted
string (tQString). Bare identifiers must be followed by an equal sign.

Add tests for bare identifier inputs that previously could panic.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
---
 model/textparse/openmetricsparse.go      |  4 +++-
 model/textparse/openmetricsparse_test.go | 24 ++++++++++++++++++++++--
 model/textparse/promparse.go             |  4 +++-
 model/textparse/promparse_test.go        | 20 ++++++++++++++++++++
 4 files changed, 48 insertions(+), 4 deletions(-)

diff --git a/model/textparse/openmetricsparse.go b/model/textparse/openmetricsparse.go
index 3a36402ef4..2f6671eb62 100644
--- a/model/textparse/openmetricsparse.go
+++ b/model/textparse/openmetricsparse.go
@@ -634,11 +634,13 @@ func (p *OpenMetricsParser) parseLVals(offsets []int, isExemplar bool) ([]int, e
 	for {
 		curTStart := p.l.start
 		curTI := p.l.i
+		var isQString bool
 		switch t {
 		case tBraceClose:
 			return offsets, nil
 		case tLName:
 		case tQString:
+			isQString = true
 		default:
 			return nil, p.parseError("expected label name", t)
 		}
@@ -647,7 +649,7 @@ func (p *OpenMetricsParser) parseLVals(offsets []int, isExemplar bool) ([]int, e
 		// A quoted string followed by a comma or brace is a metric name. Set the
 		// offsets and continue processing. If this is an exemplar, this format
 		// is not allowed.
-		if t == tComma || t == tBraceClose {
+		if isQString && (t == tComma || t == tBraceClose) {
 			if isExemplar {
 				return nil, p.parseError("expected label name", t)
 			}
diff --git a/model/textparse/openmetricsparse_test.go b/model/textparse/openmetricsparse_test.go
index 8f6393cd53..d365d8d959 100644
--- a/model/textparse/openmetricsparse_test.go
+++ b/model/textparse/openmetricsparse_test.go
@@ -941,6 +941,10 @@ func TestOpenMetricsParseErrors(t *testing.T) {
 			input: "empty_label_name{=\"\"} 0\n# EOF\n",
 			err:   "expected label name, got \"=\\\"\" (\"EQUAL\") while parsing: \"empty_label_name{=\\\"\"",
 		},
+		{
+			input: "{A}0\n# EOF\n",
+			err:   "expected equal, got \"}0\" (\"BCLOSE\") while parsing: \"{A}0\"",
+		},
 		{
 			input: "foo 1_2\n\n# EOF\n",
 			err:   "unsupported character in float while parsing: \"foo 1_2\"",
@@ -971,11 +975,11 @@ func TestOpenMetricsParseErrors(t *testing.T) {
 		},
 		{
 			input: `custom_metric_total 1 # {bb}`,
-			err:   "expected label name, got \"}\" (\"BCLOSE\") while parsing: \"custom_metric_total 1 # {bb}\"",
+			err:   "expected equal, got \"}\" (\"BCLOSE\") while parsing: \"custom_metric_total 1 # {bb}\"",
 		},
 		{
 			input: `custom_metric_total 1 # {bb, a="dd"}`,
-			err:   "expected label name, got \", \" (\"COMMA\") while parsing: \"custom_metric_total 1 # {bb, \"",
+			err:   "expected equal, got \", \" (\"COMMA\") while parsing: \"custom_metric_total 1 # {bb, \"",
 		},
 		{
 			input: `custom_metric_total 1 # {aa="bb",,cc="dd"} 1`,
@@ -1037,6 +1041,22 @@ func TestOpenMetricsParseErrors(t *testing.T) {
 	}
 }
 
+func TestOpenMetricsParseBareIdentifierInBraces(t *testing.T) {
+	require.NotPanics(t, func() {
+		p := NewOpenMetricsParser([]byte("{A} 0\n# EOF\n"), labels.NewSymbolTable(), WithOMParserSTSeriesSkipped())
+		for {
+			et, err := p.Next()
+			if err != nil {
+				break
+			}
+			if et == EntrySeries {
+				var lset labels.Labels
+				p.Labels(&lset)
+			}
+		}
+	})
+}
+
 func TestOMNullByteHandling(t *testing.T) {
 	cases := []struct {
 		input string
diff --git a/model/textparse/promparse.go b/model/textparse/promparse.go
index ada1b29013..c10a8002a7 100644
--- a/model/textparse/promparse.go
+++ b/model/textparse/promparse.go
@@ -408,11 +408,13 @@ func (p *PromParser) parseLVals() error {
 	for {
 		curTStart := p.l.start
 		curTI := p.l.i
+		var isQString bool
 		switch t {
 		case tBraceClose:
 			return nil
 		case tLName:
 		case tQString:
+			isQString = true
 		default:
 			return p.parseError("expected label name", t)
 		}
@@ -420,7 +422,7 @@ func (p *PromParser) parseLVals() error {
 		t = p.nextToken()
 		// A quoted string followed by a comma or brace is a metric name. Set the
 		// offsets and continue processing.
-		if t == tComma || t == tBraceClose {
+		if isQString && (t == tComma || t == tBraceClose) {
 			if p.offsets[0] != -1 || p.offsets[1] != -1 {
 				return fmt.Errorf("metric name already set while parsing: %q", p.l.b[p.start:p.l.i])
 			}
diff --git a/model/textparse/promparse_test.go b/model/textparse/promparse_test.go
index a398067efe..c6e0364131 100644
--- a/model/textparse/promparse_test.go
+++ b/model/textparse/promparse_test.go
@@ -510,6 +510,10 @@ func TestPromParseErrors(t *testing.T) {
 			input: "empty_label_name{=\"\"} 0",
 			err:   "expected label name, got \"=\\\"\" (\"EQUAL\") while parsing: \"empty_label_name{=\\\"\"",
 		},
+		{
+			input: "{A}0",
+			err:   "expected equal, got \"}0\" (\"BCLOSE\") while parsing: \"{A}0\"",
+		},
 		{
 			input: "foo 1_2\n",
 			err:   "unsupported character in float while parsing: \"foo 1_2\"",
@@ -550,6 +554,22 @@ func TestPromParseErrors(t *testing.T) {
 	}
 }
 
+func TestPromParseBareIdentifierInBraces(t *testing.T) {
+	require.NotPanics(t, func() {
+		p := NewPromParser([]byte("{A} 0\n"), labels.NewSymbolTable(), false)
+		for {
+			et, err := p.Next()
+			if err != nil {
+				break
+			}
+			if et == EntrySeries {
+				var lset labels.Labels
+				p.Labels(&lset)
+			}
+		}
+	})
+}
+
 func TestPromNullByteHandling(t *testing.T) {
 	cases := []struct {
 		input string