expfmt: fix nil pointer panic when parsing empty braces "{}"

The text exposition parser panics with a nil pointer dereference when it
encounters a closing brace before any metric name has been read, e.g. for
the input "{}". In startOfLine a leading "{" sets currentMetricIsInsideBraces
and jumps straight to label parsing without going through readingMetricName,
so currentMF/currentMetric are still nil when startLabelName dereferences
currentMetric on the "}" branch.

Since TextToMetricFamilies is used to decode untrusted scrape payloads, a
malicious or compromised target could crash any consumer that does not wrap
the parser in its own recover().

Guard the "}" branch on currentMF == nil and return a parse error, mirroring
the existing guard in startLabelValue. currentMF is checked rather than
currentMetric because reset only clears currentMF between parses.

Introduced in #670.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

Author: roidelapluie

expfmt/text_parse.go

@@ -339,6 +339,16 @@ func (p *TextParser) startLabelName() stateFn {
 		return nil // Unexpected end of input.
 	}
 	if p.currentByte == '}' {
+		if p.currentMF == nil {
+			// The closing brace was reached before any metric name was read,
+			// e.g. for the input "{}". There is no metric to attach labels to,
+			// so this is a malformed exposition. This mirrors the guard in
+			// startLabelValue. currentMF (not currentMetric) is checked because
+			// reset only clears currentMF between parses.
+			p.parseError("invalid metric name")
+			p.currentLabelPairs = nil
+			return nil
+		}
 		p.currentMetric.Label = append(p.currentMetric.Label, p.currentLabelPairs...)
 		p.currentLabelPairs = nil
 		if p.skipBlankTab(); p.err != nil {

expfmt/text_parse_test.go

@@ -1332,6 +1332,12 @@ request_duration_microseconds_count 2693
 `,
 			errUTF8: `text format parsing error in line 5: negative bucket population for histogram "request_duration_microseconds"`,
 		},
+		// 47: Empty braces with no metric name (regression test for a nil
+		// pointer dereference in startLabelName).
+		{
+			in:      `{} 3.14`,
+			errUTF8: "text format parsing error in line 1: invalid metric name",
+		},
 	}
 	for i, scenario := range scenarios {
 		parser.scheme = model.UTF8Validation