fix When parsing {}, the parser enters startLabelName state with p.cu…

[GuoFlight]

expfmt: panic in TextParser.startLabelName when parsing empty braces "{}"

Title: expfmt: panic in TextParser.startLabelName when parsing empty braces "{}"

What happened?

• Parsing malformed input {} causes a panic due to nil pointer dereference in startLabelName().

  How to reproduce it?

  package main

  import (
      "bytes"
      "fmt"

      dto "github.com/prometheus/client_model/go"
      "github.com/prometheus/common/expfmt"
  )

  func main() {
      buf := bytes.NewBuffer([]byte(`{}`))
      decoder := expfmt.NewDecoder(buf, expfmt.NewFormat(expfmt.TypeUnknown))
      var mf dto.MetricFamily
      if err := decoder.Decode(&mf); err != nil {
          fmt.Printf("Error: %v\n", err)
      }
  }

Stack trace:

  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x2 addr=0x30 pc=0x...]

  goroutine 1 [running]:
  github.com/prometheus/common/expfmt.(*TextParser).startLabelName(0x...)
      expfmt/text_parse.go:341 +0x44
  github.com/prometheus/common/expfmt.(*TextParser).TextToMetricFamilies(0x..., ...)
      expfmt/text_parse.go:118 +0x54
  github.com/prometheus/common/expfmt.(*textDecoder).Decode(0x..., 0x...)
      expfmt/decode.go:155 +0x74

Root cause

In expfmt/text_parse.go, the startLabelName() function accesses p.currentMetric.Label without checking if p.currentMetric is nil:

  if p.currentByte == '}' {
      p.currentMetric.Label = append(p.currentMetric.Label, p.currentLabelPairs...)  // Line 341 - PANIC here
      ...
  }

When parsing {}, the parser enters startLabelName state with p.currentMetric == nil, causing the crash.

Expected behavior

• Return a parse error instead of panicking, similar to other malformed inputs.

Suggested fix

• Add a nil check before accessing p.currentMetric:

  if p.currentByte == '}' {
      if p.currentMetric == nil {
          p.parseError("unexpected '}' without metric name")
          p.currentLabelPairs = nil
          return nil
      }
      p.currentMetric.Label = append(p.currentMetric.Label, p.currentLabelPairs...)
      ...
  }

Sources:

• https://github.com/prometheus/common
• https://raw.githubusercontent.com/prometheus/common/refs/heads/main/expfmt/text_parse.go