Data Race: exemplars share rand source without synchronization

[rodaine]

Description

exemplar.random() is used concurrently resulting in a data race (below). Access to a rand.Rand is not safe for concurrent use per the documentation:

Random numbers are generated by a Source, usually wrapped in a Rand. Both types should be used by a single goroutine at a time: sharing among multiple goroutines requires some kind of synchronization.

The resulting race:

WARNING: DATA RACE
Read at 0x00c0001ae000 by goroutine 28650:
  math/rand.(*rngSource).Uint64()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rng.go:239 +0x34
  math/rand.(*rngSource).Int63()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rng.go:234 +0x30
  math/rand.(*Rand).Int63()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rand.go:96 +0x4c
  math/rand.(*Rand).Float64()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rand.go:207 +0x38
  go.opentelemetry.io/otel/sdk/metric/internal/exemplar.random()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/exemplar/rand.go:41 +0x40
  go.opentelemetry.io/otel/sdk/metric/internal/exemplar.(*randRes).reset()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/exemplar/rand.go:142 +0x94
  go.opentelemetry.io/otel/sdk/metric/internal/exemplar.FixedSize()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/exemplar/rand.go:54 +0xf0
  go.opentelemetry.io/otel/sdk/metric.reservoirFunc.func1.2()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/exemplar.go:65 +0x34
  go.opentelemetry.io/otel/sdk/metric.reservoirFunc.func2()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/exemplar.go:82 +0x34
  go.opentelemetry.io/otel/sdk/metric/internal/aggregate.(*valueMap[go.shape.int64]).measure()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/aggregate/sum.go:47 +0x314
  go.opentelemetry.io/otel/sdk/metric/internal/aggregate.Builder[go.shape.int64].Sum.func3()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/aggregate/aggregate.go:119 +0xc8
  go.opentelemetry.io/otel/sdk/metric/internal/aggregate.Builder[go.shape.int64].filter.func2()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/aggregate/aggregate.go:72 +0xa8
  go.opentelemetry.io/otel/sdk/metric.(*int64Inst).aggregate()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/instrument.go:204 +0xf4
  go.opentelemetry.io/otel/sdk/metric.(*int64Inst).Add()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/instrument.go:194 +0xc0

Previous write at 0x00c0001ae000 by goroutine 28621:
  math/rand.(*rngSource).Uint64()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rng.go:239 +0x54
  math/rand.(*rngSource).Int63()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rng.go:234 +0x30
  math/rand.(*Rand).Int63()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rand.go:96 +0x4c
  math/rand.(*Rand).Float64()
      /Users/rodaine/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.3.darwin-arm64/src/math/rand/rand.go:207 +0x38
  go.opentelemetry.io/otel/sdk/metric/internal/exemplar.random()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/exemplar/rand.go:41 +0x40
  go.opentelemetry.io/otel/sdk/metric/internal/exemplar.(*randRes).reset()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/exemplar/rand.go:142 +0x94
  go.opentelemetry.io/otel/sdk/metric/internal/exemplar.FixedSize()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/exemplar/rand.go:54 +0xf0
  go.opentelemetry.io/otel/sdk/metric.reservoirFunc.func1.2()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/exemplar.go:65 +0x34
  go.opentelemetry.io/otel/sdk/metric.reservoirFunc.func2()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/exemplar.go:82 +0x34
  go.opentelemetry.io/otel/sdk/metric/internal/aggregate.(*valueMap[go.shape.int64]).measure()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/aggregate/sum.go:47 +0x314
  go.opentelemetry.io/otel/sdk/metric/internal/aggregate.Builder[go.shape.int64].Sum.func3()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/aggregate/aggregate.go:119 +0xc8
  go.opentelemetry.io/otel/sdk/metric/internal/aggregate.Builder[go.shape.int64].filter.func2()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/internal/aggregate/aggregate.go:72 +0xa8
  go.opentelemetry.io/otel/sdk/metric.(*int64Inst).aggregate()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/instrument.go:204 +0xf4
  go.opentelemetry.io/otel/sdk/metric.(*int64Inst).Add()
      /Users/rodaine/pkg/mod/go.opentelemetry.io/otel/sdk/metric@v1.27.0/instrument.go:194 +0xc0

Environment

• OS: macOS Sonoma 14.5 (darwin)
• Architecture: Apple M1 (arm64)
• Go Version: 1.22.3
• opentelemetry-go version: 1.27.0

Steps To Reproduce

1. Enable trace exemplars with OTEL_GO_X_EXEMPLAR
2. Have some concurrent code that uses both traces and int64 counters concurrently
3. Build (or test) with the -race flag enabled.

Expected behavior

No data race; likely via adding a mutex to exemplar.random similar to the math/rand package-level functions.