From f975a15dfde8fc37b42dd2d99b8048c3f01bd4a6 Mon Sep 17 00:00:00 2001
From: Brendan Roy <br3ndanr@gmail.com>
Date: Thu, 11 Nov 2021 17:08:10 +1100
Subject: [PATCH 1/3] change resource.Default to be evaluated on first call,
 rather than on import

---
 sdk/resource/resource.go | 33 +++++++++++++++------------------
 1 file changed, 15 insertions(+), 18 deletions(-)

diff --git a/sdk/resource/resource.go b/sdk/resource/resource.go
index 6bb2a2805aa..3aedc85fa2d 100644
--- a/sdk/resource/resource.go
+++ b/sdk/resource/resource.go
@@ -36,26 +36,11 @@ type Resource struct {
 }
 
 var (
-	emptyResource Resource
-
-	defaultResource = func(r *Resource, err error) *Resource {
-		if err != nil {
-			otel.Handle(err)
-		}
-		return r
-	}(
-		Detect(
-			context.Background(),
-			defaultServiceNameDetector{},
-			fromEnv{},
-			telemetrySDK{},
-		),
-	)
+	emptyResource   Resource
+	defaultResource *Resource
 )
 
-var (
-	errMergeConflictSchemaURL = errors.New("cannot merge resource due to conflicting Schema URL")
-)
+var errMergeConflictSchemaURL = errors.New("cannot merge resource due to conflicting Schema URL")
 
 // New returns a Resource combined from the user-provided detectors.
 func New(ctx context.Context, opts ...Option) (*Resource, error) {
@@ -211,6 +196,18 @@ func Empty() *Resource {
 // Default returns an instance of Resource with a default
 // "service.name" and OpenTelemetrySDK attributes.
 func Default() *Resource {
+	if defaultResource == nil {
+		var err error
+		defaultResource, err = Detect(
+			context.Background(),
+			defaultServiceNameDetector{},
+			fromEnv{},
+			telemetrySDK{},
+		)
+		if err != nil {
+			otel.Handle(err)
+		}
+	}
 	return defaultResource
 }
 

From a62df0c7d9f87f57ceadbbd647da7075635fabb2 Mon Sep 17 00:00:00 2001
From: Brendan Roy <br3ndanr@gmail.com>
Date: Fri, 12 Nov 2021 10:17:03 +1100
Subject: [PATCH 2/3] guard defaultResource creation with sync.Once, fallback
 to emtpyResource

---
 sdk/resource/resource.go | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/sdk/resource/resource.go b/sdk/resource/resource.go
index 3aedc85fa2d..eb0ecd2cf91 100644
--- a/sdk/resource/resource.go
+++ b/sdk/resource/resource.go
@@ -18,6 +18,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"sync"
 
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
@@ -36,8 +37,9 @@ type Resource struct {
 }
 
 var (
-	emptyResource   Resource
-	defaultResource *Resource
+	emptyResource       Resource
+	defaultResource     *Resource
+	defaultResourceOnce sync.Once
 )
 
 var errMergeConflictSchemaURL = errors.New("cannot merge resource due to conflicting Schema URL")
@@ -196,7 +198,7 @@ func Empty() *Resource {
 // Default returns an instance of Resource with a default
 // "service.name" and OpenTelemetrySDK attributes.
 func Default() *Resource {
-	if defaultResource == nil {
+	defaultResourceOnce.Do(func() {
 		var err error
 		defaultResource, err = Detect(
 			context.Background(),
@@ -207,7 +209,11 @@ func Default() *Resource {
 		if err != nil {
 			otel.Handle(err)
 		}
-	}
+		// If Detect did not return a valid resource, fall back to emptyResource.
+		if defaultResource == nil {
+			defaultResource = &emptyResource
+		}
+	})
 	return defaultResource
 }
 

From 0bee299dc7acaa4a7c62c1dba62a48d91dab7191 Mon Sep 17 00:00:00 2001
From: Brendan Roy <br3ndanr@gmail.com>
Date: Sat, 13 Nov 2021 11:17:00 +1100
Subject: [PATCH 3/3] update CHANGELOG

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5b8164147ac..03a37cbabc0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   - The following struct types moved and are replaced with type aliases, since they are exposed to the user: `Observation`, `Measurement`.
   - The No-op implementations of sync and async instruments are no longer exported, new functions `sdkapi.NewNoopAsyncInstrument()` and `sdkapi.NewNoopSyncInstrument()` are provided instead. (#2271)
 - Update the SDK `BatchSpanProcessor` to export all queued spans when `ForceFlush` is called. (#2080, #2335)
+- Change `resource.Default` to be evaluated the first time it is called, rather than on import. This allows the caller the option to update `OTEL_RESOURCE_ATTRIBUTES` first, such as with `os.Setenv`. (#2371)
 
 ### Added