envcar: add environment carrier

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Added
+
+- Add environment variables propagation carrier in `go.opentelemetry.io/contrib/propagators/envcar`. (#8442)
+
 ### Changed
 
 - Upgrade `go.opentelemetry.io/otel/semconv` to `v1.40.0`, including updates across instrumentation and detector modules. (#8631)

CODEOWNERS

@@ -64,6 +64,7 @@ processors/minsev                                                       @open-te
 propagators/autoprop/                                                   @open-telemetry/go-approvers @MrAlias
 propagators/aws/                                                        @open-telemetry/go-approvers @akats7
 propagators/b3/                                                         @open-telemetry/go-approvers @pellared
+propagators/envcar/                                                     @open-telemetry/go-approvers @Joibel @pellared
 propagators/jaeger/                                                     @open-telemetry/go-approvers @yurishkuro
 propagators/opencensus/                                                 @open-telemetry/go-approvers @dashpole
 propagators/ot/                                                         @open-telemetry/go-approvers @pellared

propagators/envcar/carrier.go

@@ -0,0 +1,103 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package envcar // import "go.opentelemetry.io/contrib/propagators/envcar"
+
+import (
+	"os"
+	"strings"
+	"sync"
+
+	"go.opentelemetry.io/otel/propagation"
+)
+
+// upperWithUnderscores converts a string so that A-Z and 0-9 and _ are kept
+// as-is, a-z is uppercased, and all other characters are replaced with _.
+func upperWithUnderscores(s string) string {
+	b := make([]byte, 0, len(s))
+	for _, r := range s {
+		switch {
+		case r >= 'A' && r <= 'Z', r >= '0' && r <= '9', r == '_':
+			b = append(b, byte(r))
+		case r >= 'a' && r <= 'z':
+			b = append(b, byte(r+'A'-'a'))
+		default:
+			b = append(b, '_')
+		}
+	}
+	return string(b)
+}
+
+// Carrier is a TextMapCarrier that uses the environment variables as a
+// storage medium for propagated key-value pairs. The keys are normalised
+// before being used to access the environment variables.
+// This is useful for propagating values that are set in the environment
+// and need to be accessed by different processes or services.
+// The keys are uppercased to avoid case sensitivity issues across different
+// operating systems and environments.
+//
+// If you do not set SetEnvFunc, [Carrier.Set] will do nothing.
+// Using [os.Setenv] here is discouraged as the environment should
+// be immutable:
+// https://opentelemetry.io/docs/specs/otel/context/env-carriers/#environment-variable-immutability
+type Carrier struct {
+	// SetEnvFunc is the function that sets the environment variable.
+	// Usually, you want to set the environment variables for processes
+	// that are spawned by the current process.
+	SetEnvFunc func(key, value string)
+	values     map[string]string
+	once       sync.Once
+}
+
+// Compile time check that Carrier implements the TextMapCarrier.
+var _ propagation.TextMapCarrier = (*Carrier)(nil)
+
+// fetch runs once on first access, and stores the environment in the
+// carrier.
+func (c *Carrier) fetch() {
+	c.once.Do(func() {
+		environ := os.Environ()
+		c.values = make(map[string]string, len(environ))
+		for _, kv := range environ {
+			kvPair := strings.SplitN(kv, "=", 2)
+			c.values[kvPair[0]] = kvPair[1]
+		}
+	})
+}
+
+// Get returns the value associated with the normalized passed key.
+// The first call to [Carrier.Get] or [Carrier.Keys] for a
+// given Carrier will read and store the values from the
+// environment and all future reads will be from that store.
+func (c *Carrier) Get(key string) string {
+	c.fetch()
+	return c.values[upperWithUnderscores(key)]
+}
+
+// Set stores the key-value pair in the environment variable.
+// The key is normalized before being used to set the
+// environment variable.
+// If SetEnvFunc is not set, this method does nothing.
+func (c *Carrier) Set(key, value string) {
+	if c.SetEnvFunc == nil {
+		return
+	}
+	k := upperWithUnderscores(key)
+	c.SetEnvFunc(k, value)
+}
+
+// Keys lists the keys stored in this carrier.
+// This returns all the keys in the environment variables.
+// The first call to [Carrier.Get] or [Carrier.Keys] for a
+// given Carrier will read and store the values from the
+// environment and all future reads will be from that store.
+// Keys are returned as is, without any normalization, but
+// this behavior is subject to change.
+func (c *Carrier) Keys() []string {
+	c.fetch()
+	keys := make([]string, 0, len(c.values))
+	for key := range c.values {
+		keys = append(keys, key)
+	}
+	return keys
+}

propagators/envcar/carrier_example_test.go

@@ -0,0 +1,81 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package envcar_test
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+
+	"go.opentelemetry.io/otel/propagation"
+	"go.opentelemetry.io/otel/trace"
+
+	"go.opentelemetry.io/contrib/propagators/envcar"
+)
+
+// This example is a go program where the environment variables are carrying the
+// trace information, and we're going to pick them up into our context.
+func ExampleCarrier_extractFromParent() {
+	// Simulate environment variables set by a parent process.
+	// In practice, these would already be set when this process starts.
+	_ = os.Setenv("TRACEPARENT", "00-0102030405060708090a0b0c0d0e0f10-0102030405060708-01")
+
+	// Create a carrier to read trace context from environment variables.
+	carrier := envcar.Carrier{}
+
+	// Extract trace context that was propagated by the parent process.
+	prop := propagation.TraceContext{}
+	ctx := prop.Extract(context.Background(), &carrier)
+
+	// The context now contains the span context from the parent.
+	spanCtx := trace.SpanContextFromContext(ctx)
+	fmt.Printf("Trace ID: %s\n", spanCtx.TraceID())
+	fmt.Printf("Span ID: %s\n", spanCtx.SpanID())
+	fmt.Printf("Sampled: %t\n", spanCtx.IsSampled())
+	// Output:
+	// Trace ID: 0102030405060708090a0b0c0d0e0f10
+	// Span ID: 0102030405060708
+	// Sampled: true
+}
+
+// This example is a go program where we have a trace and we'd like to inject it
+// into a command we're going to run.
+func ExampleCarrier_childProcess() {
+	// Create a span context with a known trace ID.
+	traceID := trace.TraceID{0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10}
+	spanID := trace.SpanID{0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08}
+	spanCtx := trace.NewSpanContext(trace.SpanContextConfig{
+		TraceID:    traceID,
+		SpanID:     spanID,
+		TraceFlags: trace.FlagsSampled,
+	})
+	ctx := trace.ContextWithSpanContext(context.Background(), spanCtx)
+
+	// Prepare a command that prints the TRACEPARENT environment variable.
+	cmd := exec.Command("printenv", "TRACEPARENT")
+	cmd.Env = os.Environ()
+
+	// Create a carrier that injects trace context into the child
+	// process's environment rather than the current process's.
+	carrier := envcar.Carrier{
+		SetEnvFunc: func(key, value string) {
+			cmd.Env = append(cmd.Env, key+"="+value)
+		},
+	}
+
+	// Inject trace context into the child's environment.
+	prop := propagation.TraceContext{}
+	prop.Inject(ctx, &carrier)
+
+	// The child process now has trace context in its environment,
+	// independent of the parent process's environment variables.
+	out, err := cmd.Output()
+	if err != nil {
+		fmt.Println("error:", err)
+		return
+	}
+	fmt.Print(string(out))
+	// Output: 00-0102030405060708090a0b0c0d0e0f10-0102030405060708-01
+}