From d5ca3fa894db3618e7397b24cc70d015f898e37b Mon Sep 17 00:00:00 2001
From: Wes Tarle <westarle@google.com>
Date: Mon, 9 Mar 2026 12:40:14 -0400
Subject: [PATCH] feat(callctx): pass logger to downstream via context

---
 v2/callctx/callctx.go      | 19 +++++++++++++++++++
 v2/callctx/callctx_test.go | 22 ++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/v2/callctx/callctx.go b/v2/callctx/callctx.go
index b28df61d0..60e6cce23 100644
--- a/v2/callctx/callctx.go
+++ b/v2/callctx/callctx.go
@@ -35,6 +35,7 @@ package callctx
 import (
 	"context"
 	"fmt"
+	"log/slog"
 )
 
 const (
@@ -125,3 +126,21 @@ func TelemetryFromContext(ctx context.Context, key string) (string, bool) {
 	val, ok := ctx.Value(telemetryKey(key)).(string)
 	return val, ok
 }
+
+// loggerKey is a private type used to store/retrieve the logger context value.
+type loggerContextKey string
+
+const loggerCKey = loggerContextKey("logger")
+
+// WithLogger injects a slog.Logger into the context. This logger will be
+// extracted by the client library or transport wrappers to emit logs.
+func WithLogger(ctx context.Context, logger *slog.Logger) context.Context {
+	return context.WithValue(ctx, loggerCKey, logger)
+}
+
+// LoggerFromContext extracts a slog.Logger from the context.
+// The returned bool indicates whether a logger was found.
+func LoggerFromContext(ctx context.Context) (*slog.Logger, bool) {
+	logger, ok := ctx.Value(loggerCKey).(*slog.Logger)
+	return logger, ok
+}
diff --git a/v2/callctx/callctx_test.go b/v2/callctx/callctx_test.go
index bbf9efbc3..c1287500e 100644
--- a/v2/callctx/callctx_test.go
+++ b/v2/callctx/callctx_test.go
@@ -30,7 +30,9 @@
 package callctx
 
 import (
+	"bytes"
 	"context"
+	"log/slog"
 	"sync"
 	"testing"
 
@@ -122,3 +124,23 @@ func TestSetHeaders_race(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+func TestLoggerFromContext(t *testing.T) {
+	ctx := context.Background()
+
+	// Should not find a logger in an empty context
+	l, ok := LoggerFromContext(ctx)
+	if ok || l != nil {
+		t.Errorf("LoggerFromContext(ctx) = %v, %v; want nil, false", l, ok)
+	}
+
+	// Should extract the exact logger that was injected
+	var logOutput bytes.Buffer
+	injectedLogger := slog.New(slog.NewTextHandler(&logOutput, nil))
+	ctx = WithLogger(ctx, injectedLogger)
+
+	extractedLogger, ok := LoggerFromContext(ctx)
+	if !ok || extractedLogger != injectedLogger {
+		t.Errorf("LoggerFromContext(ctx) = %v, %v; want %v, true", extractedLogger, ok, injectedLogger)
+	}
+}