Clarify the godoc warning for Logger.UpdateContext (#566)

README.md

@@ -755,6 +755,8 @@ Log a static string, without any context or `printf`-style templating:
 
 ## Caveats
 
+### Field duplication
+
 Note that zerolog does no de-duplication of fields. Using the same key multiple times creates multiple keys in final JSON:
 
 ```go
@@ -766,3 +768,19 @@ logger.Info().
 ```
 
 In this case, many consumers will take the last value, but this is not guaranteed; check yours if in doubt.
+
+### Concurrency safety
+
+Be careful when calling UpdateContext. It is not concurrency safe. Use the With method to create a child logger:
+
+```go
+func handler(w http.ResponseWriter, r *http.Request) {
+    // Create a child logger for concurrency safety
+    logger := log.Logger.With().Logger()
+
+    // Add context fields, for example User-Agent from HTTP headers
+    logger.UpdateContext(func(c zerolog.Context) zerolog.Context {
+        ...
+    })
+}
+```

log.go

@@ -84,6 +84,8 @@
 //
 // # Caveats
 //
+// Field duplication:
+//
 // There is no fields deduplication out-of-the-box.
 // Using the same key multiple times creates new key in final JSON each time.
 //
@@ -95,6 +97,20 @@
 //
 // In this case, many consumers will take the last value,
 // but this is not guaranteed; check yours if in doubt.
+//
+// Concurrency safety:
+//
+// Be careful when calling UpdateContext. It is not concurrency safe. Use the With method to create a child logger:
+//
+//     func handler(w http.ResponseWriter, r *http.Request) {
+//         // Create a child logger for concurrency safety
+//         logger := log.Logger.With().Logger()
+//
+//         // Add context fields, for example User-Agent from HTTP headers
+//         logger.UpdateContext(func(c zerolog.Context) zerolog.Context {
+//             ...
+//         })
+//     }
 package zerolog
 
 import (
@@ -276,7 +292,8 @@ func (l Logger) With() Context {
 
 // UpdateContext updates the internal logger's context.
 //
-// Use this method with caution. If unsure, prefer the With method.
+// Caution: This method is not concurrency safe.
+// Use the With method to create a child logger before modifying the context from concurrent goroutines.
 func (l *Logger) UpdateContext(update func(c Context) Context) {
 	if l == disabledLogger {
 		return