Can you explain what the context.WithValue function in Go is used for and how it works?

The context.WithValue function in Go is used to create a new context that contains a key-value pair. This allows passing additional data through a chain of calls without changing function signatures.

It works like this: you pass the original context, a key, and a value, and receive a new context from which you can retrieve this value using context.Value(key).

Example:

ctx := context.Background()
ctxWithValue := context.WithValue(ctx, "userID", 42)

userID := ctxWithValue.Value("userID")
fmt.Println(userID) // Outputs: 42

It is important to use unique key types to avoid conflicts, for example, by defining your own type for keys.