Why Does Nested Struct Printing Lead to Goroutine Stack Overflow in Go?

Runtime Error: Goroutine Stack Overflow Due to Nested Struct

When working with nested structs in Go, it's important to be aware of potential stack overflows. This can occur when attempting to print a nested struct using a format that relies on the String() method of the struct.

Root Cause:

Infinite recursion occurs when the String() method of a struct attempts to print the same struct as one of its fields. The %v and % v formats use the value of String() if it exists. This creates an infinite loop, leading to a stack overflow.

Example:

Consider the following nested struct and String() method:

<code class="go">type ConfigOne struct {
    // Daemon section from config file.
    Daemon daemon
}

type daemon struct {
    Loglevel int
    Logfile string
}

func (c ConfigOne) String() string {
    return fmt.Sprintf("%+v\n", c) // Uses %+v for nested structs
}</code>

When attempting to print an instance of ConfigOne using this String() method, a stack overflow error will occur:

<code class="go">c := &modules.ConfigOne{}
c.Daemon.Loglevel = 1
c.Daemon.Logfile = "/tmp/test.log"
modules.Logger.Infoln(c.String())</code>

Solution:

To avoid infinite recursion and stack overflows, the String() method should construct a string manually, specifying the desired format for the nested structs. For example:

<code class="go">func (c ConfigOne) String() string {
    return fmt.Sprintf("Loglevel: %d, Logfile: %s\n", c.Daemon.Loglevel, c.Daemon.Logfile)
}</code>

In this case, the String() method explicity formats the nested fields without using %v or % v, resolving the infinite recursion issue.

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