authMsg := &bytes.Buffer{}
authMsg.WriteByte(0x01 /* VER */)
authMsg.WriteByte(byte(len(user.Username)) /* ULEN */)
authMsg.WriteString(user.Username /* UNAME */)
authMsg.WriteByte(byte(len(user.Password)) /* PLEN */)
authMsg.WriteString(user.Password /* PASSWD */)

func (b *Buffer) Write(p []byte) (n int, err error) {
	b.lastRead = opInvalid
	m, ok := b.tryGrowByReslice(len(p))
	if !ok {
		m = b.grow(len(p))
	}
	return copy(b.buf[m:], p), nil
}

...

func (b *Buffer) grow(n int) int {
	m := b.Len()
	// If buffer is empty, reset to recover space.
	if m == 0 && b.off != 0 {
		b.Reset()
	}
	// Try to grow by means of a reslice.
	if i, ok := b.tryGrowByReslice(n); ok {
		return i
	}
	if b.buf == nil && n <= smallBufferSize {
		b.buf = make([]byte, n, smallBufferSize)
		return 0
	}
	c := cap(b.buf)
...

package main

import (
	"bytes"
	"fmt"
)

func main() {
	username := "a very very very very very very very very very very very long username"
	password := "a very very very very very very very very very very very long password"

	fmt.Println("before")

	var authMsg *bytes.Buffer

	authMsg = &bytes.Buffer{}
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteByte(0x01 /* VER */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteByte(byte(len(username)) /* ULEN */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteString(username /* UNAME */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteByte(byte(len(password)) /* PLEN */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteString(password /* PASSWD */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	// cap: 0 len: 0
	// cap: 64 len: 1
	// cap: 64 len: 2
	// cap: 128 len: 72
	// cap: 128 len: 73
	// cap: 256 len: 143

	fmt.Println("after")

	authMsgLen := 1 + 1 + len(username) + 1 + len(password)
	authMsg = bytes.NewBuffer(make([]byte, 0, authMsgLen))
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteByte(0x01 /* VER */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteByte(byte(len(username)) /* ULEN */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteString(username /* UNAME */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteByte(byte(len(password)) /* PLEN */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	authMsg.WriteString(password /* PASSWD */)
	fmt.Println("cap:", authMsg.Cap(), "len:", authMsg.Len())

	// cap: 143 len: 0
	// cap: 143 len: 1
	// cap: 143 len: 2
	// cap: 143 len: 72
	// cap: 143 len: 73
	// cap: 143 len: 143
}