middleware: add idempotency middleware

middleware/idempotency/README.md

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+# Idempotency Middleware
+
+Idempotency middleware for [Fiber](https://github.com/gofiber/fiber) allows for fault-tolerant APIs where duplicate requests — for example due to networking issues on the client-side — do not erroneously cause the same action performed multiple times on the server-side.
+
+Refer to https://datatracker.ietf.org/doc/html/draft-ietf-httpapi-idempotency-key-header-02 for a better understanding.
+
+## Table of Contents
+
+- [Idempotency Middleware](#idempotency-middleware)
+	- [Table of Contents](#table-of-contents)
+	- [Signatures](#signatures)
+	- [Examples](#examples)
+		- [Default Config](#default-config)
+		- [Custom Config](#custom-config)
+		- [Config](#config)
+		- [Default Config](#default-config-1)
+
+## Signatures
+
+```go
+func New(config ...Config) fiber.Handler
+```
+
+## Examples
+
+First import the middleware from Fiber,
+
+```go
+import (
+  "github.com/gofiber/fiber/v3"
+  "github.com/gofiber/fiber/v3/middleware/idempotency"
+)
+```
+
+Then create a Fiber app with `app := fiber.New()`.
+
+### Default Config
+
+```go
+app.Use(idempotency.New())
+```
+
+### Custom Config
+
+```go
+app.Use(idempotency.New(idempotency.Config{
+	Lifetime: 42 * time.Minute,
+	// ...
+}))
+```
+
+### Config
+
+```go
+type Config struct {
+	// Next defines a function to skip this middleware when returned true.
+	//
+	// Optional. Default: nil
+	Next func(c fiber.Ctx) bool
+
+	// Lifetime is the maximum lifetime of an idempotency key.
+	//
+	// Optional. Default: 30 * time.Minute
+	Lifetime time.Duration
+
+	// KeyHeader is the name of the header that contains the idempotency key.
+	//
+	// Optional. Default: X-Idempotency-Key
+	KeyHeader string
+	// KeyHeader is the name of the header that contains the idempotency key.
+	//
+	// Optional. Default: a function which ensures the header is 36 characters long (size of UUID).
+	KeyHeaderValidate func(string) error
+
+	// KeepResponseHeaders is a list of headers that should be kept from the original response.
+	//
+	// Optional. Default: nil (to keep all headers)
+	KeepResponseHeaders    []string
+
+	// Lock locks an idempotency key.
+	//
+	// Optional. Default: an in-memory locker for this process only
+	Lock Locker
+	// Storage stores response data by idempotency key.
+	//
+	// Optional. Default: an in-memory storage for this process only
+	Storage Storage
+
+	// MarshalFunc is the function used to marshal a Response to a []byte.
+	//
+	// Optional. Default: a marshal function which uses "encoding/gob" from the Go standard library.
+	MarshalFunc func(Response) ([]byte, error)
+	// UnmarshalFunc is the function used to unmarshal a []byte back to a Response.
+	//
+	// Optional. Default: a marshal function which uses "encoding/gob" from the Go standard library.
+	UnmarshalFunc func([]byte, *Response) error
+}
+```
+
+### Default Config
+
+```go
+var ConfigDefault = Config{
+	Next: nil,
+
+	Lifetime: 30 * time.Minute,
+
+	KeyHeader: "X-Idempotency-Key",
+	KeyHeaderValidate: func(k string) error {
+		if l, wl := len(k), 36; l != wl { // UUID length is 36 chars
+			return fmt.Errorf("%w: invalid length: %d > %d", ErrInvalidIdempotencyKey, l, wl)
+		}
+
+		return nil
+	},
+
+	KeepResponseHeaders: nil,
+
+	Lock:    NewMemoryLocker(),
+	Storage: NewMemoryStorage(),
+
+	MarshalFunc: func(res Response) ([]byte, error) {
+		var buf bytes.Buffer
+		if err := gob.
+			NewEncoder(&buf).
+			Encode(res); err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	},
+	UnmarshalFunc: func(val []byte, res *Response) error {
+		if err := gob.
+			NewDecoder(bytes.NewReader(val)).
+			Decode(res); err != nil {
+			return err
+		}
+		return nil
+	},
+}
+```

middleware/idempotency/config.go

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+package idempotency
+
+import (
+	"bytes"
+	"encoding/gob"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/gofiber/fiber/v3"
+)
+
+var (
+	ErrInvalidIdempotencyKey = errors.New("invalid idempotency key")
+)
+
+// Config defines the config for middleware.
+type Config struct {
+	// Next defines a function to skip this middleware when returned true.
+	//
+	// Optional. Default: nil
+	Next func(c fiber.Ctx) bool
+
+	// Lifetime is the maximum lifetime of an idempotency key.
+	//
+	// Optional. Default: 30 * time.Minute
+	Lifetime time.Duration
+
+	// KeyHeader is the name of the header that contains the idempotency key.
+	//
+	// Optional. Default: X-Idempotency-Key
+	KeyHeader string
+	// KeyHeader is the name of the header that contains the idempotency key.
+	//
+	// Optional. Default: a function which ensures the header is 36 characters long (size of UUID).
+	KeyHeaderValidate func(string) error
+
+	// KeepResponseHeaders is a list of headers that should be kept from the original response.
+	//
+	// Optional. Default: nil (to keep all headers)
+	KeepResponseHeaders    []string
+	keepResponseHeadersMap map[string]struct{}
+
+	// Lock locks an idempotency key.
+	//
+	// Optional. Default: an in-memory locker for this process only
+	Lock Locker
+	// Storage stores response data by idempotency key.
+	//
+	// Optional. Default: an in-memory storage for this process only
+	Storage Storage
+
+	// MarshalFunc is the function used to marshal a Response to a []byte.
+	//
+	// Optional. Default: a marshal function which uses "encoding/gob" from the Go standard library.
+	MarshalFunc func(*Response) ([]byte, error)
+	// UnmarshalFunc is the function used to unmarshal a []byte back to a Response.
+	//
+	// Optional. Default: a marshal function which uses "encoding/gob" from the Go standard library.
+	UnmarshalFunc func([]byte, *Response) error
+}
+
+// ConfigDefault is the default config
+var ConfigDefault = Config{
+	Next: nil,
+
+	Lifetime: 30 * time.Minute,
+
+	KeyHeader: "X-Idempotency-Key",
+	KeyHeaderValidate: func(k string) error {
+		if l, wl := len(k), 36; l != wl { // UUID length is 36 chars
+			return fmt.Errorf("%w: invalid length: %d > %d", ErrInvalidIdempotencyKey, l, wl)
+		}
+
+		return nil
+	},
+
+	KeepResponseHeaders: nil,
+
+	Lock:    NewMemoryLock(),
+	Storage: NewMemoryStorage(),
+
+	MarshalFunc: func(res *Response) ([]byte, error) {
+		var buf bytes.Buffer
+		if err := gob.
+			NewEncoder(&buf).
+			Encode(res); err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	},
+	UnmarshalFunc: func(val []byte, res *Response) error {
+		if err := gob.
+			NewDecoder(bytes.NewReader(val)).
+			Decode(res); err != nil {
+			return err
+		}
+		return nil
+	},
+}
+
+// Helper function to set default values
+func configDefault(config ...Config) Config {
+	// Return default config if nothing provided
+	if len(config) < 1 {
+		return ConfigDefault
+	}
+
+	// Override default config
+	cfg := config[0]
+
+	// Set default values
+
+	if cfg.Next == nil {
+		cfg.Next = ConfigDefault.Next
+	}
+
+	if cfg.Lifetime.Nanoseconds() == 0 {
+		cfg.Lifetime = ConfigDefault.Lifetime
+	}
+
+	if cfg.KeyHeader == "" {
+		cfg.KeyHeader = ConfigDefault.KeyHeader
+	}
+	if cfg.KeyHeaderValidate == nil {
+		cfg.KeyHeaderValidate = ConfigDefault.KeyHeaderValidate
+	}
+
+	if cfg.KeepResponseHeaders != nil && len(cfg.KeepResponseHeaders) == 0 {
+		cfg.KeepResponseHeaders = ConfigDefault.KeepResponseHeaders
+	}
+
+	if cfg.Lock == nil {
+		cfg.Lock = ConfigDefault.Lock
+	}
+	if cfg.Storage == nil {
+		cfg.Storage = ConfigDefault.Storage
+	}
+
+	if cfg.MarshalFunc == nil {
+		cfg.MarshalFunc = ConfigDefault.MarshalFunc
+	}
+	if cfg.UnmarshalFunc == nil {
+		cfg.UnmarshalFunc = ConfigDefault.UnmarshalFunc
+	}
+
+	return cfg
+}