96 lines
5.5 KiB
Go
96 lines
5.5 KiB
Go
// Copyright (C) MongoDB, Inc. 2022-present.
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//
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// Licensed under the Apache License, Version 2.0 (the "License"); you may
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// not use this file except in compliance with the License. You may obtain
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// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
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// Package bsoncodec provides a system for encoding values to BSON representations and decoding
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// values from BSON representations. This package considers both binary BSON and ExtendedJSON as
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// BSON representations. The types in this package enable a flexible system for handling this
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// encoding and decoding.
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//
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// The codec system is composed of two parts:
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//
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// 1) ValueEncoders and ValueDecoders that handle encoding and decoding Go values to and from BSON
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// representations.
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//
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// 2) A Registry that holds these ValueEncoders and ValueDecoders and provides methods for
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// retrieving them.
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//
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// # ValueEncoders and ValueDecoders
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//
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// The ValueEncoder interface is implemented by types that can encode a provided Go type to BSON.
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// The value to encode is provided as a reflect.Value and a bsonrw.ValueWriter is used within the
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// EncodeValue method to actually create the BSON representation. For convenience, ValueEncoderFunc
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// is provided to allow use of a function with the correct signature as a ValueEncoder. An
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// EncodeContext instance is provided to allow implementations to lookup further ValueEncoders and
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// to provide configuration information.
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//
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// The ValueDecoder interface is the inverse of the ValueEncoder. Implementations should ensure that
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// the value they receive is settable. Similar to ValueEncoderFunc, ValueDecoderFunc is provided to
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// allow the use of a function with the correct signature as a ValueDecoder. A DecodeContext
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// instance is provided and serves similar functionality to the EncodeContext.
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//
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// # Registry
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//
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// A Registry is a store for ValueEncoders, ValueDecoders, and a type map. See the Registry type
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// documentation for examples of registering various custom encoders and decoders. A Registry can
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// have three main types of codecs:
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//
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// 1. Type encoders/decoders - These can be registered using the RegisterTypeEncoder and
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// RegisterTypeDecoder methods. The registered codec will be invoked when encoding/decoding a value
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// whose type matches the registered type exactly.
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// If the registered type is an interface, the codec will be invoked when encoding or decoding
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// values whose type is the interface, but not for values with concrete types that implement the
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// interface.
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//
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// 2. Hook encoders/decoders - These can be registered using the RegisterHookEncoder and
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// RegisterHookDecoder methods. These methods only accept interface types and the registered codecs
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// will be invoked when encoding or decoding values whose types implement the interface. An example
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// of a hook defined by the driver is bson.Marshaler. The driver will call the MarshalBSON method
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// for any value whose type implements bson.Marshaler, regardless of the value's concrete type.
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//
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// 3. Type map entries - This can be used to associate a BSON type with a Go type. These type
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// associations are used when decoding into a bson.D/bson.M or a struct field of type interface{}.
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// For example, by default, BSON int32 and int64 values decode as Go int32 and int64 instances,
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// respectively, when decoding into a bson.D. The following code would change the behavior so these
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// values decode as Go int instances instead:
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//
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// intType := reflect.TypeOf(int(0))
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// registry.RegisterTypeMapEntry(bsontype.Int32, intType).RegisterTypeMapEntry(bsontype.Int64, intType)
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//
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// 4. Kind encoder/decoders - These can be registered using the RegisterDefaultEncoder and
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// RegisterDefaultDecoder methods. The registered codec will be invoked when encoding or decoding
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// values whose reflect.Kind matches the registered reflect.Kind as long as the value's type doesn't
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// match a registered type or hook encoder/decoder first. These methods should be used to change the
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// behavior for all values for a specific kind.
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//
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// # Registry Lookup Procedure
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//
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// When looking up an encoder in a Registry, the precedence rules are as follows:
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//
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// 1. A type encoder registered for the exact type of the value.
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//
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// 2. A hook encoder registered for an interface that is implemented by the value or by a pointer to
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// the value. If the value matches multiple hooks (e.g. the type implements bsoncodec.Marshaler and
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// bsoncodec.ValueMarshaler), the first one registered will be selected. Note that registries
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// constructed using bson.NewRegistry have driver-defined hooks registered for the
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// bsoncodec.Marshaler, bsoncodec.ValueMarshaler, and bsoncodec.Proxy interfaces, so those will take
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// precedence over any new hooks.
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//
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// 3. A kind encoder registered for the value's kind.
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//
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// If all of these lookups fail to find an encoder, an error of type ErrNoEncoder is returned. The
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// same precedence rules apply for decoders, with the exception that an error of type ErrNoDecoder
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// will be returned if no decoder is found.
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//
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// # DefaultValueEncoders and DefaultValueDecoders
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//
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// The DefaultValueEncoders and DefaultValueDecoders types provide a full set of ValueEncoders and
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// ValueDecoders for handling a wide range of Go types, including all of the types within the
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// primitive package. To make registering these codecs easier, a helper method on each type is
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// provided. For the DefaultValueEncoders type the method is called RegisterDefaultEncoders and for
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// the DefaultValueDecoders type the method is called RegisterDefaultDecoders, this method also
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// handles registering type map entries for each BSON type.
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package bsoncodec
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