Make a multicodec.Registry type available.

linking/cid/linksystem.go

@@ -10,12 +10,28 @@ import (
 	"github.com/ipld/go-ipld-prime/multicodec"
 )
 
+// DefaultLinkSystem returns an ipld.LinkSystem which uses cidlink.Link for ipld.Link.
+// During selection of encoders, decoders, and hashers, it examines the multicodec indicator numbers and multihash indicator numbers from the CID,
+// and uses the default global multicodec registry (see the go-ipld-prime/multicodec package) for resolving codec implementations,
+// and the default global multihash registry (see the go-multihash/core package) for resolving multihash implementations.
+//
+// No storage functions are present in the returned LinkSystem.
+// The caller can assign those themselves as desired.
 func DefaultLinkSystem() ipld.LinkSystem {
+	return LinkSystemUsingMulticodecRegistry(multicodec.DefaultRegistry)
+}
+
+// LinkSystemUsingMulticodecRegistry is similar to DefaultLinkSystem, but accepts a multicodec.Registry as a parameter.
+//
+// This can help create a LinkSystem which uses different multicodec implementations than the global registry.
+// (Sometimes this can be desired if you want some parts of a program to support a more limited suite of codecs than other parts of the program,
+// or needed to use a different multicodec registry than the global one for synchronization purposes, or etc.)
+func LinkSystemUsingMulticodecRegistry(mcReg multicodec.Registry) ipld.LinkSystem {
 	return ipld.LinkSystem{
 		EncoderChooser: func(lp ipld.LinkPrototype) (ipld.Encoder, error) {
 			switch lp2 := lp.(type) {
 			case LinkPrototype:
-				fn, err := multicodec.LookupEncoder(lp2.GetCodec())
+				fn, err := mcReg.LookupEncoder(lp2.GetCodec())
 				if err != nil {
 					return nil, err
 				}
@@ -28,7 +44,7 @@ func DefaultLinkSystem() ipld.LinkSystem {
 			lp := lnk.Prototype()
 			switch lp2 := lp.(type) {
 			case LinkPrototype:
-				fn, err := multicodec.LookupDecoder(lp2.GetCodec())
+				fn, err := mcReg.LookupDecoder(lp2.GetCodec())
 				if err != nil {
 					return nil, err
 				}

multicodec/defaultRegistry.go

@@ -0,0 +1,85 @@
+package multicodec
+
+import (
+	"github.com/ipld/go-ipld-prime"
+)
+
+// DefaultRegistry is a multicodec.Registry instance which is global to the program,
+// and is used as a default set of codecs.
+//
+// Some systems (for example, cidlink.DefaultLinkSystem) will use this default registry,
+// which makes it easier to write programs that pass fewer explicit arguments around.
+// However, these are *only* for default behaviors;
+// variations of functions which allow explicit non-default options should always be available
+// (for example, cidlink also has other LinkSystem constructor functions which accept an explicit multicodec.Registry,
+// and the LookupEncoder and LookupDecoder functions in any LinkSystem can be replaced).
+//
+// Since this registry is global, mind that there are also some necessary tradeoffs and limitations:
+// It can be difficult to control exactly what's present in this global registry
+// (Libraries may register codecs in this registry as a side-effect of importing, so even transitive dependencies can affect its content!).
+// Also, this registry is only considered safe to modify at package init time.
+// If these are concerns for your program, you can create your own multicodec.Registry values,
+// and eschew using the global default.
+var DefaultRegistry = Registry{}
+
+// RegisterEncoder updates the global DefaultRegistry to map a multicodec indicator number to the given ipld.Encoder function.
+// The encoder functions registered can be subsequently looked up using LookupEncoder.
+// It is a shortcut to the RegisterEncoder method on the global DefaultRegistry.
+//
+// Packages which implement an IPLD codec and have a multicodec number associated with them
+// are encouraged to register themselves at package init time using this function.
+// (Doing this at package init time ensures the default global registry is populated
+// without causing race conditions for application code.)
+//
+// No effort is made to detect conflicting registrations in this map.
+// If your dependency tree is such that this becomes a problem,
+// there are two ways to address this:
+// If RegisterEncoder is called with the same indicator code more than once, the last call wins.
+// In practice, this means that if an application has a strong opinion about what implementation for a certain codec,
+// then this can be done by making a Register call with that effect at init time in the application's main package.
+// This should have the desired effect because the root of the import tree has its init time effect last.
+// Alternatively, one can just avoid use of this registry entirely:
+// do this by making a LinkSystem that uses a custom EncoderChooser function.
+func RegisterEncoder(indicator uint64, encodeFunc ipld.Encoder) {
+	DefaultRegistry.RegisterEncoder(indicator, encodeFunc)
+}
+
+// LookupEncoder yields an ipld.Encoder function matching a multicodec indicator code number.
+// It is a shortcut to the LookupEncoder method on the global DefaultRegistry.
+//
+// To be available from this lookup function, an encoder must have been registered
+// for this indicator number by an earlier call to the RegisterEncoder function.
+func LookupEncoder(indicator uint64) (ipld.Encoder, error) {
+	return DefaultRegistry.LookupEncoder(indicator)
+}
+
+// RegisterDecoder updates the global DefaultRegistry a map a multicodec indicator number to the given ipld.Decoder function.
+// The decoder functions registered can be subsequently looked up using LookupDecoder.
+// It is a shortcut to the RegisterDecoder method on the global DefaultRegistry.
+//
+// Packages which implement an IPLD codec and have a multicodec number associated with them
+// are encouraged to register themselves in this map at package init time.
+// (Doing this at package init time ensures the default global registry is populated
+// without causing race conditions for application code.)
+//
+// No effort is made to detect conflicting registrations in this map.
+// If your dependency tree is such that this becomes a problem,
+// there are two ways to address this:
+// If RegisterDecoder is called with the same indicator code more than once, the last call wins.
+// In practice, this means that if an application has a strong opinion about what implementation for a certain codec,
+// then this can be done by making a Register call with that effect at init time in the application's main package.
+// This should have the desired effect because the root of the import tree has its init time effect last.
+// Alternatively, one can just avoid use of this registry entirely:
+// do this by making a LinkSystem that uses a custom DecoderChooser function.
+func RegisterDecoder(indicator uint64, decodeFunc ipld.Decoder) {
+	DefaultRegistry.RegisterDecoder(indicator, decodeFunc)
+}
+
+// LookupDecoder yields an ipld.Decoder function matching a multicodec indicator code number.
+// It is a shortcut to the LookupDecoder method on the global DefaultRegistry.
+//
+// To be available from this lookup function, an decoder must have been registered
+// for this indicator number by an earlier call to the RegisterDecoder function.
+func LookupDecoder(indicator uint64) (ipld.Decoder, error) {
+	return DefaultRegistry.LookupDecoder(indicator)
+}

multicodec/registry.go

@@ -0,0 +1,83 @@
+package multicodec
+
+import (
+	"fmt"
+
+	"github.com/ipld/go-ipld-prime"
+)
+
+// Registry is a structure for storing mappings of multicodec indicator numbers to ipld.Encoder and ipld.Decoder functions.
+//
+// The most typical usage of this structure is in combination with an ipld.LinkSystem.
+// For example, a linksystem using CIDs and a custom multicodec registry can be constructed
+// using cidlink.LinkSystemUsingMulticodecRegistry.
+//
+// Registry includes no mutexing.  If using Registry in a concurrent context, you must handle synchronization yourself.
+// (Typically, it is recommended to do initialization earlier in a program, before fanning out goroutines;
+// this avoids the need for mutexing overhead.)
+//
+// go-ipld-prime also has a default registry, which has the same methods as this structure, but are at package scope.
+// Some systems, like cidlink.DefaultLinkSystem, will use this default registry.
+// However, this default registry is global to the entire program.
+// This Registry type is for helping if you wish to make your own registry which does not share that global state.
+//
+// Multicodec indicator numbers are specified in
+// https://github.com/multiformats/multicodec/blob/master/table.csv .
+// You should not use indicator numbers which are not specified in that table
+// (however, there is nothing in this implementation that will attempt to stop you, either; please behave).
+type Registry struct {
+	encoders map[uint64]ipld.Encoder
+	decoders map[uint64]ipld.Decoder
+}
+
+func (r *Registry) ensureInit() {
+	if r.encoders != nil {
+		return
+	}
+	r.encoders = make(map[uint64]ipld.Encoder)
+	r.decoders = make(map[uint64]ipld.Decoder)
+}
+
+// RegisterEncoder updates a simple map of multicodec indicator number to ipld.Encoder function.
+// The encoder functions registered can be subsequently looked up using LookupEncoder.
+func (r *Registry) RegisterEncoder(indicator uint64, encodeFunc ipld.Encoder) {
+	r.ensureInit()
+	if encodeFunc == nil {
+		panic("not sensible to attempt to register a nil function")
+	}
+	r.encoders[indicator] = encodeFunc
+}
+
+// LookupEncoder yields an ipld.Encoder function matching a multicodec indicator code number.
+//
+// To be available from this lookup function, an encoder must have been registered
+// for this indicator number by an earlier call to the RegisterEncoder function.
+func (r *Registry) LookupEncoder(indicator uint64) (ipld.Encoder, error) {
+	encodeFunc, exists := r.encoders[indicator]
+	if !exists {
+		return nil, fmt.Errorf("no encoder registered for multicodec code %d (0x%x)", indicator, indicator)
+	}
+	return encodeFunc, nil
+}
+
+// RegisterDecoder updates a simple map of multicodec indicator number to ipld.Decoder function.
+// The decoder functions registered can be subsequently looked up using LookupDecoder.
+func (r *Registry) RegisterDecoder(indicator uint64, decodeFunc ipld.Decoder) {
+	r.ensureInit()
+	if decodeFunc == nil {
+		panic("not sensible to attempt to register a nil function")
+	}
+	r.decoders[indicator] = decodeFunc
+}
+
+// LookupDecoder yields an ipld.Decoder function matching a multicodec indicator code number.
+//
+// To be available from this lookup function, an decoder must have been registered
+// for this indicator number by an earlier call to the RegisterDecoder function.
+func (r *Registry) LookupDecoder(indicator uint64) (ipld.Decoder, error) {
+	decodeFunc, exists := r.decoders[indicator]
+	if !exists {
+		return nil, fmt.Errorf("no decoder registered for multicodec code %d (0x%x)", indicator, indicator)
+	}
+	return decodeFunc, nil
+}