docs: add documentation about getsigners (backport #17012)

docs/docs/building-modules/02-messages-and-queries.md

@@ -24,55 +24,69 @@ When a transaction is relayed from the underlying consensus engine to the Cosmos
 
 Defining Protobuf `Msg` services is the recommended way to handle messages. A Protobuf `Msg` service should be created for each module, typically in `tx.proto` (see more info about [conventions and naming](../core/05-encoding.md#faq)). It must have an RPC service method defined for each message in the module.
 
-See an example of a `Msg` service definition from `x/bank` module:
-
-```protobuf reference
-https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/proto/cosmos/bank/v1beta1/tx.proto#L13-L36
-```
 
 Each `Msg` service method must have exactly one argument, which must implement the `sdk.Msg` interface, and a Protobuf response. The naming convention is to call the RPC argument `Msg<service-rpc-name>` and the RPC response `Msg<service-rpc-name>Response`. For example:
 
 ```protobuf
   rpc Send(MsgSend) returns (MsgSendResponse);
 ```
 
-`sdk.Msg` interface is a simplified version of the Amino `LegacyMsg` interface described [below](#legacy-amino-msgs) with the `GetSigners()` method. For backwards compatibility with [Amino `LegacyMsg`s](#legacy-amino-msgs), existing `LegacyMsg` types should be used as the request parameter for `service` RPC definitions. Newer `sdk.Msg` types, which only support `service` definitions, should use canonical `Msg...` name.
-
-The Cosmos SDK uses Protobuf definitions to generate client and server code:
+See an example of a `Msg` service definition from `x/bank` module:
 
-* `MsgServer` interface defines the server API for the `Msg` service and its implementation is described as part of the [`Msg` services](./03-msg-services.md) documentation.
-* Structures are generated for all RPC request and response types.
+```protobuf reference
+https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/proto/cosmos/bank/v1beta1/tx.proto#L13-L36
+```
 
-A `RegisterMsgServer` method is also generated and should be used to register the module's `MsgServer` implementation in `RegisterServices` method from the [`AppModule` interface](./01-module-manager.md#appmodule).
+### `sdk.Msg` Interface
 
-In order for clients (CLI and grpc-gateway) to have these URLs registered, the Cosmos SDK provides the function `RegisterMsgServiceDesc(registry codectypes.InterfaceRegistry, sd *grpc.ServiceDesc)` that should be called inside module's [`RegisterInterfaces`](01-module-manager.md#appmodulebasic) method, using the proto-generated `&_Msg_serviceDesc` as `*grpc.ServiceDesc` argument.
+`sdk.Msg` is a alias of `proto.Message`. 
 
-### Legacy Amino `LegacyMsg`s
+To attach a `ValidateBasic()` method to a message then you must add methods to the type adhereing to the `HasValidateBasic`.
 
-The following way of defining messages is deprecated and using [`Msg` services](#msg-services) is preferred.
+```go reference
+https://github.com/cosmos/cosmos-sdk/blob/9c1e8b247cd47b5d3decda6e86fbc3bc996ee5d7/types/tx_msg.go#L84-L88
+```
 
-Amino `LegacyMsg`s can be defined as protobuf messages. The messages definition usually includes a list of parameters needed to process the message that will be provided by end-users when they want to create a new transaction containing said message.
+In 0.50+ signers from the `GetSigners()` call is automated via a protobuf annotation. 
 
-A `LegacyMsg` is typically accompanied by a standard constructor function, that is called from one of the [module's interface](./09-module-interfaces.md). `message`s also need to implement the `sdk.Msg` interface:
+<!-- TODO: remove this todo after https://github.com/cosmos/cosmos-sdk/pull/15548 is merged -->
 
-```go reference
-https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/types/tx_msg.go#L21-L28
+```protobuf reference 
+https://github.com/cosmos/cosmos-sdk/blob/e6848d99b55a65d014375b295bdd7f9641aac95e/proto/cosmos/bank/v1beta1/tx.proto#L41
 ```
 
-It extends `proto.Message` and contains the following methods:
+If there is a need for custom signers then there is an alternative path which can be taken. A function which returns `signing.CustomGetSigner` for a specific message can be defined. 
 
-* `GetSignBytes() []byte`: Return the canonical byte representation of the message. Used to generate a signature.
+```go
+func ProvideBankSendTransactionGetSigners() signing.CustomGetSigner {
 
-```go reference
-https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/x/auth/migrations/legacytx/stdsign.go#L21-L29
+			// Extract the signer from the signature.
+			signer, err := coretypes.LatestSigner(Tx).Sender(ethTx)
+      if err != nil {
+				return nil, err
+			}
+
+			// Return the signer in the required format.
+			return [][]byte{signer.Bytes()}, nil
+}
 ```
 
-See an example implementation of a `message` from the `gov` module:
+When using dependency injection (depinject) this can be provided to the application via the provide method.
 
-```go reference
-https://github.com/cosmos/cosmos-sdk/blob/v0.50.0-alpha.0/x/gov/types/v1/msgs.go#L103-L150
+```go
+depinject.Provide(banktypes.ProvideBankSendTransactionGetSigners)
 ```
 
+The Cosmos SDK uses Protobuf definitions to generate client and server code:
+
+* `MsgServer` interface defines the server API for the `Msg` service and its implementation is described as part of the [`Msg` services](./03-msg-services.md) documentation.
+* Structures are generated for all RPC request and response types.
+
+A `RegisterMsgServer` method is also generated and should be used to register the module's `MsgServer` implementation in `RegisterServices` method from the [`AppModule` interface](./01-module-manager.md#appmodule).
+
+In order for clients (CLI and grpc-gateway) to have these URLs registered, the Cosmos SDK provides the function `RegisterMsgServiceDesc(registry codectypes.InterfaceRegistry, sd *grpc.ServiceDesc)` that should be called inside module's [`RegisterInterfaces`](01-module-manager.md#appmodulebasic) method, using the proto-generated `&_Msg_serviceDesc` as `*grpc.ServiceDesc` argument.
+
+
 ## Queries
 
 A `query` is a request for information made by end-users of applications through an interface and processed by a full-node. A `query` is received by a full-node through its consensus engine and relayed to the application via the ABCI. It is then routed to the appropriate module via `BaseApp`'s `QueryRouter` so that it can be processed by the module's query service (./04-query-services.md). For a deeper look at the lifecycle of a `query`, click [here](../basics/02-query-lifecycle.md).