docs(adr): ADR-045: BaseApp {Check,Deliver}Tx as Middlewares (#9996)

## Description [rendered](https://github.com/cosmos/cosmos-sdk/blob/am/adr-045/docs/architecture/adr-045-check-delivertx-middlewares.md) Closes: #9953  --- ### Author Checklist *All items are required. Please add a note to the item if the item is not applicable and please add links to any relevant follow up issues.* I have... - [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title - [ ] added `!` to the type prefix if API or client breaking change - [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting)) - [ ] provided a link to the relevant issue or specification - [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules) - [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing) - [ ] added a changelog entry to `CHANGELOG.md` - [ ] included comments for [documenting Go code](https://blog.golang.org/godoc) - [ ] updated the relevant documentation or specification - [ ] reviewed "Files changed" and left comments if necessary - [ ] confirmed all CI checks have passed ### Reviewers Checklist *All items are required. Please add a note if the item is not applicable and please add your handle next to the items reviewed if you only reviewed selected items.* I have... - [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title - [ ] confirmed `!` in the type prefix if API or client breaking change - [ ] confirmed all author checklist items have been addressed - [ ] reviewed state machine logic - [ ] reviewed API design and naming - [ ] reviewed documentation is accurate - [ ] reviewed tests and test coverage - [ ] manually tested (if applicable)
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diff --git a/docs/architecture/PROCESS.md b/docs/architecture/PROCESS.md
@@ -35,7 +35,7 @@ IMPLEMENTATION STATUS is either `Implemented` or `Not Implemented`.
 #### Consensus Status
 
 ```
-DRAFT -> PROPOSED -> LAST CALL yyyy-mm-dd -> ACCEPTED | REJECTED -> SUPERSEEDED by ADR-xxx
+DRAFT -> PROPOSED -> LAST CALL yyyy-mm-dd -> ACCEPTED | REJECTED -> SUPERSEDED by ADR-xxx
                   \        |
                    \       |
                     v      v

diff --git a/docs/architecture/adr-010-modular-antehandler.md b/docs/architecture/adr-010-modular-antehandler.md
@@ -3,6 +3,11 @@
 ## Changelog
 
 - 2019 Aug 31: Initial draft
+- 2021 Sep 14: Superseded by ADR-045
+
+## Status
+
+SUPERSEDED by ADR-045
 
 ## Context
 

diff --git a/docs/architecture/adr-022-custom-panic-handling.md b/docs/architecture/adr-022-custom-panic-handling.md
@@ -3,6 +3,11 @@
 ## Changelog
 
 - 2020 Apr 24: Initial Draft
+- 2021 Sep 14: Superseded by ADR-045
+
+## Status
+
+SUPERSEDED by ADR-045
 
 ## Context
 

diff --git a/docs/architecture/adr-045-check-delivertx-middlewares.md b/docs/architecture/adr-045-check-delivertx-middlewares.md
@@ -0,0 +1,256 @@
+# ADR 045: BaseApp `{Check,Deliver}Tx` as Middlewares
+
+## Changelog
+
+- 20.08.2021: Initial draft.
+
+## Status
+
+ACCEPTED
+
+## Abstract
+
+This ADR replaces the current BaseApp `runTx` and antehandlers design with a middleware-based design.
+
+## Context
+
+BaseApp's implementation of ABCI `{Check,Deliver}Tx()` and its own `Simulate()` method call the `runTx` method under the hood, which first runs antehandlers, then executes `Msg`s. However, the [transaction Tips](https://github.com/cosmos/cosmos-sdk/issues/9406) and [refunding unused gas](https://github.com/cosmos/cosmos-sdk/issues/2150) use cases require custom logic to be run after the `Msg`s execution. There is currently no way to achieve this.
+
+An naive solution would be to add post-`Msg` hooks to BaseApp. However, the SDK team thinks in parallel about the bigger picture of making app wiring simpler ([#9181](https://github.com/cosmos/cosmos-sdk/discussions/9182)), which includes making BaseApp more lightweight and modular.
+
+## Decision
+
+We decide to transform Baseapp's implementation of ABCI `{Check,Deliver}Tx` and its own `Simulate` methods to use a middleware-based design.
+
+The two following interfaces are the base of the middleware design, and are defined in `types/tx`:
+
+```go
+type Handler interface {
+    CheckTx(ctx context.Context, tx sdk.Tx, req abci.RequestCheckTx) (abci.ResponseCheckTx, error)
+    DeliverTx(ctx context.Context, tx sdk.Tx, req abci.RequestDeliverTx) (abci.ResponseDeliverTx, error)
+    SimulateTx(ctx context.Context, tx sdk.Tx, req RequestSimulateTx) (ResponseSimulateTx, error)
+}
+
+type Middleware func(Handler) Handler
+```
+
+BaseApp holds a reference to a `tx.Handler`:
+
+```go
+type BaseApp  struct {
+    // other fields
+    txHandler tx.Handler
+}
+```
+
+Baseapp's ABCI `{Check,Deliver}Tx()` and `Simulate()` methods simply call `app.txHandler.{Check,Deliver,Simulate}Tx()` with the relevant arguments. For example, for `DeliverTx`:
+
+```go
+func (app *BaseApp) DeliverTx(req abci.RequestDeliverTx) abci.ResponseDeliverTx {
+    tx, err := app.txDecoder(req.Tx)
+    if err != nil {
+        return sdkerrors.ResponseDeliverTx(err, 0, 0, app.trace)
+    }
+
+    ctx := app.getContextForTx(runTxModeDeliver, req.Tx)
+    res, err := app.txHandler.DeliverTx(ctx, tx, req)
+    if err != nil {
+        return sdkerrors.ResponseDeliverTx(err, uint64(res.GasUsed), uint64(res.GasWanted), app.trace)
+    }
+
+    return res
+}
+```
+
+The implementations are similar for `BaseApp.CheckTx` and `BaseApp.Simulate`.
+
+`baseapp.txHandler`'s three methods' implementations can obviously be monolithic functions, but for modularity we propose a middleware composition design, where a middleware is simply a function that takes a `tx.Handler`, and returns another `tx.Handler` wrapped around the previous one.
+
+### Implementing a Middleware
+
+In practice, middlewares are created by Go function that takes as arguments some parameters needed for the middleware, and returns a `tx.Middleware`.
+
+For example, for creating an arbitrary `MyMiddleware`, we can implement:
+
+```go
+// myTxHandler is the tx.Handler of this middleware. Note that it holds a
+// reference to the next tx.Handler in the stack.
+type myTxHandler struct {
+    // next is the next tx.Handler in the middleware stack.
+    next tx.Handler
+    // some other fields that are relevant to the middleware can be added here
+}
+
+// NewMyMiddleware returns a middleware that does this and that.
+func NewMyMiddleware(arg1, arg2) tx.Middleware {
+    return func (txh tx.Handler) tx.Handler {
+        return myTxHandler{
+            next: txh,
+            // optionally, set arg1, arg2... if they are needed in the middleware
+        }
+    }
+}
+
+// Assert myTxHandler is a tx.Handler.
+var _ tx.Handler = myTxHandler{}
+
+func (txh myTxHandler) CheckTx(ctx context.Context, tx sdk.Tx, req abci.RequestCheckTx) (abci.ResponseCheckTx, error) {
+    // CheckTx specific pre-processing logic
+
+    // run the next middleware
+    res, err := txh.next.CheckTx(ctx, tx, req)
+
+    // CheckTx specific post-processing logic
+
+    return res, err
+}
+
+func (txh myTxHandler) DeliverTx(ctx context.Context, tx sdk.Tx, req abci.RequestDeliverTx) (abci.ResponseDeliverTx, error) {
+    // DeliverTx specific pre-processing logic
+
+    // run the next middleware
+    res, err := txh.next.DeliverTx(ctx, tx, req)
+
+    // DeliverTx specific post-processing logic
+
+    return res, err
+}
+
+func (txh myTxHandler) SimulateTx(ctx context.Context, tx sdk.Tx, req tx.RequestSimulateTx) (abci.ResponseSimulateTx, error) {
+    // SimulateTx specific pre-processing logic
+
+    // run the next middleware
+    res, err := txh.next.SimulateTx(ctx, tx, req)
+
+    // SimulateTx specific post-processing logic
+
+    return res, err
+}
+```
+
+### Composing Middlewares
+
+While BaseApp simply holds a reference to a `tx.Handler`, this `tx.Handler` itself is defined using a middleware stack. The SDK exposes a base (i.e. innermost) `tx.Handler` called `RunMsgsTxHandler`, which executes messages.
+
+Then, the app developer can compose multiple middlewares on top on the base `tx.Handler`. Each middleware can run pre-and-post-processing logic around its next middleware, as described in the section above. Conceptually, as an example, given the middlewares `A`, `B`, and `C` and the base `tx.Handler` `H` the stack looks like:
+
+```
+A.pre
+    B.pre
+        C.pre
+            H # The base tx.handler, for example `RunMsgsTxHandler`
+        C.post
+    B.post
+A.post
+```
+
+We define a `ComposeMiddlewares` function for composing middlewares. It takes the base handler as first argument, and middlewares in the "outer to inner" order. For the above stack, the final `tx.Handler` is:
+
+```go
+txHandler := middleware.ComposeMiddlewares(H, A, B, C)
+```
+
+The middleware is set in BaseApp via its `SetTxHandler` setter:
+
+```go
+// simapp/app.go
+
+txHandler := middleware.ComposeMiddlewares(...)
+app.SetTxHandler(txHandler)
+```
+
+The app developer can define their own middlewares, or use the SDK's pre-defined middlewares from `middleware.NewDefaultTxHandler()`.
+
+### Middlewares Maintained by the SDK
+
+While the app developer can define and compose the middlewares of their choice, the SDK provides a set of middlewares that caters for the ecosystem's most common use cases. These middlewares are:
+
+| Middleware              | Description                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
+| ----------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| RunMsgsTxHandler        | This is the base `tx.Handler`. It replaces the old baseapp's `runMsgs`, and executes a transaction's `Msg`s.                                                                                                                                                                                                                                                                                                                                                                             |
+| {Antehandlers}          | Each antehandler is converted to its own middleware. These middlewares perform signature verification, fee deductions and other validations on the incoming transaction.                                                                                                                                                                                                                                                                                                                 |
+| IndexEventsTxMiddleware | This is a simple middleware that chooses which events to index in Tendermint. Replaces `baseapp.indexEvents` (which unfortunately still exists in baseapp too, because it's used to index Begin/EndBlock events)                                                                                                                                                                                                                                                                         |
+| RecoveryTxMiddleware    | This index recovers from panics. It replaces baseapp.runTx's panic recovery described in [ADR-022](./adr-022-custom-panic-handling.md).                                                                                                                                                                                                                                                                                                                                                  |
+| GasTxMiddleware         | This replaces the [`Setup`](https://github.com/cosmos/cosmos-sdk/blob/v0.43.0/x/auth/ante/setup.go) Antehandler. It sets a GasMeter on sdk.Context. Note that before, GasMeter was set on sdk.Context inside the antehandlers, and there was some mess around the fact that antehandlers had their own panic recovery system so that the GasMeter could be read by baseapp's recovery system. Now, this mess is all removed: one middleware sets GasMeter, another one handles recovery. |
+
+### Similarities and Differences between Antehandlers and Middlewares
+
+The middleware-based design builds upon the existing antehandlers design described in [ADR-010](./adr-010-modular-antehandler.md). Even though the final decision of ADR-010 was to go with the "Simple Decorators" approach, the middleware design is actually very similar to the other [Decorator Pattern](./adr-010-modular-antehandler.md#decorator-pattern) proposal, also used in [weave](https://github.com/iov-one/weave).
+
+#### Similarities with Antehandlers
+
+- Designed as chaining/composing small modular pieces.
+- Allow code reuse for `{Check,Deliver}Tx` and for `Simulate`.
+- Set up in `app.go`, and easily customizable by app developers.
+
+#### Differences with Antehandlers
+
+- The Antehandlers are run before `Msg` execution, whereas middlewares can run before and after.
+- The middleware approach uses separate methods for `{Check,Deliver,Simulate}Tx`, whereas the antehandlers pass a `simulate bool` flag and uses the `sdkCtx.Is{Check,Recheck}Tx()` flags to determine in which transaction mode we are.
+- The middleware design lets each middleware hold a reference to the next middleware, whereas the antehandlers pass a `next` argument in the `AnteHandle` method.
+- The middleware design use Go's standard `context.Context`, whereas the antehandlers use `sdk.Context`.
+
+## Consequences
+
+### Backwards Compatibility
+
+Since this refactor removes some logic away from BaseApp and into middlewares, it introduces API-breaking changes for app developers. Most notably, instead of creating an antehandler chain in `app.go`, app developers need to create a middleware stack:
+
+```diff
+- anteHandler, err := ante.NewAnteHandler(
+-    ante.HandlerOptions{
+-        AccountKeeper:   app.AccountKeeper,
+-        BankKeeper:      app.BankKeeper,
+-        SignModeHandler: encodingConfig.TxConfig.SignModeHandler(),
+-        FeegrantKeeper:  app.FeeGrantKeeper,
+-        SigGasConsumer:  ante.DefaultSigVerificationGasConsumer,
+-    },
+-)
++txHandler, err := authmiddleware.NewDefaultTxHandler(authmiddleware.TxHandlerOptions{
++    Debug:             app.Trace(),
++    IndexEvents:       indexEvents,
++    LegacyRouter:      app.legacyRouter,
++    MsgServiceRouter:  app.msgSvcRouter,
++    LegacyAnteHandler: anteHandler,
++})
+if err != nil {
+    panic(err)
+}
+- app.SetAnteHandler(anteHandler)
++ app.SetTxHandler(txHandler)
+```
+
+Other more minor API breaking changes will also be provided in the CHANGELOG. As usual, the SDK will provide a release migration document for app developers.
+
+This ADR does not introduce any state-machine-, client- or CLI-breaking changes.
+
+### Positive
+
+- Allow custom logic to be run before an after `Msg` execution. This enables the [tips](https://github.com/cosmos/cosmos-sdk/issues/9406) and [gas refund](https://github.com/cosmos/cosmos-sdk/issues/2150) uses cases, and possibly other ones.
+- Make BaseApp more lightweight, and defer complex logic to small modular components.
+- Separate paths for `{Check,Deliver,Simulate}Tx` with different returns types. This allows for improved readability (replace `if sdkCtx.IsRecheckTx() && !simulate {...}` with separate methods) and more flexibility (e.g. returning a `priority` in `abci.ResponseCheckTx`).
+
+### Negative
+
+- It is hard to understand at first glance the state updates that would occur after a middleware runs given the `sdk.Context` and `tx`. A middleware can have an arbitrary number of nested middleware being called within its function body, each possibly doing some pre- and post-processing before calling the next middleware on the chain. Thus to understand what a middleware is doing, one must also understand what every other middleware further along the chain is also doing. This can get quite complicated to understand.
+- API-breaking changes for app developers.
+
+### Neutral
+
+No neutral consequences.
+
+## Further Discussions
+
+- [#9934](https://github.com/cosmos/cosmos-sdk/discussions/9934) Decomposing BaseApp's other ABCI methods into middlewares.
+- Replace `sdk.Tx` interface with the concrete protobuf Tx type in the `tx.Handler` methods signature.
+
+## Test Cases
+
+We update the existing baseapp and antehandlers tests to use the new middleware API, but keep the same test cases and logic, to avoid introducing regressions. Existing CLI tests will also be left untouched.
+
+For new middlewares, we introduce unit tests. Since middlewares are purposefully small, unit tests suit well.
+
+## References
+
+- Initial discussion: https://github.com/cosmos/cosmos-sdk/issues/9585
+- Implementation: [#9920 BaseApp refactor](https://github.com/cosmos/cosmos-sdk/pull/9920) and [#10028 Antehandlers migration](https://github.com/cosmos/cosmos-sdk/pull/10028)
diff --git a/x/auth/middleware/middleware.go b/x/auth/middleware/middleware.go
@@ -6,7 +6,7 @@ import (
 )
 
 // ComposeMiddlewares compose multiple middlewares on top of a tx.Handler. The
-// middleware order in the variadic arguments is from inside to outside.
+// middleware order in the variadic arguments is from outer to inner.
 //
 // Example: Given a base tx.Handler H, and two middlewares A and B, the
 // middleware stack:
-Original file line number
+Diff line change
@@ Expand Up @@
     #### Consensus Status
     ```
-    DRAFT -> PROPOSED -> LAST CALL yyyy-mm-dd -> ACCEPTED | REJECTED -> SUPERSEEDED by ADR-xxx
+    DRAFT -> PROPOSED -> LAST CALL yyyy-mm-dd -> ACCEPTED | REJECTED -> SUPERSEDED by ADR-xxx
                       \        |
                        \       |
                         v      v
@@ Expand Down @@