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@@ -43,6 +43,7 @@ Developer Notes
- [Suggestions and examples](#suggestions-and-examples)
- [Release notes](#release-notes)
- [RPC interface guidelines](#rpc-interface-guidelines)
+ - [Internal interface guidelines](#internal-interface-guidelines)
<!-- markdown-toc end -->
@@ -1100,3 +1101,124 @@ A few guidelines for introducing and reviewing new RPC interfaces:
timestamps in the documentation.
- *Rationale*: User-facing consistency.
+
+Internal interface guidelines
+-----------------------------
+
+Internal interfaces between parts of the codebase that are meant to be
+independent (node, wallet, GUI), are defined in
+[`src/interfaces/`](../src/interfaces/). The main interface classes defined
+there are [`interfaces::Chain`](../src/interfaces/chain.h), used by wallet to
+access the node's latest chain state,
+[`interfaces::Node`](../src/interfaces/node.h), used by the GUI to control the
+node, and [`interfaces::Wallet`](../src/interfaces/wallet.h), used by the GUI
+to control an individual wallet. There are also more specialized interface
+types like [`interfaces::Handler`](../src/interfaces/handler.h)
+[`interfaces::ChainClient`](../src/interfaces/chain.h) passed to and from
+various interface methods.
+
+Interface classes are written in a particular style so node, wallet, and GUI
+code doesn't need to run in the same process, and so the class declarations
+work more easily with tools and libraries supporting interprocess
+communication:
+
+- Interface classes should be abstract and have methods that are [pure
+ virtual](https://en.cppreference.com/w/cpp/language/abstract_class). This
+ allows multiple implementations to inherit from the same interface class,
+ particularly so one implementation can execute functionality in the local
+ process, and other implementations can forward calls to remote processes.
+
+- Interface method definitions should wrap existing functionality instead of
+ implementing new functionality. Any substantial new node or wallet
+ functionality should be implemented in [`src/node/`](../src/node/) or
+ [`src/wallet/`](../src/wallet/) and just exposed in
+ [`src/interfaces/`](../src/interfaces/) instead of being implemented there,
+ so it can be more modular and accessible to unit tests.
+
+- Interface method parameter and return types should either be serializable or
+ be other interface classes. Interface methods shouldn't pass references to
+ objects that can't be serialized or accessed from another process.
+
+ Examples:
+
+ ```c++
+ // Good: takes string argument and returns interface class pointer
+ virtual unique_ptr<interfaces::Wallet> loadWallet(std::string filename) = 0;
+
+ // Bad: returns CWallet reference that can't be used from another process
+ virtual CWallet& loadWallet(std::string filename) = 0;
+ ```
+
+ ```c++
+ // Good: accepts and returns primitive types
+ virtual bool findBlock(const uint256& hash, int& out_height, int64_t& out_time) = 0;
+
+ // Bad: returns pointer to internal node in a linked list inaccessible to
+ // other processes
+ virtual const CBlockIndex* findBlock(const uint256& hash) = 0;
+ ```
+
+ ```c++
+ // Good: takes plain callback type and returns interface pointer
+ using TipChangedFn = std::function<void(int block_height, int64_t block_time)>;
+ virtual std::unique_ptr<interfaces::Handler> handleTipChanged(TipChangedFn fn) = 0;
+
+ // Bad: returns boost connection specific to local process
+ using TipChangedFn = std::function<void(int block_height, int64_t block_time)>;
+ virtual boost::signals2::scoped_connection connectTipChanged(TipChangedFn fn) = 0;
+ ```
+
+- For consistency and friendliness to code generation tools, interface method
+ input and inout parameters should be ordered first and output parameters
+ should come last.
+
+ Example:
+
+ ```c++
+ // Good: error output param is last
+ virtual bool broadcastTransaction(const CTransactionRef& tx, CAmount max_fee, std::string& error) = 0;
+
+ // Bad: error output param is between input params
+ virtual bool broadcastTransaction(const CTransactionRef& tx, std::string& error, CAmount max_fee) = 0;
+ ```
+
+- For friendliness to code generation tools, interface methods should not be
+ overloaded:
+
+ Example:
+
+ ```c++
+ // Good: method names are unique
+ virtual bool disconnectByAddress(const CNetAddr& net_addr) = 0;
+ virtual bool disconnectById(NodeId id) = 0;
+
+ // Bad: methods are overloaded by type
+ virtual bool disconnect(const CNetAddr& net_addr) = 0;
+ virtual bool disconnect(NodeId id) = 0;
+ ```
+
+- For consistency and friendliness to code generation tools, interface method
+ names should be `lowerCamelCase` and standalone function names should be
+ `UpperCamelCase`.
+
+ Examples:
+
+ ```c++
+ // Good: lowerCamelCase method name
+ virtual void blockConnected(const CBlock& block, int height) = 0;
+
+ // Bad: uppercase class method
+ virtual void BlockConnected(const CBlock& block, int height) = 0;
+ ```
+
+ ```c++
+ // Good: UpperCamelCase standalone function name
+ std::unique_ptr<Node> MakeNode(LocalInit& init);
+
+ // Bad: lowercase standalone function
+ std::unique_ptr<Node> makeNode(LocalInit& init);
+ ```
+
+ Note: This last convention isn't generally followed outside of
+ [`src/interfaces/`](../src/interfaces/), though it did come up for discussion
+ before in [#14635](https://github.com/bitcoin/bitcoin/pull/14635).