Developer Notes

Coding Style

Various coding styles have been used during the history of the codebase, and the result is not very consistent. However, we're now trying to converge to a single style, so please use it in new code. Old code will be converted gradually and you are encouraged to use the provided clang-format-diff script to clean up the patch automatically before submitting a pull request.

Basic rules specified in src/.clang-format.
- Braces on new lines for namespaces, classes, functions, methods.
- Braces on the same line for everything else.
- 4 space indentation (no tabs) for every block except namespaces.
- No indentation for public/protected/private or for namespace.
- No extra spaces inside parenthesis; don't do ( this )
- No space after function names; one space after if, for and while.
- Always add braces for block statements (e.g. if, for, while).
- ++i is preferred over i++.
- static_assert is preferred over assert where possible. Generally; compile-time checking is preferred over run-time checking.
- Use CamelCase for functions/methods, and lowerCamelCase for variables.
  - GLOBAL_CONSTANTS should use UPPER_SNAKE_CASE.
  - namespaces should use lower_snake_case.
- Function names should generally start with an English command-form verb (e.g. ValidateTransaction, AddTransactionToMempool, ConnectBlock)
- Variable names should generally be nouns or past/future tense verbs. (e.g. canDoThing, signatureOperations, didThing)
- Avoid using globals, remove existing globals whenever possible.
- Class member variable names should be prepended with m_
- DO choose easily readable identifier names.
- DO favor readability over brevity.
- DO NOT use Hungarian notation.
- DO NOT use abbreviations or contractions within identifiers.
  - WRONG: mempool
  - RIGHT: MemoryPool
  - WRONG: ChangeDir
  - RIGHT: ChangeDirectory
- DO NOT use obscure acronyms, DO uppercase any acronyms.
- FINALLY, do not migrate existing code unless refactoring. It makes forwarding-porting from Bitcoin Core and Bitcoin ABC more difficult.

The naming convention roughly mirrors Microsoft Naming Conventions

C++ Coding Standards should strive to follow the LLVM Coding Standards

Code style example:

// namespaces should be lower_snake_case
namespace foo_bar_bob {

/**
 * Class is used for doing classy things.  All classes should
 * have a doxygen comment describing their PURPOSE.  That is to say,
 * why they exist.  Functional details can be determined from the code.
 * @see PerformTask()
 */
class Class {
private:
    //! memberVariable's name should be lowerCamelCase, and be a noun.
    int m_memberVariable;

public:
    /**
    * The documentation before a function or class method should follow Doxygen
    * spec. The name of the function should start with an English verb which
    * indicates the intended purpose of this code.
    *
    * The function name should be should be CamelCase.
    *
    * @param[in] s    A description
    * @param[in] n    Another argument description
    * @pre Precondition for function...
    */
    bool PerformTask(const std::string& s, int n) {
        // Use lowerChamelCase for local variables.
        bool didMore = false;

        // Comment summarizing the intended purpose of this section of code
        for (int i = 0; i < n; ++i) {
            if (!DidSomethingFail()) {
              return false;
            }
            ...
            if (IsSomethingElse()) {
                DoMore();
                didMore = true;
            } else {
                DoLess();
            }
        }

        return didMore;
    }
}
} // namespace foo

Doxygen comments

To facilitate the generation of documentation, use doxygen-compatible comment blocks for functions, methods and fields.

For example, to describe a function use:

/**
 * ... text ...
 * @param[in] arg1    A description
 * @param[in] arg2    Another argument description
 * @pre Precondition for function...
 */
bool function(int arg1, const char *arg2)

A complete list of @xxx commands can be found at https://www.doxygen.nl/manual/commands.html. As Doxygen recognizes the comments by the delimiters (/** and */ in this case), you don't need to provide any commands for a comment to be valid; just a description text is fine.

To describe a class use the same construct above the class definition:

/**
 * Alerts are for notifying old versions if they become too obsolete and
 * need to upgrade. The message is displayed in the status bar.
 * @see GetWarnings()
 */
class CAlert
{

To describe a member or variable use:

int var; //!< Detailed description after the member

or

//! Description before the member
int var;

Also OK:

///
/// ... text ...
///
bool function2(int arg1, const char *arg2)

Not OK (used plenty in the current source, but not picked up):

//
// ... text ...
//

A full list of comment syntaxes picked up by doxygen can be found at https://www.doxygen.nl/manual/docblocks.html, but if possible use one of the above styles.

To build doxygen locally to test changes to the Doxyfile or visualize your comments before landing changes:

# In the build directory, call:
doxygen doc/Doxyfile
# output goes to doc/doxygen/html/

Development tips and tricks

Compiling for debugging

Run cmake with -DCMAKE_BUILD_TYPE=Debug to add additional compiler flags that produce better debugging builds.

Compiling for gprof profiling

  cmake -GNinja .. -DENABLE_HARDENING=OFF -DENABLE_PROFIILING=gprof

debug.log

If the code is behaving strangely, take a look in the debug.log file in the data directory; error and debugging messages are written there.

The -debug=... command-line option controls debugging; running with just -debug or -debug=1 will turn on all categories (and give you a very large debug.log file).

The Qt code routes qDebug() output to debug.log under category "qt": run with -debug=qt to see it.

Writing tests

For details on unit tests, see unit-tests.md

For details on functional tests, see functional-tests.md

Writing script integration tests

Script integration tests are built using src/test/script_tests.cpp:

Uncomment the line with #define UPDATE_JSON_TESTS
Add a new TestBuilder to the script_build test to cover your test case.
ninja check-bitcoin-script_tests
Copy your newly generated test JSON from <build-dir>/src/script_tests.json.gen to src/test/data/script_tests.json.

Please commit your TestBuilder along with your generated test JSON and cleanup the uncommented #define before code review.

Testnet and Regtest modes

Run with the -testnet option to run with "play bitcoins" on the test network, if you are testing multi-machine code that needs to operate across the internet.

If you are testing something that can run on one machine, run with the -regtest option. In regression test mode, blocks can be created on-demand; see test/functional/ for tests that run in -regtest mode.

DEBUG_LOCKORDER

Bitcoin Cash Node is a multi-threaded application, and deadlocks or other multi-threading bugs can be very difficult to track down. The -DCMAKE_BUILD_TYPE=Debug cmake option adds -DDEBUG_LOCKORDER to the compiler flags. This inserts run-time checks to keep track of which locks are held, and adds warnings to the debug.log file if inconsistencies are detected.

Valgrind suppressions file

Valgrind is a programming tool for memory debugging, memory leak detection, and profiling. The repo contains a Valgrind suppressions file (valgrind.supp) which includes known Valgrind warnings in our dependencies that cannot be fixed in-tree. Example use:

$ valgrind --suppressions=contrib/valgrind.supp src/test/test_bitcoin
$ valgrind --suppressions=contrib/valgrind.supp --leak-check=full \
      --show-leak-kinds=all src/test/test_bitcoin --log_level=test_suite
$ valgrind -v --leak-check=full src/bitcoind -printtoconsole

Compiling for test coverage

LCOV can be used to generate a test coverage report based upon some test targets execution. Some packages are required to generate the coverage report: c++filt, gcov, genhtml, lcov and python3.

To install these dependencies on Debian 10:

sudo apt install binutils-common g++ lcov python3

To enable LCOV report generation during test runs:

cmake -GNinja .. -DENABLE_COVERAGE=ON
ninja coverage-check-all

A coverage report will now be accessible at ./check-all.coverage/index.html.

To include branch coverage, you can add the -DENABLE_BRANCH_COVERAGE=ON option to the cmake command line.

Sanitizers

Bitcoin Cash Node can be compiled with various "sanitizers" enabled, which add instrumentation for issues regarding things like memory safety, thread race conditions, or undefined behavior. This is controlled with the -DENABLE_SANITIZERS cmake flag, which should be a semicolon separated list of sanitizers to enable. The sanitizer list should correspond to supported -fsanitize= options in your compiler. These sanitizers have runtime overhead, so they are most useful when testing changes or producing debugging builds.

Some examples:

# Enable both the address sanitizer and the undefined behavior sanitizer
cmake -GNinja .. -DENABLE_SANITIZERS="address;undefined"

# Enable the thread sanitizer
cmake -GNinja .. -DENABLE_SANITIZERS=thread

If you are compiling with GCC you will typically need to install corresponding "san" libraries to actually compile with these flags, e.g. libasan for the address sanitizer, libtsan for the thread sanitizer, and libubsan for the undefined sanitizer. If you are missing required libraries, the cmake script will fail with an error when testing the sanitizer flags.

Note that the sanitizers will give a better output if they are run with a Debug build configuration.

There are a number of known problems for which suppressions files are provided under test/sanitizer_suppressions. These files are intended to be used with the suppressions option from the sanitizers. If you are using the check-* targets to run the tests, the suppression options are automatically set. Otherwise they need to be set manually using environment variables; refer to your compiler manual for the correct syntax.

The address sanitizer is known to fail in sha256_sse4::Transform which makes it unusable unless you also use -DCRYPTO_USE_ASM=OFF when running cmake. We would like to fix sanitizer issues, so please send pull requests if you can fix any errors found by the address sanitizer (or any other sanitizer).

Not all sanitizer options can be enabled at the same time, e.g. trying to build with `-DENABLE_SANITIZERS=="address;thread" will fail in the cmake script as these sanitizers are mutually incompatible. Refer to your compiler manual to learn more about these options and which sanitizers are supported by your compiler.

Examples:

Build and run the test suite with the address sanitizer enabled:

mkdir build_asan
cd build_asan

cmake -GNinja .. \
  -DCMAKE_BUILD_TYPE=Debug \
  -DENABLE_SANITIZERS=address \
  -DCRYPTO_USE_ASM=OFF

ninja check check-functional

Build and run the test suite with the thread sanitizer enabled (it can take a very long time to complete):

mkdir build_tsan
cd build_tsan

cmake -GNinja .. \
  -DCMAKE_BUILD_TYPE=Debug \
  -DENABLE_SANITIZERS=thread

ninja check check-functional

Build and run the test suite with the undefined sanitizer enabled:

mkdir build_ubsan
cd build_ubsan

cmake -GNinja .. \
  -DCMAKE_BUILD_TYPE=Debug \
  -DENABLE_SANITIZERS=undefined

ninja check check-functional

Additional resources:

Locking/mutex usage notes

The code is multi-threaded, and uses mutexes and the LOCK and TRY_LOCK macros to protect data structures.

Deadlocks due to inconsistent lock ordering (thread 1 locks cs_main and then cs_wallet, while thread 2 locks them in the opposite order: result, deadlock as each waits for the other to release its lock) are a problem. Compile with -DDEBUG_LOCKORDER (or use -DCMAKE_BUILD_TYPE=Debug) to get lock order inconsistencies reported in the debug.log file.

Re-architecting the core code so there are better-defined interfaces between the various components is a goal, with any necessary locking done by the components (e.g. see the self-contained CBasicKeyStore class and its cs_KeyStore lock for example).

Threads

ThreadScriptCheck : Verifies block scripts.
ThreadImport : Loads blocks from blk*.dat files or bootstrap.dat.
StartNode : Starts other threads.
ThreadDNSAddressSeed : Loads addresses of peers from the DNS.
ThreadMapPort : Universal plug-and-play startup/shutdown
ThreadSocketHandler : Sends/Receives data from peers on port 8333.
ThreadOpenAddedConnections : Opens network connections to added nodes.
ThreadOpenConnections : Initiates new connections to peers.
ThreadMessageHandler : Higher-level message handling (sending and receiving).
DumpAddresses : Dumps IP addresses of nodes to peers.dat.
ThreadRPCServer : Remote procedure call handler, listens on port 8332 for connections and services them.
Shutdown : Does an orderly shutdown of everything.

Ignoring IDE/editor files

In closed-source environments in which everyone uses the same IDE it is common to add temporary files it produces to the project-wide .gitignore file.

However, in open source software such as Bitcoin Cash Node, where everyone uses their own editors/IDE/tools, it is less common. Only you know what files your editor produces and this may change from version to version. The canonical way to do this is thus to create your local gitignore. Add this to ~/.gitconfig:

[core]
        excludesfile = /home/.../.gitignore_global

(alternatively, type the command git config --global core.excludesfile ~/.gitignore_global on a terminal)

Then put your favorite tool's temporary filenames in that file, e.g.

# NetBeans
nbproject/

Another option is to create a per-repository excludes file .git/info/exclude. These are not committed but apply only to one repository.

If a set of tools is used by the build system or scripts the repository (for example, lcov) it is perfectly acceptable to add its files to .gitignore and commit them.

Development guidelines

A few non-style-related recommendations for developers, as well as points to pay attention to for reviewers of Bitcoin Cash Node code.

Wallet

Make sure that no crashes happen with run-time option -disablewallet.
- Rationale: In RPC code that conditionally uses the wallet (such as validateaddress) it is easy to forget that global pointer pwalletMain can be NULL. See test/functional/disablewallet.py for functional tests exercising the API with -disablewallet
Include db_cxx.h (BerkeleyDB header) only when ENABLE_WALLET is set
- Rationale: Otherwise compilation of the disable-wallet build will fail in environments without BerkeleyDB

General C++

Assertions should not have side-effects
- Rationale: Even though the source code is set to refuse to compile with assertions disabled, having side-effects in assertions is unexpected and makes the code harder to understand
If you use the .h, you must link the .cpp
- Rationale: Include files define the interface for the code in implementation files. Including one but not linking the other is confusing. Please avoid that. Moving functions from the .h to the .cpp should not result in build errors
Use the RAII (Resource Acquisition Is Initialization) paradigm where possible. For example by using unique_ptr for allocations in a function.
- Rationale: This avoids memory and resource leaks, and ensures exception safety
Use std::make_unique() to construct objects owned by unique_ptrs
- Rationale: std::make_unique is concise and ensures exception safety in complex expressions.

C++ data structures

Never use the std::map [] syntax when reading from a map, but instead use .find()
- Rationale: [] does an insert (of the default element) if the item doesn't exist in the map yet. This has resulted in memory leaks in the past, as well as race conditions (expecting read-read behavior). Using [] is fine for writing to a map.
Do not compare an iterator from one data structure with an iterator of another data structure (even if of the same type)
- Rationale: Behavior is undefined. In C++ parlance this means "may reformat the universe", in practice this has resulted in at least one hard-to-debug crash bug.
Watch out for out-of-bounds vector access. &vch[vch.size()] is illegal, including &vch[0] for an empty vector. Use vch.data() and vch.data() + vch.size() instead.
Vector bounds checking is only enabled in debug mode. Do not rely on it
Make sure that constructors initialize all fields. If this is skipped for a good reason (i.e., optimization on the critical path), add an explicit comment about this
- Rationale: Ensure determinism by avoiding accidental use of uninitialized values. Also, static analyzers balk about this.
By default, declare single-argument constructors explicit.
- Rationale: This is a precaution to avoid unintended conversions that might arise when single-argument constructors are used as implicit conversion functions.
Use explicitly signed or unsigned chars, or even better uint8_t and int8_t. Do not use bare char unless it is to pass to a third-party API. This type can be signed or unsigned depending on the architecture, which can lead to interoperability problems or dangerous conditions such as out-of-bounds array accesses
Prefer explicit constructions over implicit ones that rely on 'magical' C++ behavior
- Rationale: Easier to understand what is happening, thus easier to spot mistakes, even for those that are not language lawyers
Use Span as function argument when it can operate on any range-like container.
```
void Foo(Span<const int> data);
std::vector<int> vec{1,2,3};
Foo(vec);
```
- Rationale: Compared to Foo(const vector<int>&) this avoids the need for a (potentially expensive) conversion to vector if the caller happens to have the input stored in another type of container. However, be aware of the pitfalls documented in span.h.
Initialize all non-static class members where they are defined
```
class A
{
    uint32_t m_count{0};
}
```
- Rationale: Initializing the members in the declaration makes it easy to spot uninitialized ones, and avoids accidentally reading uninitialized memory

Strings and formatting

Use std::string, avoid C string manipulation functions
- Rationale: C++ string handling is marginally safer, less scope for buffer overflows and surprises with \0 characters. Also some C string manipulations tend to act differently depending on platform, or even the user locale
Use ParseInt32, ParseInt64, ParseUInt32, ParseUInt64, ParseDouble from utilstrencodings.h for number parsing
- Rationale: These functions do overflow checking, and avoid pesky locale issues

Variable names

The shadowing warning (-Wshadow) is enabled by default. It prevents issues rising from using a different variable with the same name.

E.g. in member initializers, prepend _ to the argument name shadowing the member name:

class AddressBookPage
{
    Mode m_mode;
}

AddressBookPage::AddressBookPage(Mode _mode) :
      m_mode(_mode)
...

When using nested cycles, do not name the inner cycle variable the same as in upper cycle etc.

Please name variables so that their names do not shadow variables defined in the source code.

Threads and synchronization

Build and run tests with -DDEBUG_LOCKORDER to verify that no potential deadlocks are introduced. As of 0.12, this is defined by default when configuring with -DCMAKE_BUILD_TYPE=Debug
When using LOCK/TRY_LOCK be aware that the lock exists in the context of the current scope, so surround the statement and the code that needs the lock with braces

OK:
```
{
    TRY_LOCK(cs_vNodes, lockNodes);
    ...
}
```
Wrong:
```
TRY_LOCK(cs_vNodes, lockNodes);
{
    ...
}
```

Scripts

Shebang

Use #!/usr/bin/env bash instead of obsolete #!/bin/bash.
- Rationale: #!/bin/bash assumes it is always installed to /bin/ which can cause issues; #!/usr/bin/env bash searches the user's PATH to find the bash binary.

OK:

#!/usr/bin/env bash

Wrong:

#!/bin/bash

Source code organization

Implementation code should go into the .cpp file and not the .h, unless necessary due to template usage or when performance due to inlining is critical
- Rationale: Shorter and simpler header files are easier to read, and reduce compile time
Use only the lowercase alphanumerics (a-z0-9), underscore (_) and hyphen (-) in source code filenames.
- Rationale: grep:ing and auto-completing filenames is easier when using a consistent naming pattern. Potential problems when building on case-insensitive filesystems are avoided when using only lowercase characters in source code filenames.
Don't import anything into the global namespace (using namespace ...). Use fully specified types such as std::string.
- Rationale: Avoids symbol conflicts
Terminate namespaces with a comment (// namespace mynamespace). The comment should be placed on the same line as the brace closing the namespace, e.g.

namespace mynamespace {
    ...
} // namespace mynamespace

namespace {
    ...
} // namespace

- *Rationale*: Avoids confusion about the namespace context

Header Inclusions

Header inclusions should use angle brackets (#include <>). The include path should be relative to the src folder. e.g.: #include <qt/test/guiutiltests.h>
Native C++ headers should be preferred over C compatibility headers. e.g.: use <cstdint> instead of <stdint.h>
In order to make the code consistent, header files should be included in the following order, with each section separated by a newline:
1. In a .cpp file, the associated .h is in first position. In a test source, this is the header file under test.
2. The project headers.
3. The test headers.
4. The 3rd party libraries headers. Different libraries should be in different sections.
5. The system libraries.

All headers should be lexically ordered inside their block.

Use the include guard #pragma once as the first preprocessor directive in header files to avoid the problem of double inclusion.

GUI

Do not display or manipulate dialogs in model code (classes *Model)
- Rationale: Model classes pass through events and data from the core, they should not interact with the user. That's where View classes come in. The converse also holds: try to not directly access core data structures from Views.
Avoid adding slow or blocking code in the GUI thread. In particular do not add new interface::Node and interface::Wallet method calls, even if they may be fast now, in case they are changed to lock or communicate across processes in the future.
Prefer to offload work from the GUI thread to worker threads (see RPCExecutor in console code as an example) or take other steps (see https://doc.qt.io/archives/qq/qq27-responsive-guis.html) to keep the GUI responsive.
- Rationale: Blocking the GUI thread can increase latency, and lead to hangs and deadlocks.

Unit Tests

Test suite naming convention: The Boost test suite in file src/test/foo_tests.cpp should be named foo_tests. Test suite names must be unique.

Subtrees

Several parts of the repository are subtrees of software maintained elsewhere.

Some of these are maintained by active developers of Bitcoin Core, in which case changes should probably go directly upstream without being PRed directly against the project. They will be merged back in the next subtree merge.

Others are external projects without a tight relationship with our project. Changes to these should also be sent upstream but bugfixes may also be prudent to PR against Bitcoin Core so that they can be integrated quickly. Cosmetic changes should be purely taken upstream.

There is a tool in test/lint/git-subtree-check.sh to check a subtree directory for consistency with its upstream repository.

Current subtrees include:

src/leveldb
- Upstream at https://github.com/google/leveldb; Maintained by Google, but open important PRs to Core to avoid delay.
- Note: Follow the instructions in Upgrading LevelDB when merging upstream changes to the leveldb subtree.
src/libsecp256k1
- Upstream at https://github.com/bitcoin-core/secp256k1/; actively maintained by Core contributors.
src/crypto/ctaes
- Upstream at https://github.com/bitcoin-core/ctaes; actively maintained by Core contributors.
src/univalue
- BCHN no longer has a single upstream for src/univalue, but maintains its own univalue code based on fixes from several repositories, namely https://github.com/jgarzik/univalue, Bitcoin Core's fork of univalue and the Bitcoin ABC repository.

Upgrading LevelDB

Extra care must be taken when upgrading LevelDB. This section explains issues you must be aware of.

File Descriptor Counts

In most configurations we use the default LevelDB value for max_open_files, which is 1000 at the time of this writing. If LevelDB actually uses this many file descriptors it will cause problems with Bitcoin's select() loop, because it may cause new sockets to be created where the fd value is >= 1024. For this reason, on 64-bit Unix systems we rely on an internal LevelDB optimization that uses mmap() + close() to open table files without actually retaining references to the table file descriptors. If you are upgrading LevelDB, you must sanity check the changes to make sure that this assumption remains valid.

In addition to reviewing the upstream changes in env_posix.cc, you can use lsof to check this. For example, on Linux this command will show open .ldb file counts:

$ lsof -p $(pidof bitcoind) |\
    awk 'BEGIN { fd=0; mem=0; } /ldb$/ { if ($4 == "mem") mem++; else fd++ } END { printf "mem = %s, fd = %s\n", mem, fd}'
mem = 119, fd = 0

The mem value shows how many files are mmap'ed, and the fd value shows you many file descriptors these files are using. You should check that fd is a small number (usually 0 on 64-bit hosts).

See the notes in the SetMaxOpenFiles() function in dbwrapper.cc for more details.

Consensus Compatibility

It is possible for LevelDB changes to inadvertently change consensus compatibility between nodes. This happened in Bitcoin 0.8 (when LevelDB was first introduced). When upgrading LevelDB you should review the upstream changes to check for issues affecting consensus compatibility.

For example, if LevelDB had a bug that accidentally prevented a key from being returned in an edge case, and that bug was fixed upstream, the bug "fix" would be an incompatible consensus change. In this situation the correct behavior would be to revert the upstream fix before applying the updates to Bitcoin Cash Node's copy of LevelDB. In general you should be wary of any upstream changes affecting what data is returned from LevelDB queries.

Git and GitLab tips

See CONTRIBUTING.md for instructions on setting up your repo correctly.
Your git remote origin should be set to: git@gitlab.com:username/bitcoin-cash-node.git where username is your account name on GitLab. See CONTRIBUTING.md for details.
For resolving merge/rebase conflicts, it can be useful to enable diff3 style using git config merge.conflictstyle diff3. Instead of

<<<
yours
===
theirs
>>>

you will see

<<<
yours
|||
original
===
theirs
>>>

This may make it much clearer what caused the conflict. In this style, you can often just look at what changed between original and theirs, and mechanically apply that to yours (or the other way around).

When reviewing patches which change indentation in C++ files, use git diff -w and git show -w. This makes the diff algorithm ignore whitespace changes. This feature is also available on gitlab.com, by adding ?w=1 at the end of any URL which shows a diff.
When reviewing patches that change symbol names in many places, use git diff --word-diff. This will instead of showing the patch as deleted/added lines, show deleted/added words.
When reviewing patches that move code around, try using git diff --patience commit~:old/file.cpp commit:new/file/name.cpp, and ignoring everything except the moved body of code which should show up as neither + or - lines. In case it was not a pure move, this may even work when combined with the -w or --word-diff options described above.
When looking at other's pull requests, it may make sense to add the following section to your .git/config file:

[remote "upstream-merges"]
        url = git@gitlab.com:bitcoin-cash-node/bitcoin-cash-node.git
        fetch = +refs/merge-requests/*/head:refs/remotes/upstream-merges/merge-requests/*

This will add an upstream-merges remote to your git repository, which can be fetched using git fetch --all or git fetch upstream-merges. Afterwards, you can use upstream-merges/merge-requests/NUMBER in arguments to git show, git checkout and anywhere a commit id would be acceptable to see the changes from merge request NUMBER.

RPC interface guidelines

A few guidelines for introducing and reviewing new RPC interfaces:

Method naming: use consecutive lower-case names such as getrawtransaction and submitblock
- Rationale: Consistency with existing interface.
Argument naming: use snake case fee_delta (and not, e.g. camel case feeDelta)
- Rationale: Consistency with existing interface.
Use the JSON parser for parsing, don't manually parse integers or strings from arguments unless absolutely necessary.
- Rationale: Introduces hand-rolled string manipulation code at both the caller and callee sites, which is error prone, and it is easy to get things such as escaping wrong. JSON already supports nested data structures, no need to re-invent the wheel.
- Exception: AmountFromValue can parse amounts as string. This was introduced because many JSON parsers and formatters hard-code handling decimal numbers as floating point values, resulting in potential loss of precision. This is unacceptable for monetary values. Always use AmountFromValue and ValueFromAmount when inputting or outputting monetary values. The only exceptions to this are prioritisetransaction and getblocktemplate because their interface is specified as-is in BIP22.
Missing arguments and 'null' should be treated the same: as default values. If there is no default value, both cases should fail in the same way. The easiest way to follow this guideline is detect unspecified arguments with params[x].isNull() instead of params.size() <= x. The former returns true if the argument is either null or missing, while the latter returns true if is missing, and false if it is null.
- Rationale: Avoids surprises when switching to name-based arguments. Missing name-based arguments are passed as 'null'.
Try not to overload methods on argument type. E.g. don't make getblock(true) and getblock("hash") do different things.
- Rationale: This is impossible to use with bitcoin-cli, and can be surprising to users.
- Exception: Some RPC calls can take both an int and bool, most notably when a bool was switched to a multi-value, or due to other historical reasons. Always have false map to 0 and true to 1 in this case.
Don't forget to fill in the argument names correctly in the RPC command table.
- Rationale: If not, the call can not be used with name-based arguments.
Set okSafeMode in the RPC command table to a sensible value: safe mode is when the blockchain is regarded to be in a confused state, and the client deems it unsafe to do anything irreversible such as send. Anything that just queries should be permitted.
- Rationale: Troubleshooting a node in safe mode is difficult if half the RPCs don't work.
Add every non-string RPC argument (method, idx, name) to the table vRPCConvertParams in rpc/client.cpp.
- Rationale: bitcoin-cli and the GUI debug console use this table to determine how to convert a plaintext command line to JSON. If the types don't match, the method can be unusable from there.
A RPC method must either be a wallet method or a non-wallet method. Do not introduce new methods such as signrawtransaction that differ in behavior based on presence of a wallet.
- Rationale: as well as complicating the implementation and interfering with the introduction of multi-wallet, wallet and non-wallet code should be separated to avoid introducing circular dependencies between code units.
Try to make the RPC response a JSON object.
- Rationale: If a RPC response is not a JSON object then it is harder to avoid API breakage if new data in the response is needed.
Wallet RPCs call BlockUntilSyncedToCurrentChain to maintain consistency with getblockchaininfo's state immediately prior to the call's execution. Wallet RPCs whose behavior does not depend on the current chainstate may omit this call.
- Rationale: In previous versions of Bitcoin Core, the wallet was always in-sync with the chainstate (by virtue of them all being updated in the same cs_main lock). In order to maintain the behavior that wallet RPCs return results as of at least the highest best-known block an RPC client may be aware of prior to entering a wallet RPC call, we must block until the wallet is caught up to the chainstate as of the RPC call's entry. This also makes the API much easier for RPC clients to reason about.
Be aware of RPC method aliases and generally avoid registering the same callback function pointer for different RPCs.
- Rationale: RPC methods registered with the same function pointer will be considered aliases and only the first method name will show up in the help rpc command list.
- Exception: Using RPC method aliases may be appropriate in cases where a new RPC is replacing a deprecated RPC, to avoid both RPCs confusingly showing up in the command list.