bitcoin/doc/developer-notes.md

Developer Notes

Table of Contents

• Developer Notes
  • Coding Style (General)
  • Coding Style (C++)
  • Coding Style (Python)
  • Coding Style (Doxygen-compatible comments)
  • Development tips and tricks
    • Compiling for debugging
    • Compiling for gprof profiling
    • debug.log
    • Testnet and Regtest modes
    • DEBUG_LOCKORDER
    • Valgrind suppressions file
    • Compiling for test coverage
    • Performance profiling with perf
  • Locking/mutex usage notes
  • Threads
  • Ignoring IDE/editor files
• Development guidelines
  • General Bitcoin Core
  • Wallet
  • General C++
  • C++ data structures
  • Strings and formatting
  • Shadowing
  • Threads and synchronization
  • Scripts
    • Shebang
  • Source code organization
  • GUI
  • Subtrees
  • Scripted diffs
  • Release notes
  • RPC interface guidelines

Coding Style (General)

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, which is specified below. When writing patches, favor the new style over attempting to mimic the surrounding style, except for move-only commits.

Do not submit patches solely to modify the style of existing code.

Coding Style (C++)

• Indentation and whitespace rules as specified in src/.clang-format. You can use the provided clang-format-diff script tool to clean up patches automatically before submission.

  • Braces on new lines for 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.
  • If an if only has a single-statement then-clause, it can appear on the same line as the if, without braces. In every other case, braces are required, and the then and else clauses must appear correctly indented on a new line.

• Symbol naming conventions. These are preferred in new code, but are not required when doing so would need changes to significant pieces of existing code.

  • Variable (including function arguments) and namespace names are all lowercase and may use _ to separate words (snake_case).
    • Class member variables have a m_ prefix.
    • Global variables have a g_ prefix.
  • Constant names are all uppercase, and use _ to separate words.
  • Class names, function names, and method names are UpperCamelCase (PascalCase). Do not prefix class names with C.
  • 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.

• Miscellaneous

  • ++i is preferred over i++.
  • nullptr is preferred over NULL or (void*)0.
  • static_assert is preferred over assert where possible. Generally; compile-time checking is preferred over run-time checking.
  • enum class is preferred over enum where possible. Scoped enumerations avoid two potential pitfalls/problems with traditional C++ enumerations: implicit conversions to int, and name clashes due to enumerators being exported to the surrounding scope.

Block style example:

int g_count = 0;

namespace foo {
class Class
{
    std::string m_name;

public:
    bool Function(const std::string& s, int n)
    {
        // Comment summarising what this section of code does
        for (int i = 0; i < n; ++i) {
            int total_sum = 0;
            // When something fails, return early
            if (!Something()) return false;
            ...
            if (SomethingElse(i)) {
                total_sum += ComputeSomething(g_count);
            } else {
                DoSomething(m_name, total_sum);
            }
        }

        // Success return is usually at the end
        return true;
    }
}
} // namespace foo

Coding Style (Python)

Refer to /test/functional/README.md#style-guidelines.

Coding Style (Doxygen-compatible comments)

Bitcoin Core uses Doxygen to generate its official 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 http://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 http://www.doxygen.nl/manual/docblocks.html, but the above styles are favored.

Documentation can be generated with make docs and cleaned up with make clean-docs. The resulting files are located in doc/doxygen/html; open index.html to view the homepage.

Before running make docs, you will need to install dependencies doxygen and dot. For example, on macOS via Homebrew:

brew install doxygen --with-graphviz

Development tips and tricks

Compiling for debugging

Run configure with --enable-debug to add additional compiler flags that produce better debugging builds.

Compiling for gprof profiling

Run configure with the --enable-gprof option, then make.

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.

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 Core is a multi-threaded application, and deadlocks or other multi-threading bugs can be very difficult to track down. The --enable-debug configure 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 make check execution. LCOV must be installed on your system (e.g. the lcov package on Debian/Ubuntu).

To enable LCOV report generation during test runs:

./configure --enable-lcov
make
make cov

# A coverage report will now be accessible at `./test_bitcoin.coverage/index.html`.

Performance profiling with perf

Profiling is a good way to get a precise idea of where time is being spent in code. One tool for doing profiling on Linux platforms is called perf, and has been integrated into the functional test framework. Perf can observe a running process and sample (at some frequency) where its execution is.

Perf installation is contingent on which kernel version you're running; see this StackExchange thread for specific instructions.

Certain kernel parameters may need to be set for perf to be able to inspect the running process's stack.

$ sudo sysctl -w kernel.perf_event_paranoid=-1
$ sudo sysctl -w kernel.kptr_restrict=0

Make sure you understand the security trade-offs of setting these kernel parameters.

To profile a running bitcoind process for 60 seconds, you could use an invocation of perf record like this:

$ perf record \
    -g --call-graph dwarf --per-thread -F 140 \
    -p `pgrep bitcoind` -- sleep 60

You could then analyze the results by running:

perf report --stdio | c++filt | less

or using a graphical tool like Hotspot.

See the functional test documentation for how to invoke perf within tests.

Sanitizers

Bitcoin Core 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 --with-sanitizers configure flag, which should be a comma 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
./configure --with-sanitizers=address,undefined

# Enable the thread sanitizer
./configure --with-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 configure script will fail with a linker error when testing the sanitizer flags.

The test suite should pass cleanly with the thread and undefined sanitizers, but there are a number of known problems when using the address sanitizer. The address sanitizer is known to fail in sha256_sse4::Transform which makes it unusable unless you also use --disable-asm when running configure. 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 --with-sanitizers=address,thread will fail in the configure 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.

Additional resources:

• AddressSanitizer
• LeakSanitizer
• MemorySanitizer
• ThreadSanitizer
• UndefinedBehaviorSanitizer
• GCC Instrumentation Options
• Google Sanitizers Wiki
• Issue #12691: Enable -fsanitize flags in Travis

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 --enable-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 FillableSigningProvider 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 Core, 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 favourite 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 Core code.

General Bitcoin Core

• New features should be exposed on RPC first, then can be made available in the GUI.

  • Rationale: RPC allows for better automatic testing. The test suite for the GUI is very limited.

• Make sure pull requests pass Travis CI before merging.

  • Rationale: Makes sure that they pass thorough testing, and that the tester will keep passing on the master branch. Otherwise, all new pull requests will start failing the tests, resulting in confusion and mayhem.

  • Explanation: If the test suite is to be updated for a change, this has to be done first.

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 nullptr. 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++

For general C++ guidelines, you may refer to the C++ Core Guidelines.

Common misconceptions are clarified in those sections:

• Passing (non-)fundamental types in the C++ Core Guideline.

• 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 MakeUnique() to construct objects owned by unique_ptrs.

  • Rationale: MakeUnique is concise and ensures exception safety in complex expressions. MakeUnique is a temporary project local implementation of std::make_unique (C++14).

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++ parlor 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.

• Initialize all non-static class members where they are defined. 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. Initializing the members in the declaration makes it easy to spot uninitialized ones.

class A
{
    uint32_t m_count{0};
}

• 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.

Strings and formatting

• Be careful of LogPrint versus LogPrintf. LogPrint takes a category argument, LogPrintf does not.

  • Rationale: Confusion of these can result in runtime exceptions due to formatting mismatch, and it is easy to get wrong because of subtly similar naming.

• 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.

• Avoid using locale dependent functions if possible. You can use the provided lint-locale-dependence.sh to check for accidental use of locale dependent functions.

  • Rationale: Unnecessary locale dependence can cause bugs that are very tricky to isolate and fix.

  • These functions are known to be locale dependent: alphasort, asctime, asprintf, atof, atoi, atol, atoll, atoq, btowc, ctime, dprintf, fgetwc, fgetws, fprintf, fputwc, fputws, fscanf, fwprintf, getdate, getwc, getwchar, isalnum, isalpha, isblank, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, iswalnum, iswalpha, iswblank, iswcntrl, iswctype, iswdigit, iswgraph, iswlower, iswprint, iswpunct, iswspace, iswupper, iswxdigit, isxdigit, mblen, mbrlen, mbrtowc, mbsinit, mbsnrtowcs, mbsrtowcs, mbstowcs, mbtowc, mktime, putwc, putwchar, scanf, snprintf, sprintf, sscanf, stoi, stol, stoll, strcasecmp, strcasestr, strcoll, strfmon, strftime, strncasecmp, strptime, strtod, strtof, strtoimax, strtol, strtold, strtoll, strtoq, strtoul, strtoull, strtoumax, strtouq, strxfrm, swprintf, tolower, toupper, towctrans, towlower, towupper, ungetwc, vasprintf, vdprintf, versionsort, vfprintf, vfscanf, vfwprintf, vprintf, vscanf, vsnprintf, vsprintf, vsscanf, vswprintf, vwprintf, wcrtomb, wcscasecmp, wcscoll, wcsftime, wcsncasecmp, wcsnrtombs, wcsrtombs, wcstod, wcstof, wcstoimax, wcstol, wcstold, wcstoll, wcstombs, wcstoul, wcstoull, wcstoumax, wcswidth, wcsxfrm, wctob, wctomb, wctrans, wctype, wcwidth, wprintf

• For strprintf, LogPrint, LogPrintf formatting characters don't need size specifiers.

  • Rationale: Bitcoin Core uses tinyformat, which is type safe. Leave them out to avoid confusion.

Shadowing

Although the shadowing warning (-Wshadow) is not enabled by default (it prevents issues arising from using a different variable with the same name), please name variables so that their names do not shadow variables defined in the source code.

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

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 --enable-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.

• Every .cpp and .h file should #include every header file it directly uses classes, functions or other definitions from, even if those headers are already included indirectly through other headers.

  • Rationale: Excluding headers because they are already indirectly included results in compilation failures when those indirect dependencies change. Furthermore, it obscures what the real code dependencies are.

• 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.

• Use #include <primitives/transaction.h> bracket syntax instead of #include "primitives/transactions.h" quote syntax.

  • Rationale: Bracket syntax is less ambiguous because the preprocessor searches a fixed list of include directories without taking location of the source file into account. This allows quoted includes to stand out more when the location of the source file actually is relevant.

• Use include guards to avoid the problem of double inclusion. The header file foo/bar.h should use the include guard identifier BITCOIN_FOO_BAR_H, e.g.

#ifndef BITCOIN_FOO_BAR_H
#define BITCOIN_FOO_BAR_H
...
#endif // BITCOIN_FOO_BAR_H

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 interfaces::Node and interfaces::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.

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

  • Upstream at https://github.com/bitcoin-core/univalue ; actively maintained by Core contributors, deviates from upstream https://github.com/jgarzik/univalue

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's copy of LevelDB. In general, you should be wary of any upstream changes affecting what data is returned from LevelDB queries.

Scripted diffs

For reformatting and refactoring commits where the changes can be easily automated using a bash script, we use scripted-diff commits. The bash script is included in the commit message and our Travis CI job checks that the result of the script is identical to the commit. This aids reviewers since they can verify that the script does exactly what it's supposed to do. It is also helpful for rebasing (since the same script can just be re-run on the new master commit).

To create a scripted-diff:

• start the commit message with scripted-diff: (and then a description of the diff on the same line)
• in the commit message include the bash script between lines containing just the following text:
  • -BEGIN VERIFY SCRIPT-
  • -END VERIFY SCRIPT-

The scripted-diff is verified by the tool test/lint/commit-script-check.sh. The tool's default behavior, when supplied with a commit is to verify all scripted-diffs from the beginning of time up to said commit. Internally, the tool passes the first supplied argument to git rev-list --reverse to determine which commits to verify script-diffs for, ignoring commits that don't conform to the commit message format described above.

For development, it might be more convenient to verify all scripted-diffs in a range A..B, for example:

test/lint/commit-script-check.sh origin/master..HEAD

Commit bb81e173 is an example of a scripted-diff.

Release notes

Release notes should be written for any PR that:

• introduces a notable new feature
• fixes a significant bug
• changes an API or configuration model
• makes any other visible change to the end-user experience.

Release notes should be added to a PR-specific release note file at /doc/release-notes-<PR number>.md to avoid conflicts between multiple PRs. All release-notes* files are merged into a single /doc/release-notes.md file prior to the release.

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 the existing interface.

• Argument naming: use snake case fee_delta (and not, e.g. camel case feeDelta)

  • Rationale: Consistency with the 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 to 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 that differ in behavior based on the 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.