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diff --git a/doc/developer-notes.md b/doc/developer-notes.md index 77ab9cccbe..540f2e8b84 100644 --- a/doc/developer-notes.md +++ b/doc/developer-notes.md @@ -1,6 +1,43 @@ Developer Notes =============== +<!-- markdown-toc start --> +**Table of Contents** + +- [Developer Notes](#developer-notes) + - [Coding Style](#coding-style) + - [Doxygen comments](#doxygen-comments) + - [Development tips and tricks](#development-tips-and-tricks) + - [Compiling for debugging](#compiling-for-debugging) + - [Compiling for gprof profiling](#compiling-for-gprof-profiling) + - [debug.log](#debuglog) + - [Testnet and Regtest modes](#testnet-and-regtest-modes) + - [DEBUG_LOCKORDER](#debug_lockorder) + - [Valgrind suppressions file](#valgrind-suppressions-file) + - [Compiling for test coverage](#compiling-for-test-coverage) + - [Locking/mutex usage notes](#lockingmutex-usage-notes) + - [Threads](#threads) + - [Ignoring IDE/editor files](#ignoring-ideeditor-files) +- [Development guidelines](#development-guidelines) + - [General Bitcoin Core](#general-bitcoin-core) + - [Wallet](#wallet) + - [General C++](#general-c) + - [C++ data structures](#c-data-structures) + - [Strings and formatting](#strings-and-formatting) + - [Variable names](#variable-names) + - [Threads and synchronization](#threads-and-synchronization) + - [Source code organization](#source-code-organization) + - [GUI](#gui) + - [Subtrees](#subtrees) + - [Git and GitHub tips](#git-and-github-tips) + - [Scripted diffs](#scripted-diffs) + - [RPC interface guidelines](#rpc-interface-guidelines) + +<!-- markdown-toc end --> + +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, which is specified below. When writing patches, favor the new @@ -34,11 +71,14 @@ code. - 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`. - **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: ```c++ @@ -137,43 +177,44 @@ Documentation can be generated with `make docs` and cleaned up with `make clean- Development tips and tricks --------------------------- -**compiling for debugging** +### Compiling for debugging -Run configure with the --enable-debug option, then make. Or run configure with -CXXFLAGS="-g -ggdb -O0" or whatever debug flags you need. +Run configure with `--enable-debug` to add additional compiler flags that +produce better debugging builds. -**compiling for gprof profiling** +### Compiling for gprof profiling -Run configure with the --enable-gprof option, then make. +Run configure with the `--enable-gprof` option, then make. -**debug.log** +### 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 +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 +The Qt code routes `qDebug()` output to debug.log under category "qt": run with `-debug=qt` to see it. -**testnet and regtest modes** +### Testnet and Regtest modes -Run with the -testnet option to run with "play bitcoins" on the test network, if you +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. +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/](/test/functional) for tests +that run in `-regtest` mode. -**DEBUG_LOCKORDER** +### DEBUG_LOCKORDER -Bitcoin Core is a multithreaded application, and deadlocks or other multithreading bugs -can be very difficult to track down. Compiling with -DDEBUG_LOCKORDER (configure -CXXFLAGS="-DDEBUG_LOCKORDER -g") inserts run-time checks to keep track of which locks -are held, and adds warnings to the debug.log file if inconsistencies are detected. +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 suppressions file Valgrind is a programming tool for memory debugging, memory leak detection, and profiling. The repo contains a Valgrind suppressions file @@ -188,7 +229,7 @@ $ valgrind --suppressions=contrib/valgrind.supp --leak-check=full \ $ valgrind -v --leak-check=full src/bitcoind -printtoconsole ``` -**compiling for test coverage** +### 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 @@ -204,22 +245,73 @@ make cov # A coverage report will now be accessible at `./test_bitcoin.coverage/index.html`. ``` +**Sanitizers** + +Bitcoin 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: + +```bash +# 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](/src/crypto/sha256_sse4.cpp) 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](https://clang.llvm.org/docs/AddressSanitizer.html) + * [LeakSanitizer](https://clang.llvm.org/docs/LeakSanitizer.html) + * [MemorySanitizer](https://clang.llvm.org/docs/MemorySanitizer.html) + * [ThreadSanitizer](https://clang.llvm.org/docs/ThreadSanitizer.html) + * [UndefinedBehaviorSanitizer](https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html) + * [GCC Instrumentation Options](https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html) + * [Google Sanitizers Wiki](https://github.com/google/sanitizers/wiki) + * [Issue #12691: Enable -fsanitize flags in Travis](https://github.com/bitcoin/bitcoin/issues/12691) + Locking/mutex usage notes ------------------------- The code is multi-threaded, and uses mutexes and the -LOCK/TRY_LOCK macros to protect data structures. +`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 to get lock order -inconsistencies reported in the debug.log file. +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 CKeyStore class -and its cs_KeyStore lock for example). +done by the components (e.g. see the self-contained `CBasicKeyStore` class +and its `cs_KeyStore` lock for example). Threads ------- @@ -552,7 +644,10 @@ 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 + - 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](#upgrading-leveldb) when + merging upstream changes to the leveldb subtree. - src/libsecp256k1 - Upstream at https://github.com/bitcoin-core/secp256k1/ ; actively maintaned by Core contributors. @@ -563,6 +658,52 @@ Current subtrees include: - src/univalue - Upstream at https://github.com/jgarzik/univalue ; report important PRs to Core to avoid delay. +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: + +```bash +$ 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. Git and GitHub tips --------------------- |