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zcashd/doc/developer-notes.md

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Coding

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.

  • Basic rules specified in src/.clang-format. Use a recent clang-format-3.5 to format automatically.
    • 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 namespaces.
    • No extra spaces inside parenthesis; don't do ( this )
    • No space after function names; one space after if, for and while.

Block style example:

namespace foo
{
class Class
{
    bool Function(char* psz, int n)
    {
        // Comment summarising what this section of code does
        for (int i = 0; i < n; i++) {
            // When something fails, return early
            if (!Something())
                return false;
            ...
        }

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

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 http://www.stack.nl/~dimitri/doxygen/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.stack.nl/~dimitri/doxygen/manual/docblocks.html, but if possible use one of the above styles.

Development tips and tricks

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.

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).

testnet and regtest modes

Run with the -testnet option to run with "play zcash" 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 qa/rpc-tests/ for tests that run in -regtest mode.

DEBUG_LOCKORDER

Zcash 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.

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:

# 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:

Locking/mutex usage notes

The code is multi-threaded, and uses mutexes and the LOCK/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.

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).

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 8233.

  • 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.

  • ThreadFlushWalletDB : Close the wallet.dat file if it hasn't been used in 500ms.

  • ThreadRPCServer : Remote procedure call handler, listens on port 8232 for connections and services them.

  • ZcashMiner : Generates zcash (if wallet is enabled).

  • Shutdown : Does an orderly shutdown of everything.

Pull Request Terminology

Concept ACK - Agree with the idea and overall direction, but have neither reviewed nor tested the code changes.

utACK (untested ACK) - Reviewed and agree with the code changes but haven't actually tested them.

Tested ACK - Reviewed the code changes and have verified the functionality or bug fix.

ACK - A loose ACK can be confusing. It's best to avoid them unless it's a documentation/comment only change in which case there is nothing to test/verify; therefore the tested/untested distinction is not there.

NACK - Disagree with the code changes/concept. Should be accompanied by an explanation.

See the Development Guidelines documentation for preferred workflows, information on continuous integration and release versioning.

Strings and formatting

  • 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

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

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

Subtrees

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

Some of these are maintained by active developers of Zcash or 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 Zcash and/or 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 (instructions) to check a subtree directory for consistency with its upstream repository.

Current subtrees include:

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

Commit ccd074a5 is an example of a scripted-diff.