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Getting started with PHP core development - phpCE 2017

8dfcb5f1b3cd5397f19780e2319694da?s=47 Arnout Boks
November 04, 2017

Getting started with PHP core development - phpCE 2017

Joind.in: https://joind.in/talk/3e087

While PHP’s open source community is thriving, only a relatively small number of PHP developers contribute to the number one tool they use on a daily basis: PHP itself. Although it may seem daunting to help out on a project written in C, requiring compilation and linking, getting started is actually pretty easy.

In this talk I would like to share my own recent experiences of beginning to contribute to PHP itself. We will cover some practical tips, workflows and resources, so you can start giving back to your programming language straight away!

8dfcb5f1b3cd5397f19780e2319694da?s=128

Arnout Boks

November 04, 2017
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  1. Getting started with PHP core development @arnoutboks Arnout Boks #phpce17

    04 nov 2017
  2. @arnoutboks #phpce17 PHP Open Source projects

  3. @arnoutboks #phpce17 PHP Open Source projects 4175 contributors 418 contributors

    689 contributors 1497 contributors
  4. @arnoutboks #phpce17 PHP Open Source projects 4175 contributors 418 contributors

    689 contributors 1497 contributors
  5. @arnoutboks #phpce17 PHP Open Source projects 4175 contributors 418 contributors

    689 contributors 1497 contributors 497 contributors?
  6. @arnoutboks #phpce17 We all use PHP intensively Why not contribute

    something back?
  7. @arnoutboks #phpce17 The catch

  8. @arnoutboks #phpce17 The core

  9. @arnoutboks #phpce17 The core

  10. @arnoutboks #phpce17 My own journey to the core

  11. @arnoutboks #phpce17 Once upon a time… <something xmlns:foo="http://www.example.com"> <foo:bar />

    <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagName( "bar" )->length;
  12. @arnoutboks #phpce17 Once upon a time… <something xmlns:foo="http://www.example.com"> <foo:bar />

    <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagName( "bar" )->length; // returns 2
  13. @arnoutboks #phpce17 Once upon a time… <something xmlns:foo="http://www.example.com"> <foo:bar />

    <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagNameNS( "http://www.example.com", "bar" )->length;
  14. @arnoutboks #phpce17 Once upon a time… <something xmlns:foo="http://www.example.com"> <foo:bar />

    <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagNameNS( "http://www.example.com", "bar" )->length; // returns 1
  15. @arnoutboks #phpce17 Once upon a time… <something xmlns:foo="http://www.example.com"> <foo:bar />

    <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagNameNS( "", "bar" )->length;
  16. @arnoutboks #phpce17 Once upon a time… <something xmlns:foo="http://www.example.com"> <foo:bar />

    <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagNameNS( "", "bar" )->length; // returns 0!
  17. @arnoutboks #phpce17 Once upon a time…

  18. Reporting bugs Gotta squash them all

  19. @arnoutboks #phpce17 bugs.php.net PHP bug tracker

  20. @arnoutboks #phpce17 Write good bug reports https://bugs.php.net/how-to-report.php

  21. @arnoutboks #phpce17 Search for existing bugs

  22. @arnoutboks #phpce17 Sharpen bug details Report different platform/version: Don’t post

    “me too” comments – Do post other relevant details
  23. @arnoutboks #phpce17 Help triaging bugs! https://bugs.php.net/random

  24. Building PHP from git Living on the edge

  25. @arnoutboks #phpce17 1. Install dependencies • git • build-essential •

    autoconf • automake • libtool • re2c • bison • libxml2-dev Using apt-get, yum or similar
  26. @arnoutboks #phpce17 2. Clone php-src $ git clone https://github.com/php/php- src.git

  27. @arnoutboks #phpce17 3. Generate ./configure script $ ./buildconf Uses autoconf

    to build ./configure from: • configure.ac • acinclude.m4 • sapi/*/config.m4 • ext/*/config.m4
  28. @arnoutboks #phpce17 4. Run ./configure script $ ./configure Configure a

    default PHP build: $ ./configure --disable-cgi --disable-dom --enable-opcache --with-sqlite3=/path /to/sqlite3 Customize extensions/SAPIs: $ ./configure --help See all options
  29. @arnoutboks #phpce17 5. Compile $ make $ make –j`nproc` To

    use all available cores (faster):
  30. @arnoutboks #phpce17 6. Run! $ sapi/cli/php --version Binaries are in

    sapi/cli/php and sapi/cgi/php-cgi:
  31. @arnoutboks #phpce17 Subsequent builds $ make Just run Recompiles only

    files that have changed
  32. @arnoutboks #phpce17 Cleanup scripts $ make clean # try again

    $ make distclean # try again $ ./buildconf –force $ ./configure # try again If compilation fails, try (subsequently): Might be needed after git pull/git checkout
  33. @arnoutboks #phpce17 https://wiki.php.net/ internals/windows/stepbystepbuild Building for Windows

  34. @arnoutboks #phpce17

  35. @arnoutboks #phpce17 Vagrant boxes for compiling PHP core

  36. @arnoutboks #phpce17 ramsey/vagrant-php-src-dev

  37. @arnoutboks #phpce17 rlerdorf/php7dev

  38. Running tests for PHP Human-driven CI

  39. @arnoutboks #phpce17 Running tests $ sapi/cli/php run-tests.php –p `pwd`/sapi/cli/php

  40. @arnoutboks #phpce17 Running tests $ sapi/cli/php run-tests.php –p `pwd`/sapi/cli/php $

    sapi/cli/php run-tests.php -P
  41. @arnoutboks #phpce17 Running tests $ sapi/cli/php run-tests.php –p `pwd`/sapi/cli/php $

    sapi/cli/php run-tests.php -P $ make test
  42. @arnoutboks #phpce17 Running tests $ make test […] PASS Trivial

    "Hello World" test [tests/basic/001.phpt] SKIP Check libcurl config on windows [ext/curl/tests/check_win_config.phpt] reason: for windows only FAIL Compiled regex cache limit [ext/pcre/tests/cache_limit.phpt] XFAIL Inconsistencies when accessing protected members [Zend/tests/access_modifiers_008.phpt] XFAIL REASON: Discussion: http://marc.info/?l=php- internals&m=120221184420957&w=2 […10000 more lines of output…]
  43. @arnoutboks #phpce17 Running tests $ make test […] ========================================= TEST

    RESULT SUMMARY ----------------------------------------- Number of tests : 14896 12520 Tests skipped : 2376 ( 16.0%) -------- Tests warned : 2 ( 0.0%) ( 0.0%) Tests failed : 175 ( 1.2%) ( 1.4%) Expected fail : 44 ( 0.3%) ( 0.4%) Tests passed : 12299 ( 82.6%) ( 98.2%) ----------------------------------------- Time taken : 2937 seconds
  44. @arnoutboks #phpce17 Running tests $ make test […] You may

    have found a problem in PHP. This report can be automatically sent to the PHP QA team at http://qa.php.net/reports and http://news.php.net/php.qa.reports This gives us a better understanding of PHP's behavior. If you don't want to send the report immediately you can choose option "s" to save it. You can then email it to qa- reports@lists.php.net later. Do you want to send this report now? [Yns]:|
  45. @arnoutboks #phpce17 Options for running tests $ make test TESTS=path/to/my/test.phpt

    $ make test TESTS=ext/dom $ make test TESTS="--verbose ext/dom" $ make test TESTS=--help
  46. Writing tests for PHP “the best bug report”

  47. @arnoutboks #phpce17 Steps to reproduce for bug #67474 Test script:

    --------------- $doc = new DOMDocument(); $doc->loadXML('<root xmlns:x="x"><a/><x:a/></root>'); $list = $doc->getElementsByTagNameNS('', 'a'); echo $list->length; Expected result: ---------------- 1 Actual result: -------------- 0
  48. @arnoutboks #phpce17 Testing tools My PHP projects: • Written in

    PHP • Tests written in PHP (PHPUnit, PHPSpec, CodeCeption) The PHP core: • Written in C • Tests written in …
  49. @arnoutboks #phpce17 PHP is tested using PHPT tests No C

    skills required
  50. @arnoutboks #phpce17 PHPT tests • Basically plaintext with sections •

    Some sections (can) contain PHP code • Let PHP print some output… • …and check against expected output
  51. @arnoutboks #phpce17 Very simple PHPT test --TEST-- Basic arithmetic -

    addition --FILE-- <?php var_dump(42 + 1); ?> --EXPECT-- int(43)
  52. @arnoutboks #phpce17 Very simple PHPT test --TEST-- Basic arithmetic -

    addition --FILE-- <?php var_dump(42 + 1); ?> --EXPECT-- int(43)
  53. @arnoutboks #phpce17 Folder structure tests/ and Zend/tests/ tests for the

    Zend engine ext/standard/tests /<group>/ tests for ‘standard’ extension (array, string, filesystem functions, etc.) ext/<name>/tests/ tests for extensions sapi/<name>/tests/ tests for specific SAPI’s (CGI, CLI, PHP-FPM, etc.)
  54. @arnoutboks #phpce17 Naming PHPT files bug<bugid>.phpt tests for bugs <func>_basic[<no>].phpt

    <func>_variation[<no>].phpt <func>_error[<no>].phpt tests for specific function: basic behavior, variations (edge cases, etc.) and errors <extname><no>.phpt general tests for extension
  55. @arnoutboks #phpce17 PHPT test for bug #67474 --TEST-- Bug #67474

    getElementsByTagNameNS and default namespace --FILE-- <?php $doc = new DOMDocument(); $doc->loadXML('<root xmlns:x="x"><a/><x:a/></root>'); $list = $doc->getElementsByTagNameNS('', 'a'); var_dump($list->length); ?> --EXPECT-- int(1)
  56. @arnoutboks #phpce17 Running our PHPT test $ make test TESTS=ext/dom/tests/bug67474.phpt

    […] ============================================= FAILED TEST SUMMARY --------------------------------------------- Bug #67474 getElementsByTagNameNS and default namespace [ext/dom/tests/bug67474.phpt] =============================================
  57. @arnoutboks #phpce17 Running our PHPT test Files generated for our

    failing test: bug67474.php contents of the FILE section bug67474.sh shell script for running the .php file bug67474.exp expected output bug67474.out actual output bug67474.diff diff between expected and actual bug67474.log log file, including the expected and actual output
  58. @arnoutboks #phpce17 Running our PHPT test $ cat ext/dom/tests/bug67474.log ----

    EXPECTED OUTPUT int(1) ---- ACTUAL OUTPUT int(0) ---- FAILED
  59. @arnoutboks #phpce17 Running our PHPT test

  60. @arnoutboks #phpce17 Sections in PHPT files SKIPIF: Skips the test

    if the code in SKIPIF generates output containing ‘skip’ somewhere --SKIPIF-- <?php if(!extension_loaded('dom')) { die('skip dom extension not available'); } ?>
  61. @arnoutboks #phpce17 Sections in PHPT files Tip: Extensions usually have

    a skipif.inc shared between tests: --SKIPIF-- <?php require_once('skipif.inc'); ?>
  62. @arnoutboks #phpce17 Sections in PHPT files GET, POST, PUT, COOKIE,

    HEADERS: Simulate HTTP input to the PHP script in FILE --GET-- foo=bar&baz=qux --COOKIE-- foo=bar;baz=qux --FILE-- <?php var_dump($_GET['foo'] . $_COOKIE['foo']); ?> --EXPECT-- string(6) "barbar"
  63. @arnoutboks #phpce17 Sections in PHPT files EXPECTF, EXPECTREGEX: Expect the

    output to match a certain format --FILE-- <?php $fp = fopen(__FILE__, 'r'); var_dump($fp); ?> --EXPECTF-- resource(%d) of type (stream)
  64. @arnoutboks #phpce17 Sections in PHPT files CLEAN: Cleans up after

    the test --FILE-- <?php touch(__DIR__ . '/foo.tmp'); var_dump(file_exists(__DIR__ . '/foo.tmp')); ?> --EXPECT-- bool(true) --CLEAN-- <?php unlink(__DIR__ . '/foo.tmp'); ?>
  65. @arnoutboks #phpce17 Sections in PHPT files INI: Specifies custom php.ini

    directives for the test --INI-- precision=7 --FILE-- <?php var_dump(pi()); ?> --EXPECT-- float(3.141593)
  66. @arnoutboks #phpce17 Sections in PHPT files XFAIL: Indicates the test

    is expected to fail, and gives an explanation why This is usually used for hard-to-fix bugs or bugs in upstream code. --XFAIL-- See bug #xxxx
  67. @arnoutboks #phpce17 Easy way to get started without C skills

  68. @arnoutboks #phpce17 Finding untested code gcov.php.net

  69. @arnoutboks #phpce17 PHP TestFest 2017 • September-December 2017 • Participate

    through a local User Group or by yourself • Write tests for PHP core • Win prizes! See phptestfest.org
  70. @arnoutboks #phpce17 Resources • https://qa.php.net/write-test.php • https://www.sammyk.me/compiling-php-from-source- writing-tests-for-php-source • https://phptestfest.org/tutorials/

    • http://www.phpinternalsbook.com/#testing-php- source • https://github.com/herdphp/docker-phpqa • https://gist.github.com/SammyK/4a5cf70d0973731d0c 85b151a323ea2d
  71. Editing the PHP source Your own custom PHP

  72. @arnoutboks #phpce17 A shot at fixing #67474: recap <something xmlns:foo="http://www.example.com">

    <foo:bar /> <bar /> </something> <?php $doc = /* … (DOMDocument) */; $doc->getElementsByTagNameNS( "", "bar" )->length; // returns 0!
  73. @arnoutboks #phpce17 A shot at fixing #67474 php-src/ext/dom/ • attr.c

    • document.c • dom_ce.h • dom_fe.h • element.c • node.c • php_dom.c • xpath.c • …
  74. @arnoutboks #phpce17 A shot at fixing #67474 const zend_function_entry php_dom_document_class_functions[]

    = { // ... PHP_FALIAS(createAttributeNS, dom_document_create_attribute_ns, arginfo_dom_document_create_attribute_ns) PHP_FALIAS(getElementsByTagNameNS, dom_document_get_elements_by_tag_name_ns, arginfo_dom_document_get_elements_by_tag_name_ns) PHP_FALIAS(getElementById, dom_document_get_element_by_id, arginfo_dom_document_get_element_by_id) // ... PHP_ME(domdocument, __construct, arginfo_dom_document_construct, ZEND_ACC_PUBLIC) PHP_FE_END };
  75. @arnoutboks #phpce17 A shot at fixing #67474

  76. @arnoutboks #phpce17 A shot at fixing #67474 const zend_function_entry php_dom_document_class_functions[]

    = { // ... PHP_FALIAS(createAttributeNS, dom_document_create_attribute_ns, arginfo_dom_document_create_attribute_ns) PHP_FALIAS(getElementsByTagNameNS, dom_document_get_elements_by_tag_name_ns, arginfo_dom_document_get_elements_by_tag_name_ns) PHP_FALIAS(getElementById, dom_document_get_element_by_id, arginfo_dom_document_get_element_by_id) // ... PHP_ME(domdocument, __construct, arginfo_dom_document_construct, ZEND_ACC_PUBLIC) PHP_FE_END };
  77. @arnoutboks #phpce17 A shot at fixing #67474 PHP_FUNCTION(dom_document_get_elements_by_tag_name_ns) { //

    ... if (zend_parse_method_parameters(ZEND_NUM_ARGS(), getThis(), "Oss", &id, dom_document_class_entry, &uri, &uri_len, &name, &name_len) == FAILURE) { return; } // ... local = xmlCharStrndup(name, name_len); nsuri = xmlCharStrndup(uri, uri_len); dom_namednode_iter(intern, 0, namednode, NULL, local, nsuri); }
  78. @arnoutboks #phpce17 zend_parse_parameters (‘ZPP’) PHP_FUNCTION(dom_document_get_elements_by_tag_name_ns) { // ... if (zend_parse_method_parameters(ZEND_NUM_ARGS(),

    getThis(), "Oss", &id, dom_document_class_entry, &uri, &uri_len, &name, &name_len) == FAILURE) { return; } // ... local = xmlCharStrndup(name, name_len); nsuri = xmlCharStrndup(uri, uri_len); dom_namednode_iter(intern, 0, namednode, NULL, local, nsuri); } s: string s: string O: object of given class
  79. @arnoutboks #phpce17 zend_parse_parameters (‘ZPP’) Specifier Type a array b boolean

    d double/float l long/int o object (any type) O object (specific type) r resource s string z mixed
  80. @arnoutboks #phpce17 zend_parse_parameters (‘ZPP’) Modifier Effect * variable number of

    arguments (0 or more) + variable number of arguments (1 or more) ! optional argument | remaining arguments are optional See README.PARAMETER_PARSING_API
  81. @arnoutboks #phpce17 A shot at fixing #67474 PHP_FUNCTION(dom_document_get_elements_by_tag_name_ns) { //

    ... if (zend_parse_method_parameters(ZEND_NUM_ARGS(), getThis(), "Oss", &id, dom_document_class_entry, &uri, &uri_len, &name, &name_len) == FAILURE) { return; } // ... local = xmlCharStrndup(name, name_len); nsuri = xmlCharStrndup(uri, uri_len); dom_namednode_iter(intern, 0, namednode, NULL, local, nsuri); }
  82. @arnoutboks #phpce17 A shot at fixing #67474 xmlNode *dom_get_elements_by_tag_name_ns_raw( xmlNodePtr

    nodep, char *ns, char *local /* ... */ ) { // ... if ( ns == NULL || (nodep->ns != NULL && ( xmlStrEqual(nodep->ns->href, (xmlChar *)ns) || xmlStrEqual((xmlChar *)"*", (xmlChar *)ns) ))) { { } // ... } Eventually:
  83. @arnoutboks #phpce17 A shot at fixing #67474 xmlNode *dom_get_elements_by_tag_name_ns_raw( xmlNodePtr

    nodep, char *ns, char *local /* ... */ ) { // ... if ( ns == NULL || (nodep->ns != NULL && ( xmlStrEqual(nodep->ns->href, (xmlChar *)ns) || xmlStrEqual((xmlChar *)"*", (xmlChar *)ns) ))) { { } // ... } Eventually: element to ‘test’ filter namespace and local name
  84. @arnoutboks #phpce17 A shot at fixing #67474 xmlNode *dom_get_elements_by_tag_name_ns_raw( xmlNodePtr

    nodep, char *ns, char *local /* ... */ ) { // ... if ( ns == NULL || (nodep->ns != NULL && ( xmlStrEqual(nodep->ns->href, (xmlChar *)ns) || xmlStrEqual((xmlChar *)"*", (xmlChar *)ns) ))) { { } // ... } Eventually: “no namespace filter”
  85. @arnoutboks #phpce17 A shot at fixing #67474 xmlNode *dom_get_elements_by_tag_name_ns_raw( xmlNodePtr

    nodep, char *ns, char *local /* ... */ ) { // ... if ( ns == NULL || (nodep->ns != NULL && ( xmlStrEqual(nodep->ns->href, (xmlChar *)ns) || xmlStrEqual((xmlChar *)"*", (xmlChar *)ns) ))) { { } // ... } Eventually: “node has a namespace”, and “it matches the filter” or “filter is a wildcard”
  86. @arnoutboks #phpce17 A shot at fixing #67474 xmlNode *dom_get_elements_by_tag_name_ns_raw( xmlNodePtr

    nodep, char *ns, char *local /* ... */ ) { // ... if ( ns == NULL || (nodep->ns == NULL && !strcmp(ns, "")) || (nodep->ns != NULL && ( xmlStrEqual(nodep->ns->href, (xmlChar *)ns) || xmlStrEqual((xmlChar *)"*", (xmlChar *)ns) ))) { { } // ... } Fix: “node has empty namespace”, and “filter is empty namespace”
  87. @arnoutboks #phpce17 A shot at fixing #67474 $ make test

    TESTS=ext/dom/tests/bug67474.phpt […] ============================================= Running selected tests. PASS Bug #67474 getElementsByTagNameNS and default namespace [ext/dom/tests/bug67474.phpt] ============================================= Number of tests : 1 1 Tests passed : 1 (100.0%) (100.0%)
  88. @arnoutboks #phpce17 PR for #67474 Submit bug fixes to the

    oldest supported branch: http://php.net/supported-versions.php
  89. @arnoutboks #phpce17 PR for #67474: 2 hours later…

  90. @arnoutboks #phpce17 It’s not THAT difficult • Read past the

    C details • See where variables come from and go to • Try and see what happens • Rely on the tests
  91. @arnoutboks #phpce17 Bug fixes vs. features • Bug fixes •

    PR against oldest supported branch • No RFC needed • Refer to bug in bug tracker • Features • PR against master • Formal RFC needed
  92. @arnoutboks #phpce17 RFC Process • Introduce idea, measure initial reaction

    • Formal proposal • Discussion period • Voting See https://wiki.php.net/rfc/howto & https://benramsey.com/talks/2015/06/dpc-contributing-core/
  93. @arnoutboks #phpce17 Resources • http://php.net/internals • https://wiki.php.net/internals • https://wiki.php.net/internals/references •

    http://www.phpinternalsbook.com/ • http://blog.jpauli.tech/ • https://nikic.github.io/ • https://blog.ircmaxell.com/search/label/PHP-Internals • https://www.sammyk.me/how-to-find-and-patch-a- bug-in-php-source-php-internals • https://externals.io
  94. @arnoutboks #phpce17 Some time later…

  95. Documentation for PHP Contributing to the manual

  96. @arnoutboks #phpce17 edit.php.net PHP Docbook Online Editor ‘Easiest’ way to

    get started (if you can figure out how it works)
  97. @arnoutboks #phpce17 edit.php.net

  98. @arnoutboks #phpce17 edit.php.net

  99. @arnoutboks #phpce17 edit.php.net

  100. @arnoutboks #phpce17 1. Find the correct file Under “All files”

    in the left menu Top-level directory per language • reference/<ext|group>/ • functions/<function>.xml • <class>.xml • <class>/<method>.xml • language/<feature>.xml
  101. @arnoutboks #phpce17 1. Find the correct file Tip: Use the

    “edit” link from the documentation itself
  102. @arnoutboks #phpce17 2. Make your changes Documentation is written using

    DocBook XML http://tdg.docbook.org/tdg/5.2/
  103. @arnoutboks #phpce17 2. Make your changes Documentation is written using

    DocBook XML (or just peek in other docs)
  104. @arnoutboks #phpce17 3. Preview changes Seems broken at the moment

  105. @arnoutboks #phpce17 4. Submit as patch

  106. @arnoutboks #phpce17 Tip: submit a docs patch instead of user

    note
  107. @arnoutboks #phpce17 Documentation karma Commit changes yourself? Apply for docs

    karma! http://doc.php.net/tutorial/ joining.php
  108. @arnoutboks #phpce17 Don’t like edit.php.net? Make it better!

  109. @arnoutboks #phpce17 Don’t like edit.php.net? Or set up your own

    development environment: https://www.sammyk.me/how-to-contribute-to-php- documentation
  110. @arnoutboks #phpce17 Where to help? • Bugs of type “Doc”

    in bug tracker • check-missing-docs.php • “Failures to meet strict standards” (in edit.php.net sidebar) • Translations • Missing translations • Out-of-date translations • Translations needing review • Translations with errors
  111. @arnoutboks #phpce17 Where to help?

  112. Recap Ways to contribute to PHP

  113. @arnoutboks #phpce17 Numerous ways to contribute • Bug reports •

    Bug triaging • Running tests • Writing tests • Bug fixes • New features • Documentation patches • Documentation translations • Tools
  114. @arnoutboks #phpce17 Why contribute? • Get ‘own’ bugs fixed •

    Fix ‘own’ bugs • Make PHP better • Make PHP more popular • Learn PHP • Learn C • Give back to the community
  115. @arnoutboks #phpce17 You don’t have to be an expert to

    contribute to the PHP core
  116. @arnoutboks #phpce17 Feedback & Questions @arnoutboks @arnoutboks @aboks Arnout Boks

    Please leave your feedback on joind.in: https://joind.in/talk/3e087
  117. @arnoutboks #phpce17 Image Credits • https://photojournal.jpl.nasa.gov/jpeg/PIA19058.jpg • https://www.flickr.com/photos/vivianejl/22990832 • https://www.flickr.com/photos/dhuiz/13609080315/

    • https://www.flickr.com/photos/gotovan/8671348572 • https://www.flickr.com/photos/sidelong/246816211 • https://www.flickr.com/photos/pezon64/23764268828 • https://www.flickr.com/photos/crdot/6855538268/ • https://www.flickr.com/photos/borkurdotnet/9682277597