composer/Plugin/Scaffold/Interpolator.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold;
+
+/**
+ * Injects config values from an associative array into a string.
+ *
+ * @internal
+ */
+class Interpolator {
+
+  /**
+   * The character sequence that identifies the start of a token.
+   *
+   * @var string
+   */
+  protected $startToken;
+
+  /**
+   * The character sequence that identifies the end of a token.
+   *
+   * @var string
+   */
+  protected $endToken;
+
+  /**
+   * The associative array of replacements.
+   *
+   * @var array
+   */
+  protected $data = [];
+
+  /**
+   * Interpolator constructor.
+   *
+   * @param string $start_token
+   *   The start marker for a token, e.g. '['.
+   * @param string $end_token
+   *   The end marker for a token, e.g. ']'.
+   */
+  public function __construct($start_token = '\\[', $end_token = '\\]') {
+    $this->startToken = $start_token;
+    $this->endToken = $end_token;
+  }
+
+  /**
+   * Sets the data set to use when interpolating.
+   *
+   * @param array $data
+   *   The key:value pairs to use when interpolating.
+   *
+   * @return $this
+   */
+  public function setData(array $data) {
+    $this->data = $data;
+    return $this;
+  }
+
+  /**
+   * Adds to the data set to use when interpolating.
+   *
+   * @param array $data
+   *   The key:value pairs to use when interpolating.
+   *
+   * @return $this
+   */
+  public function addData(array $data) {
+    $this->data = array_merge($this->data, $data);
+    return $this;
+  }
+
+  /**
+   * Replaces tokens in a string with values from an associative array.
+   *
+   * Tokens are surrounded by delimiters, e.g. square brackets "[key]". The
+   * characters that surround the key may be defined when the Interpolator is
+   * constructed.
+   *
+   * Example:
+   * If the message is 'Hello, [user.name]', then the value of the user.name
+   * item is fetched from the array, and the token [user.name] is replaced with
+   * the result.
+   *
+   * @param string $message
+   *   Message containing tokens to be replaced.
+   * @param array $extra
+   *   Data to use for interpolation in addition to whatever was provided to
+   *   self::setData().
+   * @param string|bool $default
+   *   (optional) The value to substitute for tokens that are not found in the
+   *   data. If FALSE, then missing tokens are not replaced. Defaults to an
+   *   empty string.
+   *
+   * @return string
+   *   The message after replacements have been made.
+   */
+  public function interpolate($message, array $extra = [], $default = '') {
+    $data = $extra + $this->data;
+    $replacements = $this->replacements($message, $data, $default);
+    return strtr($message, $replacements);
+  }
+
+  /**
+   * Finds the tokens that exist in a message and builds a replacement array.
+   *
+   * All of the replacements in the data array are looked up given the token
+   * keys from the provided message. Keys that do not exist in the configuration
+   * are replaced with the default value.
+   *
+   * @param string $message
+   *   String with tokens.
+   * @param array $data
+   *   Data to use for interpolation.
+   * @param string $default
+   *   (optional) The value to substitute for tokens that are not found in the
+   *   data. If FALSE, then missing tokens are not replaced. Defaults to an
+   *   empty string.
+   *
+   * @return string[]
+   *   An array of replacements to make. Keyed by tokens and the replacements
+   *   are the values.
+   */
+  protected function replacements($message, array $data, $default = '') {
+    $tokens = $this->findTokens($message);
+    $replacements = [];
+    foreach ($tokens as $sourceText => $key) {
+      $replacement_text = array_key_exists($key, $data) ? $data[$key] : $default;
+      if ($replacement_text !== FALSE) {
+        $replacements[$sourceText] = $replacement_text;
+      }
+    }
+    return $replacements;
+  }
+
+  /**
+   * Finds all of the tokens in the provided message.
+   *
+   * @param string $message
+   *   String with tokens.
+   *
+   * @return string[]
+   *   map of token to key, e.g. {{key}} => key
+   */
+  protected function findTokens($message) {
+    $reg_ex = '#' . $this->startToken . '([a-zA-Z0-9._-]+)' . $this->endToken . '#';
+    if (!preg_match_all($reg_ex, $message, $matches, PREG_SET_ORDER)) {
+      return [];
+    }
+    $tokens = [];
+    foreach ($matches as $matchSet) {
+      [$sourceText, $key] = $matchSet;
+      $tokens[$sourceText] = $key;
+    }
+    return $tokens;
+  }
+
+}

composer/Plugin/Scaffold/LICENSE.txt

+        GNU GENERAL PUBLIC LICENSE
+           Version 2, June 1991
+
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+          Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+License is intended to guarantee your freedom to share and change free
+software--to make sure the software is free for all its users.  This
+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
+using it.  (Some other Free Software Foundation software is covered by
+the GNU Lesser General Public License instead.)  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+this service if you wish), that you receive source code or can get it
+if you want it, that you can change the software or use pieces of it
+in new free programs; and that you know you can do these things.
+
+  To protect your rights, we need to make restrictions that forbid
+anyone to deny you these rights or to ask you to surrender the rights.
+These restrictions translate to certain responsibilities for you if you
+distribute copies of the software, or if you modify it.
+
+  For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must give the recipients all the rights that
+you have.  You must make sure that they, too, receive or can get the
+source code.  And you must show them these terms so they know their
+rights.
+
+  We protect your rights with two steps: (1) copyright the software, and
+(2) offer you this license which gives you legal permission to copy,
+distribute and/or modify the software.
+
+  Also, for each author's protection and ours, we want to make certain
+that everyone understands that there is no warranty for this free
+software.  If the software is modified by someone else and passed on, we
+want its recipients to know that what they have is not the original, so
+that any problems introduced by others will not reflect on the original
+authors' reputations.
+
+  Finally, any free program is threatened constantly by software
+patents.  We wish to avoid the danger that redistributors of a free
+program will individually obtain patent licenses, in effect making the
+program proprietary.  To prevent this, we have made it clear that any
+patent must be licensed for everyone's free use or not licensed at all.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.
+
+        GNU GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License applies to any program or other work which contains
+a notice placed by the copyright holder saying it may be distributed
+under the terms of this General Public License.  The "Program", below,
+refers to any such program or work, and a "work based on the Program"
+means either the Program or any derivative work under copyright law:
+that is to say, a work containing the Program or a portion of it,
+either verbatim or with modifications and/or translated into another
+language.  (Hereinafter, translation is included without limitation in
+the term "modification".)  Each licensee is addressed as "you".
+
+Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running the Program is not restricted, and the output from the Program
+is covered only if its contents constitute a work based on the
+Program (independent of having been made by running the Program).
+Whether that is true depends on what the Program does.
+
+  1. You may copy and distribute verbatim copies of the Program's
+source code as you receive it, in any medium, provided that you
+conspicuously and appropriately publish on each copy an appropriate
+copyright notice and disclaimer of warranty; keep intact all the
+notices that refer to this License and to the absence of any warranty;
+and give any other recipients of the Program a copy of this License
+along with the Program.
+
+You may charge a fee for the physical act of transferring a copy, and
+you may at your option offer warranty protection in exchange for a fee.
+
+  2. You may modify your copy or copies of the Program or any portion
+of it, thus forming a work based on the Program, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) You must cause the modified files to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    b) You must cause any work that you distribute or publish, that in
+    whole or in part contains or is derived from the Program or any
+    part thereof, to be licensed as a whole at no charge to all third
+    parties under the terms of this License.
+
+    c) If the modified program normally reads commands interactively
+    when run, you must cause it, when started running for such
+    interactive use in the most ordinary way, to print or display an
+    announcement including an appropriate copyright notice and a
+    notice that there is no warranty (or else, saying that you provide
+    a warranty) and that users may redistribute the program under
+    these conditions, and telling the user how to view a copy of this
+    License.  (Exception: if the Program itself is interactive but
+    does not normally print such an announcement, your work based on
+    the Program is not required to print an announcement.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Program,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Program, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Program.
+
+In addition, mere aggregation of another work not based on the Program
+with the Program (or with a work based on the Program) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may copy and distribute the Program (or a work based on it,
+under Section 2) in object code or executable form under the terms of
+Sections 1 and 2 above provided that you also do one of the following:
+
+    a) Accompany it with the complete corresponding machine-readable
+    source code, which must be distributed under the terms of Sections
+    1 and 2 above on a medium customarily used for software interchange; or,
+
+    b) Accompany it with a written offer, valid for at least three
+    years, to give any third party, for a charge no more than your
+    cost of physically performing source distribution, a complete
+    machine-readable copy of the corresponding source code, to be
+    distributed under the terms of Sections 1 and 2 above on a medium
+    customarily used for software interchange; or,
+
+    c) Accompany it with the information you received as to the offer
+    to distribute corresponding source code.  (This alternative is
+    allowed only for noncommercial distribution and only if you
+    received the program in object code or executable form with such
+    an offer, in accord with Subsection b above.)
+
+The source code for a work means the preferred form of the work for
+making modifications to it.  For an executable work, complete source
+code means all the source code for all modules it contains, plus any
+associated interface definition files, plus the scripts used to
+control compilation and installation of the executable.  However, as a
+special exception, the source code distributed need not include
+anything that is normally distributed (in either source or binary
+form) with the major components (compiler, kernel, and so on) of the
+operating system on which the executable runs, unless that component
+itself accompanies the executable.
+
+If distribution of executable or object code is made by offering
+access to copy from a designated place, then offering equivalent
+access to copy the source code from the same place counts as
+distribution of the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  4. You may not copy, modify, sublicense, or distribute the Program
+except as expressly provided under this License.  Any attempt
+otherwise to copy, modify, sublicense or distribute the Program is
+void, and will automatically terminate your rights under this License.
+However, parties who have received copies, or rights, from you under
+this License will not have their licenses terminated so long as such
+parties remain in full compliance.
+
+  5. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Program or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Program (or any work based on the
+Program), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Program or works based on it.
+
+  6. Each time you redistribute the Program (or any work based on the
+Program), the recipient automatically receives a license from the
+original licensor to copy, distribute or modify the Program subject to
+these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties to
+this License.
+
+  7. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Program at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Program by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Program.
+
+If any portion of this section is held invalid or unenforceable under
+any particular circumstance, the balance of the section is intended to
+apply and the section as a whole is intended to apply in other
+circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system, which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  8. If the distribution and/or use of the Program is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Program under this License
+may add an explicit geographical distribution limitation excluding
+those countries, so that distribution is permitted only in or among
+countries not thus excluded.  In such case, this License incorporates
+the limitation as if written in the body of this License.
+
+  9. The Free Software Foundation may publish revised and/or new versions
+of the General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Program
+specifies a version number of this License which applies to it and "any
+later version", you have the option of following the terms and conditions
+either of that version or of any later version published by the Free
+Software Foundation.  If the Program does not specify a version number of
+this License, you may choose any version ever published by the Free Software
+Foundation.
+
+  10. If you wish to incorporate parts of the Program into other free
+programs whose distribution conditions are different, write to the author
+to ask for permission.  For software which is copyrighted by the Free
+Software Foundation, write to the Free Software Foundation; we sometimes
+make exceptions for this.  Our decision will be guided by the two goals
+of preserving the free status of all derivatives of our free software and
+of promoting the sharing and reuse of software generally.
+
+          NO WARRANTY
+
+  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
+FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
+OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
+PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
+OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
+TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
+PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
+REPAIR OR CORRECTION.
+
+  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
+REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
+INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
+OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
+TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
+YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
+PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGES.
+
+         END OF TERMS AND CONDITIONS
+
+      How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+Also add information on how to contact you by electronic and paper mail.
+
+If the program is interactive, make it output a short notice like this
+when it starts in an interactive mode:
+
+    Gnomovision version 69, Copyright (C) year name of author
+    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, the commands you use may
+be called something other than `show w' and `show c'; they could even be
+mouse-clicks or menu items--whatever suits your program.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the program, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the program
+  `Gnomovision' (which makes passes at compilers) written by James Hacker.
+
+  <signature of Ty Coon>, 1 April 1989
+  Ty Coon, President of Vice
+
+This General Public License does not permit incorporating your program into
+proprietary programs.  If your program is a subroutine library, you may
+consider it more useful to permit linking proprietary applications with the
+library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.

composer/Plugin/Scaffold/ManageGitIgnore.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold;
+
+use Composer\IO\IOInterface;
+
+/**
+ * Manage the .gitignore file.
+ *
+ * @internal
+ */
+class ManageGitIgnore {
+
+  /**
+   * Composer's I/O service.
+   *
+   * @var \Composer\IO\IOInterface
+   */
+  protected $io;
+
+  /**
+   * The directory where the project is located.
+   *
+   * @var string
+   */
+  protected $dir;
+
+  /**
+   * ManageGitIgnore constructor.
+   *
+   * @param \Composer\IO\IOInterface $io
+   *   The Composer IO interface.
+   * @param string $dir
+   *   The directory where the project is located.
+   */
+  public function __construct(IOInterface $io, $dir) {
+    $this->io = $io;
+    $this->dir = $dir;
+  }
+
+  /**
+   * Manages gitignore files.
+   *
+   * @param \Drupal\Composer\Plugin\Scaffold\Operations\ScaffoldResult[] $files
+   *   A list of scaffold results, each of which holds a path and whether
+   *   or not that file is managed.
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldOptions $options
+   *   Configuration options from the composer.json extras section.
+   */
+  public function manageIgnored(array $files, ScaffoldOptions $options) {
+    if (!$this->managementOfGitIgnoreEnabled($options)) {
+      return;
+    }
+
+    // Accumulate entries to add to .gitignore, sorted into buckets based on the
+    // location of the .gitignore file the entry should be added to.
+    $add_to_git_ignore = [];
+    foreach ($files as $scaffoldResult) {
+      $path = $scaffoldResult->destination()->fullPath();
+      $is_ignored = Git::checkIgnore($this->io, $path, $this->dir);
+      if (!$is_ignored) {
+        $is_tracked = Git::checkTracked($this->io, $path, $this->dir);
+        if (!$is_tracked && $scaffoldResult->isManaged()) {
+          $dir = realpath(dirname($path));
+          $name = basename($path);
+          $add_to_git_ignore[$dir][] = '/' . $name;
+        }
+      }
+    }
+    // Write out the .gitignore files one at a time.
+    foreach ($add_to_git_ignore as $dir => $entries) {
+      $this->addToGitIgnore($dir, $entries);
+    }
+  }
+
+  /**
+   * Determines whether we should manage gitignore files.
+   *
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldOptions $options
+   *   Configuration options from the composer.json extras section.
+   *
+   * @return bool
+   *   Whether or not gitignore files should be managed.
+   */
+  protected function managementOfGitIgnoreEnabled(ScaffoldOptions $options) {
+    // If the composer.json stipulates whether gitignore is managed or not, then
+    // follow its recommendation.
+    if ($options->hasGitIgnore()) {
+      return $options->gitIgnore();
+    }
+
+    // Do not manage .gitignore if there is no repository here.
+    if (!Git::isRepository($this->io, $this->dir)) {
+      return FALSE;
+    }
+
+    // If the composer.json did not specify whether or not .gitignore files
+    // should be managed, then manage them if the vendor directory is ignored.
+    return Git::checkIgnore($this->io, 'vendor', $this->dir);
+  }
+
+  /**
+   * Adds a set of entries to the specified .gitignore file.
+   *
+   * @param string $dir
+   *   Path to directory where gitignore should be written.
+   * @param string[] $entries
+   *   Entries to write to .gitignore file.
+   */
+  protected function addToGitIgnore($dir, array $entries) {
+    sort($entries);
+    $git_ignore_path = $dir . '/.gitignore';
+    $contents = '';
+
+    // Appending to existing .gitignore files.
+    if (file_exists($git_ignore_path)) {
+      $contents = file_get_contents($git_ignore_path);
+      if (!empty($contents) && !str_ends_with($contents, "\n")) {
+        $contents .= "\n";
+      }
+    }
+
+    $contents .= implode("\n", $entries);
+    file_put_contents($git_ignore_path, $contents);
+  }
+
+}

composer/Plugin/Scaffold/ManageOptions.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold;
+
+use Composer\Composer;
+use Composer\Package\PackageInterface;
+use Composer\Util\Filesystem;
+
+/**
+ * Per-project options from the 'extras' section of the composer.json file.
+ *
+ * Projects that describe scaffold files do so via their scaffold options.
+ * This data is pulled from the 'drupal-scaffold' portion of the extras
+ * section of the project data.
+ *
+ * @internal
+ */
+class ManageOptions {
+
+  /**
+   * The Composer service.
+   *
+   * @var \Composer\Composer
+   */
+  protected $composer;
+
+  /**
+   * ManageOptions constructor.
+   *
+   * @param \Composer\Composer $composer
+   *   The Composer service.
+   */
+  public function __construct(Composer $composer) {
+    $this->composer = $composer;
+  }
+
+  /**
+   * Gets the root-level scaffold options for this project.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\ScaffoldOptions
+   *   The scaffold options object.
+   */
+  public function getOptions() {
+    return $this->packageOptions($this->composer->getPackage());
+  }
+
+  /**
+   * Gets the scaffold options for the stipulated project.
+   *
+   * @param \Composer\Package\PackageInterface $package
+   *   The package to fetch the scaffold options from.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\ScaffoldOptions
+   *   The scaffold options object.
+   */
+  public function packageOptions(PackageInterface $package) {
+    return ScaffoldOptions::create($package->getExtra());
+  }
+
+  /**
+   * Creates an interpolator for the 'locations' element.
+   *
+   * The interpolator returned will replace a path string with the tokens
+   * defined in the 'locations' element.
+   *
+   * Note that only the root package may define locations.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\Interpolator
+   *   Interpolator that will do replacements in a string using tokens in
+   *   'locations' element.
+   */
+  public function getLocationReplacements() {
+    return (new Interpolator())->setData($this->ensureLocations());
+  }
+
+  /**
+   * Ensures that all of the locations defined in the scaffold files exist.
+   *
+   * Create them on the filesystem if they do not.
+   */
+  protected function ensureLocations() {
+    $fs = new Filesystem();
+    $locations = $this->getOptions()->locations() + ['web_root' => './'];
+    $locations = array_map(function ($location) use ($fs) {
+      $fs->ensureDirectoryExists($location);
+      $location = realpath($location);
+      return $location;
+    }, $locations);
+    return $locations;
+  }
+
+}

composer/Plugin/Scaffold/Operations/AbstractOperation.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold\Operations;
+
+use Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath;
+
+/**
+ * Provides default behaviors for operations.
+ *
+ * @internal
+ */
+abstract class AbstractOperation implements OperationInterface {
+
+  /**
+   * Cached contents of scaffold file to be written to disk.
+   *
+   * @var string
+   */
+  protected $contents;
+
+  /**
+   * {@inheritdoc}
+   */
+  final public function contents() {
+    if (!isset($this->contents)) {
+      $this->contents = $this->generateContents();
+    }
+    return $this->contents;
+  }
+
+  /**
+   * Load the scaffold contents or otherwise generate what is needed.
+   *
+   * @return string
+   *   The contents of the scaffold file.
+   */
+  abstract protected function generateContents();
+
+  /**
+   * {@inheritdoc}
+   */
+  public function scaffoldOverExistingTarget(OperationInterface $existing_target) {
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function scaffoldAtNewLocation(ScaffoldFilePath $destination) {
+    return $this;
+  }
+
+}

composer/Plugin/Scaffold/Operations/AppendOp.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold\Operations;
+
+use Composer\IO\IOInterface;
+use Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath;
+use Drupal\Composer\Plugin\Scaffold\ScaffoldOptions;
+
+/**
+ * Scaffold operation to add to the beginning and/or end of a scaffold file.
+ *
+ * @internal
+ */
+class AppendOp extends AbstractOperation {
+
+  /**
+   * Identifies Append operations.
+   */
+  const ID = 'append';
+
+  /**
+   * Path to the source file to prepend, if any.
+   *
+   * @var \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath
+   */
+  protected $prepend;
+
+  /**
+   * Path to the source file to append, if any.
+   *
+   * @var \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath
+   */
+  protected $append;
+
+  /**
+   * Path to the default data to use when appending to an empty file.
+   *
+   * @var \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath
+   */
+  protected $default;
+
+  /**
+   * An indicator of whether the file we are appending to is managed or not.
+   *
+   * @var bool
+   */
+  protected $managed;
+
+  /**
+   * An indicator of whether we are allowed to append to a non-scaffolded file.
+   *
+   * @var bool
+   */
+  protected $forceAppend;
+
+  /**
+   * The contents from the file that we are prepending / appending to.
+   *
+   * @var string
+   */
+  protected $originalContents;
+
+  /**
+   * Constructs an AppendOp.
+   *
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath|null $prepend_path
+   *   (optional) The relative path to the prepend file.
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath|null $append_path
+   *   (optional) The relative path to the append file.
+   * @param bool $force_append
+   *   (optional) TRUE if is okay to append to a file that was not scaffolded.
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath|null $default_path
+   *   (optional) The relative path to the default data.
+   */
+  public function __construct(
+    ?ScaffoldFilePath $prepend_path = NULL,
+    ?ScaffoldFilePath $append_path = NULL,
+    $force_append = FALSE,
+    ?ScaffoldFilePath $default_path = NULL,
+  ) {
+    $this->forceAppend = $force_append;
+    $this->prepend = $prepend_path;
+    $this->append = $append_path;
+    $this->default = $default_path;
+    $this->managed = TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function generateContents() {
+    // Fetch the prepend contents, if provided.
+    $prepend_contents = '';
+    if (!empty($this->prepend)) {
+      $prepend_contents = file_get_contents($this->prepend->fullPath()) . "\n";
+    }
+    // Fetch the append contents, if provided.
+    $append_contents = '';
+    if (!empty($this->append)) {
+      $append_contents = "\n" . file_get_contents($this->append->fullPath());
+    }
+
+    // Get the original contents, or the default data if the original is empty.
+    $original_contents = $this->originalContents;
+    if (empty($original_contents) && !empty($this->default)) {
+      $original_contents = file_get_contents($this->default->fullPath());
+    }
+
+    // Attach it all together.
+    return $prepend_contents . $original_contents . $append_contents;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function process(ScaffoldFilePath $destination, IOInterface $io, ScaffoldOptions $options) {
+    $destination_path = $destination->fullPath();
+    $interpolator = $destination->getInterpolator();
+
+    // Be extra-noisy of creating a new file or appending to a non-scaffold
+    // file. Note that if the file already has the append contents, then the
+    // OperationFactory will make a SkipOp instead, and we will not get here.
+    if (!$this->managed) {
+      $message = '  - <info>NOTICE</info> Modifying existing file at <info>[dest-rel-path]</info>.';
+      if (!file_exists($destination_path)) {
+        $message = '  - <info>NOTICE</info> Creating a new file at <info>[dest-rel-path]</info>.';
+      }
+      $message .= ' Examine the contents and ensure that it came out correctly.';
+      $io->write($interpolator->interpolate($message));
+    }
+
+    // Notify that we are prepending, if there is prepend data.
+    if (!empty($this->prepend)) {
+      $this->prepend->addInterpolationData($interpolator, 'prepend');
+      $io->write($interpolator->interpolate("  - Prepend to <info>[dest-rel-path]</info> from <info>[prepend-rel-path]</info>"));
+    }
+    // Notify that we are appending, if there is append data.
+    if (!empty($this->append)) {
+      $this->append->addInterpolationData($interpolator, 'append');
+      $io->write($interpolator->interpolate("  - Append to <info>[dest-rel-path]</info> from <info>[append-rel-path]</info>"));
+    }
+
+    // Write the resulting data
+    file_put_contents($destination_path, $this->contents());
+
+    // Return a ScaffoldResult with knowledge of whether this file is managed.
+    return new ScaffoldResult($destination, $this->managed);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function scaffoldOverExistingTarget(OperationInterface $existing_target) {
+    $this->originalContents = $existing_target->contents();
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function scaffoldAtNewLocation(ScaffoldFilePath $destination) {
+    // If there is no existing scaffold file at the target location, then any
+    // append we do will be to an unmanaged file.
+    $this->managed = FALSE;
+
+    // Default: do not allow an append over a file that was not scaffolded.
+    if (!$this->forceAppend) {
+      $message = "  - Skip <info>[dest-rel-path]</info>: cannot append to a path that was not scaffolded unless 'force-append' property is set.";
+      return new SkipOp($message);
+    }
+
+    // If the target file does not exist, then we will allow the append to
+    // happen if we have default data to provide for it.
+    if (!file_exists($destination->fullPath())) {
+      if (!empty($this->default)) {
+        return $this;
+      }
+      $message = "  - Skip <info>[dest-rel-path]</info>: no file exists at the target path, and no default data provided.";
+      return new SkipOp($message);
+    }
+
+    // If the target file DOES exist, and it already contains the append/prepend
+    // data, then we will skip the operation.
+    $existingData = file_get_contents($destination->fullPath());
+    if ($this->existingFileHasData($existingData, $this->append) || $this->existingFileHasData($existingData, $this->prepend)) {
+      $message = "  - Skip <info>[dest-rel-path]</info>: the file already has the append/prepend data.";
+      return new SkipOp($message);
+    }
+
+    // Cache the original data to use during append.
+    $this->originalContents = $existingData;
+
+    return $this;
+  }
+
+  /**
+   * Check to see if the append/prepend data has already been applied.
+   *
+   * @param string $contents
+   *   The contents of the target file.
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath $data_path
+   *   The path to the data to append or prepend.
+   *
+   * @return bool
+   *   'TRUE' if the append/prepend data already exists in contents.
+   */
+  protected function existingFileHasData($contents, $data_path) {
+    if (empty($data_path)) {
+      return FALSE;
+    }
+    $data = file_get_contents($data_path->fullPath());
+
+    return str_contains($contents, $data);
+  }
+
+}

composer/Plugin/Scaffold/Operations/OperationFactory.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold\Operations;
+
+use Composer\Composer;
+use Composer\Package\PackageInterface;
+use Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath;
+
+/**
+ * Create Scaffold operation objects based on provided metadata.
+ *
+ * @internal
+ */
+class OperationFactory {
+
+  /**
+   * The Composer service.
+   *
+   * @var \Composer\Composer
+   */
+  protected $composer;
+
+  /**
+   * OperationFactory constructor.
+   *
+   * @param \Composer\Composer $composer
+   *   Reference to the 'Composer' object, since the Scaffold Operation Factory
+   *   is also responsible for evaluating relative package paths as it creates
+   *   scaffold operations.
+   */
+  public function __construct(Composer $composer) {
+    $this->composer = $composer;
+  }
+
+  /**
+   * Creates a scaffolding operation object as determined by the metadata.
+   *
+   * @param \Composer\Package\PackageInterface $package
+   *   The package that relative paths will be relative from.
+   * @param OperationData $operation_data
+   *   The parameter data for this operation object; varies by operation type.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\Operations\OperationInterface
+   *   The scaffolding operation object (skip, replace, etc.)
+   *
+   * @throws \RuntimeException
+   *   Exception thrown when parameter data does not identify a known scaffold
+   *   operation.
+   */
+  public function create(PackageInterface $package, OperationData $operation_data) {
+    switch ($operation_data->mode()) {
+      case SkipOp::ID:
+        return new SkipOp();
+
+      case ReplaceOp::ID:
+        return $this->createReplaceOp($package, $operation_data);
+
+      case AppendOp::ID:
+        return $this->createAppendOp($package, $operation_data);
+    }
+    throw new \RuntimeException("Unknown scaffold operation mode <comment>{$operation_data->mode()}</comment>.");
+  }
+
+  /**
+   * Creates a 'replace' scaffold op.
+   *
+   * Replace ops may copy or symlink, depending on settings.
+   *
+   * @param \Composer\Package\PackageInterface $package
+   *   The package that relative paths will be relative from.
+   * @param OperationData $operation_data
+   *   The parameter data for this operation object, i.e. the relative 'path'.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\Operations\OperationInterface
+   *   A scaffold replace operation object.
+   */
+  protected function createReplaceOp(PackageInterface $package, OperationData $operation_data) {
+    if (!$operation_data->hasPath()) {
+      throw new \RuntimeException("'path' component required for 'replace' operations.");
+    }
+    $package_name = $package->getName();
+    $package_path = $this->getPackagePath($package);
+    $source = ScaffoldFilePath::sourcePath($package_name, $package_path, $operation_data->destination(), $operation_data->path());
+    $op = new ReplaceOp($source, $operation_data->overwrite());
+    return $op;
+  }
+
+  /**
+   * Creates an 'append' (or 'prepend') scaffold op.
+   *
+   * @param \Composer\Package\PackageInterface $package
+   *   The package that relative paths will be relative from.
+   * @param OperationData $operation_data
+   *   The parameter data for this operation object, i.e. the relative 'path'.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\Operations\OperationInterface
+   *   A scaffold replace operation object.
+   */
+  protected function createAppendOp(PackageInterface $package, OperationData $operation_data) {
+    $package_name = $package->getName();
+    $package_path = $this->getPackagePath($package);
+    $prepend_source_file = NULL;
+    $append_source_file = NULL;
+    $default_data_file = NULL;
+    if ($operation_data->hasPrepend()) {
+      $prepend_source_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $operation_data->destination(), $operation_data->prepend());
+    }
+    if ($operation_data->hasAppend()) {
+      $append_source_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $operation_data->destination(), $operation_data->append());
+    }
+    if ($operation_data->hasDefault()) {
+      $default_data_file = ScaffoldFilePath::sourcePath($package_name, $package_path, $operation_data->destination(), $operation_data->default());
+    }
+    if (!$this->hasContent($prepend_source_file) && !$this->hasContent($append_source_file)) {
+      $message = '  - Keep <info>[dest-rel-path]</info> unchanged: no content to prepend / append was provided.';
+      return new SkipOp($message);
+    }
+
+    return new AppendOp($prepend_source_file, $append_source_file, $operation_data->forceAppend(), $default_data_file);
+  }
+
+  /**
+   * Checks to see if the specified scaffold file exists and has content.
+   *
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath|null $file
+   *   (optional) Scaffold file to check.
+   *
+   * @return bool
+   *   True if the file exists and has content.
+   */
+  protected function hasContent(?ScaffoldFilePath $file = NULL) {
+    if (!$file) {
+      return FALSE;
+    }
+    $path = $file->fullPath();
+    return is_file($path) && (filesize($path) > 0);
+  }
+
+  /**
+   * Gets the file path of a package.
+   *
+   * Note that if we call getInstallPath on the root package, we get the
+   * wrong answer (the installation manager thinks our package is in
+   * vendor). We therefore add special checking for this case.
+   *
+   * @param \Composer\Package\PackageInterface $package
+   *   The package.
+   *
+   * @return string
+   *   The file path.
+   */
+  protected function getPackagePath(PackageInterface $package) {
+    if ($package->getName() == $this->composer->getPackage()->getName()) {
+      // This will respect the --working-dir option if Composer is invoked with
+      // it. There is no API or method to determine the filesystem path of
+      // a package's composer.json file.
+      return getcwd();
+    }
+    return $this->composer->getInstallationManager()->getInstallPath($package);
+  }
+
+}

composer/Plugin/Scaffold/Operations/ScaffoldResult.php

+<?php
+
+namespace Drupal\Composer\Plugin\Scaffold\Operations;
+
+use Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath;
+
+/**
+ * Record the result of a scaffold operation.
+ *
+ * @internal
+ */
+class ScaffoldResult {
+
+  /**
+   * The path to the scaffold file that was processed.
+   *
+   * @var \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath
+   */
+  protected $destination;
+
+  /**
+   * Indicates if this scaffold file is managed by the scaffold command.
+   *
+   * @var bool
+   */
+  protected $managed;
+
+  /**
+   * ScaffoldResult constructor.
+   *
+   * @param \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath $destination
+   *   The path to the scaffold file that was processed.
+   * @param bool $isManaged
+   *   (optional) Whether this result is managed. Defaults to FALSE.
+   */
+  public function __construct(ScaffoldFilePath $destination, $isManaged = FALSE) {
+    $this->destination = $destination;
+    $this->managed = $isManaged;
+  }
+
+  /**
+   * Determines whether this scaffold file is managed.
+   *
+   * @return bool
+   *   TRUE if this scaffold file is managed, FALSE if not.
+   */
+  public function isManaged() {
+    return $this->managed;
+  }
+
+  /**
+   * Gets the destination scaffold file that this result refers to.
+   *
+   * @return \Drupal\Composer\Plugin\Scaffold\ScaffoldFilePath
+   *   The destination path for the scaffold result.
+   */
+  public function destination() {
+    return $this->destination;
+  }
+
+}