What is a patch?

The patch is a program that updates text files according to instructions in a separate file called a patch file. The patch file (also called a patch for short) is a text file that consists of a list of differences and is produced by running the related diff program with the original and updated file as arguments. Updating files with a patch is often referred to as applying the patch or simply patching the files.

How do patches work?

Patch files are text files that note the following:

• The file(s) to be changed
• The line number to begin the change and the number of lines to be changed
• The new code to swap in.

How to create a custom patch?

Think twice before creating a patch, as there are better ways to customize functionality. Try implementing your task in other ways, e.g., by creating a plugin.

1. Create a patches/composer directory in your local project
2. Identify the GitHub commit or pull request to use for the patch. This example uses the 2d31571 commit linked to Magento 2 GitHub issue #6474
3. Append the .patch or the .diff extensions to the commit URL
4. Save the page as a file in the patches/composer directory. For example, github-issue-6474.diff
5. Edit the file and remove app/code/<VENDOR>/<PACKAGE> from all paths so that they are relative to the vendor/<VENDOR>/<PACKAGE> directory.

The following example shows the previously mentioned diff file after removing all instances of app/code/<VENDOR>/<PACKAGE>:

diff --git a/view/frontend/web/js/view/payment/iframe.js
 b/view/frontend/web/js/view/payment/iframe.js

index c8a6fef58d31..7d01c195791e 100644
--- a/view/frontend/web/js/view/payment/iframe.js
+++ b/view/frontend/web/js/view/payment/iframe.js
@@ -154,6 +154,7 @@ define(
              */
              clearTimeout: function () {
                  clearTimeout(this.timeoutId);
                  this.fail();
                  return this;
            },

How to apply patches using Composer?

1. Open your command line application and navigate to your project directory
2. Add the cweagans/composer-patches plugin to the composer.json file: composer require cweagans/composer-patches (Magento now uses the Quality Patch Tool)
3. Edit the composer.json file and add the following section to specify:
   • Module: “magento/module-payment”
   • Title: “MAGETWO-56934: Checkout page freezes when ordering with Authorize.net with invalid credit card”
   • Path to patch: “patches/composer/github-issue-6474.diff”.

"extra": {
      "composer-exit-on-patch-failure": true,
      "patches": {
          "magento/module-payment": {
              "MAGETWO-56934: Checkout page freezes when ordering with Authorize.net with invalid credit card": "patches/composer/github-issue-6474.diff"
          }
      }
  }

You must create multiple patch files targeting multiple modules if a patch affects numerous modules.

4. Apply the patch. Use the -v option only if you want to see debugging information: composer -v install
5. Update the composer.lock file. The lock file tracks which patches have been applied to each Composer package in an object: composer update --lock.

Warning! If line endings do not match the patch and your files, the patching won’t work. This is a silent error, so check the line endings if something goes wrong.

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