🩹 Plaster and semantical patching

Plaster is an experimental tool being introduced in Brave to allow us to apply changes to upstream sources files, by relying on regex transformations for to search for patterns and apply substitutions.

Why 🩹 Plaster

The two traditional approaches to introduce changes to Chromium have been git patches (i.e. patches/), and language overrides (i.e. chromium_src/). The use of patches is in general avoided whenever possible, as they tend to easily run into conflicts when rebasing Brave onto a newer Chromium version. With language overrides, although more flexible than patches, they tend to be invisible in the source code target, hard to interpret, and lead to many cases of unintended replacements.

Using regexes, Plaster avoids the brittleness of patch files, while providing matching mechanisms that are more flexible than the methods currently employed for macro replacement. This also means a language-agnostic way to approach source changes semantically, and in place. Plaster changes can be both seen in place, as well as audit as patch files.

How does it work

Plaster files are placed under rewrite/, using a .toml extension, and they are supposed to match the path for the file being plastered.

Warning

At the moment, only plaster files to sources in Chromium's src repo are supported.

Each plaster file will be used to apply changes into a given source, and then generate a patch for the effected changes.

For example, imagine you want to add changes to chrome/browser/autocomplete/autocomplete_classifier_factory.cc. You would care about the following files.

Plaster file: brave/rewrite/chrome/browser/autocomplete/autocomplete_classifier_factory.cc.toml
Source file: chrome/browser/autocomplete/autocomplete_classifier_factory.cc
Patch file brave/patches/chrome-browser-autocomplete-autocomplete_classifier_factory.cc.patch

Creating a plaster file

A plaster file is a toml file used to list regexes operations to be applied in the corresponding source, based on its path. The following is plaster file to apply a few changes to chrome/browser/autocomplete/autocomplete_classifier_factory.cc.

Creating the source file with vscode:

code rewrite/chrome/browser/autocomplete/autocomplete_classifier_factory.cc.toml

This file below has two substitutions to be applied on its source: The first adds a header to the list of headers in the source. The second one replaces all occurrences of ChromeAutocompleteSchemeClassifier with BraveAutocompleteSchemeClassifier.

# Copyright (c) 2025 The Brave Authors. All rights reserved.
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this file,
# You can obtain one at https://mozilla.org/MPL/2.0/.

[[substitution]]
description = 'Adding header for BraveAutocompleteSchemeClassifier'
re_pattern = '(#include "[\s\S]*)'
replace = '#include "brave/browser/autocomplete/brave_autocomplete_scheme_classifier.h"\n\1'

[[substitution]]
description = 'Patching in BraveAutocompleteSchemeClassifier'
pattern = 'ChromeAutocompleteSchemeClassifier'
replace = 'BraveAutocompleteSchemeClassifier'

The basic format for a `[[substitution]]` entry is as follow:

```toml
[[substition]]
description = ''
# One of either pattern or re_pattern must be specified
pattern = ''  # non-regex pattern (string will be escaped)
re_pattern = ''  # regex pattern
replace = ''
re_flags = []  # These are traditional python regex flags.
count = 1  # use 0 to ignore the match count and replace all

To apply this plaster file, just run plaster.py:

tools/cr/plaster.py apply

Running plaster.py will cause the substitutions to be applied in chrome/browser/autocomplete/autocomplete_classifier_factory.cc, but it will cause the creation/update of the corresponding patch file for this source. For exmample, it may produce a diff file like this:

diff --git a/patches/chrome-browser-autocomplete-autocomplete_classifier_factory.cc.patch b/patches/chrome-browser-autocomplete-autocomplete_classifier_factory.cc.patch
new file mode 100644
index 00000000000..63703e6940b
--- /dev/null
+++ b/patches/chrome-browser-autocomplete-autocomplete_classifier_factory.cc.patch
@@ -0,0 +1,21 @@
+diff --git a/chrome/browser/autocomplete/autocomplete_classifier_factory.cc b/chrome/browser/autocomplete/autocomplete_classifier_factory.cc
+index db73283fbc4f5146bb42e2a8abfcdfa4567ab252..f241c6c5f0611dcbcad3e468a7595be75324a8ba 100644
+--- a/chrome/browser/autocomplete/autocomplete_classifier_factory.cc
++++ b/chrome/browser/autocomplete/autocomplete_classifier_factory.cc
+@@ -16,6 +16,7 @@
+ #include "components/omnibox/browser/autocomplete_classifier.h"
+ #include "components/omnibox/browser/autocomplete_controller.h"
+ #include "extensions/buildflags/buildflags.h"
++#include "brave/browser/autocomplete/brave_autocomplete_scheme_classifier.h"
+
+ #if BUILDFLAG(ENABLE_EXTENSIONS)
+ #include "extensions/browser/extension_system_provider.h"
+@@ -43,7 +44,7 @@ std::unique_ptr<KeyedService> AutocompleteClassifierFactory::BuildInstanceFor(
+       std::make_unique<AutocompleteController>(
+           std::make_unique<ChromeAutocompleteProviderClient>(profile),
+           AutocompleteClassifier::DefaultOmniboxProviders()),
+-      std::make_unique<ChromeAutocompleteSchemeClassifier>(profile));
++      std::make_unique<BraveAutocompleteSchemeClassifier>(profile));
+ }
+
+ AutocompleteClassifierFactory::AutocompleteClassifierFactory()

The produced patch is supposed to be committed in the repository as usual.

Checking if files are up-to-date

You can verify if all plaster files are applied, with Chromium sources updated and patch files checked under path with the following command:

tools/cr/plaster.py check

This is the equivalent of a dry run of plaster.py apply.

Best practices

See https://github.com/brave-experiments/brave-core-tools/blob/master/docs/best-practices/plaster.md

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