From 9a959a0c454448353d2d9c00511a353f874d8d9d Mon Sep 17 00:00:00 2001
From: John Hawthorn <john@hawthorn.email>
Date: Mon, 15 Apr 2019 15:01:12 -0700
Subject: [PATCH] Avoid string allocations in BacktraceCleaner

Our backtraces tend to be fairly large so this operates over quite a few
strings.

Previously, each filter would call `String#sub`, which always returns a
new string object, and uses a new string buffer any time it modifies the
string.

String#slice allows Ruby to share the same internal string buffer, which
should be faster and use less memory. In cases where changes aren't
necessary we can also reuse the same string object.

This should not change behaviour except for the last filter which
changes from sub(/\.\//, "") to sub(/\A\.\//, ""), which I think was the
original intention.
---
 railties/lib/rails/backtrace_cleaner.rb | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/railties/lib/rails/backtrace_cleaner.rb b/railties/lib/rails/backtrace_cleaner.rb
index 7c2eb1dc42..6ac963e1ef 100644
--- a/railties/lib/rails/backtrace_cleaner.rb
+++ b/railties/lib/rails/backtrace_cleaner.rb
@@ -4,18 +4,25 @@ require "active_support/backtrace_cleaner"
 
 module Rails
   class BacktraceCleaner < ActiveSupport::BacktraceCleaner
-    APP_DIRS_PATTERN = /^\/?(app|config|lib|test|\(\w*\))/
+    APP_DIRS_PATTERN = /\A\/?(?:app|config|lib|test|\(\w*\))/
     RENDER_TEMPLATE_PATTERN = /:in `.*_\w+_{2,3}\d+_\d+'/
-    EMPTY_STRING = ""
-    SLASH        = "/"
-    DOT_SLASH    = "./"
 
     def initialize
       super
       @root = "#{Rails.root}/"
-      add_filter { |line| line.sub(@root, EMPTY_STRING) }
-      add_filter { |line| line.sub(RENDER_TEMPLATE_PATTERN, EMPTY_STRING) }
-      add_filter { |line| line.sub(DOT_SLASH, SLASH) } # for tests
+      add_filter do |line|
+        line.start_with?(@root) ? line.from(@root.size) : line
+      end
+      add_filter do |line|
+        if RENDER_TEMPLATE_PATTERN.match?(line)
+          line.sub(RENDER_TEMPLATE_PATTERN, "")
+        else
+          line
+        end
+      end
+      add_filter do |line|
+        line.start_with?("./") ? line.from(1) : line
+      end
       add_silencer { |line| !APP_DIRS_PATTERN.match?(line) }
     end
   end