Replace standalone bundle with a fork-based runner in the file-by-file CI part

script/ci_parts/run_specs_file_by_file

@@ -6,22 +6,15 @@ source "script/ci_parts/setup_env" "test" $1 $2
 # of entire gems when just one spec file is run.
 unset COVERAGE
 
-# Setup standalone binstubs for RSpec that we use below.
-bundle install --standalone && bundle binstubs rspec-core --standalone
-
 function run_gem_specs_file_by_file() {
   gem=$1
   pushd $gem
-    for file in `find spec -iname '*_spec.rb'`; do
-      echo "Running $file"
-
-      # Note: here we avoid using `bundle exec`, opting for a binstub instead.
-      # Our `bundle install` command (with `--standalone` and `binstubs` options) creates the
-      # rspec binstub in a way where it won't actually load bundler at runtime. This is
-      # *slightly* faster. Not enough to usually matter, but it adds up when we boot rspec
-      # once for each spec file as we do here!
-      ../bin/rspec $file -b --format progress --no-profile
-    done
+    # Note: here we avoid booting `rspec` (and paying for `bundle exec`) once per spec
+    # file. The runner script boots bundler and rspec-core a single time and then runs
+    # each spec file in a `fork`, so every file still executes in its own isolated
+    # process. That saves 1-2 seconds per spec file, which adds up over the hundreds of
+    # spec files this build part covers.
+    ../script/rspec_file_by_file `find spec -iname '*_spec.rb'`
   popd
 }
 

script/rspec_file_by_file

@@ -0,0 +1,50 @@
+#!/usr/bin/env ruby
+
+# Copyright 2024 - 2026 Block, Inc.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+# frozen_string_literal: true
+
+# Runs each given spec file in a separate, isolated process -- equivalent to booting
+# `rspec <file>` once per file, but far cheaper: bundler and rspec-core are loaded only
+# once, in this parent process, and each spec file then runs in a `fork` of it.
+#
+# The parent never loads any ElasticGraph code or spec code, so each child starts from
+# the same clean slate as a freshly booted `rspec` process. That preserves what
+# file-by-file runs are meant to verify: that every spec file requires everything it
+# needs and can run in isolation.
+#
+# Unlike our previous approach (`bundle install --standalone` with binstubs, which skips
+# RubyGems gem activation entirely), children inherit a normally-activated bundle, so
+# gems that consult `Gem.loaded_specs` behave the same as they do in production. See
+# https://github.com/elastic/elastic-transport-ruby/pull/128 for an example of a gem
+# that breaks when loaded without activation.
+
+invocation_env = ENV.to_h
+
+ENV["BUNDLE_GEMFILE"] ||= ::File.expand_path("../Gemfile", __dir__)
+require "bundler/setup"
+require "rspec/core"
+
+# Booting bundler above mutated ENV (BUNDLE_GEMFILE, RUBYOPT, etc.) and captured that
+# mutated state in `Bundler::ORIGINAL_ENV`, which children inherit. A freshly booted
+# `rspec` process wouldn't have any of that: it would load bundler lazily (if at all)
+# with the unmodified environment, so e.g. `Bundler.with_original_env` in a spec would
+# restore the clean invocation environment for shelled-out commands. Restore both here
+# so forked children observe the same environment a fresh process would.
+ENV.replace(invocation_env)
+::Bundler::ORIGINAL_ENV.replace(invocation_env)
+
+ARGV.each do |spec_file|
+  puts "Running #{spec_file}"
+
+  pid = fork do
+    exit ::RSpec::Core::Runner.run([spec_file, "-b", "--format", "progress", "--no-profile"])
+  end
+
+  _, status = ::Process.wait2(pid)
+  exit(status.exitstatus || 1) unless status.success?
+end