Set WEB_CONCURRENCY_SET_BY=heroku/ruby if WEB_CONCURRENCY gets auto-set (heroku#1700)

* Set WEB_CONCURRENCY_SET_BY=heroku/ruby if WEB_CONCURRENCY gets auto-set

The Ruby buildpack sets the `WEB_CONCURRENCY` env var at app boot based on the
current dyno size if the (deprecated) `SENSIBLE_DEFAULTS` mode is enabled, so
long as the user hasn't specified a custom value via the app's config vars.

Now, in addition to setting the `WEB_CONCURRENCY` env var, the buildpack will
also set `WEB_CONCURRENCY_SET_BY=heroku/ruby` to allow the app, other boot time
scripts, or humans more easily differentiate between a user and a buildpack
provided `WEB_CONCURRENCY` (and determine which buildpack actually set it).

In addition to assisting with debugging, this allows for UX improvements in
other buildpacks such as the PHP buildpack, which can now check for
`WEB_CONCURRENCY_SET_BY` in its boot time Apache/Nginx `heroku-php-...` scripts
to ignore `WEB_CONCURRENCY` in cases where users have ordered the buildpacks on
their app in the wrong order (the "primary" language is supposed to be listed
last, otherwise the wrong concurrency value will be used).

See also:
heroku/heroku-buildpack-nodejs#932
heroku/heroku-buildpack-python#2015

GUS-W-20882005

* Track SENSIBLE_DEFAULTS usage

This feature is deprecated, let's see how many people still rely on it.

---------

Signed-off-by: Richard Schneeman <rschneeman@salesforce.com>
Co-authored-by: Richard Schneeman <rschneeman@salesforce.com>

Author: dzuelke

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## [Unreleased]
 
+- Set `WEB_CONCURRENCY_SET_BY=heroku/ruby` if `WEB_CONCURRENCY` gets set in (deprecated) `SENSIBLE_DEFAULTS` mode (https://github.com/heroku/heroku-buildpack-ruby/pull/1700)
 - Default Node.js version now 24.13.0 (https://github.com/heroku/heroku-buildpack-ruby/pull/1704)
 
 ## [v344] - 2026-01-14

lib/language_pack/ruby.rb

@@ -113,7 +113,7 @@ def compile
     best_practice_warnings
     warn_outdated_ruby
     default_config_vars = self.class.default_config_vars(metadata: @metadata, ruby_version: @ruby_version, bundler: bundler, environment_name: environment_name)
-    setup_profiled(ruby_layer_path: "$HOME", gem_layer_path: "$HOME", ruby_version: @ruby_version, default_config_vars: default_config_vars) # $HOME is set to /app at run time
+    setup_profiled(ruby_layer_path: "$HOME", gem_layer_path: "$HOME", ruby_version: @ruby_version, default_config_vars: default_config_vars, report: @report) # $HOME is set to /app at run time
     setup_export(app_path: app_path, ruby_version: @ruby_version, default_config_vars: default_config_vars)
     cleanup
     super
@@ -240,18 +240,22 @@ def set_default_web_concurrency
       256)
         export HEROKU_RAM_LIMIT_MB=${HEROKU_RAM_LIMIT_MB:-512}
         export WEB_CONCURRENCY=${WEB_CONCURRENCY:-2}
+        export WEB_CONCURRENCY_SET_BY=heroku/ruby
         ;;
       512)
         export HEROKU_RAM_LIMIT_MB=${HEROKU_RAM_LIMIT_MB:-1024}
         export WEB_CONCURRENCY=${WEB_CONCURRENCY:-4}
+        export WEB_CONCURRENCY_SET_BY=heroku/ruby
         ;;
       16384)
         export HEROKU_RAM_LIMIT_MB=${HEROKU_RAM_LIMIT_MB:-2560}
         export WEB_CONCURRENCY=${WEB_CONCURRENCY:-8}
+        export WEB_CONCURRENCY_SET_BY=heroku/ruby
         ;;
       32768)
         export HEROKU_RAM_LIMIT_MB=${HEROKU_RAM_LIMIT_MB:-6144}
         export WEB_CONCURRENCY=${WEB_CONCURRENCY:-16}
+        export WEB_CONCURRENCY_SET_BY=heroku/ruby
         ;;
       *)
         ;;
@@ -335,7 +339,7 @@ def setup_export(app_path: , ruby_version: , default_config_vars: )
   end
 
   # sets up the profile.d script for this buildpack
-  def setup_profiled(ruby_layer_path: , gem_layer_path:, ruby_version: , default_config_vars: )
+  def setup_profiled(ruby_layer_path: , gem_layer_path:, ruby_version: , default_config_vars: , report:)
     profiled_path = []
 
     default_config_vars.each do |key, value|
@@ -361,6 +365,7 @@ def setup_profiled(ruby_layer_path: , gem_layer_path:, ruby_version: , default_c
     set_env_default "MALLOC_ARENA_MAX", "2"     if default_malloc_arena_max?
 
     web_concurrency = env("SENSIBLE_DEFAULTS") ? set_default_web_concurrency : ""
+    report.capture("web_concurrency.sensible_defaults" => web_concurrency.empty?)
     add_to_profiled(web_concurrency, filename: "WEB_CONCURRENCY.sh", mode: "w") # always write that file, even if its empty (meaning no defaults apply), for interop with other buildpacks - and we overwrite the file rather than appending (which is the default)
 
     # TODO handle JRUBY

lib/language_pack/test/ruby.rb

@@ -15,7 +15,7 @@ def compile
     install_binaries
     prepare_tests
     default_config_vars = self.class.default_config_vars(metadata: @metadata, ruby_version: @ruby_version, bundler: bundler, environment_name: environment_name)
-    setup_profiled(ruby_layer_path: "$HOME", gem_layer_path: "$HOME", ruby_version: @ruby_version, default_config_vars: default_config_vars) # $HOME is set to /app at run time
+    setup_profiled(ruby_layer_path: "$HOME", gem_layer_path: "$HOME", ruby_version: @ruby_version, default_config_vars: default_config_vars, report: @report) # $HOME is set to /app at run time
     setup_export(app_path: app_path, ruby_version: @ruby_version, default_config_vars: default_config_vars)
     super
   end

spec/hatchet/ruby_spec.rb

@@ -381,4 +381,12 @@
       expect(app.run("echo $WEB_CONCURRENCY", :heroku => {:env => "WEB_CONCURRENCY=0"}).strip).to eq("0")
     end
   end
+
+  it "with SENSIBLE_DEFAULTS on sets environment variables" do
+    Hatchet::Runner.new('default_ruby', stack: DEFAULT_STACK, config: { "SENSIBLE_DEFAULTS" => "1" }).deploy do |app|
+      expect(app.run("cat .profile.d/WEB_CONCURRENCY.sh").strip).not_to be_empty
+      expect(app.run("echo $HEROKU_RAM_LIMIT_MB $WEB_CONCURRENCY $WEB_CONCURRENCY_SET_BY").strip).to eq("512 2 heroku/ruby")
+      expect(app.run("echo $WEB_CONCURRENCY", :heroku => {:env => "WEB_CONCURRENCY=3"}).strip).to eq("3")
+    end
+  end
 end