Remove enable_shared_helpers_command_timeout feature flag (#14125)

* Remove enable_shared_helpers_command_timeout feature flag from shared_helpers_spec.rb

Co-authored-by: pavera <660677+pavera@users.noreply.github.com>

* Remove enable_shared_helpers_command_timeout experiment mocks from all spec files

Co-authored-by: pavera <660677+pavera@users.noreply.github.com>

* Fix Sorbet type errors by defaulting stdout/stderr to empty strings

Co-authored-by: pavera <660677+pavera@users.noreply.github.com>

* Fix Sorbet type errors: use T.let for stdout/stderr type narrowing

Replace `||=` pattern with explicit T.let to properly narrow
T.nilable(String) to String, satisfying Sorbet strict type checking.

Co-authored-by: pavera <660677+pavera@users.noreply.github.com>

* Fix CI failures: update Open3 mocks to SharedHelpers and handle EPIPE

- github_actions/update_checker_spec.rb: Replace Open3.capture2e mocks
  with Dependabot::SharedHelpers.run_shell_command mocks since the
  feature flag removal means run_shell_command now always uses
  CommandHelpers.capture3_with_timeout (which uses Open3.popen3 internally)
- command_helpers.rb: Catch Errno::EPIPE during stdin.write in
  capture3_with_timeout - this occurs when the process exits before
  reading its input (e.g. the `false` builtin used in npm tests)

Co-authored-by: pavera <660677+pavera@users.noreply.github.com>

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Remove process startup error handling

Removed error handling for process startup failure.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pavera <660677+pavera@users.noreply.github.com>
Co-authored-by: Tom Christensen <pavera@github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

bun/spec/dependabot/bun/file_updater_spec.rb

@@ -57,8 +57,6 @@
 
   before do
     FileUtils.mkdir_p(tmp_path)
-    allow(Dependabot::Experiments).to receive(:enabled?)
-      .with(:enable_shared_helpers_command_timeout).and_return(true)
     allow(Dependabot::Experiments).to receive(:enabled?)
       .with(:avoid_duplicate_updates_package_json).and_return(false)
   end

bun/spec/dependabot/bun/update_checker/version_resolver_spec.rb

@@ -75,8 +75,6 @@
       .to_return(status: 200, body: opentelemetry_api_registry_response)
     stub_request(:get, opentelemetry_context_async_hooks_registry_listing_url)
       .to_return(status: 200, body: opentelemetry_context_async_hooks_registry_response)
-    allow(Dependabot::Experiments).to receive(:enabled?)
-      .with(:enable_shared_helpers_command_timeout).and_return(true)
   end
 
   after do

bun/spec/dependabot/bun/update_checker_spec.rb

@@ -63,8 +63,6 @@
       .to_return(status: 200, body: registry_response)
     stub_request(:head, "#{registry_base}/#{dependency_name}/-/#{unscoped_dependency_name}-#{target_version}.tgz")
       .to_return(status: 200)
-    allow(Dependabot::Experiments).to receive(:enabled?)
-      .with(:enable_shared_helpers_command_timeout).and_return(true)
   end
 
   after do

common/lib/dependabot/command_helpers.rb

@@ -103,7 +103,11 @@ def self.capture3_with_timeout(
           Dependabot.logger.info("Started process PID: #{pid} with command: #{sanitized_env_cmd.join(' ')}")
 
           # Write to stdin if input data is provided
-          stdin&.write(stdin_data) if stdin_data
+          begin
+            stdin&.write(stdin_data) if stdin_data
+          rescue Errno::EPIPE
+            # Process exited before reading stdin - continue to collect output
+          end
           stdin&.close
 
           stdout_io.sync = true

common/lib/dependabot/shared_helpers.rb

@@ -163,15 +163,13 @@ def self.run_helper_subprocess(
       end
 
       env_cmd = [env, cmd].compact
-      if Experiments.enabled?(:enable_shared_helpers_command_timeout)
-        stdout, stderr, process = CommandHelpers.capture3_with_timeout(
-          env_cmd,
-          stdin_data: stdin_data,
-          timeout: timeout
-        )
-      else
-        stdout, stderr, process = T.unsafe(Open3).capture3(*env_cmd, stdin_data: stdin_data)
-      end
+      raw_stdout, raw_stderr, process = CommandHelpers.capture3_with_timeout(
+        env_cmd,
+        stdin_data: stdin_data,
+        timeout: timeout
+      )
+      stdout = T.let(raw_stdout || "", String)
+      stderr = T.let(raw_stderr || "", String)
       time_taken = Time.now - start
 
       if ENV["DEBUG_HELPERS"] == "true"
@@ -480,22 +478,16 @@ def self.run_shell_command(
       opts[:chdir] = cwd if cwd
 
       env_cmd = [env || {}, cmd, opts].compact
-      if Experiments.enabled?(:enable_shared_helpers_command_timeout)
-        kwargs = {
-          stderr_to_stdout: stderr_to_stdout,
-          timeout: timeout
-        }
-        kwargs[:output_observer] = output_observer if output_observer
-
-        stdout, stderr, process = CommandHelpers.capture3_with_timeout(
-          env_cmd,
-          **kwargs
-        )
-      elsif stderr_to_stdout
-        stdout, process = Open3.capture2e(env || {}, cmd, opts)
-      else
-        stdout, stderr, process = Open3.capture3(env || {}, cmd, opts)
-      end
+      kwargs = {
+        stderr_to_stdout: stderr_to_stdout,
+        timeout: timeout
+      }
+      kwargs[:output_observer] = output_observer if output_observer
+
+      stdout, stderr, process = CommandHelpers.capture3_with_timeout(
+        env_cmd,
+        **kwargs
+      )
 
       time_taken = Time.now - start