spec_helper: add check for unexpected network calls

Any test that is not tagged as :needs_network and that makes
a call to an unapproved method in the `Utils::Curl` module
will raise an error unless that method gets mocked somehow.

Library/Homebrew/test/spec_helper.rb

@@ -61,6 +61,11 @@
   HOMEBREW_TEMP,
 ].freeze
 
+# These are methods in `Utils::Curl` that we know won't make network
+# requests and can be used even when :needs_network is not set.
+ALLOWED_UTILS_CURL_METHODS = [:curl_args].freeze
+DISALLOWED_UTILS_CURL_METHODS = (Utils::Curl.methods(false) - ALLOWED_UTILS_CURL_METHODS).freeze
+
 # Make `instance_double` and `class_double`
 # work when type-checking is active.
 RSpec::Sorbet.allow_doubles!
@@ -166,6 +171,22 @@
     skip "Requires network connection." unless ENV["HOMEBREW_TEST_ONLINE"]
   end
 
+  config.before do |example|
+    next if example.metadata.key?(:needs_network)
+
+    DISALLOWED_UTILS_CURL_METHODS.each do |method|
+      allow(Utils::Curl).to receive(method).and_raise(<<~ERROR)
+        Unexpected call to Utils::Curl.#{method} without setting :needs_network.
+
+        If this curl method never makes network requests, add it to the
+        ALLOWED_UTILS_CURL_METHODS array in the spec_helper.rb file.
+
+        If this curl method won't make network requests this time in particular,
+        set :allow_utils_curl on the failing spec.
+      ERROR
+    end
+  end
+
   config.before(:each, :needs_svn) do
     svn_shim = HOMEBREW_SHIMS_PATH/"shared/svn"
     skip "Subversion is not installed." unless quiet_system svn_shim, "--version"