Update cached formula json file when needed

Library/Homebrew/api/formula.rb

@@ -16,6 +16,11 @@ module Homebrew
         end
         alias generic_formula_api_path formula_api_path
 
+        sig { returns(String) }
+        def cached_formula_json_file
+          HOMEBREW_CACHE_API/"#{formula_api_path}.json"
+        end
+
         sig { params(name: String).returns(Hash) }
         def fetch(name)
           Homebrew::API.fetch "#{formula_api_path}/#{name}.json"
@@ -24,13 +29,29 @@ module Homebrew
         sig { returns(Array) }
         def all_formulae
           @all_formulae ||= begin
-            json_formulae = JSON.parse((HOMEBREW_CACHE_API/"#{formula_api_path}.json").read)
+            download_formulae_if_needed
+
+            json_formulae = JSON.parse(cached_formula_json_file.read)
 
             json_formulae.to_h do |json_formula|
               [json_formula["name"], json_formula.except("name")]
             end
           end
         end
+
+        private
+
+        sig { void }
+        def download_formulae_if_needed
+          curl_args = %w[--compressed --silent https://formulae.brew.sh/api/formula.json]
+          if cached_formula_json_file.exist?
+            last_modified = cached_formula_json_file.mtime.utc
+            last_modified = last_modified.strftime("%a, %d %b %Y %H:%M:%S GMT")
+            curl_args = ["--header", "If-Modified-Since: #{last_modified}", *curl_args]
+          end
+
+          curl_download(*curl_args, to: HOMEBREW_CACHE_API/"#{formula_api_path}.json", max_time: 5)
+        end
       end
     end
   end

Library/Homebrew/cmd/update.sh

@@ -752,7 +752,7 @@ EOS
     curl \
       "${CURL_DISABLE_CURLRC_ARGS[@]}" \
       --fail --compressed --silent --max-time 5 \
-      --location --output "${HOMEBREW_CACHE}/api/formula.json" \
+      --location --remote-time --output "${HOMEBREW_CACHE}/api/formula.json" \
       --user-agent "${HOMEBREW_USER_AGENT_CURL}" \
       "https://formulae.brew.sh/api/formula.json"
     # TODO: we probably want to print an error if this fails.