Fix db:check-schema in forks and shallow clones

The `regenerate-schema` script attempts to find the merge base between
the source and branch SHA to determine the right `db/structure.sql` to
apply the migrations in the merge request. However, to get the merge
base, we need the target SHA and the history between the source and
target SHAs. Instead of downloading it via a curl request, we fetch the
target branch and check out the `db/structure.sql` from there.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/216214

scripts/regenerate-schema

@@ -2,7 +2,7 @@
 
 # frozen_string_literal: true
 
-require 'net/http'
+require 'open3'
 require 'uri'
 
 class SchemaRegenerator
@@ -56,35 +56,15 @@ class SchemaRegenerator
   # Get clean schema from remote servers
   #
   # This script might run in CI, using a shallow clone, so to checkout
-  # the file, download it from the server.
+  # the file, fetch the target branch from the server.
   def remote_checkout_clean_schema
     return false unless project_url
+    return false unless target_project_url
 
-    uri = URI.join("#{project_url}/", 'raw/', "#{merge_base}/", FILENAME)
+    run %Q[git remote add target_project #{target_project_url}.git]
+    run %Q[git fetch target_project #{target_branch}:#{target_branch}]
 
-    download_schema(uri)
-  end
-
-  ##
-  # Download the schema from the given +uri+.
-  def download_schema(uri)
-    puts "Downloading #{uri}..."
-
-    Net::HTTP.start(uri.host, uri.port, use_ssl: true) do |http|
-      request = Net::HTTP::Get.new(uri.request_uri)
-      http.read_timeout = 500
-      http.request(request) do |response|
-        raise("Failed to download file: #{response.code} #{response.message}") if response.code.to_i != 200
-
-        File.open(FILENAME, 'w') do |io|
-          response.read_body do |chunk|
-            io.write(chunk)
-          end
-        end
-      end
-    end
-
-    true
+    local_checkout_clean_schema
   end
 
   ##
@@ -150,15 +130,17 @@ class SchemaRegenerator
   # When the command failed an exception is raised.
   def run(cmd)
     puts "\e[32m$ #{cmd}\e[37m"
-    ret = system(cmd)
-    puts "\e[0m"
-    raise("Command failed") unless ret
+    stdout_str, stderr_str, status = Open3.capture3(cmd)
+    puts "#{stdout_str}#{stderr_str}\e[0m"
+    raise("Command failed: #{stderr_str}") unless status.success?
+
+    stdout_str
   end
 
   ##
   # Return the base commit between source and target branch.
   def merge_base
-    @merge_base ||= `git merge-base #{target_branch} #{source_ref}`.chomp
+    @merge_base ||= run("git merge-base #{target_branch} #{source_ref}").chomp
   end
 
   ##
@@ -179,11 +161,17 @@ class SchemaRegenerator
   end
 
   ##
-  # Return the project URL from CI environment variable.
+  # Return the source project URL from CI environment variable.
   def project_url
     ENV['CI_PROJECT_URL']
   end
 
+  ##
+  # Return the target project URL from CI environment variable.
+  def target_project_url
+    ENV['CI_MERGE_REQUEST_PROJECT_URL']
+  end
+
   ##
   # Return whether the script is running from CI
   def ci?