pr-pull: check for conflicts with long running builds

This change will prevent us having to run some long running builds
multiple times and to rely on luck to get things merged without conflicts.

The check takes less than 30 secondes on my local setup.

Library/Homebrew/dev-cmd/pr-pull.rb

@@ -368,6 +368,45 @@ module Homebrew
     end
   end
 
+  def pr_check_conflicts(name, tap_remote_repo, pr)
+    hash_template = proc.new { |h, k| h[k] = [] }
+    long_build_pr_files = GitHub.search_issues(
+      "org:#{name}", repo: tap_remote_repo, state: "open", label: "\"no long build conflict\""
+    ).each_with_object(Hash.new(hash_template)) do |long_build_pr, hash|
+      number = long_build_pr["number"]
+      GitHub.get_pull_request_changed_files(name, tap_remote_repo, number).each do |file|
+        key = file["filename"]
+        hash[key] << number
+      end
+    end
+
+    this_pr_files = GitHub.get_pull_request_changed_files(name, tap_remote_repo, pr)
+
+    conflicts = this_pr_files.each_with_object(Hash.new(hash_template)) do |file, hash|
+      filename = file["filename"]
+      next unless long_build_pr_files.key?(filename)
+
+      long_build_pr_files[filename].each do |pr_number|
+        key = "#{tap_remote_repo}/pull/#{pr_number}"
+        hash[key] << filename
+      end
+    end
+    return if conflicts.blank?
+
+    # Raise an error, display the conflicting PR. For example:
+    # Error: You are trying to merge a pull request that conflicts with a long running build in:
+    #  {
+    #    "homebrew-core/pull/98809": [
+    #     "Formula/icu4c.rb",
+    #     "Formula/node@10.rb"
+    #    ]
+    #  }
+    odie <<~EOS
+      You are trying to merge a pull request that conflicts with a long running build in:
+        #{JSON.pretty_generate(conflicts)}
+    EOS
+  end
+
   def pr_pull
     args = pr_pull_args.parse
 
@@ -397,6 +436,8 @@ module Homebrew
         opoo "Current branch is #{tap.path.git_branch}: do you need to pull inside #{tap.path.git_origin_branch}?"
       end
 
+      pr_check_conflicts(user, repo, pr)
+
       ohai "Fetching #{tap} pull request ##{pr}"
       Dir.mktmpdir pr do |dir|
         cd dir do

Library/Homebrew/utils/github.rb

@@ -479,8 +479,9 @@ module GitHub
 
   def check_for_duplicate_pull_requests(name, tap_remote_repo, state:, file:, args:, version: nil)
     pull_requests = fetch_pull_requests(name, tap_remote_repo, state: state, version: version).select do |pr|
-      pr_files = API.open_rest(url_to("repos", tap_remote_repo, "pulls", pr["number"], "files"))
-      pr_files.any? { |f| f["filename"] == file }
+      get_pull_request_changed_files(
+        name, tap_remote_repo, pr["number"]
+      ).any? { |f| f["filename"] == file }
     end
     return if pull_requests.blank?
 
@@ -501,6 +502,10 @@ module GitHub
     end
   end
 
+  def get_pull_request_changed_files(name, tap_remote_repo, pr)
+    API.open_rest(url_to("repos", name, tap_remote_repo, "pulls", pr, "files"))
+  end
+
   def forked_repo_info!(tap_remote_repo, org: nil)
     response = create_fork(tap_remote_repo, org: org)
     # GitHub API responds immediately but fork takes a few seconds to be ready.