[68047] Prevent FrozenError in Attachments::ExtractFulltextJob

There is a bug in the plaintext gem in its `Plaintext::Resolve#text`: it
executes external commands to extract text from files. This can return
`nil` when it has no output, and it's then converted to a string using
`to_s`.

`nil.to_s` returns a frozen empty string since ruby 2.7. When it then
calls `text.gsub!(/\s+/m, ' ')` later in the `#text` method, the
`FrozenError` occurs.

The fix is to patch the `Plaintext::Resolver#text` method to create a
mutable copy of the text if it's frozen.

lib/open_project/patches/plaintext_resolver.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+module OpenProject::Patches::PlaintextResolverPatch
+  # identical to the original implementation but with a mutable copy of the text
+  # if it's frozen.
+  def text
+    if handler = find_handler and
+        text = handler.text(@file, max_size: max_plaintext_bytes)
+
+      # Handle frozen strings by creating a mutable copy
+      text = text.dup if text.frozen?
+      text.gsub!(/\s+/m, " ")
+      text.strip!
+      text.mb_chars.compose.limit(max_plaintext_bytes).to_s
+    end
+  end
+end
+
+Plaintext::Resolver.prepend(OpenProject::Patches::PlaintextResolverPatch)

spec/workers/attachments/extract_fulltext_job_spec.rb

@@ -48,11 +48,13 @@ RSpec.describe Attachments::ExtractFulltextJob, type: :job do
 
   let(:text) { "lorem ipsum" }
   let(:attachment) do
-    create(:attachment).tap do |attachment|
-      allow(Attachment)
-        .to receive(:find_by)
-              .with(id: attachment.id)
-              .and_return(attachment)
+    create(:attachment)
+  end
+  let(:plaintext_file_handler) do
+    Plaintext::Resolver.file_handlers.find { |h| h.accept? attachment.content_type }.tap do |plaintext_file_handler|
+      if plaintext_file_handler.nil?
+        fail "Plaintext::FileHandler not found for content type #{attachment.content_type}"
+      end
     end
   end
 
@@ -73,7 +75,7 @@ RSpec.describe Attachments::ExtractFulltextJob, type: :job do
 
   context "with successful text extraction" do
     before do
-      allow_any_instance_of(Plaintext::Resolver).to receive(:text).and_return(text)
+      allow(plaintext_file_handler).to receive(:text).and_return(text)
     end
 
     context "when the attachment is readable" do
@@ -102,8 +104,26 @@ RSpec.describe Attachments::ExtractFulltextJob, type: :job do
     end
   end
 
+  context "when the plaintext handler returns a frozen string (Bug #68047)" do
+    let(:frozen_text) { "frozen string".freeze } # rubocop:disable Style/RedundantFreeze
+
+    before do
+      allow(plaintext_file_handler).to receive(:text).and_return(frozen_text)
+    end
+
+    it "updates the attachment's DB record with fulltext, fulltext_tsv, and file_tsv" do
+      perform_enqueued_jobs
+      expect(extracted_attributes["fulltext"]).to eq frozen_text
+      expect(extracted_attributes["fulltext_tsv"].size).to be > 0
+      expect(extracted_attributes["file_tsv"].size).to be > 0
+    end
+  end
+
   context "with the file not readable" do
     before do
+      allow(Attachment)
+        .to receive(:find_by).with(id: attachment.id)
+        .and_return(attachment)
       allow(attachment).to receive(:readable?).and_return(false)
     end
 
@@ -120,7 +140,7 @@ RSpec.describe Attachments::ExtractFulltextJob, type: :job do
     let(:logger) { Rails.logger }
 
     before do
-      allow_any_instance_of(Plaintext::Resolver).to receive(:text).and_raise(exception_message) # rubocop:disable RSpec/AnyInstance
+      allow(plaintext_file_handler).to receive(:text).and_raise(exception_message)
 
       allow(logger).to receive(:error)