fix: lock agent quota checks (#15029)

# Pull Request Template

## Description

Locks the agent quota check to the account row while creating account
users. This fixes a race where concurrent agent-create requests could
all observe the same remaining seat before any `account_users` row was
inserted.

The API continues to return the existing `402 Account limit exceeded.
Please purchase more licenses` response when the limit is reached. Bulk
create now preflights the requested email count while holding the
account lock, then creates each agent through the same locked builder
path. The Enterprise custom-role hook now no-ops when create did not
produce an agent.

Fixes:
[CW-7039](https://linear.app/chatwoot/issue/CW-7039/race-condition-in-agent-creation-bypasses-plan-agent-seat-limit)

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

- `POSTGRES_DATABASE=chatwoot_test_c20f_agent_quota REDIS_DB=9 bundle
exec rspec spec/builders/agent_builder_spec.rb
spec/enterprise/builders/agent_builder_spec.rb
spec/controllers/api/v1/accounts/agents_controller_spec.rb
spec/enterprise/controllers/api/v1/accounts/agents_controller_spec.rb
spec/enterprise/controllers/enterprise/api/v1/accounts/agents_controller_spec.rb`
- `bundle exec rubocop app/builders/agent_builder.rb
app/controllers/api/v1/accounts/agents_controller.rb
enterprise/app/controllers/enterprise/api/v1/accounts/agents_controller.rb
spec/builders/agent_builder_spec.rb
spec/enterprise/controllers/api/v1/accounts/agents_controller_spec.rb`
- `git diff --check`
- One-off threaded Rails validation with 8 concurrent `AgentBuilder`
calls against an account with one remaining seat: `created: 1`,
`limited: 7`, final `count=2`, `limit=2`.

## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/builders/agent_builder.rb

@@ -2,6 +2,14 @@
 # It initializes with necessary attributes and provides a perform method
 # to create a user and account user in a transaction.
 class AgentBuilder
+  LIMIT_EXCEEDED_MESSAGE = 'Account limit exceeded. Please purchase more licenses'.freeze
+
+  class LimitExceededError < StandardError
+    def initialize
+      super(AgentBuilder::LIMIT_EXCEEDED_MESSAGE)
+    end
+  end
+
   # Initializes an AgentBuilder with necessary attributes.
   # @param email [String] the email of the user.
   # @param name [String] the name of the user.
@@ -14,15 +22,23 @@ class AgentBuilder
   # Creates a user and account user in a transaction.
   # @return [User] the created user.
   def perform
-    ActiveRecord::Base.transaction do
-      @user = find_or_create_user
-      create_account_user
+    account.with_lock do
+      raise LimitExceededError unless can_add_agent?
+
+      ActiveRecord::Base.transaction do
+        @user = find_or_create_user
+        create_account_user
+      end
     end
     @user
   end
 
   private
 
+  def can_add_agent?
+    account.usage_limits[:agents] > account.account_users.count
+  end
+
   # Finds a user by email or creates a new one with a temporary password.
   # @return [User] the found or created user.
   def find_or_create_user

app/controllers/api/v1/accounts/agents_controller.rb

@@ -1,8 +1,6 @@
 class Api::V1::Accounts::AgentsController < Api::V1::Accounts::BaseController
   before_action :fetch_agent, except: [:create, :index, :bulk_create]
   before_action :check_authorization
-  before_action :validate_limit, only: [:create]
-  before_action :validate_limit_for_bulk_create, only: [:bulk_create]
 
   def index
     @agents = agents
@@ -20,6 +18,8 @@ class Api::V1::Accounts::AgentsController < Api::V1::Accounts::BaseController
     )
 
     @agent = builder.perform
+  rescue AgentBuilder::LimitExceededError => e
+    render_payment_required(e.message)
   end
 
   def update
@@ -36,25 +36,13 @@ class Api::V1::Accounts::AgentsController < Api::V1::Accounts::BaseController
   def bulk_create
     emails = params[:emails]
 
-    emails.each do |email|
-      builder = AgentBuilder.new(
-        email: email,
-        name: email.split('@').first,
-        inviter: current_user,
-        account: Current.account
-      )
-      begin
-        builder.perform
-      rescue ActiveRecord::RecordInvalid => e
-        Rails.logger.info "[Agent#bulk_create] ignoring email #{email}, errors: #{e.record.errors}"
-      end
-    end
-
+    bulk_create_agents(emails)
     # This endpoint is used to bulk create agents during onboarding
     # onboarding_step key in present in Current account custom attributes, since this is a one time operation
-    Current.account.custom_attributes.delete('onboarding_step')
-    Current.account.save!
+    clear_onboarding_step
     head :ok
+  rescue AgentBuilder::LimitExceededError => e
+    render_payment_required(e.message)
   end
 
   private
@@ -87,22 +75,33 @@ class Api::V1::Accounts::AgentsController < Api::V1::Accounts::BaseController
     @agents ||= Current.account.users.order_by_full_name.includes(:account_users, { avatar_attachment: [:blob] })
   end
 
-  def validate_limit_for_bulk_create
-    limit_available = params[:emails].count <= available_agent_count
+  def bulk_create_agents(emails)
+    Current.account.with_lock do
+      raise AgentBuilder::LimitExceededError if emails.count > available_agent_count
 
-    render_payment_required('Account limit exceeded. Please purchase more licenses') unless limit_available
+      emails.each { |email| create_agent_from_email(email) }
+    end
   end
 
-  def validate_limit
-    render_payment_required('Account limit exceeded. Please purchase more licenses') unless can_add_agent?
+  def create_agent_from_email(email)
+    builder = AgentBuilder.new(
+      email: email,
+      name: email.split('@').first,
+      inviter: current_user,
+      account: Current.account
+    )
+    builder.perform
+  rescue ActiveRecord::RecordInvalid => e
+    Rails.logger.info "[Agent#bulk_create] ignoring email #{email}, errors: #{e.record.errors}"
+  end
+
+  def clear_onboarding_step
+    Current.account.custom_attributes.delete('onboarding_step')
+    Current.account.save!
   end
 
   def available_agent_count
-    Current.account.usage_limits[:agents] - agents.count
-  end
-
-  def can_add_agent?
-    available_agent_count.positive?
+    Current.account.usage_limits[:agents] - Current.account.account_users.count
   end
 
   def delete_user_record(agent)

enterprise/app/controllers/enterprise/api/v1/accounts/agents_controller.rb

@@ -1,6 +1,8 @@
 module Enterprise::Api::V1::Accounts::AgentsController
   def create
     super
+    return if @agent.blank?
+
     associate_agent_with_custom_role
   end
 

spec/builders/agent_builder_spec.rb

@@ -23,6 +23,12 @@ RSpec.describe AgentBuilder, type: :model do
   end
 
   describe '#perform' do
+    it 'locks the account while checking and creating the agent' do
+      expect(account).to receive(:with_lock).and_call_original
+
+      agent_builder.perform
+    end
+
     context 'when user does not exist' do
       it 'creates a new user' do
         expect { agent_builder.perform }.to change(User, :count).by(1)
@@ -67,5 +73,17 @@ RSpec.describe AgentBuilder, type: :model do
         expect(user.encrypted_password).not_to be_empty
       end
     end
+
+    context 'when the account has reached its agent limit' do
+      before do
+        allow(account).to receive(:usage_limits).and_return({ agents: account.account_users.count })
+      end
+
+      it 'raises a limit exceeded error without creating a user' do
+        expect { agent_builder.perform }.to raise_error(described_class::LimitExceededError, described_class::LIMIT_EXCEEDED_MESSAGE)
+
+        expect(User.from_email(email)).to be_nil
+      end
+    end
   end
 end

spec/enterprise/controllers/api/v1/accounts/agents_controller_spec.rb

@@ -21,6 +21,27 @@ RSpec.describe 'Agents API', type: :request do
         expect(response).to have_http_status(:payment_required)
         expect(response.body).to include('Account limit exceeded. Please purchase more licenses')
       end
+
+      it 'prevents adding an agent if the last seat is consumed before creation' do
+        account.update!(limits: { agents: account.account_users.count + 1 })
+        competing_agent_created = false
+
+        allow(AgentBuilder).to receive(:new).and_wrap_original do |method, *args|
+          unless competing_agent_created
+            create(:user, account: account, role: :agent)
+            competing_agent_created = true
+          end
+
+          method.call(*args)
+        end
+
+        post "/api/v1/accounts/#{account.id}/agents", params: params, headers: admin.create_new_auth_token, as: :json
+
+        expect(response).to have_http_status(:payment_required)
+        expect(response.body).to include('Account limit exceeded. Please purchase more licenses')
+        expect(User.from_email(params[:email])).to be_nil
+        expect(account.account_users.count).to eq(account.usage_limits[:agents])
+      end
     end
   end