log_innodb_status is not safe

deadlock_retry.gemspec

@@ -2,7 +2,7 @@
 
 Gem::Specification.new do |s|
   s.name = %q{deadlock_retry}
-  s.version = "1.1.0"
+  s.version = "1.1.1"
   s.authors = ["Jamis Buck", "Mike Perham"]
   s.description = s.summary = %q{Provides automatic deadlock retry and logging functionality for ActiveRecord and MySQL}
   s.email = %q{mperham@gmail.com}

lib/deadlock_retry.rb

@@ -1,5 +1,4 @@
 module DeadlockRetry
-
   def self.included(base)
     base.extend(ClassMethods)
     base.class_eval do
@@ -9,6 +8,16 @@ class << self
     end
   end
 
+  @@innodb_status_available = nil
+
+  def self.innodb_status_available?
+    @@innodb_status_available
+  end
+
+  def self.innodb_status_available=(bool)
+    @@innodb_status_available = bool
+  end
+
   module ClassMethods
     DEADLOCK_ERROR_MESSAGES = [
       "Deadlock found when trying to get lock",
@@ -17,9 +26,12 @@ module ClassMethods
 
     MAXIMUM_RETRIES_ON_DEADLOCK = 3
 
+
     def transaction_with_deadlock_handling(*objects, &block)
       retry_count = 0
 
+      check_innodb_status_available
+
       begin
         transaction_without_deadlock_handling(*objects, &block)
       rescue ActiveRecord::StatementInvalid => error
@@ -28,7 +40,7 @@ def transaction_with_deadlock_handling(*objects, &block)
           raise if retry_count >= MAXIMUM_RETRIES_ON_DEADLOCK
           retry_count += 1
           logger.info "Deadlock detected on retry #{retry_count}, restarting transaction"
-          log_innodb_status
+          log_innodb_status if DeadlockRetry.innodb_status_available?
           retry
         else
           raise
@@ -43,10 +55,27 @@ def in_nested_transaction?
       connection.open_transactions != 0
     end
 
+    def show_innodb_status
+       self.connection.select_value("show innodb status")
+    end
+
+    # Should we try to log innodb status -- if we don't have permission to,
+    # we actually break in-flight transactions, silently (!)
+    def check_innodb_status_available
+      return unless DeadlockRetry.innodb_status_available?.nil?
+
+      begin
+        show_innodb_status
+        DeadlockRetry.innodb_status_available = true
+      rescue
+        DeadlockRetry.innodb_status_available = false
+      end
+    end
+
     def log_innodb_status
       # show innodb status is the only way to get visiblity into why
       # the transaction deadlocked.  log it.
-      lines = connection.select_value("show innodb status")
+      lines = show_innodb_status
       logger.warn "INNODB Status follows:"
       lines.each_line do |line|
         logger.warn line
@@ -59,4 +88,4 @@ def log_innodb_status
   end
 end
 
-ActiveRecord::Base.send(:include, DeadlockRetry)
+ActiveRecord::Base.send(:include, DeadlockRetry)

test/deadlock_retry_test.rb

@@ -32,13 +32,24 @@ def self.logger
     @logger ||= Logger.new(nil)
   end
 
+  def self.show_innodb_status
+    []
+  end
+
+  def self.reset_innodb_status_availability
+    DeadlockRetry.innodb_status_availability = nil
+  end
+
   include DeadlockRetry
 end
 
 class DeadlockRetryTest < Test::Unit::TestCase
   DEADLOCK_ERROR = "MySQL::Error: Deadlock found when trying to get lock"
   TIMEOUT_ERROR = "MySQL::Error: Lock wait timeout exceeded"
 
+  def setup
+  end
+
   def test_no_errors
     assert_equal :success, MockModel.transaction { :success }
   end
@@ -71,6 +82,13 @@ def test_included_by_default
     assert ActiveRecord::Base.respond_to?(:transaction_with_deadlock_handling)
   end
 
+  def test_innodb_status_availability
+    DeadlockRetry.innodb_status_available = nil
+    MockModel.transaction {}
+    assert_equal true, DeadlockRetry.innodb_status_available?
+  end
+
+
   def test_error_in_nested_transaction_should_retry_outermost_transaction
     tries = 0
     errors = 0
@@ -84,7 +102,7 @@ def test_error_in_nested_transaction_should_retry_outermost_transaction
         end
       end
     end
-  
+
     assert_equal 4, tries
   end
 end