Make Database#table_exists? on PostgreSQL handle lock or statement ti…

…meout errors as evidence the table exists (Fixes #2106)

This checks the exception message so it works with the postgres
adapter with both pg and pure-ruby postgres drivers, and with
the jdbc/postgresql adapter.

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Make Database#table_exists? on PostgreSQL handle lock or statement timeout errors as evidence the table exists (jeremyevans) (#2106)
+
 * Work around DateTime.jd fractional second bug on JRuby in named_timezones extension (jeremyevans)
 
 * Support fractional times and timestamps on SQLAnywhere (jeremyevans)

lib/sequel/adapters/shared/postgres.rb

@@ -1092,6 +1092,14 @@ def _set_constraints_sql(type, opts)
         sql << type
       end
 
+      # Consider lock or statement timeout errors as evidence that the table exists
+      # but is locked.
+      def _table_exists?(ds)
+        super
+      rescue DatabaseError => e    
+        raise e unless /canceling statement due to (?:statement|lock) timeout/ =~ e.message 
+      end
+
       def alter_table_add_column_sql(table, op)
         "ADD COLUMN#{' IF NOT EXISTS' if op[:if_not_exists]} #{column_definition_sql(op)}"
       end

spec/adapters/postgres_spec.rb

@@ -74,6 +74,27 @@ def c.exec_prepared(*); super; nil end
     Sequel.datetime_class = Time
   end
 
+  it "should return true for table_exists? if table exists but is locked" do
+    begin
+      @db.create_table(:test){Integer :id}
+      t = []
+      @db[:test].lock('ACCESS EXCLUSIVE') do
+        Thread.new do
+          @db.run "SET lock_timeout = 1"
+          t << @db.table_exists?(:test)
+         end.join
+        Thread.new do
+          @db.run "SET statement_timeout = 1"
+          t << @db.table_exists?(:test)
+         end.join
+      end
+      t.must_equal [true, true]
+    ensure
+      # Must disconnect to ensure connections with lock/statement timeout are not left in the pool
+      @db.disconnect
+    end
+  end
+
   it "should be able to handle various types of IN/NOT IN queries" do
     ds = @db.select(1)
     ds.where(2=>[2, 3]).wont_be_empty