Fixes #224.

Postgres only Issue:
Jobs that meet all of the following conditions will result in the database connection entering an invalid and
unrecoverable state, requiring a rollback before any further database requests (reads or writes) can be made:

1. Use SolidQueue's 'limits_concurrency' macro
2. Are enqueued inside an application-level transaction
3. The limits_concurrency key already exists in the Semaphore table (i.e., this is job 2 - N for a given key)

SolidQueue uses the following design pattern to implement the conflict detection of the limits_concurrency
macro which works 100% correctly, 100% of the time, with MySQL and SQLite:
  begin
    Semaphore.create!(...)
      no_conflict_path()
    rescue ActiveRecord::RecordNotUnique
      handle_concurrency_conflict()
    end

The problem is Postgres:
It's not possible to rescue and recover from an insert failing due to an conflict on unique index (or any other
database constraint). Until a rollback is executed, the database connection is in an invalid state and unusable.

Possible Solutions:
1. Nested transactions
   + Easiest to implement
   + Portable across all three standard Rails databases
   - Has significant performance issues, especially for databases under load or replicated databases with
     long-running queries (Postgres specific)
   - Requires using Raise and Rescue for flow of control (performance, less than ideal coding practice)
   - Requires issuing a rollback (performance, additional command that must round trip from the client to
     database and back)

2. Insert into the Semaphore table using 'INSERT INTO table (..) VALUES (...) ON CONFLICT DO NOTHING' syntax
   + ANSI standard syntax (not a Postgres 'one off' solution)
   + Rails natively supports identifying database adaptors that support this syntax, making the implementation
     portable and maintainable
   + Supported by Postgres and allows this issue to be addressed
   + Does not require Raise and Rescue for flow of control
   + Performant (native, fast path database functionality)

Solution:
Leverage Rails connection adaptors allowing code to easily identifying supported database features/functionality to
implement strategy #2 (INSERT ON CONFLICT) for those databases that support it (i.e., Postgres) and leave the
original implementation of rescuing RecordNotUnique for those that do not.

Note: Although I'd love to take credit for the "quality" of this implementation, all that credit belongs to @rosa who's
excellent feedback on an earlier implementation resulted in this significantly better implementation.

app/models/solid_queue/semaphore.rb

@@ -17,6 +17,17 @@ def signal(job)
       def signal_all(jobs)
         Proxy.signal_all(jobs)
       end
+
+      # Requires a unique index on key
+      def create_unique_by(attributes)
+        if connection.supports_insert_conflict_target?
+          insert({ **attributes }, unique_by: :key).any?
+        else
+          create!(**attributes)
+        end
+      rescue ActiveRecord::RecordNotUnique
+        false
+      end
     end
 
     class Proxy
@@ -44,35 +55,22 @@ def signal
 
         attr_accessor :job
 
-        def attempt_creation_with_insert_on_conflict
-          results = Semaphore.insert({ key: key, value: limit - 1, expires_at: expires_at }, unique_by: :key)
-
-          if results.length.zero?
-            limit == 1 ? false : attempt_decrement
-          else
+        def attempt_creation
+          if Semaphore.create_unique_by(key: key, value: limit - 1, expires_at: expires_at)
             true
+          else
+            check_limit_or_decrement
           end
         end
 
-        def attempt_creation_with_create_and_exception_handling
-          Semaphore.create!(key: key, value: limit - 1, expires_at: expires_at)
-          true
-        rescue ActiveRecord::RecordNotUnique
-          limit == 1 ? false : attempt_decrement
-        end
-
-        if ActiveRecord::Base.connection.adapter_name == "PostgreSQL"
-          alias attempt_creation attempt_creation_with_insert_on_conflict
-        else
-          alias attempt_creation attempt_creation_with_create_and_exception_handling
-        end
+        def check_limit_or_decrement = limit == 1 ? false : attempt_decrement
 
         def attempt_decrement
-          Semaphore.available.where(key: key).update_all([ "value = value - 1, expires_at = ?", expires_at ]) > 0
+          Semaphore.available.where(key: key).update_all(["value = value - 1, expires_at = ?", expires_at]) > 0
         end
 
         def attempt_increment
-          Semaphore.where(key: key, value: ...limit).update_all([ "value = value + 1, expires_at = ?", expires_at ]) > 0
+          Semaphore.where(key: key, value: ...limit).update_all(["value = value + 1, expires_at = ?", expires_at]) > 0
         end
 
         def key

test/integration/concurrency_controls_test.rb

@@ -83,7 +83,7 @@ class ConcurrencyControlsTest < ActiveSupport::TestCase
 
     # C would have started in the beginning, seeing the status empty, and would finish after
     # all other jobs, so it'll do the last update with only itself
-    assert_stored_sequence(@result, [ "C" ])
+    assert_stored_sequence(@result, ["C"])
   end
 
   test "run several jobs over the same record sequentially, with some of them failing" do
@@ -99,7 +99,7 @@ class ConcurrencyControlsTest < ActiveSupport::TestCase
     wait_for_jobs_to_finish_for(3.seconds)
     assert_equal 3, SolidQueue::FailedExecution.count
 
-    assert_stored_sequence @result, [ "B", "D", "F" ] + ("G".."K").to_a
+    assert_stored_sequence @result, ["B", "D", "F"] + ("G".."K").to_a
   end
 
   test "rely on dispatcher to unblock blocked executions with an available semaphore" do
@@ -133,7 +133,7 @@ class ConcurrencyControlsTest < ActiveSupport::TestCase
 
     # We can't ensure the order between B and C, because it depends on which worker wins when
     # unblocking, as one will try to unblock B and another C
-    assert_stored_sequence @result, ("A".."K").to_a, [ "A", "C", "B" ] + ("D".."K").to_a
+    assert_stored_sequence @result, ("A".."K").to_a, ["A", "C", "B"] + ("D".."K").to_a
   end
 
   test "rely on dispatcher to unblock blocked executions with an expired semaphore" do
@@ -165,7 +165,7 @@ class ConcurrencyControlsTest < ActiveSupport::TestCase
 
     # We can't ensure the order between B and C, because it depends on which worker wins when
     # unblocking, as one will try to unblock B and another C
-    assert_stored_sequence @result, ("A".."K").to_a, [ "A", "C", "B" ] + ("D".."K").to_a
+    assert_stored_sequence @result, ("A".."K").to_a, ["A", "C", "B"] + ("D".."K").to_a
   end
 
   test "don't block claimed executions that get released" do
@@ -183,44 +183,20 @@ class ConcurrencyControlsTest < ActiveSupport::TestCase
     assert job.reload.ready?
   end
 
-  if ActiveRecord::Base.connection.adapter_name == "PostgreSQL"
-    test "insert_with_unique_by_has_same_database_results_as_create!_with_exception_handling" do
-      key = "key", limit = 1, expires_at = 1.minute.from_now
+  test "verify transactions remain valid after Job creation conflicts via limits_concurrency" do
+    ActiveRecord::Base.transaction do
+      SequentialUpdateResultJob.perform_later(@result, name: "A", pause: 0.2.seconds)
+      SequentialUpdateResultJob.perform_later(@result, name: "B")
 
-      assert SolidQueue::Semaphore.count == 0
-
-      SolidQueue::Semaphore.insert({ key: key, value: limit - 1, expires_at: expires_at }, unique_by: :key)
-      assert SolidQueue::Semaphore.count == 1
-
-      SolidQueue::Semaphore.insert({ key: key, value: limit - 1, expires_at: expires_at }, unique_by: :key)
-      assert SolidQueue::Semaphore.count == 1
-
-      SolidQueue::Semaphore.delete_all
-      assert SolidQueue::Semaphore.count == 0
-
-      SolidQueue::Semaphore.create!(key: key, value: limit - 1, expires_at: expires_at)
-      assert SolidQueue::Semaphore.count == 1
-
-      assert_raises ActiveRecord::RecordNotUnique do
-        SolidQueue::Semaphore.create!(key: key, value: limit - 1, expires_at: expires_at)
+      begin
+        assert_equal 2, SolidQueue::Job.count
+        assert true, "Transaction state valid"
+      rescue ActiveRecord::StatementInvalid
+        assert false, "Transaction state unexpectedly invalid"
       end
     end
   end
 
-  test "confirm correct version of attempt_creation by database adaptor" do
-    proxy = SolidQueue::Semaphore::Proxy.new(true)
-
-    aliased_method = proxy.method(:attempt_creation)
-
-    if ActiveRecord::Base.connection.adapter_name == "PostgreSQL"
-      original_method = proxy.method(:attempt_creation_with_insert_on_conflict)
-    else
-      original_method = proxy.method(:attempt_creation_with_create_and_exception_handling)
-    end
-
-    assert_equal original_method.name, aliased_method.original_name, "The alias maps to the correct original method"
-  end
-
   private
 
     def assert_stored_sequence(result, *sequences)