Fixed #26500 -- Added SKIP LOCKED support to select_for_update().

Thanks Tim for the review.

7 files changed, 23 insertions, 6 deletions

diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index 484b272294..9bb0ed6848 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -36,6 +36,7 @@ class BaseDatabaseFeatures(object):
     allow_sliced_subqueries = True
     has_select_for_update = False
     has_select_for_update_nowait = False
+    has_select_for_update_skip_locked = False
 
     supports_select_related = True
 
diff --git a/django/db/backends/base/operations.py b/django/db/backends/base/operations.py
index 4e5814e58f..1ed11b178e 100644
--- a/django/db/backends/base/operations.py
+++ b/django/db/backends/base/operations.py
@@ -177,12 +177,14 @@ class BaseDatabaseOperations(object):
         """
         return []
 
-    def for_update_sql(self, nowait=False):
+    def for_update_sql(self, nowait=False, skip_locked=False):
         """
         Returns the FOR UPDATE SQL clause to lock rows for an update operation.
         """
         if nowait:
             return 'FOR UPDATE NOWAIT'
+        elif skip_locked:
+            return 'FOR UPDATE SKIP LOCKED'
         else:
             return 'FOR UPDATE'
 
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index 84e18cf9ba..956f7a051b 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -13,6 +13,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     uses_savepoints = True
     has_select_for_update = True
     has_select_for_update_nowait = True
+    has_select_for_update_skip_locked = True
     can_return_id_from_insert = True
     allow_sliced_subqueries = False
     supports_subqueries_in_group_by = False
diff --git a/django/db/backends/postgresql/features.py b/django/db/backends/postgresql/features.py
index 218a7645de..a9e1c77480 100644
--- a/django/db/backends/postgresql/features.py
+++ b/django/db/backends/postgresql/features.py
@@ -1,5 +1,6 @@
 from django.db.backends.base.features import BaseDatabaseFeatures
 from django.db.utils import InterfaceError
+from django.utils.functional import cached_property
 
 
 class DatabaseFeatures(BaseDatabaseFeatures):
@@ -31,3 +32,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     greatest_least_ignores_nulls = True
     can_clone_databases = True
     supports_temporal_subtraction = True
+
+    @cached_property
+    def has_select_for_update_skip_locked(self):
+        return self.connection.pg_version >= 90500
diff --git a/django/db/models/query.py b/django/db/models/query.py
index 07f1fc414f..0a0965fe47 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -835,15 +835,18 @@ class QuerySet(object):
         else:
             return self._filter_or_exclude(None, **filter_obj)
 
-    def select_for_update(self, nowait=False):
+    def select_for_update(self, nowait=False, skip_locked=False):
         """
         Returns a new QuerySet instance that will select objects with a
         FOR UPDATE lock.
         """
+        if nowait and skip_locked:
+            raise ValueError('The nowait option cannot be used with skip_locked.')
         obj = self._clone()
         obj._for_write = True
         obj.query.select_for_update = True
         obj.query.select_for_update_nowait = nowait
+        obj.query.select_for_update_skip_locked = skip_locked
         return obj
 
     def select_related(self, *fields):
diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index cedb140143..5eaac1fc74 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -445,13 +445,16 @@ class SQLCompiler(object):
                         "select_for_update cannot be used outside of a transaction."
                     )
 
-                # If we've been asked for a NOWAIT query but the backend does
-                # not support it, raise a DatabaseError otherwise we could get
-                # an unexpected deadlock.
                 nowait = self.query.select_for_update_nowait
+                skip_locked = self.query.select_for_update_skip_locked
+                # If we've been asked for a NOWAIT/SKIP LOCKED query but the
+                # backend does not support it, raise a DatabaseError otherwise
+                # we could get an unexpected deadlock.
                 if nowait and not self.connection.features.has_select_for_update_nowait:
                     raise DatabaseError('NOWAIT is not supported on this database backend.')
-                result.append(self.connection.ops.for_update_sql(nowait=nowait))
+                elif skip_locked and not self.connection.features.has_select_for_update_skip_locked:
+                    raise DatabaseError('SKIP LOCKED is not supported on this database backend.')
+                result.append(self.connection.ops.for_update_sql(nowait=nowait, skip_locked=skip_locked))
 
             return ' '.join(result), tuple(params)
         finally:
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index d182242ce4..426087f8af 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -167,6 +167,7 @@ class Query(object):
         self.distinct_fields = []
         self.select_for_update = False
         self.select_for_update_nowait = False
+        self.select_for_update_skip_locked = False
 
         self.select_related = False
         # Arbitrary limit for select_related to prevents infinite recursion.
@@ -286,6 +287,7 @@ class Query(object):
         obj.distinct_fields = self.distinct_fields[:]
         obj.select_for_update = self.select_for_update
         obj.select_for_update_nowait = self.select_for_update_nowait
+        obj.select_for_update_skip_locked = self.select_for_update_skip_locked
         obj.select_related = self.select_related
         obj.values_select = self.values_select[:]
         obj._annotations = self._annotations.copy() if self._annotations is not None else None