zone: Fix zone configuration application bug

There was a bug that allowed zone configuration application on indexes
to leak into the zone configurations for partitions, due to a subtlety in
ZoneConfig.GetSubzone. This PR fixes the bug with zone configuration
application and adds a test.

This PR is necessary for cockroachdb#40493 to land.

An example of this is as follows:

```
CREATE TABLE infect (x INT PRIMARY KEY);
ALTER TABLE infect PARTITION BY LIST (x) ( PARTITION p1 VALUES IN (1));
ALTER INDEX infect@primary CONFIGURE ZONE USING num_replicas=5;
ALTER PARTITION p1 OF TABLE infect CONFIGURE ZONE USING
constraints='[+dc=dc1]';
```
Before, the zone configuration for p1 would *also have* num_replicas=5
set, which should not be the case. This PR ensures that the zone
configuration for p1 only has constraints set.

Release Justification: Important bug fix.

Release note (bug fix): Fixing bug where zone configuration application
on indexes could leak into configurations on partitions.

pkg/ccl/cliccl/cli_test.go

@@ -215,7 +215,7 @@ func Example_cclzone() {
 	// gc:
 	//   ttlseconds: 90000
 	// num_replicas: 3
-	// constraints: [+zone=us-east-1a, +ssd]
+	// constraints: []
 	// lease_preferences: []
 	// zone ls
 	// .default

pkg/ccl/logictestccl/testdata/logic_test/zone

@@ -342,6 +342,21 @@ test.t38074@i ALTER INDEX t38074@i CONFIGURE ZONE USING
                                    constraints = '[]',
                                    lease_preferences = '[]'
 
+# Test that index configurations don't infect partition configurations.
+# Specifically we are testing that values written to infect@primary's
+# zone configuration does not appear in partition p1 of infect@primary's zone config.
+statement ok
+CREATE TABLE infect (x INT PRIMARY KEY);
+ALTER TABLE infect PARTITION BY LIST (x) ( PARTITION p1 VALUES IN (1));
+ALTER INDEX infect@primary CONFIGURE ZONE USING num_replicas=5;
+ALTER PARTITION p1 OF TABLE infect CONFIGURE ZONE USING constraints='[+dc=dc1]'
+
+query T
+SELECT config_sql FROM [SHOW ZONE CONFIGURATIONS] WHERE zone_name = 'test.infect.p1'
+----
+ALTER PARTITION p1 OF INDEX test.public.infect@primary CONFIGURE ZONE USING
+  constraints = '[+dc=dc1]'
+
 # ------------------------------------------------------------------------------
 # Regression test for #36348; place this at the bottom of this file.
 # ------------------------------------------------------------------------------

pkg/config/zone.go

@@ -653,6 +653,19 @@ func (z *ZoneConfig) GetSubzone(indexID uint32, partition string) *Subzone {
 	return nil
 }
 
+// GetSubzoneExact is similar to GetSubzone but does not find the most specific
+// subzone that applies to a specified index and partition, as it finds either the
+// exact config that applies, or returns nil.
+func (z *ZoneConfig) GetSubzoneExact(indexID uint32, partition string) *Subzone {
+	for _, s := range z.Subzones {
+		if s.IndexID == indexID && s.PartitionName == partition {
+			copySubzone := s
+			return &copySubzone
+		}
+	}
+	return nil
+}
+
 // GetSubzoneForKeySuffix returns the ZoneConfig for the subzone that contains
 // keySuffix, if it exists.
 func (z ZoneConfig) GetSubzoneForKeySuffix(keySuffix []byte) *Subzone {

pkg/sql/set_zone_config.go

@@ -286,7 +286,7 @@ func (n *setZoneConfigNode) startExec(params runParams) error {
 
 	var partialSubzone *config.Subzone
 	if index != nil {
-		partialSubzone = partialZone.GetSubzone(uint32(index.ID), partition)
+		partialSubzone = partialZone.GetSubzoneExact(uint32(index.ID), partition)
 		if partialSubzone == nil {
 			partialSubzone = &config.Subzone{Config: *config.NewZoneConfig()}
 		}