add new compute_ctl spec flag

. If it is set, drop all the subscriptions from the compute node
before it starts. To avoid race on compute start, use new GUC neon.disable_logical_replication_subscribers
to temporarily disable logical replication workers until we drop the subscriptions.

compute_tools/src/compute.rs

@@ -47,8 +47,7 @@ use crate::spec_apply::ApplySpecPhase::{
 };
 use crate::spec_apply::PerDatabasePhase;
 use crate::spec_apply::PerDatabasePhase::{
-    ChangeSchemaPerms, DeleteDBRoleReferences, DropSubscriptionsForDeletedDatabases,
-    HandleAnonExtension,
+    ChangeSchemaPerms, DeleteDBRoleReferences, DropLogicalSubscriptions, HandleAnonExtension,
 };
 use crate::spec_apply::{apply_operations, MutableApplyContext, DB};
 use crate::sync_sk::{check_if_synced, ping_safekeeper};
@@ -996,7 +995,7 @@ impl ComputeNode {
                         jwks_roles.clone(),
                         concurrency_token.clone(),
                         db,
-                        [DropSubscriptionsForDeletedDatabases].to_vec(),
+                        [DropLogicalSubscriptions].to_vec(),
                     );
 
                     Ok(spawn(fut))
@@ -1064,19 +1063,25 @@ impl ComputeNode {
                     }
 
                     let conf = Arc::new(conf);
+                    let mut phases = vec![
+                        DeleteDBRoleReferences,
+                        ChangeSchemaPerms,
+                        HandleAnonExtension,
+                    ];
+
+                    if spec.drop_subscriptions_before_start {
+                        info!("Adding DropLogicalSubscriptions phase because drop_subscriptions_before_start is set");
+                        phases.push(DropLogicalSubscriptions);
+                    }
+
                     let fut = Self::apply_spec_sql_db(
                         spec.clone(),
                         conf,
                         ctx.clone(),
                         jwks_roles.clone(),
                         concurrency_token.clone(),
                         db,
-                        [
-                            DeleteDBRoleReferences,
-                            ChangeSchemaPerms,
-                            HandleAnonExtension,
-                        ]
-                        .to_vec(),
+                        phases,
                     );
 
                     Ok(spawn(fut))
@@ -1463,6 +1468,16 @@ impl ComputeNode {
                         Ok(())
                     },
                 )?;
+
+                let postgresql_conf_path = pgdata_path.join("postgresql.conf");
+                if config::line_in_file(
+                    &postgresql_conf_path,
+                    "neon.disable_logical_replication_subscribers=false",
+                )? {
+                    info!("updated postgresql.conf to set neon.disable_logical_replication_subscribers=false");
+                } else {
+                    info!("postgresql.conf is up-to-date");
+                }
                 self.pg_reload_conf()?;
             }
             self.post_apply_config()?;

compute_tools/src/config.rs

@@ -129,6 +129,13 @@ pub fn write_postgres_conf(
 
     writeln!(file, "neon.extension_server_port={}", extension_server_port)?;
 
+    if spec.drop_subscriptions_before_start {
+        writeln!(file, "neon.disable_logical_replication_subscribers=true")?;
+    } else {
+        // be explicit about the default value
+        writeln!(file, "neon.disable_logical_replication_subscribers=false")?;
+    }
+
     // This is essential to keep this line at the end of the file,
     // because it is intended to override any settings above.
     writeln!(file, "include_if_exists = 'compute_ctl_temp_override.conf'")?;

compute_tools/src/spec_apply.rs

@@ -47,7 +47,7 @@ pub enum PerDatabasePhase {
     DeleteDBRoleReferences,
     ChangeSchemaPerms,
     HandleAnonExtension,
-    DropSubscriptionsForDeletedDatabases,
+    DropLogicalSubscriptions,
 }
 
 #[derive(Clone, Debug)]
@@ -331,7 +331,7 @@ async fn get_operations<'a>(
                             // NB: there could be other db states, which prevent us from dropping
                             // the database. For example, if db is used by any active subscription
                             // or replication slot.
-                            // Such cases are handled in the DropSubscriptionsForDeletedDatabases
+                            // Such cases are handled in the DropLogicalSubscriptions
                             // phase. We do all the cleanup before actually dropping the database.
                             let drop_db_query: String = format!(
                                 "DROP DATABASE IF EXISTS {} WITH (FORCE)",
@@ -444,7 +444,7 @@ async fn get_operations<'a>(
         }
         ApplySpecPhase::RunInEachDatabase { db, subphase } => {
             match subphase {
-                PerDatabasePhase::DropSubscriptionsForDeletedDatabases => {
+                PerDatabasePhase::DropLogicalSubscriptions => {
                     match &db {
                         DB::UserDB(db) => {
                             let drop_subscription_query: String = format!(

control_plane/src/bin/neon_local.rs

@@ -1357,6 +1357,7 @@ async fn handle_endpoint(subcmd: &EndpointCmd, env: &local_env::LocalEnv) -> Res
                 args.pg_version,
                 mode,
                 !args.update_catalog,
+                false,
             )?;
         }
         EndpointCmd::Start(args) => {

control_plane/src/endpoint.rs

@@ -76,6 +76,7 @@ pub struct EndpointConf {
     http_port: u16,
     pg_version: u32,
     skip_pg_catalog_updates: bool,
+    drop_subscriptions_before_start: bool,
     features: Vec<ComputeFeature>,
 }
 
@@ -143,6 +144,7 @@ impl ComputeControlPlane {
         pg_version: u32,
         mode: ComputeMode,
         skip_pg_catalog_updates: bool,
+        drop_subscriptions_before_start: bool,
     ) -> Result<Arc<Endpoint>> {
         let pg_port = pg_port.unwrap_or_else(|| self.get_port());
         let http_port = http_port.unwrap_or_else(|| self.get_port() + 1);
@@ -162,6 +164,7 @@ impl ComputeControlPlane {
             // with this we basically test a case of waking up an idle compute, where
             // we also skip catalog updates in the cloud.
             skip_pg_catalog_updates,
+            drop_subscriptions_before_start,
             features: vec![],
         });
 
@@ -177,6 +180,7 @@ impl ComputeControlPlane {
                 pg_port,
                 pg_version,
                 skip_pg_catalog_updates,
+                drop_subscriptions_before_start,
                 features: vec![],
             })?,
         )?;
@@ -240,6 +244,7 @@ pub struct Endpoint {
     // Optimizations
     skip_pg_catalog_updates: bool,
 
+    drop_subscriptions_before_start: bool,
     // Feature flags
     features: Vec<ComputeFeature>,
 }
@@ -291,6 +296,7 @@ impl Endpoint {
             tenant_id: conf.tenant_id,
             pg_version: conf.pg_version,
             skip_pg_catalog_updates: conf.skip_pg_catalog_updates,
+            drop_subscriptions_before_start: conf.drop_subscriptions_before_start,
             features: conf.features,
         })
     }
@@ -607,6 +613,18 @@ impl Endpoint {
                         restrict_conn: false,
                         invalid: false,
                     }]
+                }
+                // This is a hack to enable testing of the drop_subscriptions_before_start feature.
+                // Why don't we just always set the database vector properly here
+                // so that the code matched real cplane logic?
+                else if self.drop_subscriptions_before_start {
+                    vec![Database {
+                        name: PgIdent::from_str("postgres").unwrap(),
+                        owner: PgIdent::from_str("cloud_admin").unwrap(),
+                        options: None,
+                        restrict_conn: false,
+                        invalid: false,
+                    }]
                 } else {
                     Vec::new()
                 },
@@ -625,6 +643,7 @@ impl Endpoint {
             shard_stripe_size: Some(shard_stripe_size),
             local_proxy_config: None,
             reconfigure_concurrency: 1,
+            drop_subscriptions_before_start: self.drop_subscriptions_before_start,
         };
         let spec_path = self.endpoint_path().join("spec.json");
         std::fs::write(spec_path, serde_json::to_string_pretty(&spec)?)?;

libs/compute_api/src/spec.rs

@@ -138,6 +138,13 @@ pub struct ComputeSpec {
     /// enough spare connections for reconfiguration process to succeed.
     #[serde(default = "default_reconfigure_concurrency")]
     pub reconfigure_concurrency: usize,
+
+    /// If set to true, the compute_ctl will drop all subscriptions before starting the
+    /// compute. This is needed when we start an endpoint on a branch, so that child
+    /// would not compete with parent branch subscriptions
+    /// over the same replication content from publisher.
+    #[serde(default)] // Default false
+    pub drop_subscriptions_before_start: bool,
 }
 
 /// Feature flag to signal `compute_ctl` to enable certain experimental functionality.

pgxn/neon/logical_replication_monitor.c

@@ -237,11 +237,8 @@ LogicalSlotsMonitorMain(Datum main_arg)
 		XLogRecPtr	cutoff_lsn;
 
 		/* In case of a SIGHUP, just reload the configuration. */
-		if (ConfigReloadPending)
-		{
-			ConfigReloadPending = false;
-			ProcessConfigFile(PGC_SIGHUP);
-		}
+
+		ProcessConfigFile(PGC_SIGHUP);
 
 		/* Get the cutoff LSN */
 		cutoff_lsn = get_snapshots_cutoff_lsn();

pgxn/neon/neon.c

@@ -19,6 +19,7 @@
 #include "access/xlogrecovery.h"
 #endif
 #include "replication/logical.h"
+#include "replication/logicallauncher.h"
 #include "replication/slot.h"
 #include "replication/walsender.h"
 #include "storage/proc.h"
@@ -434,6 +435,15 @@ _PG_init(void)
 
 	restore_running_xacts_callback = RestoreRunningXactsFromClog;
 
+	DefineCustomBoolVariable(
+							"neon.disable_logical_replication_subscribers",
+							"Disables incomming logical replication",
+							NULL,
+							&disable_logical_replication_subscribers,
+							false,
+							PGC_SIGHUP,
+							0,
+							NULL, NULL, NULL);
 
 	DefineCustomBoolVariable(
 							"neon.allow_replica_misconfig",