pg_basebackup-write-dbname.v0006.patch

diff --git a/doc/src/sgml/ref/pg_basebackup.sgml b/doc/src/sgml/ref/pg_basebackup.sgml
index 88c689e725..903da73df7 100644
--- a/doc/src/sgml/ref/pg_basebackup.sgml
+++ b/doc/src/sgml/ref/pg_basebackup.sgml
@@ -243,7 +243,10 @@ PostgreSQL documentation
         The <filename>postgresql.auto.conf</filename> file will record the connection
         settings and, if specified, the replication slot
         that <application>pg_basebackup</application> is using, so that
-        streaming replication will use the same settings later on.
+        streaming replication or <link linkend="logicaldecoding-replication-slots-synchronization">
+        logical replication slot synchronization</link> will use the same
+        settings later on. The dbname will be recorded only if the dbname was
+        specified explicitly in the connection string.
        </para>
 
       </listitem>
@@ -807,7 +810,9 @@ PostgreSQL documentation
         client applications, but because <application>pg_basebackup</application>
         doesn't connect to any particular database in the cluster, any database
         name in the connection string will be ignored
-        by <productname>PostgreSQL</productname>. Middleware, or proxies, used in
+        by <productname>PostgreSQL</productname>. Middleware, proxies, or
+        <link linkend="logicaldecoding-replication-slots-synchronization">
+        logical replication slot synchronization</link> used in
         connecting to <productname>PostgreSQL</productname> might however
         utilize the value.
        </para>
diff --git a/src/bin/pg_basebackup/pg_basebackup.c b/src/bin/pg_basebackup/pg_basebackup.c
index 3a9940097c..2826af1252 100644
--- a/src/bin/pg_basebackup/pg_basebackup.c
+++ b/src/bin/pg_basebackup/pg_basebackup.c
@@ -1807,10 +1807,15 @@ BaseBackup(char *compression_algorithm, char *compression_detail,
 	}
 
 	/*
-	 * Build contents of configuration file if requested
+	 * Build contents of configuration file if requested.
+	 *
+	 * The dbname dbname is either of 1) the value written in the connection
+	 * string, 2) specified as PGDATABASE or 3) the value written in the
+	 * service file.
 	 */
 	if (writerecoveryconf)
-		recoveryconfcontents = GenerateRecoveryConfig(conn, replication_slot);
+		recoveryconfcontents = GenerateRecoveryConfig(conn, replication_slot,
+													  GetDbnameFromConnectionStringOrDefaults());
 
 	/*
 	 * Run IDENTIFY_SYSTEM so we can get the timeline
diff --git a/src/bin/pg_basebackup/streamutil.c b/src/bin/pg_basebackup/streamutil.c
index 56d1b15951..733eb89d49 100644
--- a/src/bin/pg_basebackup/streamutil.c
+++ b/src/bin/pg_basebackup/streamutil.c
@@ -34,6 +34,8 @@
 int			WalSegSz;
 
 static bool RetrieveDataDirCreatePerm(PGconn *conn);
+static void FindDbnameFromPQconninfoOptions(PQconninfoOption *conn_opts,
+											char **dbname);
 
 /* SHOW command for replication connection was introduced in version 10 */
 #define MINIMUM_VERSION_FOR_SHOW_CMD 100000
@@ -267,6 +269,71 @@ GetConnection(void)
 	return tmpconn;
 }
 
+/*
+ * FindDbnameFromPQconninfoOptions
+ *
+ * This is a helper function for GetDbnameFromConnectionStringOrDefaults().
+ * Read all parameters in PQconninfoOption and extract the dbname to the
+ * argument.
+ */
+static void
+FindDbnameFromPQconninfoOptions(PQconninfoOption *conn_opts, char **dbname)
+{
+	Assert(dbname != NULL);
+
+	for (PQconninfoOption *conn_opt = conn_opts; conn_opt->keyword != NULL;
+		 conn_opt++)
+	{
+		if ((strcmp(conn_opt->keyword, "dbname") == 0) &&
+			conn_opt->val != NULL && conn_opt->val[0] != '\0')
+			*dbname = pg_strdup(conn_opt->val);
+	}
+}
+
+/*
+ * GetDbnameFromConnectionStringOrDefaults
+ *
+ * If database is specified either in the connection string (higher priority)
+ * or enviroment variables, then return the last database name specified.
+ * If database is not specified, then return NULL.
+ */
+char *
+GetDbnameFromConnectionStringOrDefaults(void)
+{
+	PQconninfoOption *conn_opts = NULL;
+	char	   *err_msg = NULL;
+	char	   *dbname = NULL;
+
+	/* Parse the connection string first because this has higher priority */
+	if (connection_string)
+	{
+		conn_opts = PQconninfoParse(connection_string, &err_msg);
+		if (conn_opts == NULL)
+			pg_fatal("%s", err_msg);
+
+		FindDbnameFromPQconninfoOptions(conn_opts, &dbname);
+	}
+
+	/* Skip till the cleanup phase when found */
+	if (dbname)
+		goto cleanup;
+
+	/*
+	 * If we reach here, the connection string does not contain the dbname.
+	 * Let's check default settings, especially environment variables and the
+	 * service file.
+	 */
+	conn_opts = PQconndefaults();
+	if (conn_opts == NULL)
+		pg_fatal("%s", err_msg);
+
+	FindDbnameFromPQconninfoOptions(conn_opts, &dbname);
+
+cleanup:
+	PQconninfoFree(conn_opts);
+	return dbname;
+}
+
 /*
  * From version 10, explicitly set wal segment size using SHOW wal_segment_size
  * since ControlFile is not accessible here.
diff --git a/src/bin/pg_basebackup/streamutil.h b/src/bin/pg_basebackup/streamutil.h
index 7a3dd98da3..406c0070b2 100644
--- a/src/bin/pg_basebackup/streamutil.h
+++ b/src/bin/pg_basebackup/streamutil.h
@@ -31,6 +31,8 @@ extern PGconn *conn;
 
 extern PGconn *GetConnection(void);
 
+extern char *GetDbnameFromConnectionStringOrDefaults(void);
+
 /* Replication commands */
 extern bool CreateReplicationSlot(PGconn *conn, const char *slot_name,
 								  const char *plugin, bool is_temporary,
diff --git a/src/bin/pg_rewind/pg_rewind.c b/src/bin/pg_rewind/pg_rewind.c
index bde90bf60b..8449ae78ef 100644
--- a/src/bin/pg_rewind/pg_rewind.c
+++ b/src/bin/pg_rewind/pg_rewind.c
@@ -451,7 +451,7 @@ main(int argc, char **argv)
 		pg_log_info("no rewind required");
 		if (writerecoveryconf && !dry_run)
 			WriteRecoveryConfig(conn, datadir_target,
-								GenerateRecoveryConfig(conn, NULL));
+								GenerateRecoveryConfig(conn, NULL, NULL));
 		exit(0);
 	}
 
@@ -525,7 +525,7 @@ main(int argc, char **argv)
 	/* Also update the standby configuration, if requested. */
 	if (writerecoveryconf && !dry_run)
 		WriteRecoveryConfig(conn, datadir_target,
-							GenerateRecoveryConfig(conn, NULL));
+							GenerateRecoveryConfig(conn, NULL, NULL));
 
 	/* don't need the source connection anymore */
 	source->destroy(source);
diff --git a/src/fe_utils/recovery_gen.c b/src/fe_utils/recovery_gen.c
index 63c78c986c..5904f0840a 100644
--- a/src/fe_utils/recovery_gen.c
+++ b/src/fe_utils/recovery_gen.c
@@ -18,9 +18,15 @@ static char *escape_quotes(const char *src);
 /*
  * Write recovery configuration contents into a fresh PQExpBuffer, and
  * return it.
+ *
+ * This can also accept the dbname, which should be written in the
+ * primary_conninfo. The walreciever process will ignore this value, but it is
+ * essential for the slotsync worker. It connects to the specified database on
+ * the primary server.
  */
 PQExpBuffer
-GenerateRecoveryConfig(PGconn *pgconn, const char *replication_slot)
+GenerateRecoveryConfig(PGconn *pgconn, const char *replication_slot,
+					   char *dbname)
 {
 	PQconninfoOption *connOptions;
 	PQExpBufferData conninfo_buf;
@@ -66,6 +72,20 @@ GenerateRecoveryConfig(PGconn *pgconn, const char *replication_slot)
 		appendPQExpBuffer(&conninfo_buf, "%s=", opt->keyword);
 		appendConnStrVal(&conninfo_buf, opt->val);
 	}
+
+	if (dbname)
+	{
+		/*
+		 * If dbname is specified in the connection, append the dbname. This
+		 * will be used later for logical replication slot synchronization.
+		 */
+		if (conninfo_buf.len != 0)
+			appendPQExpBufferChar(&conninfo_buf, ' ');
+
+		appendPQExpBuffer(&conninfo_buf, "%s=", "dbname");
+		appendConnStrVal(&conninfo_buf, dbname);
+	}
+
 	if (PQExpBufferDataBroken(conninfo_buf))
 		pg_fatal("out of memory");
 
diff --git a/src/include/fe_utils/recovery_gen.h b/src/include/fe_utils/recovery_gen.h
index f1b760604b..73c1aa8e59 100644
--- a/src/include/fe_utils/recovery_gen.h
+++ b/src/include/fe_utils/recovery_gen.h
@@ -21,7 +21,8 @@
 #define MINIMUM_VERSION_FOR_RECOVERY_GUC 120000
 
 extern PQExpBuffer GenerateRecoveryConfig(PGconn *pgconn,
-										  const char *replication_slot);
+										  const char *replication_slot,
+										  char *dbname);
 extern void WriteRecoveryConfig(PGconn *pgconn, const char *target_dir,
 								PQExpBuffer contents);