Introduce the 'force' option for the Drop Database command.

This new option terminates the other sessions connected to the target
database and then drop it.  To terminate other sessions, the current user
must have desired permissions (same as pg_terminate_backend()).  We don't
allow to terminate the sessions if prepared transactions, active logical
replication slots or subscriptions are present in the target database.

Author: Pavel Stehule with changes by me
Reviewed-by: Dilip Kumar, Vignesh C, Ibrar Ahmed, Anthony Nowocien,
Ryan Lambert and Amit Kapila
Discussion: https://postgr.es/m/CAP_rwwmLJJbn70vLOZFpxGw3XD7nLB_7+NKz46H5EOO2k5H7OQ@mail.gmail.com

12 files changed, 219 insertions, 10 deletions

diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 6f28859f730..446813f0f0b 100644
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -810,7 +810,7 @@ createdb_failure_callback(int code, Datum arg)
  * DROP DATABASE
  */
 void
-dropdb(const char *dbname, bool missing_ok)
+dropdb(const char *dbname, bool missing_ok, bool force)
 {
 	Oid			db_id;
 	bool		db_istemplate;
@@ -910,6 +910,14 @@ dropdb(const char *dbname, bool missing_ok)
 								  "There are %d subscriptions.",
 								  nsubscriptions, nsubscriptions)));
 
+
+	/*
+	 * Attempt to terminate all existing connections to the target database if
+	 * the user has requested to do so.
+	 */
+	if (force)
+		TerminateOtherDBBackends(db_id);
+
 	/*
 	 * Check for other backends in the target database.  (Because we hold the
 	 * database lock, no new ones can start after this.)
@@ -1430,6 +1438,30 @@ movedb_failure_callback(int code, Datum arg)
 	(void) rmtree(dstpath, true);
 }
 
+/*
+ * Process options and call dropdb function.
+ */
+void
+DropDatabase(ParseState *pstate, DropdbStmt *stmt)
+{
+	bool		force = false;
+	ListCell   *lc;
+
+	foreach(lc, stmt->options)
+	{
+		DefElem    *opt = (DefElem *) lfirst(lc);
+
+		if (strcmp(opt->defname, "force") == 0)
+			force = true;
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_SYNTAX_ERROR),
+					 errmsg("unrecognized DROP DATABASE option \"%s\"", opt->defname),
+					 parser_errposition(pstate, opt->location)));
+	}
+
+	dropdb(stmt->dbname, stmt->missing_ok, force);
+}
 
 /*
  * ALTER DATABASE name ...
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 3432bb921dd..2f267e4bb65 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -3868,6 +3868,7 @@ _copyDropdbStmt(const DropdbStmt *from)
 
 	COPY_STRING_FIELD(dbname);
 	COPY_SCALAR_FIELD(missing_ok);
+	COPY_NODE_FIELD(options);
 
 	return newnode;
 }
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index 18cb0143733..da0e1d139ac 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -1676,6 +1676,7 @@ _equalDropdbStmt(const DropdbStmt *a, const DropdbStmt *b)
 {
 	COMPARE_STRING_FIELD(dbname);
 	COMPARE_SCALAR_FIELD(missing_ok);
+	COMPARE_NODE_FIELD(options);
 
 	return true;
 }
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 3f67aaf30ea..2f7bd662e8a 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -310,6 +310,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 %type <defelt>	vac_analyze_option_elem
 %type <list>	vac_analyze_option_list
 %type <node>	vac_analyze_option_arg
+%type <defelt>	drop_option
 %type <boolean>	opt_or_replace
 				opt_grant_grant_option opt_grant_admin_option
 				opt_nowait opt_if_exists opt_with_data
@@ -406,6 +407,7 @@ static Node *makeRecursiveViewSelect(char *relname, List *aliases, Node *query);
 				TriggerTransitions TriggerReferencing
 				publication_name_list
 				vacuum_relation_list opt_vacuum_relation_list
+				drop_option_list
 
 %type <list>	group_by_list
 %type <node>	group_by_item empty_grouping_set rollup_clause cube_clause
@@ -10213,7 +10215,7 @@ AlterDatabaseSetStmt:
 
 /*****************************************************************************
  *
- *		DROP DATABASE [ IF EXISTS ]
+ *		DROP DATABASE [ IF EXISTS ] dbname [ [ WITH ] ( options ) ]
  *
  * This is implicitly CASCADE, no need for drop behavior
  *****************************************************************************/
@@ -10223,6 +10225,7 @@ DropdbStmt: DROP DATABASE database_name
 					DropdbStmt *n = makeNode(DropdbStmt);
 					n->dbname = $3;
 					n->missing_ok = false;
+					n->options = NULL;
 					$$ = (Node *)n;
 				}
 			| DROP DATABASE IF_P EXISTS database_name
@@ -10230,10 +10233,48 @@ DropdbStmt: DROP DATABASE database_name
 					DropdbStmt *n = makeNode(DropdbStmt);
 					n->dbname = $5;
 					n->missing_ok = true;
+					n->options = NULL;
 					$$ = (Node *)n;
 				}
+			| DROP DATABASE database_name opt_with '(' drop_option_list ')'
+				{
+					DropdbStmt *n = makeNode(DropdbStmt);
+					n->dbname = $3;
+					n->missing_ok = false;
+					n->options = $6;
+					$$ = (Node *)n;
+				}
+			| DROP DATABASE IF_P EXISTS database_name opt_with '(' drop_option_list ')'
+				{
+					DropdbStmt *n = makeNode(DropdbStmt);
+					n->dbname = $5;
+					n->missing_ok = true;
+					n->options = $8;
+					$$ = (Node *)n;
+				}
+		;
+
+drop_option_list:
+			drop_option
+				{
+					$$ = list_make1((Node *) $1);
+				}
+			| drop_option_list ',' drop_option
+				{
+					$$ = lappend($1, (Node *) $3);
+				}
 		;
 
+/*
+ * Currently only the FORCE option is supported, but the syntax is designed
+ * to be extensible so that we can add more options in the future if required.
+ */
+drop_option:
+			FORCE
+				{
+					$$ = makeDefElem("force", NULL, @1);
+				}
+		;
 
 /*****************************************************************************
  *
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 3da53074b18..13bcbe77de7 100644
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -52,6 +52,8 @@
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "catalog/catalog.h"
+#include "catalog/pg_authid.h"
+#include "commands/dbcommands.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/proc.h"
@@ -2971,6 +2973,118 @@ CountOtherDBBackends(Oid databaseId, int *nbackends, int *nprepared)
 }
 
 /*
+ * Terminate existing connections to the specified database. This routine
+ * is used by the DROP DATABASE command when user has asked to forcefully
+ * drop the database.
+ *
+ * The current backend is always ignored; it is caller's responsibility to
+ * check whether the current backend uses the given DB, if it's important.
+ *
+ * It doesn't allow to terminate the connections even if there is a one
+ * backend with the prepared transaction in the target database.
+ */
+void
+TerminateOtherDBBackends(Oid databaseId)
+{
+	ProcArrayStruct *arrayP = procArray;
+	List	   *pids = NIL;
+	int			nprepared = 0;
+	int			i;
+
+	LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+	for (i = 0; i < procArray->numProcs; i++)
+	{
+		int			pgprocno = arrayP->pgprocnos[i];
+		PGPROC	   *proc = &allProcs[pgprocno];
+
+		if (proc->databaseId != databaseId)
+			continue;
+		if (proc == MyProc)
+			continue;
+
+		if (proc->pid != 0)
+			pids = lappend_int(pids, proc->pid);
+		else
+			nprepared++;
+	}
+
+	LWLockRelease(ProcArrayLock);
+
+	if (nprepared > 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_IN_USE),
+				 errmsg("database \"%s\" is being used by prepared transaction",
+						get_database_name(databaseId)),
+				 errdetail_plural("There is %d prepared transaction using the database.",
+								  "There are %d prepared transactions using the database.",
+								  nprepared,
+								  nprepared)));
+
+	if (pids)
+	{
+		ListCell   *lc;
+
+		/*
+		 * Check whether we have the necessary rights to terminate other
+		 * sessions.  We don't terminate any session untill we ensure that we
+		 * have rights on all the sessions to be terminated.  These checks are
+		 * the same as we do in pg_terminate_backend.
+		 *
+		 * In this case we don't raise some warnings - like "PID %d is not a
+		 * PostgreSQL server process", because for us already finished session
+		 * is not a problem.
+		 */
+		foreach(lc, pids)
+		{
+			int			pid = lfirst_int(lc);
+			PGPROC	   *proc = BackendPidGetProc(pid);
+
+			if (proc != NULL)
+			{
+				/* Only allow superusers to signal superuser-owned backends. */
+				if (superuser_arg(proc->roleId) && !superuser())
+					ereport(ERROR,
+							(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+							 (errmsg("must be a superuser to terminate superuser process"))));
+
+				/* Users can signal backends they have role membership in. */
+				if (!has_privs_of_role(GetUserId(), proc->roleId) &&
+					!has_privs_of_role(GetUserId(), DEFAULT_ROLE_SIGNAL_BACKENDID))
+					ereport(ERROR,
+							(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+							 (errmsg("must be a member of the role whose process is being terminated or member of pg_signal_backend"))));
+			}
+		}
+
+		/*
+		 * There's a race condition here: once we release the ProcArrayLock,
+		 * it's possible for the session to exit before we issue kill.  That
+		 * race condition possibility seems too unlikely to worry about.  See
+		 * pg_signal_backend.
+		 */
+		foreach(lc, pids)
+		{
+			int			pid = lfirst_int(lc);
+			PGPROC	   *proc = BackendPidGetProc(pid);
+
+			if (proc != NULL)
+			{
+				/*
+				 * If we have setsid(), signal the backend's whole process
+				 * group
+				 */
+#ifdef HAVE_SETSID
+				(void) kill(-pid, SIGTERM);
+#else
+				(void) kill(pid, SIGTERM);
+#endif
+			}
+		}
+	}
+}
+
+/*
  * ProcArraySetReplicationSlotXmin
  *
  * Install limits to future computations of the xmin horizon to prevent vacuum
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index 6787d8e66d3..3a03ca7e2f0 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -595,13 +595,9 @@ standard_ProcessUtility(PlannedStmt *pstmt,
 			break;
 
 		case T_DropdbStmt:
-			{
-				DropdbStmt *stmt = (DropdbStmt *) parsetree;
-
-				/* no event triggers for global objects */
-				PreventInTransactionBlock(isTopLevel, "DROP DATABASE");
-				dropdb(stmt->dbname, stmt->missing_ok);
-			}
+			/* no event triggers for global objects */
+			PreventInTransactionBlock(isTopLevel, "DROP DATABASE");
+			DropDatabase(pstate, (DropdbStmt *) parsetree);
 			break;
 
 			/* Query-level asynchronous notification */
diff --git a/src/bin/psql/tab-complete.c b/src/bin/psql/tab-complete.c
index 2b1e3cda4a4..98c917bf7ae 100644
--- a/src/bin/psql/tab-complete.c
+++ b/src/bin/psql/tab-complete.c
@@ -2844,6 +2844,10 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH_FUNCTION_ARG(prev2_wd);
 	else if (Matches("DROP", "FOREIGN"))
 		COMPLETE_WITH("DATA WRAPPER", "TABLE");
+	else if (Matches("DROP", "DATABASE", MatchAny))
+		COMPLETE_WITH("WITH (");
+	else if (HeadMatches("DROP", "DATABASE") && (ends_with(prev_wd, '(')))
+		COMPLETE_WITH("FORCE");
 
 	/* DROP INDEX */
 	else if (Matches("DROP", "INDEX"))
diff --git a/src/include/commands/dbcommands.h b/src/include/commands/dbcommands.h
index 154c8157ee2..d1e91a24559 100644
--- a/src/include/commands/dbcommands.h
+++ b/src/include/commands/dbcommands.h
@@ -20,7 +20,8 @@
 #include "nodes/parsenodes.h"
 
 extern Oid	createdb(ParseState *pstate, const CreatedbStmt *stmt);
-extern void dropdb(const char *dbname, bool missing_ok);
+extern void dropdb(const char *dbname, bool missing_ok, bool force);
+extern void DropDatabase(ParseState *pstate, DropdbStmt *stmt);
 extern ObjectAddress RenameDatabase(const char *oldname, const char *newname);
 extern Oid	AlterDatabase(ParseState *pstate, AlterDatabaseStmt *stmt, bool isTopLevel);
 extern Oid	AlterDatabaseSet(AlterDatabaseSetStmt *stmt);
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index d93a79a5547..ff626cbe61e 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -3145,6 +3145,7 @@ typedef struct DropdbStmt
 	NodeTag		type;
 	char	   *dbname;			/* database to drop */
 	bool		missing_ok;		/* skip error if db is missing? */
+	List	   *options;		/* currently only FORCE is supported */
 } DropdbStmt;
 
 /* ----------------------
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index da8b672096f..8f67b860e72 100644
--- a/src/include/storage/procarray.h
+++ b/src/include/storage/procarray.h
@@ -113,6 +113,7 @@ extern void CancelDBBackends(Oid databaseid, ProcSignalReason sigmode, bool conf
 extern int	CountUserBackends(Oid roleid);
 extern bool CountOtherDBBackends(Oid databaseId,
 								 int *nbackends, int *nprepared);
+extern void TerminateOtherDBBackends(Oid databaseId);
 
 extern void XidCacheRemoveRunningXids(TransactionId xid,
 									  int nxids, const TransactionId *xids,
diff --git a/src/test/regress/expected/drop_if_exists.out b/src/test/regress/expected/drop_if_exists.out
index 6a174677177..5e44c2c3cea 100644
--- a/src/test/regress/expected/drop_if_exists.out
+++ b/src/test/regress/expected/drop_if_exists.out
@@ -330,3 +330,13 @@ HINT:  Specify the argument list to select the routine unambiguously.
 -- cleanup
 DROP PROCEDURE test_ambiguous_procname(int);
 DROP PROCEDURE test_ambiguous_procname(text);
+-- This test checks both the functionality of 'if exists' and the syntax
+-- of the drop database command.
+drop database test_database_exists (force);
+ERROR:  database "test_database_exists" does not exist
+drop database test_database_exists with (force);
+ERROR:  database "test_database_exists" does not exist
+drop database if exists test_database_exists (force);
+NOTICE:  database "test_database_exists" does not exist, skipping
+drop database if exists test_database_exists with (force);
+NOTICE:  database "test_database_exists" does not exist, skipping
diff --git a/src/test/regress/sql/drop_if_exists.sql b/src/test/regress/sql/drop_if_exists.sql
index 8a791b1ef2d..ac6168b91f8 100644
--- a/src/test/regress/sql/drop_if_exists.sql
+++ b/src/test/regress/sql/drop_if_exists.sql
@@ -295,3 +295,10 @@ DROP ROUTINE IF EXISTS test_ambiguous_procname;
 -- cleanup
 DROP PROCEDURE test_ambiguous_procname(int);
 DROP PROCEDURE test_ambiguous_procname(text);
+
+-- This test checks both the functionality of 'if exists' and the syntax
+-- of the drop database command.
+drop database test_database_exists (force);
+drop database test_database_exists with (force);
+drop database if exists test_database_exists (force);
+drop database if exists test_database_exists with (force);