Prevent internal error caused by concurrent ALTER FUNCTION

Previously, concurrent ALTER FUNCTION commands could fail with an
internal error "tuple concurrently updated". This occurred
because multiple sessions attempted to modify the same catalog tuple
simultaneously.

To prevent this, ensure that an exclusive lock on the function object
is acquired earlier in the process.

Additionally, if the target function is dropped by another session while
waiting for the lock, an appropriate error is raised to indicate the
object no longer exists.

Author: yugo-n

src/backend/commands/functioncmds.c

@@ -61,6 +61,7 @@
 #include "parser/parse_func.h"
 #include "parser/parse_type.h"
 #include "pgstat.h"
+#include "storage/lmgr.h"
 #include "tcop/pquery.h"
 #include "tcop/utility.h"
 #include "utils/acl.h"
@@ -1383,9 +1384,21 @@ AlterFunction(ParseState *pstate, AlterFunctionStmt *stmt)
 
 	ObjectAddressSet(address, ProcedureRelationId, funcOid);
 
+	/* Lock the function so nobody else can do anything with it. */
+	LockDatabaseObject(ProcedureRelationId, funcOid, 0, AccessExclusiveLock);
+
+	/*
+	 * It is possible that by the time we acquire the lock on function,
+	 * concurrent DDL has removed it. We can test this by checking the
+	 * existence of function. We get the tuple again to avoid the risk
+	 * of function definition getting changed.
+	 */
 	tup = SearchSysCacheCopy1(PROCOID, ObjectIdGetDatum(funcOid));
-	if (!HeapTupleIsValid(tup)) /* should not happen */
-		elog(ERROR, "cache lookup failed for function %u", funcOid);
+	if (!HeapTupleIsValid(tup))
+		ereport(ERROR,
+				errcode(ERRCODE_UNDEFINED_OBJECT),
+				errmsg("function \"%s\" does not exist",
+					   NameListToString(stmt->func->objname)));
 
 	procForm = (Form_pg_proc) GETSTRUCT(tup);