What happens when one query inserts and one alters the same table concurrently?

What will happen if one or more processes are trying to append data to the table and one or more processes are trying to alter the table at the same time?

Let's say we have a table called db.table(partitioned on band column):

band |name |age
---------------------------
 1        abc   18
 1        xyz   20
 2        aaa   18
 2        bbb   19
 3        ccc   22
 4        ddd   28

Alter command:

spark.sql("alter table db.table add columns (year_of_birth int, salary double, city string)")

Data append:

df = spark.sql("select 5 as band, 'fff' as name, 33 as age")

df.write.partitionBy('band').mode('append').saveAsTable('db.table')

Here df.write and spark.sql(alter command) are executing at the same time by multiple processes. What is the behaviour of spark in this scenario?