Alternator has a constant, max_table_name_length=222, which is currently
used for two different things:

1. Limiting the length of the name allowed for Alternator table.
2. Limiting the length of some auxiliary tables the user is not aware
   of, such as a materialized view (whose name is tablename:indexname)
   or (in the next patch) CDC log table.

In principle, there is no reason why these two limits need to be identical -
we could lower the table name limit to, say, 192, but still allow the
tablename:indexname to be even longer, up to 222 - i.e., allow creating
materialized views even on tables whose name has 192 characters.

So in this patch we split this variable into two, max_table_name_length
and max_auxiliary_table_name_length. At the moment, both are still set
to the same value - 222. In a following patch we plan to lower
max_table_name_length but leave max_auxiliary_table_name_length at 222.

Signed-off-by: Nadav Har'El <[email protected]>

Currently, in Alternator it is possible to create a table whose name has
222 characters, and then trying to add Streams to that table results in
an attempt to create a CDC log table with the same name plus a
15-character suffix "_scylla_cdc_log", which resulted (Ref scylladb#24598) in
an IO-error and a Scylla shutdown.

This patch adds code to the Stream-adding operations (both CreateTable
and UpdateTable) that validates that the table's name, plus that 15
character suffix, doesn't exceed max_auxiliary_table_name_length, i.e.,
222.

After this patch, if you have a table whose name is between 207 and 222
characters, attempting to enable Streams on it will fail with:

 "Streams cannot be added if the table name is longer than 207 characters."

Note that in the future, if we lower max_table_name_length to below 207,
e.g., to 192, then it will always be possible to add a stream to any
legal table, and the new checks we had here will be mostly redundant.
But only "mostly" - not entirely: Checking in UpdateTable is still
important because of the possibility that an upgrading user might have
a pre-existing table whose name is longer than the new limit, and might
try to enable Streams.

After this patch, the crash reported in scylladb#24598 can no longer happen, so
in this sense the bug is solved. However, we still want to lower
max_table_name_length from 222 to 192, so that it will always be
possible to enable streams on any table with a legal name length.
We'll do this in the next patch.

Signed-off-by: Nadav Har'El <[email protected]>

Currently, Alternator allows creating a table with a name up to 222
(max_table_name_length) characters in length. But if you do create
a table with such a long name, you can have some difficulties later:
You you will not be able to add Streams or GSI or LSI to that table,
because 222 is also the absolute maximum length Scylla tables can have
and the auxilliary tables we want to create (CDC log, materialized views)
will go over this absolute limit (max_auxiliary_table_name_length).

This is not nice. DynamoDB users assume that after successfully
creating a table, they can later - perhaps much later - decide to
add Streams or GSI to it, and today if they chose extremely long
names, they won't be able to do this.

So in this patch, we lower max_table_name_length from 222 to 192.
A user will not be able to create tables with longer names, but
the good news is that once successfully creating a table, it will
always be possible to enable Streams on it (the CDC log table has an
extra 15 bytes in its name, and 192 + 15 is less than 222), and it
will be possible to add GSIs with short enough names (if the GSI
name is 29 or less, 192 + 29 + 1 = 222).

This patch is a trivial one-line code change, but also includes the
corrected documentation of the limits, and a fix for one test that
previously checked that a table name with length 222 was allowed -
and now needs to check 192 because 222 is no longer allowed.

Note that if a user has existing tables and upgrades Scylla, it
is possible that some pre-existing Alternator tables might have
lengths over 192 (up to 222). This is fine - in the previous patches
we made sure that even in this case, all operations will still work
correctly on these old tables (by not not validating the name!), and
we also made sure that attempting to enable Streams may fail when
the name is too long (we do not remove those old checks in this patch,
and don't plan to remove them in the forseeable future).

Note that the limit we chose - 192 characters - is identical to the
table name limit we recently chose in CQL. It's nicer that we don't
need to memorize two different limits for Alternator and CQL.

Signed-off-by: Nadav Har'El <[email protected]>