refactoring suggestion - migration table name is hardcoded in both pkg/spanner and cmd/

[fcostin]

while reviewing the code I noticed a minor issue with internal code structure, does not impact functionality of wrench tool:

Currently, it is not clear which layer of the code is responsible for choosing the default migration table name:

1. the public API exposed by pkg/spanner Client has a tableName parameter on most operations that interact with the migration table, with the exception of TruncateAllTables, which hardcodes the table name instead of letting the caller control it:

   wrench/pkg/spanner/client.go

   Line 134 in 9f346f2

   if t.TableName == "SchemaMigrations" {

2. cmd/migrate.go also hardcodes the migration table name:

   wrench/cmd/migrate.go

   Line 36 in 9f346f2

   migrationTableName = "SchemaMigrations"

The code structure could be slightly cleaner if only one layer of the code was responsible for choosing the migration table name.

I can think of a few options for refactoring to clarify this, but some have potential downsides of changing the API of exported methods of pkg/spanner Client . If it is important to not make breaking changes to pkg/spanner Client API (if users are using that as a library from their projects without using wrench command line tool) then it might be best not to do that.

Suggested options:

option	description	code structure	breaking change to wrench tool?	breaking change to exported wrench pkg/spanner Client API?
a	current structure	slightly unclear	NA, no change	NA, no change
b	add tableName to TruncateAllTables	slightly clearer	no change	breaking change!
c	add TruncateAllTablesV2 with tableName, deprecate TruncateAllTables	slightly clearer	no change	no change
d	remove tableName from pkg/spanner Client, allow custom migration table name to be set using pkg/spanner Config	slightly clearer	no change	breaking change!

[fcostin]

I have raised a draft PR to explore what "option d" suggested above could look like: #51

that proposal has downside of being a breaking change to pkg/spanner Client API.

[asetty]

Kind of on topic here, I have a use case where I'd like to manage migration separately for multiple sets of tables colocated in the same DB. We could add a migration table flag to the CLI tool to support this. I started making changes on a branch in my fork, but will need to add some testing probably.

Also some error checking for maybe:

1. migrations table is valid ( in case of typos)
2. all tables modified in migration do not conflict with other migration table's tables

One idea I have is creating a owners table that maps migration table to DB table in order to check this.
I may create a separate issue, but would be interested if someone replied here. It would be nice to merge your PR #72 first