add Filename to MigrationPgError

[davidmdm]

Simply put, when using tern/migrate to run migrations, if a migration fails the error is returned and the PgError message is printed to the user. It would be useful to show to the user which of their migration files caused the error.

Intended use:

if err := m.Migrate(ctx); err != nil {
  var migrationErr migrate.MigrationPgError
  if errors.As(err, &migrationErr) {
    fmt.Fprintf(os.Stderr, "failed to perform migration(s): %s: %v\n", migrationErr.Filename, migrationErr.PgError)
    os.Exit(1)  
  }
  // handle error
}

[davidmdm]

@jackc

I didn't want to change anything or cause any tests to break, so I added the Filename property only, so that it could be used by programs that use tern/migrate.

Would you be open to changing the MigrationPgError Error to something like this?

type MigrationPgError struct {
    Filename string
    Sql      string
    *pgconn.PgError
}

func (e MigrationPgError) Error() string {
    if e.Filename == "" {
        return e.PgError.Error()
    }
    return fmt.Sprintf("%s: %s", e.Filename, e.PgError.Error())
}

[jackc]

While migrations typically do come from files it isn't strictly required, so I would suggest that instead of Filename the field should be named Name or MigrationName.

As far as changing MigrationPgError.Error(), I think that is okay as long we can keep the existing error output for the CLI tool.

[davidmdm]

@jackc

Here is what I did in this last commit:

• changed Filename -> MigrationName
• added the new Error() string method for MigrationPgError that included MigrationName in output
• added an Unwrap() error method for MigrationPgError is someone wants to get the PgError directly.
• made sure tern logs the PgError output for backward comparability.

[jackc]

LGTM

[davidmdm]

@jackc

Can you tag master with this new commit? I want to use this new error context in my tern scripts.

Much appreciated :)

[jackc]

Done.