worker not task should concern temporal directory to save the dumped files

[july2993]

Feature Request

Is your feature request related to a problem? Please describe:

ref: https://docs.pingcap.com/tidb-data-migration/dev/task-configuration-file-full

# Configuration arguments of the Loader processing unit.
loaders:
  global:                            # The configuration name of the processing unit.
    pool-size: 16                    # The number of threads that execute Mydumper SQL files concurrently in Loader (16 by default).
    # The directory output by Mydumper that Loader reads ("./dumped_data" by default). Directories for different tasks of the same instance must be different (Mydumper outputs the SQL file based on the directory).
    dir: "./dumped_data"

A task is a data migration job might scheduled to some worker, above is a slice of task configuration, we could see there's a dir in task configuration.

But we do not know which worker this task will be run on. Different workers may be deployed in different environment, and dir is not general for all of them.

Describe the feature you'd like:

Remove the dir item in task config.

And add a temporal directory config item in worker config if need, or we only support saving in a directory by convention.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

1. loader config in task config file:

   dm/dm/config/task.go

   Line 321 in 84b12c5

   Loaders map[string]*LoaderConfig `yaml:"loaders" toml:"loaders" json:"loaders"`

2. worker config:

   dm/dm/worker/config.go

   Line 68 in 84b12c5

   type Config struct {

3. loader config in subtask config file, which the worker indeed use:

   dm/dm/config/subtask.go

   Line 175 in 84b12c5

   LoaderConfig // Loader configuration

One implementation is move dir to 2. , and successfully initiate it in 3.