Allow /dev/null as the source path

I am adding ✨ Allow a callback function for the Source path parameter Active as a related issue. I do not see an easy way to solve this problem with the proposed resolution from that issue, but maybe there is a way.

MR 14 is a simple, 1-line change.

I must have been doing something wrong, because the one-line change seemed to be working yesterday. On further testing, it does not.

The current MR adds an empty file (empty.csv) to the codebase. If the path is set to the specific string '/dev/null', then the constructor replaces that (in $this->configuration) with the path to the empty file and also sets header_offset. The result is that the source plugin returns zero rows, without errors, warnings, nor exceptions.

Other benefits.

With the feature branch, I can roll back a migration:

$ ddev drush mr import_csv_terms
 [notice] Rolled back 7 items - done with 'import_csv_terms'
$ ddev drush ms import_csv_terms
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------- 
  Group                     Migration ID       Status   Total   Imported   Unprocessed   Message Count   Last Imported  
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------- 
  Import CSV (import_csv)   import_csv_terms   Idle     0       0          0             0                              
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------- 

With the 8.x-3.x branch, I cannot roll back a migration unless it specifies an actual path:

$ ddev drush mr import_csv_terms

In UnavailableFeature.php line 40:
                                    
  stream does not support seeking.  
                                    

Failed to run drush mr import_csv_terms: exit status 1

With the feature branch, when I check the status of my migrations, I get this:

$ ddev drush ms
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------------- 
  Group                     Migration ID       Status   Total   Imported   Unprocessed   Message Count   Last Imported        
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------------- 
  Import CSV (import_csv)   import_csv_terms   Idle     0       7          -7            0               2024-10-14 13:40:21  
  Import CSV (import_csv)   import_csv_users   Idle     0       3          -3            0               2024-10-14 13:40:21  
  Import CSV (import_csv)   import_csv_nodes   Idle     0       3          -3            0               2024-10-14 13:40:21  
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- ---------------------

With the 8.x-3.x branch, I get errors (but the same information):

$ ddev drush ms
 [error]  Could not retrieve source count from import_csv_terms: stream does not support seeking. 
 [error]  Could not retrieve source count from import_csv_users: stream does not support seeking. 
 [error]  Could not retrieve source count from import_csv_nodes: stream does not support seeking. 
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------------- 
  Group                     Migration ID       Status   Total   Imported   Unprocessed   Message Count   Last Imported        
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- --------------------- 
  Import CSV (import_csv)   import_csv_terms   Idle     N/A     7          N/A           0               2024-10-14 13:40:21  
  Import CSV (import_csv)   import_csv_users   Idle     N/A     3          N/A           0               2024-10-14 13:40:21  
  Import CSV (import_csv)   import_csv_nodes   Idle     N/A     3          N/A           0               2024-10-14 13:40:21  
 ------------------------- ------------------ -------- ------- ---------- ------------- --------------- ---------------------

Thanks for the contributions. This makes a lot of sense.

Automatically closed - issue fixed for 2 weeks with no activity.