Add a per mount flag to not create a node on O_CREAT in open

[Chris Johns]

The libio rtems_filesystem_eval_path_context_t context uses a token left after a path evaluation to indicate the path to the leaf node is valid and the path leaf does not exist. System calls such as mknod, mkdir and open uses this to return EEXIST or in the case of open to create a S_IFREG node for a file. Some file systems do not support creating an S_IFREG node and the open handler is required to do this.

[ An aside, libio should leave the EEXIST detection to the file system and its evaluation handlers. The EEXIST error is one of many that could be returned yet it is specifically singled out using a fragile interface of a path token remaining. Anyway back to the change ... ]

The open call has:

  if ( rtems_filesystem_eval_path_has_token( &ctx ) ) {
    create_regular_file( &ctx, mode );
  }

and create_regular_file makes a node:

  rv = rtems_filesystem_mknod(
    currentloc,
    token,
    tokenlen,
    S_IFREG | mode,
    0
  );

then wipes the evaluation context of any real and valid flags and settings:

    rtems_filesystem_eval_path_set_flags( ctx, 0 );
    rtems_filesystem_eval_path_set_path( ctx, token, tokenlen );
    rtems_filesystem_eval_path_continue( ctx );

then continues the path evaluation. The continues path evaluation is suppose to find the new node. If a file system does not support making a regular file node and the mknod call is stubbed out to lie about making the node continuing the path evaluation fails with a not found error. I cannot see a way to code around this with the current libio code.

I propose adding a bool flag called no_reg_make_node to the rtems_filesystem_mount_table_entry_t struct that defaults to false. If true open does not call create_regular_file.

[Chris Johns <chrisj@…>]

In 822cad89/rtems:

libcsupport: Add no_regular_file_mknod as a mount option to the mount table

• Add the bool flag no_regular_file_mknod to the mount table so a file system can indicate creating regular files is not done by use the mknod handler. The file system will handle creating a file node in the open handler.

• Note, the mount option is an enum which means there is only one exclusive option supported. As a result no encapsulation is provided and file systems need to set no_regular_file_mknod directly.

Closes #4222