Fdinfo engine
Masters and slaves
- A file may be referred by several file descriptors. The descriptors may belong to a single process or to several processes.
- Group of descriptors referring to the same file is called shared. One of them called master, others are slaves.
- Every descriptor is discribed via struct fdinfo_list_entry.
- Shared file descriptors are distributed between tasks using scm_rights. See send_fds() and receive_fds() for the details.
Per-process files restore
Every file types is described via structure file_desc. We sequentially call file_desc::ops::open(struct file_desc *d, int *new_fd) method for every master file of a process until all masters are restored. The open methods may return three values:
- 0 -- restore of the master file is successefuly finished;
- 1 -- restore is in progress or it can't be started yet, because of it depends on another files, so the method should be called once again;
- -1 -- restore failed.
Right after a file is open at first time, the open method must return fd value in new_fd argument. This allows the common code to send this master to other processes to reopen the master as a slave as soon as possible. The same time, returning of not-negative new_fd does not mean, that the master is restored. The open() callback may return not-negative new_fd and "1" as return value at the same time.
Example. Restore of connected unix socket by open() method.
- 1)Open a socket, write its file descriptor to new_fd and return 1.
- 2)Check if peer socket is open and bound. If it's not so, then return 1 and repeat step "2" in next time.
- 3)Connect to the peer and return 0.
Note: it's also possible to go to step "2" right after new_fd is written.