Re: [PATCH v5 02/35] api-lockfile: expand the documentation
From: Jonathan Nieder <hidden>
Date: 2016-06-15 23:02:32
Michael Haggerty wrote:
Document a couple more functions and the flags argument as used by hold_lock_file_for_update() and hold_lock_file_for_append().
Thanks. [...]
quoted hunk ↗ jump to hunk
--- a/Documentation/technical/api-lockfile.txt +++ b/Documentation/technical/api-lockfile.txt@@ -28,9 +28,39 @@ hold_lock_file_for_update:: the final destination (e.g. `$GIT_DIR/index`) and a flag `die_on_error`. Attempt to create a lockfile for the destination and return the file descriptor for writing - to the file. If `die_on_error` flag is true, it dies if - a lock is already taken for the file; otherwise it - returns a negative integer to the caller on failure. + to the file. The flags parameter is a combination of ++ +--
Context: this document has structure
lockfile API
============
Explanation of purpose (nice!).
The functions
-------------
Quick descriptions of each of the four functions
`hold_lock_file_for_update`, `commit_lock_file`,
`rollback_lock_file`, `close_lock_file`.
Reminder about lifetime of the lock_file structure.
Description of cleanup convention (thou shalt either
commit or roll back; if you forget to, the atexit
handler will roll back for you).
Long warning about the harder use cases. The above
"thou shalt" was a lie --- you can also
close_lock_file if you know what you're doing
[jn: why is that function part of the public API?].
What's nice about the existing structure is that you can get
a sense of how to use the API at a glance. Would there be a
way to add this extra information while preserving that property?
E.g.:
lockfile API
============
Nice brief explanation of purpose ("is this the API
I want to use?"), as before.
Calling sequence
----------------
The caller:
* Allocates a variable `struct lock_file lock` in the bss
section or heap. Because the `lock_file` structure is used
in an `atexit(3)` handler, its storage has to stay
throughout the life of the program. It cannot be an auto
variable allocated on the stack.
* Attempts to create a lockfile by passing that variable and
the filename of the final destination (e.g. `$GIT_DIR/index`)
to `hold_lock_file_for_update` or `hold_lock_file_for_append`.
+
If the `die_on_error` flag is true, git dies if a lock is
already taken for the file.
* Writes new content for the destination file by writing to
`lock->fd`.
When finished writing, the caller can:
* Close the file descriptor and rename the lockfile to
its final destination by calling `commit_lock_file`.
* Close the file descriptor and remove the lockfile by
calling `rollback_lock_file`.
* Close the file descriptor without removing or renaming
the lockfile by calling `close_lock_file`.
If you do not call one of `commit_lock_file`,
`rollback_lock_file`, and `close_lock_file` and instead
simply `exit(3)` from the program, an `atexit(3)` handler will
close and remove the lockfile.
You should never call `close(2)` on `lock->fd` yourself~
Otherwise the ...
Error handling
--------------
Functions return 0 on success, -1 on failure. errno is?
isn't? meaningful on error.
... description of unable_to_lock_error and unable_to_lock_die
here ...
Flags
-----
LOCK_NODEREF::
Usually symbolic links in the destination path are
resolved and the lockfile is created by adding ".lock"
to the resolved path. If `LOCK_NODEREF` is set, then
the lockfile is created by adding ".lock" to the path
argument itself.
What is the user-visible effect of that flag? When would I want
to pass that flag, and when wouldn't I?
LOCK_DIE_ON_ERROR::
If a lock is already taken for the file, `die()` with
an error message. If this option is not specified,
trying to hold a lock file that is already taken will
return -1 to the caller.
Sensible?
Jonathan