From bc3863eaf3e4ae92eac8cd3ce0ca9dcb8915fc36 Mon Sep 17 00:00:00 2001 From: Laurent Bercot Date: Mon, 11 Sep 2023 08:13:01 +0000 Subject: Better s6-setlock; delete the s6lock subsystem Signed-off-by: Laurent Bercot --- doc/libs6/lock.html | 239 ---------------------------------------------------- 1 file changed, 239 deletions(-) delete mode 100644 doc/libs6/lock.html (limited to 'doc/libs6/lock.html') diff --git a/doc/libs6/lock.html b/doc/libs6/lock.html deleted file mode 100644 index 1cc896d..0000000 --- a/doc/libs6/lock.html +++ /dev/null @@ -1,239 +0,0 @@ - - - - - - s6: the s6lock library interface - - - - - - -

-libs6
-s6
-Software
-skarnet.org -

- -

The `s6lock` library interface

- -

General information

- -

- s6lock is a C interface to timed locks. Unix natively provides -locks, but the locking primitives are synchronous, so either they are -unbounded in execution time or they require polling. s6lock provides -poll-free locks that can timeout during attempted acquisition. -

- -

Programming

- -

Check the s6/lock.h header -for the prototypes. The functions documented here are -often simplified macros, for instance relying on the STAMP global variable -to hold the current time. Fully reentrant functions with more control -options are usually available.
Given the nature of the s6lock library, it makes sense to use a -s6lockd service concurrently -accessed by several applications using such locks to gate shared -resources.
If you're not using an s6lockd service, -make sure your application is not disturbed by children it doesn't -know it has. Using nonblocking waits, ignoring pids you don't know, and -using a -self-pipe -if your application is built around an event loop, are good programming -practices.

- -

Starting and ending a session

- -

-s6lock_t a = S6LOCK_ZERO ;
-tain_t deadline ;
-
-tain_now_g() ;
-tain_addsec_g(&deadline, 2)
-
-char const *path = S6LOCK_IPCPATH ;
-s6lock_start_g(&a, path, &deadline) ;
-// char const *lockdir = "/tmp/lock" ;
-// s6lock_startf_g(&a, lockdir, &deadline) ;
-

- -

-s6lock_start_g starts a session by connecting to an s6lockd service -listening on path. The working directory is set by the administrator -of the service.
-s6lock_startf_g starts a session with an s6lockd process as a child, -using lockdir as its working directory. -
-a is an s6lock_t structure that must be declared in the stack and -initialized to S6LOCK_ZERO. -If the session initialization fails, the function returns 0 and errno is set; -else the function returns 1. -

-If the absolute time deadline is reached and the function -has not returned yet, it immediately returns 0 with errno set to ETIMEDOUT. - -Only local interprocess communications are involved; unless your system is -heavily overloaded, the function should return near-instantly. One or two -seconds of delay between the current time and deadline should be -enough: if the function takes more than that to return, then there is a -problem with the underlying processes. -

- -

- You can have more than one session open in parallel, by declaring -several distinct s6lock_t structures and calling -s6lock_startf_g (or s6lock_start_g) more than once. -However, one single session can handle -virtually as many concurrent locks as your application needs, so -opening several sessions is only useful if you need to acquire locks -in various distinct lock directories. -

- -

-s6lock_end(&a) ;
-

- -

-s6lock_end frees all the resources used by the session. The -a structure is then reusable for another session. -

- -

Acquiring and releasing locks

- -

-uint16_t id ;
-char const *file = "lockfile" ;
-tain_t limit ;
-tain_t deadline ;
-
-int r = s6lock_acquire_sh_g (&a, &id, file, &limit, &deadline) ;
-/* int r = s6lock_acquire_ex_g (&a, &id, file, &limit, &deadline) ; */
-r = s6lock_release_g(&a, id, &deadline) ;
-

- -

-s6lock_acquire_sh_g instructs the -s6lockd daemon, related to the open -session represented by the a handle, to try and acquire a -shared lock on the -file file located under that daemon's working directory -(typically /var/lock). file will be interpreted as -relative to the daemon's working directory even if it starts with a -slash; however, slashes in the middle of file are likely to -result in an error. -

- -

-limit and deadline are two absolute dates. -deadline is a deadline for the execution of the -function: if by deadline the function has not returned, -then it instantly returns 0 and sets errno to ETIMEDOUT. The -function is normally near-instantaneous, so deadline can -be very close in the future and serves only as a protection against -malicious servers. limit is the acquisition deadline: if -by limit the daemon still has not been able to acquire a lock -on file, then it will report a timeout to the client. -

- -

-The function returns 1 in case of success, or 0 if an error occurs, -with errno set to a suitable value. If it succeeds, then a 16-bit -number is stored into *id; this number serves as an identifier -for this lock. -

- -

-s6lock_acquire_ex_g works just like s6lock_acquire_sh_g, -except that the daemon tries to acquire an exclusive lock. -

- -

-s6lock_release_g releases the lock identified by id. -It normally returns 1. It can return 0 with errno set to a suitable -value if it fails. id is not valid after the corresponding -lock has been released. The function normally returns instantly, with -deadline as a safeguard. -

- -

Asynchronously waiting for locks

- -

- (from now on, the functions are listed with their prototypes instead -of usage examples.) -

- -

-int s6lock_fd (s6lock_t const *a)
-

- -

- Returns a file descriptor to select on for reading. Do not -read() it though. -

- -

-int s6lock_update (s6lock_t *a)
-

- -

- Call this function whenever the fd checks readability: it will -update a's internal structures with information from the -s6lockd daemon. It returns -1 if an error -occurs; in case of success, it returns the number of identifiers for -which something happened. -

- -

- When s6lock_update returns, -genalloc_s(uint16_t, &a->list) points to an array of -genalloc_len(uint16_t, &a->list) 16-bit unsigned -integers. Those integers are ids waiting to be passed to -s6lock_check. -

- -

-int s6lock_check (s6lock_t *a, uint16_t id, char *what)
-

- -

- Checks whether the lock identified by id has -been acquired. Use after a call to s6lock_update(). -

- -

If an error occurred, returns -1 and sets errno. The error -number may have been transmitted from -s6lockd.
If the lock has not been acquired yet, returns 0.
If the lock has been acquired, returns 1.

- -

Synchronously waiting for locks

- -

- int s6lock_wait_or_g (s6lock_t *a, uint16_t const *idlist, unsigned int n, tain_t const *deadline)
-Synchronously waits for one of the locks represented by the array pointed to -by idlist of length n to be acquired. Returns -1 if it fails, -or a nonnegative number on success, which is the index in idlist of the -acquired lock's id. If no result has been obtained by deadline, the -function returns -1 ETIMEDOUT. -

- -

- int s6lock_wait_and_g (s6lock_t *a, uint16_t const *idlist, unsigned int n, tain_t const *deadline)
-Synchronously waits for all of the locks represented by the array pointed to -by idlist of length n to be acquired. Returns -1 if it fails and -0 if it succeeds. If no result has been obtained by deadline, the -function returns -1 ETIMEDOUT. -

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