Lock file

Purpose: Locks file exclusively.

 lock-file <file path> id <lock id> status <status>

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lock-file attempts to create file with the full path of <file path> (deleting it if existed), and exclusively lock it. If successful, no other process can do the same unless current process ends or calls unlock-file. This statement is non-blocking, thus you can check in a sleepy loop for success (see pause-program).

<file path> should be either an existing or non-existing file with a valid file path. If existing, it will be deleted.

<lock id> (in "id" clause) is a file descriptor associated with locked file.

<status> (in "status" clause) represents the status of file locking: GG_OKAY if successfully locked, GG_ERR_FAILED if cannot lock (likely because another process holds a lock), GG_ERR_INVALID if the path of the file is invalid (i.e. if it is empty), GG_ERR_CREATE if lock file cannot be created.

Generally, this statement is used for process control. A process would use lock-file to start working on a job that only one process can do at a time; once done, by using unlock-file statement, another process will be able to issue a successful lock-file call. Typically, lock-file is issued in a sleepy loop (see pause-program), waiting for a resource to be released.

Note that file lock is not carried across children processes (i.e. if your process creates children, such as with exec-program, then such children must obtain their own lock). In addition, if a process serving a request terminates before the request could issue unlock-file, the lock will be automatically released.

You can use any file name (likely choose a name that signifies the purpose of a lock, as long as you have permissions to create it), and create any number of them. This way you can create as many "binary semaphore"-like objects as you like.

Examples

 %% /locktest public
     // Get application home directory
     get-app directory to dir
     // Create lock file name
     write-string fname
     @<<p-out dir>>/.lock
     end-write-string
     // Enter loop in which something is done, OR, program waits for others to complete their work before doing its own
     set-number lockid
     start-loop
         // Attempt to lock file
         lock-file fname id lockid status lockst
         // Check the status of locking
         if-true lockst equal GG_OKAY
             // File successfully locked, simulate working for 20 seconds
             @WORKING
             pause-program 20000
             @DONE
             // Exit while loop
             break-loop
         else-if lockst equal GG_ERR_FAILED
             // Another process holds the lock, wait, try again for 1 second
             pause-program 1000
             @WAITING
             continue-loop
         else-if lockst equal GG_ERR_OPEN or lockst equal GG_ERR_INVALID
             // Errors
             @BAD LOCK
             exit-handler
         end-if
     end-loop
     // Once done, unlock file
     unlock-file id lockid
 %%

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