Changeset 31717


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Timestamp:
05/30/17 21:25:17 (3 years ago)
Author:
ak19
Message:

Finally finished first draft of SafeProcess.java Readme. Added more sections besides filling in the remaining ones that previously only had section headings.

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  • main/trunk/gli/src/org/greenstone/gatherer/util/Readme_Using_SafeProcess.txt

    r31713 r31717  
    1 SAFEPROCESS README
     1First drafts of this document
     2ak19, 30 May 2017
     3
     4=========================================
     5    SAFEPROCESS.JAVA README
     6=========================================
    27
    38GS Java developers should use SafeProcess.java in place of directly using Java's Process class, unless any issues are discovered with SafeProcess.java hereafter.
     
    813- greenstone3/src/java/org/greenstone/util/SafeProcess.java
    914
     15
     16This document contains the following sections
     17    A. WHY WE SHOULD GO THROUGH SAFEPROCESS INSTEAD OF USING JAVA'S PROCESS DIRECTLY
     18    B. MODEL OF SAFEPROCESS THREADS AND INTERRUPT BEHAVIOUR
     19    C. USAGE: DEBUGGING
     20    D. USAGE: INSTANTIATION & DEFAULT RUNPROCESS VARIANTS
     21    E. INTERNAL IMPLEMENTATION DETAILS - USEFUL IF CUSTOMISING SAFEPROCESS' BEHAVIOUR OR CALLING CANCEL ON SAFEPROCESS
     22    F. USAGE: CUSTOMISING
     23    G. USAGE: CANCELLING
     24    H. USAGE: IMPLEMENTING HOOKS AROUND CANCELLING OR PROCESS' END
     25    I. USEFUL STATIC METHODS AND STATIC INNER CLASSES
     26    J. OTHER USEFUL INSTANCE METHODS
     27    K. LINKS AND NOTES ON PROCESSES, PROCESS STREAMS, INTERRUPTING AND DESTROYING PROCESSES
     28    L. USEFUL SYNCHRONISATION NOTES AND SUGGESTED READING ON THREADS AND THREAD SAFETY
     29    M. TICKETS, COMMITS, TAGS
     30    N. GLI vs GS3 CODE's SAFEPROCESS.JAVA - THE DIFFERENCES
     31    O. FUTURE WORK, IMPROVEMENTS
     32
    1033_______________________________________________________________________________
    11 WHY WE SHOULD GO THROUGH SAFEPROCESS INSTEAD OF USING JAVA'S PROCESS DIRECTLY
     34A. WHY WE SHOULD GO THROUGH SAFEPROCESS INSTEAD OF USING JAVA'S PROCESS DIRECTLY
    1235_______________________________________________________________________________
    1336
     
    2043
    2144
    22 _____________________________________________________
    23 MODEL OF SAFEPROCESS THREADS AND INTERRUPT BEHAVIOUR
    24 _____________________________________________________
     45_________________________________________________________
     46B. MODEL OF SAFEPROCESS THREADS AND INTERRUPT BEHAVIOUR
     47_________________________________________________________
    2548
    2649This section is especially IMPORTANT TO READ IF you're thinking of customising SafeProcess' handling of the internal Process' iostreams, since each of them are always handled in their own distinct worker threads, and you need to remember to deal with ThreadSafety issues.
     
    5275
    5376__________________________________________________
    54 USAGE: DEBUGGING
    55 __________________________________________________
    56 
    57 There's a lot of info that a SafeProcess instance will log during its execution and termination phases, including Exceptions but also very basic things, such as the command that the SafeProcess instance is running.
    58 
    59 By default, such debugging-related logging is turned off. You can turn it on by adjusting the static SafeProcess.DEBUG = 1 and recompiling.
     77C. USAGE: DEBUGGING
     78__________________________________________________
     79
     80There's a lot of info that a SafeProcess instance will log during its execution and termination phases, including Exceptions but also very basic things, such as the command that the SafeProcess instance is running and other messages.
     81
     82Exceptions are always logged, but debugging related messages can be turned off or on. By default, it's turned off. You can turn it on by adjusting the static SafeProcess.DEBUG = 1 and recompiling.
    6083
    6184For GS3, the log output uses log4j.
     
    6487If you're using SafeProcess yourself and wish to log debug statements that are related to how well you're using SafeProcess, you can call one of the static SafeProcess.log() functions yourself. Remember to set the static SafeProcess.DEBUG = 1.
    6588
    66 __________________________________________________
    67 USAGE: INSTANTIATION & DEFAULT RUNPROCESS VARIANTS
    68 __________________________________________________
     89
     90There are 4 variants of the log() function:
     91
     92    - public static void log(String msg)
     93    logs the message if debugging is on, DEBUG = 1
     94
     95    - public static void log(Exception e)
     96    logs the stacktrace for the exception, regardless of if debugging is on
     97
     98    - public static void log(String msg, Exception e)
     99    logs the message and logs the stacktrace for the exception, regardless of if debugging is on
     100
     101    - public static void log(String msg, Exception e, boolean printStackTrace)
     102    if debugging is on or if the printStackTrace parameter is true, both the message and the stacktrace for the exception are logged.
     103    if debugging is off and the printStackTrace parameter is false, nothing is logged. Call this variant if an exception is expected and you only want to log it in debugging mode. An example of an expected exception are some InterruptedExceptions that may occur over the course of SafeProcess
     104
     105_______________________________________________________
     106D. USAGE: INSTANTIATION & DEFAULT RUNPROCESS VARIANTS
     107_______________________________________________________
    69108
    70109Usage of the SafeProcess class generally follows the following sequence:
     
    156195The exit value will also be -1 if you call getExitValue() before setting off the SafeProcess via any of the runProcess() variants.
    157196
    158 _________________________________________________________________________________________________________________
    159 INTERNAL IMPLEMENTATION DETAILS - USEFUL IF CUSTOMISING SAFEPROCESS' BEHAVIOUR OR CALLING CANCEL ON SAFEPROCESS
    160 _________________________________________________________________________________________________________________
     197_____________________________________________________________________________________________________________________
     198E. INTERNAL IMPLEMENTATION DETAILS - USEFUL IF CUSTOMISING SAFEPROCESS' BEHAVIOUR OR CALLING CANCEL ON SAFEPROCESS
     199_____________________________________________________________________________________________________________________
    161200
    162201This section is for understanding how SafeProcess runs a Java Process internally. These methods are not public and their details are hidden away in the code.
     
    239278
    240279__________________________________________________
    241 USAGE: CUSTOMISING
     280F. USAGE: CUSTOMISING
    242281__________________________________________________
    243282
     
    318357
    319358If, during the execution of the primary thread, you want to do some complex things during a cancel operation, such as or before and after a process is destroyed, then
    320 - implement SafeProcess.MainProcessHandler (see below)
     359- implement SafeProcess.MainProcessHandler. See the Interface definition below. For further details, refer to the sections "CANCELLING" and "IMPLEMENTING HOOKS AROUND CANCELLING OR PROCESS' END".
    321360- configure your SafeProcess instance by calling setMainHandler(MainProcessHandler mph) on it *before* calling a runProcess() method on that instance.
    322361- For EXAMPLES OF USE, see GLI's DownloadJob.java.
     
    370409    }
    371410
    372 
    373 __________________________________________________
    374 USAGE: CANCELLING
    375 __________________________________________________
     411__________________________________________________
     412G. USAGE: CANCELLING
     413__________________________________________________
     414
     415SafeProcess provides a way to cancel an internal Process that is run by SafeProcess, and which will ensure that the internal process and worker threads all safely terminate and tidily clean up after themselves.
     416
     417As explained, SafeProcess internally accomplishes this by sending an InterruptedException on the primary thread that's running the internal Process. An InterruptedException is sent only when SafeProcess is in an "interruptible" phase, as no interruption is necessary at any other time such as cleanup.
     418
     419Only a thread *external* to the SafeProcess' primary thread, such as a GUI thread, can call cancelRunningProcess() on a reference to the SafeProcess instance. Two variants are available:
     420
     421- boolean cancelRunningProcess()
     422- boolean cancelRunningProcess(boolean forceWaitUntilInterruptible)
     423
     424The boolean value returned is true if an interrupt had to be sent, or false if one didn't need to be sent. An interrupt does not need to be sent if
     425    - the process was never yet run by the time cancel was called or
     426    - the internal Process has already naturally terminated and the SafeProcess has moved on to some stage of the cleanup phase by the time cancel was called.
     427
     428If you want to inspect the return value, a good variable name is sentInterrupt. For example,
     429    boolean sentInterrupt = mySafeProc.cancelRunningProcess();
     430
     431
     432Programming pattern:
     433A graphical interface may have a Cancel button associated with an actionPerformed() method, which will be run from a GUI/EventDispatchThread when the user presses cancel. Either directly or indirectly, actionPerfomed() during a cancel action needs to lead to one of the cancelRunningProcess() method variants being called on the SafeProcess instance.
     434
     435The variant you choose depends on whether the thread that invokes cancel on the SafeProcess is an EventDispatchThread (a GUI thread), or else, whether you are willing to wait until SafeProcess is interruptible. If you're calling the cancel method from a GUI thread or otherwise don't care to wait, call the zero-argument variant. Otherwise, you can call the single argument version and pass in true for forceWaitUntilInterruptible.
     436
     437
     438Usage:
     439- Usually, there is no need to bother waiting for the uninterruptible phase to finish, so you can call the zero-argument version of cancelRunningProcess(), and continue as before.
     440
     441- If you're calling from a GUI thread and want to disable your cancel button during the uninterruptible phase, however short this may be, in order to give proper feedback to the user, then you can implement the hooks of the SafeProcess.MainHandler interface as explained in the next section.
     442
     443- Call the cancelRunningProcess(forceWaitUntilInterruptible=true) variant if you're not calling from a GUI thread and you want to wait until SafeProcess becomes interruptible again (if you happened to have invoked the cancel method during an uninterruptible phase).
     444
     445
     446Further details:
     447Calling cancelRunningProcess(forceWaitUntilInterruptible=true) forces the method to only return when the SafeProcess has internally become "interruptible". There is a very brief phase at the start of a SafeProcess' cleanup period that blocks until the worker threads have cleanly terminated. This phase should not be interrupted (as would have happened upon an InterruptedException being sent just then), as the uninterruptible phase happens after the internal process has finished and the process is properly terminating.
     448
     449If the cancel method happened to be called during this "blocking" phase and if this phase were to theoretically take long and if cancelRunningProcess() were not to return immediately if the calling thread is a GUI thread, then your graphical interface, or at least the cancel button, would have been unresponsive until the cancelRunningProcess() method finally returned. But this uninterruptible phase is short, and it always terminates, so calling cancelRunningProcess() is sufficient. And it always returns immediately if the caller is a GUI thread. The zero-argument variant of cancelRunningProecss() internally calls cancelRunningProcess(forceWaitUntilInterruptible=false) and causes the method to return immediately: either cancelRunningProcess was called when SafeProcess was interruptible and it sends an InterruptedException to the primary thread and returns, or cancelRunningProcess got called when SafeProcess was in an uninterruptible phase. And if the caller was a GUI thread (or if forceWaitUntilInterruptible was set to false), the cancelRunningProcess() method would also return immediately.
     450
    376451
    377452For EXAMPLES OF USE, see GLI's GShell.java and DownloadJob.java.
    378453
    379 __________________________________________________
    380 USAGE: IMPLEMENTING HOOKS AROUND CANCELLING
    381 __________________________________________________
    382 
    383 For EXAMPLES OF USE, see GLI's DownloadJob.java.
    384 
    385 __________________________________________________
    386 USEFUL STATIC METHODS AND STATIC INNER CLASSES
    387 __________________________________________________
     454_________________________________________________________________
     455H. USAGE: IMPLEMENTING HOOKS AROUND CANCELLING OR PROCESS' END
     456_________________________________________________________________
     457
     458- Implement the SafeProcess.MainProcessHandler Interface, see the decription below and the section CUSTOMISING.
     459- Write the hooks you want.
     460- If you don't really intend to override beforeWaitingForStreamsToEnd(boolean forciblyTerminating) or afterStreamsEnded(boolean forciblyTerminating), then return the boolean parameter forciblyTerminating.
     461- For EXAMPLES OF USE, see GLI's DownloadJob.java.
     462
     463
     464The SafeProcess.MainProcessHandler provides hooks into various key stages of the SafeProcess instance's life cycle. Specifically, the clean up and termination stages of its life cycle. Currently, all stages that you can write hooks for come after the internal process has come to an end either naturally or prematurely.
     465
     466
     467If your code needs to do some special processes during any of these stages, override them.
     468
     469The stages are:
     470
     471    1. boolean beforeWaitingForStreamsToEnd(boolean forciblyTerminating)
     472
     473    Called after Process.waitFor() (when the internal Process has naturally or prematurely ended) and before the worker threads are join()ed (when the cleanup phase has begun, which always occurs, regardless of whether the Process had ended naturally or was cancelled). The parameter forciblyTerminating is true if the Process had been cancelled during an interruptible phase, or if an unexpected exception had occurred. In both cases, the internal Process object may need to be destroy()ed. But if the Process had only been cancelled during an uninterruptible phase, it means it had already naturally terminated and has started cleaning up. In such cases, the internal Process object does not need to be destroy()ed and forciblyTerminating will be false.
     474
     475    If not overriding, return the parameter. If overriding, even if the parameter is true, you may determine the process has come to a natural end after all. In that case, the process need not be destroyed, so you'd return false. For an example, see GLI's DownloadJob.java.
     476
     477    If you want to disable your cancel button temporarily while the join() is taking place, this is the method to do it.
     478
     479
     480    2. boolean afterStreamsEnded(boolean forciblyTerminating)
     481
     482    Called after the worker threads are join()ed, which is the start of the clean up phase. This method is also always called and the parameter indicates whether it's so far been determined that the process needs to be destroyed or not. If not overriding, return the parameter.
     483
     484    If you want to re-enable your cancel button since the join() have by now taken place, this is the method to do so.
     485
     486
     487    3. void beforeProcessDestroy()
     488    4. void afterProcessDestroy()
     489
     490    These two methods are only called if destroy() needed to be called on the internal Process object (if the Process did not come to a natural end for any reason, including being cancelled during an interruptible phase or if an unexpected exception occurred).
     491
     492
     493    5. void doneCleanup(boolean wasForciblyTerminated)
     494
     495    This method is always called at the very end of the SafeProcess' lifecycle, regardless of whether the internal Process naturally terminated or needed to be destroyed as happens upon premature termination. If the process needed to be destroyed, then the parameter wasForciblyTerminated would be true.
     496
     497
     498___________________________________________________
     499I. USEFUL STATIC METHODS AND STATIC INNER CLASSES
     500___________________________________________________
    388501
    389502public static boolean isAvailable(String programName)
    390503    - Runs the `which` cmd over the program and returns true or false
    391     `which` is included in winbin for Windows, and is part of unix systems
     504    - `which` is included in winbin for Windows, and is part of unix systems
    392505
    393506static public boolean processRunning(Process process)
    394     - returns true if the Process is currently running and hasn't come to an end.
    395     - It does not
     507    - returns true if the internal Process is currently running and hasn't come to an end. That is, this method returns true if Process.waitFor() is still blocking
     508    - If the SafeProcess is in its cleanup/termination phase, then this method will return false, even though SafeProcess is still doing stuff.
     509    - If you want to know when SafeProcess has finished in its entirety, call the instance method processRunning() on the SafeProcess object.
    396510
    397511public static long getProcessID(Process p)
     
    411525
    412526public static boolean closeSocket(Socket resourceHandle)
    413     - will attempt to cleanly close your Socket, logging on Exception. In Java 6, Sockets didn't yet implement Closeable,
    414     so this method is to ensure backwards compatibility
     527    - will attempt to cleanly close your Socket, logging on Exception.
     528    - From Java 7 onwards, calling closeResource() would suffice on Sockets too. But in Java 6, Sockets didn't yet implement Closeable, so this method is to ensure backwards compatibility.
    415529
    416530public static class OutputStreamGobbler extends Thread
     
    423537
    424538
    425 Package access:
    426 static methods to prematurely terminate any process denoted by processID and any subprocesses it may have launched:
    427     static boolean killUnixProcessWithID(long processID)
    428     static void killWinProcessWithID(long processID)
    429 
    430 
    431 __________________________________________________
    432 OTHER USEFUL INSTANCE METHODS
     539Package access: static methods to prematurely terminate any process denoted by processID and any subprocesses it may have launched
     540
     541static void killWinProcessWithID(long processID)
     542
     543static boolean killUnixProcessWithID(long processID)
     544    - for Mac/Linux
     545
     546
     547__________________________________________________
     548J. OTHER USEFUL INSTANCE METHODS
    433549__________________________________________________
    434550
     
    437553
    438554public boolean cancelRunningProcess()
     555public boolean cancelRunningProcess(boolean forceWaitUntilInterruptible)
    439556    - cancel the SafeProcess instance
    440557    - untimately synchronized, so threadsafe
    441 __________________________________________________
    442 USEFUL SYNCHRONISATION NOTES
    443 __________________________________________________
    444 [Also add in links]
    445 
    446 
    447 __________________________________________________
    448 TICKETS, COMMITS, TAGS
    449 __________________________________________________
    450 
    451 
     558    - returns true if the process were still running so that an interrupt needed to be sent to terminate it, returns false if sending an interrupt was unnecessary because the process had already entered the start of its termination phase when cancel was called. Will also return false if the process was never run by the time cancel was called.
     559    - usually you want to call the zero-argument version
     560    - See the section CANCELLING for further details
     561
     562_________________________________________________________________________________________________
     563K. LINKS AND NOTES ON PROCESSES, PROCESS STREAMS, INTERRUPTING AND DESTROYING PROCESSES
     564_________________________________________________________________________________________________
     565
     566Processes and streams
     567
     568- http://www.javaworld.com/article/2071275/core-java/when-runtime-exec---won-t.html?page=2
     569explains why SafeProcess.java is implemented with worker threads to handle a Process' iostreams.
     570- http://steveliles.github.io/invoking_processes_from_java.html
     571- http://mark.koli.ch/leaky-pipes-remember-to-close-your-streams-when-using-javas-runtimegetruntimeexec
     572
     573
     574Interrupting a Process:
     575
     576- http://stackoverflow.com/questions/2126997/who-is-calling-the-java-thread-interrupt-method-if-im-not
     577- http://stackoverflow.com/questions/13623445/future-cancel-method-is-not-working?noredirect=1&lq=1
     578- http://stackoverflow.com/questions/3976344/handling-interruptedexception-in-java
     579- http://stackoverflow.com/questions/4906799/why-invoke-thread-currentthread-interrupt-when-catch-any-interruptexception
     580- http://stackoverflow.com/questions/4906799/why-invoke-thread-currentthread-interrupt-when-catch-any-interruptexception
     581- https://praveer09.github.io/technology/2015/12/06/understanding-thread-interruption-in-java/
     582
     583
     584Destroying a process and subprocesses on various OS:
     585
     586- Need process id, pid, for which Java Native Access libraries (jar files) are required.
     587http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program
     588- To find the processID of the process launched by SafeProcess, need to use Java Native Access (JNA) jars, available jna.jar and jna-platform.jar.
     589    http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program
     590    http://stackoverflow.com/questions/35842/how-can-a-java-program-get-its-own-process-id
     591    http://www.golesny.de/p/code/javagetpid
     592    https://github.com/java-native-access/jna/blob/master/www/GettingStarted.md
     593    We're using JNA v 4.1.0, downloaded from https://mvnrepository.com/artifact/net.java.dev.jna/jna
     594
     595WINDOWS NOTES:
     596- Can't artificially send Ctrl-C: stackoverflow.com/questions/1835885/send-ctrl-c-to-process-open-by-java
     597
     598 On Windows, p.destroy() terminates process p that Java launched,
     599 but does not terminate any processes that p may have launched. Presumably since they didn't form a proper process tree.
     600    https://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/e3cb7532-87f6-4ae3-9d80-a3afc8b9d437/how-to-kill-a-process-tree-in-cc-on-windows-platform?forum=vclanguage
     601    https://msdn.microsoft.com/en-us/library/windows/desktop/ms684161(v=vs.85).aspx
     602
     603 Searching for: "forcibly terminate external process launched by Java on Windows"   
     604 Not possible: stackoverflow.com/questions/1835885/send-ctrl-c-to-process-open-by-java
     605 But can use taskkill or tskill or wmic commands to terminate a process by processID
     606 stackoverflow.com/questions/912889/how-to-send-interrupt-key-sequence-to-a-java-process
     607 Taskkill command can kill by Image Name, such as all running perl, e.g. taskkill /f /im perl.exe
     608 But what if we kill perl instances not launched by GS?
     609    /f Specifies to forcefully terminate the process(es). We need this flag switched on to kill childprocesses.
     610    /t Terminates the specified process and any child processes which were started by it.
     611            /t didn't work to terminate subprocesses. Maybe since the process wasn't launched as
     612            a properly constructed processtree.
     613    /im is the image name (the name of the program), see Image Name column in Win Task Manager.
     614   
     615 We don't want to kill all perl running processes.
     616 Another option is to use wmic, available since Windows XP, to kill a process based on its command
     617 which we sort of know (SafeProcess.command) and which can be seen in TaskManager under the
     618 "Command Line" column of the Processes tab.
     619    https://superuser.com/questions/52159/kill-a-process-with-a-specific-command-line-from-command-line
     620 The following works kill any Command Line that matches -site localsite lucene-jdbm-demo
     621    C:>wmic PATH win32_process Where "CommandLine like '%-site%localsite%%lucene-jdbm-demo%'" Call Terminate
     622 "WMIC Wildcard Search using 'like' and %"
     623    https://codeslammer.wordpress.com/2009/02/21/wmic-wildcard-search-using-like-and/
     624 However, we're not even guaranteed that every perl command GS launches will contain the collection name
     625 Nor do we want to kill all perl processes that GS launches with bin\windows\perl\bin\perl, though this works:
     626    wmic PATH win32_process Where "CommandLine like '%bin%windows%perl%bin%perl%'" Call Terminate
     627 The above could kill GS perl processes we don't intend to terminate, as they're not spawned by the particular
     628 Process we're trying to terminate from the root down.
     629   
     630 Solution: We can use taskkill or the longstanding tskill or wmic to kill a process by ID. Since we can
     631 kill an external process that SafeProcess launched OK, and only have trouble killing any child processes
     632 it launched, we need to know the pids of the child processes.
     633 
     634 We can use Windows' wmic to discover the childpids of a process whose id we know.
     635 And we can use JNA to get the process ID of the external process that SafeProcess launched.
     636 
     637 See links further above on how to find the processID of the process launched by SafeProcess on various OS,
     638 and where to find the JNA jars needed for this.
     639 
     640 WMIC can show us a list of parent process id and process id of running processes, and then we can
     641 kill those child processes with a specific process id.
     642    https://superuser.com/questions/851692/track-which-program-launches-a-certain-process
     643    http://stackoverflow.com/questions/7486717/finding-parent-process-id-on-windows
     644 WMIC can get us the pids of all childprocesses launched by parent process denoted by parent pid.
     645 And vice versa:
     646    if you know the parent pid and want to know all the pids of the child processes spawned:
     647        wmic process where (parentprocessid=596) get processid
     648    if you know a child process id and want to know the parent's id:
     649        wmic process where (processid=180) get parentprocessid
     650 
     651 The above is the current solution.
     652 
     653 Linux ps equivalent on Windows is "tasklist", see
     654    http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program
     655
     656
     657MAC/UNIX NOTES:
     658- Kill signals, their names and numerical equivalents: http://www.faqs.org/qa/qa-831.html
     659- Killing a processtree (a process group) on Unix:
     660https://stackoverflow.com/questions/8533377/why-child-process-still-alive-after-parent-process-was-killed-in-linux
     661Works on Linux but not Mac: kill -TERM -pid
     662Works on Mac but not Linux: pkill -TERM -P pid did work
     663- https://stackoverflow.com/questions/28332888/return-value-of-kill
     664    "kill returns an exit code of 0 (true) if the process still existed it and was killed.
     665    kill returns an exit code of 1 (false) if the kill failed, probably because the process was no longer running."
     666- https://superuser.com/questions/343031/sigterm-with-a-keyboard-shortcut
     667Ctrl-C sends a SIGNINT, not SIGTERM or SIGKILL. And on Ctrl-C, "the signal is sent to the foreground *process group*."
     668- https://linux.die.net/man/1/kill (manual)
     669- https://unix.stackexchange.com/questions/117227/why-pidof-and-pgrep-are-behaving-differently
     670- https://unix.stackexchange.com/questions/67635/elegantly-get-list-of-children-processes
     671- https://stackoverflow.com/questions/994033/mac-os-x-quickest-way-to-kill-quit-an-entire-process-tree-from-within-a-cocoa-a
     672- https://unix.stackexchange.com/questions/132224/is-it-possible-to-get-process-group-id-from-proc
     673- https://unix.stackexchange.com/questions/99112/default-exit-code-when-process-is-terminated
     674
     675_______________________________________________________________________________________
     676L. USEFUL SYNCHRONISATION NOTES AND SUGGESTED READING ON THREADS AND THREAD SAFETY
     677_______________________________________________________________________________________
     678
     679Things worth reading on Concurrency and Thread safety:
     680
     681- Deitel and Deitel's newer editions of their Java books have updated their chapter on Concurrency.
     682- http://docs.oracle.com/javase/tutorial/uiswing/concurrency/
     683series of Java articles on concurrency again.
     684- https://docs.oracle.com/javase/tutorial/essential/concurrency/atomicvars.html
     685On "primitive" like variables that provide instrinsic locks.
     686
     687
     688Notes on synchronization:
     689
     690- You can declare methods are synchronized. For example,
     691    public synchronized void doStuff()
     692This implicitly synchronizes on the *instance* of the class on which this method is called.
     693
     694- Within any methods, you can synchronize on specific objects to just lock on them. You have a synchronized code block within which you can manipulate the object.
     695
     696Keep synchronized blocks and methods as short as you can.
     697There are some subtle issues related to using synchronized methods, as explained below, that can result in deadlocks.
     698
     699
     700More reading:
     701
     702- http://stackoverflow.com/questions/574240/is-there-an-advantage-to-use-a-synchronized-method-instead-of-a-synchronized-blo
     703
     704       "Not only do synchronized methods not lock the whole class, but they don't lock the whole instance either. Unsynchronized methods in the class may still proceed on the instance."
     705       "Only the syncronized methods are locked. If there are fields you use within synced methods that are accessed by unsynced methods, you can run into race conditions."
     706       
     707       "synchronizing on "this" is considered in some circles to be an anti-pattern. The unintended consequence is that outside of the class someone can lock on an object reference that is equal to "this" and prevent other threads from passing the barriers within the class potentially creating a deadlock situation. Creating a "private final Object = new Object();" variable purely for locking purposes is the often used solution. Here's another question relating directly to this issue.
     708
     709- http://stackoverflow.com/questions/442564/avoid-synchronizedthis-in-java?lq=1"
     710
     711       "A private lock is a defensive mechanism, which is never a bad idea.
     712
     713       Also, as you alluded to, private locks can control granularity. One set of operations on an object might be totally unrelated to another but synchronized(this) will mutually exclude access to all of them."
     714
     715
     716- http://stackoverflow.com/questions/8393883/is-synchronized-keyword-exception-safe
     717     "In any scoped thread-safe block, the moment you get out of it, the thread-safety is gone."
     718     "In case of an exception the lock will be released."
     719
     720- http://stackoverflow.com/questions/8259479/should-i-synchronize-listener-notifications-or-not
     721     "Use a CopyOnWriteArrayList for your listener arrays."
     722     "If you use the CopyOnWriteArrayList, then you don't have to synchronize when iterating."
     723     "CopyOnWriteArrayList is thread-safe, so there is no need to synchronize."
     724
     725     "Use a ConcurrentLinkedQueue<Listener> ... for this kind of problems: adding, removing and iterating simultaneously on a collection.
     726     A precision : this solution prevents a listener from being called from the very moment it is deregistered."
     727     "It means that you start iterating, an element is added, it will be called, another is removed, it won't, all this in the same iteration cycle.
     728     It's the best of both world: ensuring synchronization, while being fine grained on who gets called and who's not."
     729
     730Examples of using CopyOnWriteArrayList: in GLI's shell/GShell.java and GS3's gsdl3/build/CollectionConstructor.java
     731
     732- http://stackoverflow.com/questions/8260205/when-a-listener-is-removed-is-it-okay-that-the-event-be-called-on-that-listener
     733
     734- http://stackoverflow.com/questions/2282166/java-synchronizing-on-primitives
     735     
     736     1. You can't lock on a primitive and
     737     2. Don't lock on a Long unless you're careful how you construct them. Long values created by autoboxing or Long.valueOf() in a certain range are guaranteed to be the same across the JVM which means other threads could be locking on the same exact Long object and giving you cross-talk. This can be a subtle concurrency bug (similar to locking on intern'ed strings).
     738
     739     Cross-talk:
     740     "In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another."
     741
     742
     743__________________________________________________
     744M. TICKETS, COMMITS, TAGS
     745__________________________________________________
     746
     747The following was already documented in ticket http://trac.greenstone.org/ticket/895
     748
     749    Other Java classes of GLI and GS3 src code were shifted from using Java's Process class directly to using our new SafeProcess class.
     750
     751    GLI src code classes that were changed:
     752    /Scratch/ak19/gs3-svn-15Nov2016/gli>fgrep -rl "Process" src
     753
     754    + src/org/greenstone/gatherer/util/SafeProcess.java
     755    + src/org/greenstone/gatherer/gui/FormatConversionDialog.java
     756    + src/org/greenstone/gatherer/gui/DownloadPane.java
     757    + src/org/greenstone/gatherer/util/GS3ServerThread.java
     758    + 2 src/org/greenstone/gatherer/Gatherer.java [2 MORE but don't want to mess with those]
     759    + 1 src/org/greenstone/gatherer/download/ServerInfoDialog.java
     760    + 2 src/org/greenstone/gatherer/greenstone/Classifiers.java
     761    + 2 src/org/greenstone/gatherer/greenstone/Plugins.java
     762    + 1 src/org/greenstone/gatherer/collection/ScriptOptions.java
     763    + !! src/org/greenstone/gatherer/util/ExternalProgram.java && file/FileAssociationManager.java
     764
     765    After updating GS3 src code to using SafeProcess, returned to GLI:
     766    - VERY HARD:
     767     + 1 src/org/greenstone/gatherer/shell/GShell.java
     768    - VERY, VERY HARD:
     769     + 2 src/org/greenstone/gatherer/download/DownloadJob.java
     770
     771
     772    GS3 src code classes that were changed:
     773    > fgrep -r "Process" . | grep -v ".svn" | grep -v "~" | less
     774
     775    + 2 ./java/org/greenstone/admin/guiext/Command.java
     776    X ./java/org/greenstone/gsdl3/util/Processing.java
     777    + MANY OCCURRENCES ./java/org/greenstone/gsdl3/service/MapRetrieve.java
     778    + Uses Processing 2 times: ./java/org/greenstone/gsdl3/util/GDBMWrapper.java
     779    + ./java/org/greenstone/util/BrowserLauncher.java
     780    + ./java/org/greenstone/util/RunTarget.java
     781
     782    In the GLI and GS3 src files modified to use SafeProcess above, only the modifications to GS3's Command.java, MapRetrieve.java and GDMBWrapper.java are untested. All others have been tested, at least on Linux. Many have also been tested on Windows.
     783
     784    Tested GLI's collection building (uses GShell.java's modifications) and DownloadJob.java on all 3 OS. Both have a Cancel button that should cancel any processes and subprocesses launched. Before the modifications, subprocesses would still be running on Windows and during (full-)import on the Mac when cancel was pressed. But now have added code to SafeProcess to issue OS specific system calls to terminate the process and its subprocesses successfully on Windows and Mac too.
     785
     786    Also tested GS3 user comments and the online GS3's doc editor's meta editing followed by rebuilding. All worked successfully on Linux, Windows and Mac with the modifications to use SafeProcess.
     787
     788    The modifications to GLI started with GS3ServerThread.java and other *easily* changed classes in
     789    - http://trac.greenstone.org/changeset/31582 (beginning of GLI modification)
     790    - until http://trac.greenstone.org/changeset/31665 (GS3 src code modification).
     791
     792    The version of GS3 and GLI Java code before the GS3 src was modified is at
     793    - http://trac.greenstone.org/browser/main/tags/GS3-src-SafeProcess
     794
     795    Next, GLI's GShell.java and any associated files were modified along with SafeProcess, to support cancelling.
     796
     797    Then, around 22 May, GLI's DownloadJob.java and any associated class files were modified along with SafeProcess to ensure subprocesses were safely terminated on Windows when the download action was cancelled OR when building (GShell.java) was cancelled. The Mac modifications to SafeProcess to ensure subprocesses were terminated was finished on 26 May 2017.
     798
     799    The tagged version of the GS3 and GLI Java code for when all the changes related to using SafeProcess were made to GS3 and GLI code is at
     800    - http://trac.greenstone.org/browser/main/tags/UsingSafeProcess
     801
     802_________________________________________________________
     803N. GLI vs GS3 CODE's SAFEPROCESS.JAVA - THE DIFFERENCES
     804_________________________________________________________
     805
     806- package name
     807    GLI: package org.greenstone.gatherer.util;
     808    GS3: package org.greenstone.util;
     809- imports:
     810    GLI: import org.greenstone.gatherer.DebugStream;
     811- GS3 uses Misc.java and GLI uses Utility.java for determining the OS and the appropriate newline character representation for the OS.
     812- GS3 uses its Logger object for logging messages, GLI uses System.err (Debugstream code is presently commented out).
     813
     814
     815    $ diff gli/src/org/greenstone/gatherer/util/SafeProcess.java src/java/org/greenstone/util/SafeProcess.java
     816    -----------------------------------------------------------------------------------------------------------
     817    1c1
     818    < package org.greenstone.gatherer.util;
     819    ---
     820    > package org.greenstone.util;
     821    27c27
     822    < import org.greenstone.gatherer.DebugStream;
     823    ---
     824    > //import org.greenstone.gatherer.DebugStream;
     825    56c56
     826    <     ///static Logger logger = Logger.getLogger(org.greenstone.util.SafeProcess.class.getName());
     827    ---
     828    >     static Logger logger = Logger.getLogger(org.greenstone.util.SafeProcess.class.getName());
     829    153a154
     830    >
     831    816c817
     832    <   if(Utility.isMac()) {
     833    ---
     834    >   if(Misc.isMac()) {
     835    880c881
     836    <     if(Utility.isWindows()) {
     837    ---
     838    >     if(Misc.isWindows()) {
     839    909c910
     840    <   if(!Utility.isMac() && canSkipExtraWorkIfLinux) {
     841    ---
     842    >   if(!Misc.isMac() && canSkipExtraWorkIfLinux) {
     843    1284c1285
     844    <           outputstr.append(Utility.NEWLINE); // "\n" is system dependent (Win must be "\r\n")
     845    ---
     846    >           outputstr.append(Misc.NEWLINE); // "\n" is system dependent (Win must be "\r\n")
     847    1381c1382
     848    <       /*if(Utility.isWindows()) {
     849    ---
     850    >       /*if(Misc.isWindows()) {
     851    1416c1417
     852    <   //logger.info(msg);
     853    ---
     854    >   logger.info(msg);
     855    1418c1419
     856    <   System.err.println(msg);
     857    ---
     858    >   //System.err.println(msg);
     859    1424c1425
     860    <   //logger.error(msg, e);
     861    ---
     862    >   logger.error(msg, e);
     863    1426,1427c1427,1428
     864    <   System.err.println(msg);
     865    <   e.printStackTrace();
     866    ---
     867    >   //System.err.println(msg);
     868    >   //e.printStackTrace();
     869    1434c1435
     870    <   //logger.error(e);
     871    ---
     872    >   logger.error(e);
     873    1436c1437
     874    <   e.printStackTrace();
     875    ---
     876    >   //e.printStackTrace();
     877
     878    -----------------------------------------------------------------------------------------------------------
     879
     880
     881__________________________________________________
     882O. FUTURE WORK, IMPROVEMENTS
     883__________________________________________________
     884
     885- On Windows, Perl could launch processes as proper ProcessTrees: http://search.cpan.org/~gsar/libwin32-0.191/
     886Then killing the root process will kill child processes naturally, and we won't need to call OS commands to terminate a process.
     887- Need to find a Mac (or Unix) equivalent way creating Process Tree in Perl too.
     888
     889Eventually, instead of running a windows/mac or even linux command to kill the process ourselves, investigate if it is useful to change over to use
     890    https://github.com/flapdoodle-oss/de.flapdoodle.embed.process/blob/master/src/main/java/de/flapdoodle/embed/process/runtime/Processes.java
     891- mentioned by http://stackoverflow.com/questions/4750470/how-to-get-pid-of-process-ive-just-started-within-java-program
     892- works with Apache license, http://www.apache.org/licenses/LICENSE-2.0
     893- This is a Java class that uses JNA to terminate processes. It also has the getProcessID() method.
     894- However, does it kill subprocesses? (The OS specific commands presently issued in SafeProcess.destroyProcess() ensure subprocesses are killed.)
     895
     896
     897Currently, SafeProcess does Java things the way that older Java versions recognise and accept, not only for backwards compatibility with earlier versions of Java, but in order to minimise the number of drastic changes from porting the existing Greenstone GLI and GS3 code to using SafeProcess.
     898
     899- In future, think of changing the method doRuntimeExec() over to using ProcessBuilder, instead of Runtime.exec(). ProcessBuilder seems to have been introduced from Java 5.
     900https://docs.oracle.com/javase/7/docs/api/java/lang/ProcessBuilder.html
     901See also https://zeroturnaround.com/rebellabs/how-to-deal-with-subprocesses-in-java/
     902which suggests using Apache Common Exec to launch processes and says what will be forthcoming in Java 9
     903
     904- Instead of creating and running Threads the usual way, should we use features of newer Java versions like the Concurrency API and Executor which uses Threadpools, and let Executor handle these details? Improvements would be with multicore systems
     905https://stackoverflow.com/questions/3330430/does-java-have-support-for-multicore-processors-parallel-processing
     906
     907
     908
     909
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