Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > Python > Re: How to safely maintain a status file

Reply
Thread Tools

Re: How to safely maintain a status file

 
 
Hans Mulder
Guest
Posts: n/a
 
      07-12-2012
On 12/07/12 14:30:41, Laszlo Nagy wrote:
>> You are contradicting yourself. Either the OS is providing a fully
>> atomic rename or it doesn't. All POSIX compatible OS provide an atomic
>> rename functionality that renames the file atomically or fails without
>> loosing the target side. On POSIX OS it doesn't matter if the target
>> exists.


> This is not a contradiction. Although the rename operation is atomic,
> the whole "change status" process is not. It is because there are two
> operations: #1 delete old status file and #2. rename the new status
> file. And because there are two operations, there is still a race
> condition. I see no contradiction here.


On Posix systems, you can avoid the race condition. The trick is to
skip step #1. The rename will implicitly delete the old file, and
it will still be atomic. The whole process now consists of a single
stop, so the whole process is now atomic.

>> You don't need locks or any other fancy stuff. You just need to make
>> sure that you flush the data and metadata correctly to the disk and
>> force a re-write of the directory inode, too. It's a standard pattern on
>> POSIX platforms and well documented in e.g. the maildir RFC.


> It is not entirely true. We are talking about two processes. One is
> reading a file, another one is writting it. They can run at the same
> time, so flushing disk cache forcedly won't help.


On Posix systems, it will work, and be atomic, even if one process is
reading the old status file while another process is writing the new
one. The old file will be atomically removed from the directory by
the rename operation; it will continue to exists on the hard drive, so
that the reading process can continue reading it. The old file will
be deleted when the reader closes it. Or, if the system crashed before
the old file is closed, it will deleted when the system is restarted.

On Windows, things are very different.

Hope this helps,

-- HansM


 
Reply With Quote
 
 
 
 
Laszlo Nagy
Guest
Posts: n/a
 
      07-12-2012

>> This is not a contradiction. Although the rename operation is atomic,
>> the whole "change status" process is not. It is because there are two
>> operations: #1 delete old status file and #2. rename the new status
>> file. And because there are two operations, there is still a race
>> condition. I see no contradiction here.

> On Posix systems, you can avoid the race condition. The trick is to
> skip step #1. The rename will implicitly delete the old file, and
> it will still be atomic. The whole process now consists of a single
> stop, so the whole process is now atomic.

Well, I didn't know that this is going to work. At least it does not
work on Windows 7 (which should be POSIX compatible?)

>>> f = open("test.txt","wb+")
>>> f.close()
>>> f2 = open("test2.txt","wb+")
>>> f2.close()
>>> import os
>>> os.rename("test2.txt","test.txt")

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
WindowsError: [Error 183] File already exists
>>>


I have also tried this on FreeBSD and it worked.

Now, let's go back to the original question:

>>>This works well on Linux but Windows raises an error when status_file already exists.


It SEEMS that the op wanted a solution for Windows....


 
Reply With Quote
 
 
 
 
Rick Johnson
Guest
Posts: n/a
 
      07-13-2012
On Jul 12, 2:39*pm, Christian Heimes <(E-Mail Removed)> wrote:
> Windows's file system layer is not POSIX compatible. For example
> you can't remove or replace a file while it is opened by a process.


Sounds like a reasonable fail-safe to me. Not much unlike a car
ignition that will not allow starting the engine if the transmission
is in any *other* gear besides "park" or "neutral", OR a governor (be
it mechanical or electrical) that will not allow the engine RPMs to
exceed a maximum safe limit, OR even, ABS systems which "pulse" the
brakes to prevent overzealous operators from loosing road-to-tire
traction when decelerating the vehicle.

You could say: "Hey, if someone is dumb enough to shoot themselves in
the foot then let them"... however, sometimes fail-safes not only save
the dummy from a life of limps, they also prevent catastrophic
"collateral damage" to rest of us.
 
Reply With Quote
 
Steven D'Aprano
Guest
Posts: n/a
 
      07-13-2012
On Fri, 13 Jul 2012 12:12:01 +1000, Chris Angelico wrote:

> On Fri, Jul 13, 2012 at 11:20 AM, Rick Johnson
> <(E-Mail Removed)> wrote:
>> On Jul 12, 2:39 pm, Christian Heimes <(E-Mail Removed)> wrote:
>>> Windows's file system layer is not POSIX compatible. For example you
>>> can't remove or replace a file while it is opened by a process.

>>
>> Sounds like a reasonable fail-safe to me.


Rick has obviously never tried to open a file for reading when somebody
else has it opened, also for reading, and discovered that despite Windows
being allegedly a multi-user operating system, you can't actually have
multiple users read the same files at the same time.

(At least not unless the application takes steps to allow it.)

Or tried to back-up files while some application has got them opened. Or
open a file while an anti-virus scanner is oh-so-slooooowly scanning it.

Opening files for exclusive read *by default* is a pointless and silly
limitation. It's also unsafe: if a process opens a file for exclusive
read, and then dies, *no other process* can close that file.

At least on POSIX systems, not even root can override a mandatory
exclusive lock (it would be pretty pointless if it could), so a rogue or
buggy program could wreck havoc with mandatory exclusive file locks.
That's why Linux, by default, treats exclusive file locks as advisory
(cooperative), not mandatory.

In general, file locking is harder than it sounds, with many traps for
the unwary, and of course the semantics are dependent on both the
operating system and the file system.

https://en.wikipedia.org/wiki/File_locking


> POSIX says that files and file names are independent. I can open a file
> based on its name, delete the file based on its name, and still have the
> open file there. When it's closed, it'll be wiped from the disk.


One neat trick is to open a file, then delete it from disk while it is
still open. So long as your process is still running, you can write to
this ghost file, as normal, but no other process can (easily) see it. And
when your process ends, the file contents is automatically deleted.

This is remarkably similar to what Python does with namespaces and dicts:

# create a fake "file system"
ns = {'a': [], 'b': [], 'c': []}
# open a file
myfile = ns['a']
# write to it
myfile.append('some data')
# delete it from the "file system"
del ns['a']
# but I can still read and write to it
myfile.append('more data')
print(myfile[0])
# but anyone else will get an error if they try
another_file = ns['a']


--
Steven
 
Reply With Quote
 
Gene Heskett
Guest
Posts: n/a
 
      07-13-2012
On Thursday 12 July 2012 23:21:16 Steven D'Aprano did opine:

> On Fri, 13 Jul 2012 12:12:01 +1000, Chris Angelico wrote:
> > On Fri, Jul 13, 2012 at 11:20 AM, Rick Johnson
> >
> > <(E-Mail Removed)> wrote:
> >> On Jul 12, 2:39 pm, Christian Heimes <(E-Mail Removed)> wrote:
> >>> Windows's file system layer is not POSIX compatible. For example you
> >>> can't remove or replace a file while it is opened by a process.
> >>
> >> Sounds like a reasonable fail-safe to me.

>
> Rick has obviously never tried to open a file for reading when somebody
> else has it opened, also for reading, and discovered that despite
> Windows being allegedly a multi-user operating system, you can't
> actually have multiple users read the same files at the same time.
>

Chuckle. That was one of the 'features' that os9 on the trs-80 color
computer had back in the 80's, and it was clean and well done because of
the locking model the random block file manager had in OS9 for 6809 cpu's,
no relation to the Mac OS9 other than a similar name. That color computer
has a separate, text only video card I could plug in and display on an 80
column amber screen monitor.

When I wanted to impress the visiting frogs, I often did something I have
never been able to do on any other operating system since, start assembling
a long assembly language file on one of the screens on the color monitor,
hit the clear key to advance to the amber screen and start a listing on it
of the assemblers output listing file.

Because the file locking was applied only to the sector (256 bytes on that
machine) being written at the instant, the listing would fly by till it
caught up with the assemblers output, running into the lock and then
dutifully following along, one sector behind the assemblers output, until
the assembly was finished. That was in 1986 folks, and in the year of our
Lord 2012, 26 years later, I still cannot do that in linux. When I ask why
not, the replies seem to think I'm from outer space. Its apparently a
concept that is not even attempted to be understood by the linux code
carvers.

Something is drastically wrong with that picture IMO.

> (At least not unless the application takes steps to allow it.)
>
> Or tried to back-up files while some application has got them opened.


That in fact, ran me out of the amiga business in 1999, a 30Gb drive failed
on my full blown 040 + 64 megs of dram A2000. When the warranty drive
arrived is when I found that due to file locks on the startup files, all of
them involved with the booting of that machine, my high priced Diavolo Pro
backup tapes didn't contain a single one of those files. The linux box
with Red Hat 5.0 on it that I had built in late 1998 to see what linux was
all about found space under that desk yet that evening and I never looked
back.

> Or
> open a file while an anti-virus scanner is oh-so-slooooowly scanning it.
>
> Opening files for exclusive read *by default* is a pointless and silly
> limitation. It's also unsafe: if a process opens a file for exclusive
> read, and then dies, *no other process* can close that file.
>
> At least on POSIX systems, not even root can override a mandatory
> exclusive lock (it would be pretty pointless if it could), so a rogue or
> buggy program could wreck havoc with mandatory exclusive file locks.
> That's why Linux, by default, treats exclusive file locks as advisory
> (cooperative), not mandatory.
>
> In general, file locking is harder than it sounds, with many traps for
> the unwary, and of course the semantics are dependent on both the
> operating system and the file system.
>
> https://en.wikipedia.org/wiki/File_locking
>
> > POSIX says that files and file names are independent. I can open a
> > file based on its name, delete the file based on its name, and still
> > have the open file there. When it's closed, it'll be wiped from the
> > disk.

>
> One neat trick is to open a file, then delete it from disk while it is
> still open. So long as your process is still running, you can write to
> this ghost file, as normal, but no other process can (easily) see it.
> And when your process ends, the file contents is automatically deleted.
>
> This is remarkably similar to what Python does with namespaces and
> dicts:
>
> # create a fake "file system"
> ns = {'a': [], 'b': [], 'c': []}
> # open a file
> myfile = ns['a']
> # write to it
> myfile.append('some data')
> # delete it from the "file system"
> del ns['a']
> # but I can still read and write to it
> myfile.append('more data')
> print(myfile[0])
> # but anyone else will get an error if they try
> another_file = ns['a']


Cheers, Gene
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene> is up!
You just wait, I'll sin till I blow up!
-- Dylan Thomas
 
Reply With Quote
 
Steven D'Aprano
Guest
Posts: n/a
 
      07-13-2012
On Thu, 12 Jul 2012 23:49:02 -0400, Gene Heskett wrote:

> When I wanted to impress the visiting frogs, I often did something I
> have never been able to do on any other operating system since, start
> assembling a long assembly language file on one of the screens on the
> color monitor, hit the clear key to advance to the amber screen and
> start a listing on it of the assemblers output listing file.
>
> Because the file locking was applied only to the sector (256 bytes on
> that machine) being written at the instant, the listing would fly by
> till it caught up with the assemblers output, running into the lock and
> then dutifully following along, one sector behind the assemblers output,
> until the assembly was finished. That was in 1986 folks, and in the
> year of our Lord 2012, 26 years later, I still cannot do that in linux.


Um, what you are describing sounds functionally equivalent to what
tail -f does.


> When I ask why not, the replies seem to think I'm from outer space. Its
> apparently a concept that is not even attempted to be understood by the
> linux code carvers.


You could certainly create a pair of cooperative programs, one which
keeps a lock on only the last block of the file, and a tail-like reader
which honours that lock. But why bother? Just have the assembler append
to the file, and let people use any reader they like, such as tail.

Or have I misunderstood you?



--
Steven
 
Reply With Quote
 
rantingrickjohnson@gmail.com
Guest
Posts: n/a
 
      07-13-2012
On Thursday, July 12, 2012 10:13:47 PM UTC-5, Steven D'Aprano wrote:
> Rick has obviously never tried to open a file for reading when somebody
> else has it opened, also for reading, and discovered that despite Windows
> being allegedly a multi-user operating system, you can't actually have
> multiple users read the same files at the same time.


You misread my response. My comment was direct result of Christian stating:

(paraphrase) "On some systems you are not permitted to delete a file whilstthe file is open "

....which seems to be consistent to me. Why would *anybody* want to delete afile whilst the file is open? Bringing back the car analogy again: Would you consider jumping from a moving vehicle a consistent interaction with theinterface of a vehicle? Of course not. The interface for a vehicle is simple and consistent:

1. You enter the vehicle at location A
2. The vehicle transports you to location B
3. You exit the vehicle

At no time during the trip would anyone expect you to leap from the vehicle.. But when you delete open files, you are essentially leaping from the moving vehicle! This behavior goes against all expectations of consistency in an API -- and against all sanity when riding in a vehicle!

> Opening files for exclusive read *by default* is a pointless and silly
> limitation. It's also unsafe: if a process opens a file for exclusive
> read, and then dies, *no other process* can close that file.


Oh come on. Are you actually going to use "errors" or "unintended consequences", or even "Acts of God" to defend your argument? Okay. Okay. I suppose "IF" the car spontaneously combusted "THEN" the passengers would be wise tojump out, leaving the vehicle to the whims of inertia.

> One neat trick is to open a file, then delete it from disk while it is
> still open. So long as your process is still running, you can write to
> this ghost file, as normal, but no other process can (easily) see it. And
> when your process ends, the file contents is automatically deleted.


Well "neat tricks" aside, I am of the firm belief that deleting files should never be possible whilst they are open.

* Opening files requires that data exist on disk
* Reading and writing files requires an open file obj
* Closing files requires an open file object
* And deleting files requires that the file NOT be open

Would you also entertain the idea of reading or writing files that do not exist? (not including pseudo file objs like StringIO of course!).

Summary: Neat tricks and Easter eggs are real hoot, but consistency in APIsis the key.
 
Reply With Quote
 
Steven D'Aprano
Guest
Posts: n/a
 
      07-13-2012
On Thu, 12 Jul 2012 21:26:20 -0700, rantingrickjohnson wrote:

> On Thursday, July 12, 2012 10:13:47 PM UTC-5, Steven D'Aprano wrote:
>> Rick has obviously never tried to open a file for reading when somebody
>> else has it opened, also for reading, and discovered that despite
>> Windows being allegedly a multi-user operating system, you can't
>> actually have multiple users read the same files at the same time.

>
> You misread my response. My comment was direct result of Christian
> stating:
>
> (paraphrase) "On some systems you are not permitted to delete a file
> whilst the file is open "
>
> ...which seems to be consistent to me. Why would *anybody* want to
> delete a file whilst the file is open?


Because it is useful and a sensible thing to do.

Why should one misbehaved application, keeping a file open, be allowed to
hold every other application, and the file system, hostage?

This is one of the many poor decisions which makes Windows so vulnerable
to viruses and malware. If malware can arrange to keep itself open, you
can't delete it. Thanks guys!


> Bringing back the car analogy
> again: Would you consider jumping from a moving vehicle a consistent
> interaction with the interface of a vehicle? Of course not. The
> interface for a vehicle is simple and consistent:
>
> 1. You enter the vehicle at location A
> 2. The vehicle transports you to location B
> 3. You exit the vehicle


Amusingly, you neglected to specify "the vehicle stops" -- and rightly
so, because of course having to stop the vehicle is not a *necessary*
condition for exiting it, as tens of thousands of stunt men and women can
attest.

Not to mention people parachuting out of an airplane, pirates or
commandos boarding a moving ship, pedestrians transferring from a slow
moving walkway to a faster moving walkway, farmers jumping off a trailer
while it is still being towed behind a tractor (and jumping back on
again), and Bruce Willis in "Red" in very possibly the best slow-motion
action sequence in the history of Hollywood.

http://www.youtube.com/watch?v=xonMpj2YyDU


> At no time during the trip would anyone expect you to leap from the
> vehicle.


Expected or not, you can do so.


> But when you delete open files, you are essentially leaping
> from the moving vehicle! This behavior goes against all expectations of
> consistency in an API -- and against all sanity when riding in a
> vehicle!


Fortunately, files on a file system are not cars, and deleting open files
is a perfectly reasonable thing to do, no more frightening than in Python
deleting a reference to an object using the del statement. Imagine how
stupid it would be if this happened:


py> x = 42
py> y = x
py> del y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
DeleteError: cannot delete reference to object '42' until no other
references to it exist


Fortunately, Python doesn't do that -- it tracks when the object is no
longer being accessed, and only then physically reclaims the memory used.
And so it is on POSIX file systems: the file system keeps track of when
the file on disk is no longer being accessed, and only then physically
reclaims the blocks being used. Until then, deleting the file merely
unlinks the file name from the blocks on disk, in the same way that
"del y" merely unlinks the name y from the object 42.


>> Opening files for exclusive read *by default* is a pointless and silly
>> limitation. It's also unsafe: if a process opens a file for
>> exclusive read, and then dies, *no other process* can close that file.

>
> Oh come on. Are you actually going to use "errors" or "unintended
> consequences", or even "Acts of God" to defend your argument?


Features have to be judged by their actual consequences, not some
unrealistic sense of theoretical purity. The actual consequences of
mandatory exclusive file locking is, *it sucks*.

Windows users are used to having to reboot their server every few days
because something is broken, so they might not mind rebooting it because
some file is locked in a mandatory open state and not even the operating
system can unlock it. But for those with proper operating systems who
expect months of uninterrupted service, mandatory locking is a problem to
be avoided, not a feature.


> Okay.
> Okay. I suppose "IF" the car spontaneously combusted "THEN" the
> passengers would be wise to jump out, leaving the vehicle to the whims
> of inertia.


In this analogy, is the car the file name, the inode, or the directory?
Are the passengers the file name(s), or the file contents, or the inode?
Is the driver meant to be the file system? If I have a hard link to the
file, does that mean the passengers are in two cars at once, or two lots
of passengers in the same car?


>> One neat trick is to open a file, then delete it from disk while it is
>> still open. So long as your process is still running, you can write to
>> this ghost file, as normal, but no other process can (easily) see it.
>> And when your process ends, the file contents is automatically deleted.

>
> Well "neat tricks" aside, I am of the firm belief that deleting files
> should never be possible whilst they are open.


[condescension = ON]

Good for you Rick. Having strongly held opinions on things you have only
a limited understanding about is your right as an American.

[condescension = OFF]


> * Opening files requires that data exist on disk
> * Reading and writing files requires an open file obj


You have missed a step in jumping from files on disk to open file objects.

Open file objects do not necessarily correspond to files on disk. For
example, in standard Pascal, file objects are purely in-memory constructs
emulating files on a tape drive, with no relationship to on-disk files.

(Any half-decent Pascal compiler or interpreter will *also* give you ways
to access real files on disk, but that isn't covered by the standard.)

Even when the file object does come from an actual disk file, we can
conclude that before you can open a file for reading, it must exist; but
having opened it, there is no *necessary* requirement that it *remains*
on disk. If you try to read from an open file object whose underlying
file has been deleted, there are three perfectly reasonable behaviours:

- you get an error;

- it is the same result as if the file has been truncated to zero bytes;

- deleting the file only deletes the *name*, not contents, until the
last open file handle is shut, and then the contents are deleted.


> * Closing files requires an open file object


Naturally; but open file objects don't require that the on-disk file
still exists.


> * And deleting files requires that the file NOT be open


Not at all.


> Would you also entertain the idea of reading or writing files that do
> not exist? (not including pseudo file objs like StringIO of course!).


Define "file" and "exist". Because you are conflating at least three
different things:

a file name
an inode (blocks on a disk)
a file object

Of course it is useful to break the abstraction that file objects must be
files on disk. StringIO is one such example. Standard Pascal file objects
is another. Likewise, it is useful to be able to read from an inode that
is no longer connected to a file name.

So, absolutely, yes, it is useful to be able to read and write from files
that don't exist, under some circumstances.


> Summary: Neat tricks and Easter eggs are real hoot, but consistency
> in APIs is the key.


A foolish consistency is the hobgoblin of little minds.


--
Steven
 
Reply With Quote
 
Prasad, Ramit
Guest
Posts: n/a
 
      07-13-2012
> Well "neat tricks" aside, I am of the firm belief that deleting files should

> never be possible whilst they are open.


This is oneof the few instances I think Windows does something better
than OS X. Windows will check before you attempt to delete (i.e. move
to Recycling Bin) while OS X will move a file to Trash quite happily
onlytell me it cannot remove the file when I try to empty the Trash.

Ramit


Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
712 Main Street | Houston, TX 77002
work phone: 713 - 216 - 5423

--


This email is confidential and subject to important disclaimers and
conditions including on offers for the purchase or sale of
securities, accuracy and completeness of information, viruses,
confidentiality, legal privilege, and legal entity disclaimers,
available at http://www.jpmorgan.com/pages/disclosures/email.
 
Reply With Quote
 
Chris Gonnerman
Guest
Posts: n/a
 
      07-13-2012
On 07/13/2012 11:00 AM, Prasad, Ramit wrote:
>> Well "neat tricks" aside, I am of the firm belief that deleting files should
>> never be possible whilst they are open.

> This is one of the few instances I think Windows does something better
> than OS X. Windows will check before you attempt to delete (i.e. move
> to Recycling Bin) while OS X will move a file to Trash quite happily
> only tell me it cannot remove the file when I try to empty the Trash.

While I was trained in the Unix way, and believe it is entirely
appropriate to delete an open file. Even if I my program is the opener.
It's just too handy to have temp files that disappear on their own.

As opposed to periodically going to %TEMP% and deleting them manually. Gah.

-- Chris.
 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Re: How to safely maintain a status file Plumo Python 1 07-12-2012 05:46 PM
Re: How to safely maintain a status file Plumo Python 1 07-09-2012 08:47 PM
Re: How to safely maintain a status file Laszlo Nagy Python 1 07-09-2012 08:57 AM
Re: How to safely maintain a status file Dennis Lee Bieber Python 1 07-09-2012 05:58 AM
How to safely maintain a status file Richard Baron Penman Python 0 07-08-2012 11:29 AM



Advertisments