I think this question hasn't been answered for my use-case.
We wish to detect if the user has changed a file without re-reading its contents for the purposes of caching a computation result based on the file contents. Our program is a long-running one that lets the user click a button to perform a computation based on data entered in the program and data stored in external files (sorry, I can't be more specific than that). The external data needs to be read, processed and various data structures need to be built based on it, so we try to cache those between computations to speed up re-computes when the user changes the data in the program itself, but not the data in the external files. However, if the external file has changed, we have to re-read that.
For each external resource we're checking if the modification time and file size have changed, but that's not really all that robust and can lead to user frustration if they have e.g. fileA and fileB with the same size and timestamp and copy or fileA to fileC, use fileC as an external resource, and then copy fileB to fileC. The system preserves the modification time of the original file and the sizes are the same, so we don't re-read the external resource.
Our program runs on Windows, macOS and Linux, is written in C++ and we're perfectly OK with using platform-specific code to detect file changes. We're interested in the most robust way to detect if the contents of a file identified by a file path have changed without actually reading the file itself.
I've made this answer a community wiki so others can add their ideas for the various platforms listed in the question.
Linux
MacOS
Windows
Option 1
Set up a thread that watches the directory containing the file. When the directory changes, you'll have to check if the file you care about has actually changed. That may mean opening and re-reading the file, (e.g., to compute the current checksum). But since you have to do this only after a change notification, this overhead may be acceptable.
I believe (but have not verified) that if someone copies a same-size, same-timestamp file over an existing file, you'll get a directory change notification.
Option 2
Hold the file open with an opportunistic lock. This involves creating the lock with a call to DeviceIoControl and then issuing a blocking call to GetOverlappedResult, which will unblock when another process attempts to change the file. Your program can the release the lock, allowing the other process to update the file, and know that the file is being changed.
Related
I have code that performs following steps:
open file
write data
set file timestamps (via SetFileInformationByHandle(FileBasicInfo))
close file
When file is stored on certain NAS devices (and accessed via share) it's modification time ends up being set to current time.
According to Process Monitor Close() in step 4 results in a Write (local cache gets flushed/pushed to NAS device) that (seemingly) updates file's mtime on server.
If I add FlushFileBuffers() (or sleep for few seconds) between steps 2 and 3 -- everything is fine.
Is this a bug in SMB implementation of this NAS device (Dell EMC Isilon) or SetFileInformationByHandle() never promised anything?
What is the best way to deal with this situation? I would really like to avoid having to call FlushFileBuffers()...
Edit: Great... :-/ It looks like for executables (and only executables) atime (last access time) gets screwed up too (in the same way). Only these are harder to reproduce -- need to run this logic few times. Could be some antivirus... I am still investigating.
Edit 2: According to procmon access time gets updated by EXPLORER.EXE -- when it sees an executable, it can't resist opening it and reading portions of it (probably extracting the icon).
You can't really do anything -- I guess Isilon's SMB implementation doesn't support certain things (that would've preserved timestamps).
I simply added FlushFileBuffers() before SetFileInformationByHandle() and made sure there are no related race conditions in my code.
I'm writing a Qt/C++ program that does long-running simulations, and to guard against data loss, I wrote some simple autosave behaviour. The program periodically saves to the user's temp directory (using QDir::temp()), and if the program closes gracefully, this file is deleted. If the program starts up and sees the file in that directory, it assumes a previous instance crashed or was forcibly ended, and it prompts the user about loading it.
Now here is the complication - I'd like this functionality to work properly even if multiple instances of the program are used at once. So when the program loads, it can't just look for the presence of an autosave file. If it finds one, it needs to determine if that file was created by a running instance (in which case, there's nothing wrong and nothing to be done) or if it has been left over by a instance that crashed or was forcibly ended (in which case it should prompt the user about loading it).
My program is for Windows/Mac/Linux, so what would be the best way to implement this using Qt or otherwise in a cross-platform fashion?
Edit:
The comments suggested the use of the process identifier, which I can get using QCoreApplication::applicationPid(). I like this idea, but when the program loads and sees a file with a certain PID in the name, how can it look at the other running instances (if any) to see if there is a match?
You can simply use QSaveFile which, as the documentation states:-
The QSaveFile class provides an interface for safely writing to files.
QSaveFile is an I/O device for writing text and binary files, without losing existing data if the writing operation fails.
While writing, the contents will be written to a temporary file, and if no error happened, commit() will move it to the final file. This ensures that no data at the final file is lost in case an error happens while writing, and no partially-written file is ever present at the final location. Always use QSaveFile when saving entire documents to disk.
As for multiple instances, you just need to reflect that in the filename.
So often my applications want to save files to load again later. Having recently got unlucky with a crash, I want to write the operation in such a way that I am guaranteed to either have the new data, or the original data, but no a corrupted mess.
My first idea was to do something along the lines of (to save a file called example.dat):
Come up with a unique file name for the target directory, e.g. example.dat.tmp
Create that file and write my data to it.
Delete the original file (example.dat)
Rename ("Move") the temp file to where the original was (example.dat.tmp -> example.dat).
Then at load time the application can follow the following rules:
If no "example.dat" and no "example.dat.tmp", first run / new project, so load in the defaults / create new file.
If "example.dat" and no "example.dat.tmp", then load example.dat (normal load case)
If "example.dat.tmp" exists offer the user the chance to potentially recover data. If "example.dat" also exists, do not overwrite it without explicit user constant.
However, having done a little research, I found that as well as OS caching which I may be able to override with the file flush methods, some disk drives still then cache internally and may even lie to the OS saying they are done, so 4. could complete, the write is not actually written, and if the system goes down I have lost my data...
I am not sure the disk problem is actually solvable by an application, but are the general rules above the correct thing to do? Should I keep an old recovery copy of the file for longer to be sure, what are the guidelines regarding such things (e.g. acceptable disk usage, should the user choose, where to put such files, etc.).
Also how should I avoid potential conflict the user and other programs for "example.dat.tmp". I recall seeing a "~example.dat" sometimes from some other software, is that a better convention?
If the disk drives report back to the OS that the data is
physically on the disk, and it's not, then there's not much you
can do about it. A lot of disks do cache a certain number of
writes, and report them done, but such disks should have
a battery backup, and finish the physical writes no matter what
(and they won't loose data in case of a system crash, since they
won't even see it).
For the rest, you say you've done some research, so you no doubt
know that you can't use std::ofstream (nor FILE*) for this;
you have to do the actual writes at the system level, and open
the files with special attributes for them to ensure full
synchronization. Otherwise, the operations can stick around in
the OS buffering for a while. And that as far as I know,
there's no way of ensuring such synchronization for a rename.
(But I'm not sure that it's necessary, if you always keep two
versions: my usual convention in such cases is to write to
a file "example.dat.new", then when I'm done writing, delete
any file named "example.dat.bak", rename "example.dat" to
"example.dat.bak", and then rename "example.dat.new" to
"example.dat". Given this, you should be able to figure out
what did or did not happen, and find the correct file
(interactively, if need be, or insert an initial line with the
timestamp).
You should lock the actual data file while you write its substitute, if there's a chance that a different process could be going through the same protocol that you are describing.
You can use flock for the file lock.
As for your temp file name, you could make your process ID part of it, for instance "example.dat.3124," No other simultaneously-running process would generate the same name.
Alright so to start this is strictly for Windows and I'd prefer to use C++ over .NET but I'm not opposed to boost::filesystem although if it can be avoided in favor of straight Windows API I'd prefer that.
Now the scenario is an application on another machine I can't change is going to create files in a particular directory on the machine that I need to make backups of and do some extra processing. Currently I've made a little application which will sit and listen for change notifications in a target directory using FindFirstChangeNotification and FindNextChangeNotification windows APIs.
The problem is that while I can get notified when new files are created in the directory, modified, size changes, etc it only notifies once and does not specifically tell me which files. I've looked at ReadDirectoryChangesW as well but it's the same story there except that I can get slightly more specific information.
Now I can scan the directory and try to acquire locks or open the files to determine what specifically changed from the last notification and whether they are available for further use but in the case of copying a large file I've found this isn't good enough as the file won't be ready to be manipulated and I won't get any other notifications after the first so there is no way to tell when it's actually done copying unless after the first notification I continually try to acquire locks until it succeeds.
The only other thing I can think of that would be less hackish would be to have some kind of end token file but since I don't have control over the application creating the files in the first place I don't see how I'd go about doing that and it's still not ideal.
Any suggestions?
This is a fairly common problem and one that doesn't have an easy answer. Acquiring locks is one of the best options when you cannot change the thing at the remote end. Another I have seen is to watch the file at intervals until the size doesn't change for an interval or two.
Other strategies include writing a no-byte file as a trigger when the main file is complete and writing to a temp directory then moving the complete file to the real destination. But to be reliable, it must be the sender who controls this. As the receiver, you are constrained to watching the directory and waiting for the file to settle.
It looks like ReadDirectoryChangesW is going to be your best bet. For each file copy operation, you should be receiving FILE_ACTION_ADDED followed by a bunch of FILE_ACTION_MODIFIED notifications. On the last FILE_ACTION_MODIFIED notification, the file should no longer be locked by the copying process. So, if you try to acquire a lock after each FILE_ACTION_MODIFIED of the copy, it should fail until the copy completes. It's not a particularly elegant solution, but there doesn't seem to be any notifications available for when a file copy completes.
You can process the data once the file is closed, right? So the task is to track when the file is closed. This can be done using file system filter driver. You can write your own or you can use our CallbackFilter product.
Windows Win32 C++ question about flushing file activity to disk.
I have an external application (ran using CreateProcess) which does some file creation. i.e., when it returns it will have created a file with some content.
How can I ensure that the file the process created was really flushed to disk, before I proceed?
By this I mean not the C++ buffers but really flushing disk (e.g. FlushFileBuffers).
Remember that I don't have access to any file HANDLE - this is all of course hidden inside the external process.
I guess I could open up a handle of my own to the file and then use FlushFileBuffers, but it's not clear this would work (since my handle doesn't actually contain anything which needs flushing).
Finally, I want this to run in non-admin userspace so I cannot use FlushFileBuffers on a whole volume.
Any ideas?
UPDATE: Why do I think this is a problem?
I'm working on a data backup application. Essentially it has to create some files as described. It then has to update it's internal DB (using SQLite embedded DB).
I recently had a data corruption issue which occurred during a bluescreen (the cause of which was unrelated to my app).
What I'm concerned about is application integrity during a system crash. And yes, I do care about this because this app is a data backup app.
The use case I'm concerned about is this:
A small data file is created using external process. This write is waiting in the OS cache to be written to disk.
I update the DB and commit. This is a disk activity. This write is also waiting in the OS cache.
A system failure occurs.
As I see it, we're now in a potential race condition. If "1" gets flushed and "2" doesn't then we're fine (as the DB transact wasn't then committed). If neither gets flushed or both get flushed then we're also OK.
As I understand it, the writes will be non-deterministic. i.e., I'm not aware that the OS will guarantee to write "1" before "2". (Am I wrong?)
So, if "2" gets flushed, but "1" doesn't then we have a problem.
What I observed was that the DB was correctly updated, but that the file had garbage in: the last 2 thirds of the data was binary "zeroes". Now, I don't know what it looks like when you have a file part flushed at the time of bluescreen, but I wouldn't be surprised if it looked like that.
Can I guarantee this is the cause? No I cannot guarantee this. I'm just speculating. It could just be that the file was "naturally" corrupted due to disk failure or as a result of the blue screen.
With regards to performance, this is something I believe I can deal with.
For example, the default behaviour of SQLite is to do a full file flush (using FlushFileBuffers) every time you commit a transaction. They are quite clear that if you don't do this then at the time of system crash, you might have a corrupted DB.
Also, I believe I can mitigate the performance hit by only flushing at "checkpoints". For example, writing 50 files, flushing the lot and then writing to the DB.
How likely is all this to be a problem? Beats me. But then my app might well be archiving at or around the time of system failure so it might be more likely that you think.
Hope that explains why I wan't to do this.
Why would you want this? The OS will make sure that the data is flushed to the disk in due time. If you access it, it will either return the data from the cache or from disk, so this is transparent for you.
If you need some safety in case of disaster, then you must call FlushFileBuffers, for example by creating a process with admin rights after running the external process. But that can severely impact the performance of the whole machine.
Your only other option is to modify the source of the other process.
[EDIT] The most simple solution is probably to copy the file in your process and then flush the copy (since you have the handle). Save the copy under a name which says "not committed in the database".
Then update the database. Write into the database, "updated from file ...". If this entry already exists next time, don't update the database and skip this step.
Flush the database to disk.
Rename the file to "file has been processed into database". Rename is an atomic operation (so it either happens or not).
If you can't think of a good filename for the different states, then use subfolders and move the file between them.
Well, there are no attractive options here. There is no documented way to retrieve the file handle you need from the process. Although there are undocumented ones, go there (via DuplicateHandle) only with careful consideration.
Yes, calling FlushFileBuffers on a volume handle is the documented way. You can avoid the privilege problem by letting a service make the call. Talk to it from your app with one of the standard process interop mechanisms. A named pipe whose name is prefixed with Global\ is probably the easiest way to get that going.
After your update I think http://sqlite.org/atomiccommit.html gives you the answers you need.
The way SQLite ensures that everything is flushed to disc works. So it works for you as well - take a look at the source.