I want to create a temporary file/directory in node.js. To do this, I'm attempting a simple algorithm:
Generate a file name based on pid, time, and random chars
Check if file exists
if yes: return to step 1 and repeat
if not: create the file and return it
Here's the problem: The node.js documentation for fs.exists explicitly states that fs.exists should not be used, and instead one should just use fs.open and catch a potential error:
http://nodejs.org/docs/latest/api/fs.html#fs_fs_exists_path_callback
In my case, I am not interested in opening the file if it exists, I am strictly trying to find a filename that does not yet exist. Is there a way I can go about this that does not use fs.exists? Alternatively, if I do use fs.exists, should I be worried that this method will become deprecated in the future?
Use fs.open with the 'wx' flags instead so that you'll create the file if it doesn't exist, and return an error if it already exists.
That way you eliminate the (albeit minute) possibility that the file is created between a fs.exists check and an fs.open call to create the file.
Related
I have an open std::fs::File, and I want to get it's filename, e.g. as a PathBuf. How do I do that?
The simple solution would be to just save the path used in the call to File::open. Unfortunately, this does not work for me. I am trying to write a program that reads log files, and the program that writes the logs keep changing the filenames as part of it's log rotation. So the file may very well have been renamed since it was opened. This is on Linux, so renaming open files is possible.
How do I get around this issue, and get the current filename of an open file?
On a typical Unix filesystem, a file may have multiple filenames at once, or even none at all. The file metadata is stored in an inode, which has a unique inode number, and this inode number can be linked from any number of directory entries. However, there are no reverse links from the inode back to the directory entries.
Given an open File object in Rust, you can get the inode number using the ino() method. If you know the directory the log file is in, you can use std::fs::read_dir() to iterate over all entries in that directory, and each entry will also have an ino() method, so you can find the one(s) matching your open file object. Of course this approach is subject to race conditions – the directory entry may already be gone again once you try to do anything with it.
On linux, files handles held by the current process can be found under /proc/self/fd. These look and act like symlinks to the original files (though I think they may technically be something else - perhaps someone who knows more can chip in).
You can therefore recover the (possibly changed) file name by constructing the correct path in /proc/self/fd using your file descriptor, and then following the symlink back to the filesystem.
This snippet shows the steps:
use std::fs::read_link;
use std::os::unix::io::AsRawFd;
use std::path::PathBuf;
// if f is your std::fs::File
// first construct the path to the symlink under /proc
let path_in_proc = PathBuf::from(format!("/proc/self/fd/{}", f.as_raw_fd()));
// ...and follow it back to the original file
let new_file_name = read_link(path_in_proc).unwrap();
I have a file and I am reading it through chokidar. I am also writing to the same file in some another function using fs.writeFileSync.
I want to emit the watcher.on('change') function only when the file is changed manually (i.e. not through WriteFileSync). Is there a way of determining that ?
No, there isn't, not to my knowledge.
You can only know the file was changed, not what process/... changed it.
As I review these file system flags, I'm I correct in concluding that there is no flag you can pass to fs.promises.writeFile that will automatically create all missing directories leading up to a filename? If not, which flag does this?
I don't like solutions that check for the existence of the folders first before attempting writeFile, because after the folders are created that check happens every time you write to a file in that folder.
In my program, after the folders are created once, it should always be there, so it seems more efficient to only create the folders if there is an exception. However, I'm hoping there is a flag that avoids all this micro-management.
If a flag for auto-creating the folders doesn't exist for writeFile, then I'd like to attempt writeFile first, and then (only if there is an exception) create the folders recursively.
fs.promises.writeFile() does not automatically create the directory structure for you. That must exist first.
If you want to automatically create the path because you received an error indicative of a path problem, you can use fs.promises.mkdir() and pass the recursive flag.
And you could, of course, create your own wrapper function that calls fs.promises.writeFile() and if it gets whatever error you get when the path doesn't exist (you'd have to test to see exactly what that error is), then call fs.promises.mkdir() and then repeat the fs.promises.writeFile(). It could all be wrapped in your own utility function.
I am using the latest version of pyRevit, v45.
I'm writing some info in temporary files with
myTempFile = script.get_instance_data_file("id")
This creates a file named pyRevit_2018_xxxx_id.tmp in which I store useful info. If I'm not mistaken, the "xxxx" part is changing every time I reload Revit. Now, I need to get access to this information from another pyRevit script.
How can I retrieve the name of the temp file I need to read? In other words, how do I access "myTempFile" from within the second script, which has no idea of the name of "myTempFile"?
I guess I can share somehow that variable between my script, but what's the proper way to do this? I know this must be a very basic programming question, but I'm indeed not a programmer ;)
Thanks a lot,
Arnaud.
Ok, I realise now that my variables in the 1st script cease to exist after its execution.
So for now I wrote the file name in another file, of which I know the name.. That works.
But if there's a cleaner way to do this, I'd be glad to learn ;)
Arnaud
pyrevit.script module provides 4 different methods for creating temporary files based on their use case:
get_instance_data_file:
for data files marked with Revit instance pid. This means that scripts running on another instance will not see this temp file.
http://pyrevit.readthedocs.io/en/latest/pyrevit/script.html#pyrevit.script.get_instance_data_file
get_universal_data_file:
for temp files accessible to all Revit instances and versions
http://pyrevit.readthedocs.io/en/latest/pyrevit/script.html#pyrevit.script.get_universal_data_file
get_data_file:
Base method to get a standard temp file for current revit version
http://pyrevit.readthedocs.io/en/latest/pyrevit/script.html#pyrevit.script.get_data_file
get_document_data_file:
temp file marked with active document (so scripts working on another document will not see this)
http://pyrevit.readthedocs.io/en/latest/pyrevit/script.html#pyrevit.script.get_document_data_file
Each method uses a pattern to create the temp file name. So as long as the call to the method is the same of different scripts, the method generates the same file name.
Example:
Script 1:
from pyrevit import script
tfile = script.get_data_file('mydata')
Script 2:
from pyrevit import script
tempfile = script.get_data_file('mydata')
In this example tempfile = tfile since the file id is the same.
There is documentation on each so make sure you take a look at those and pick the flavor that serves your purpose.
I have a rather peculiar file format to work with:
Every line begins with the checksum of its content, followed by a new-line-character.
It looks like this:
[CHECKSUM OF LINE_1][LINE_1]\n
[CHECKSUM OF LINE_2][LINE_2]\n
[CHECKSUM OF LINE_3][LINE_3]\n
...
My goal: To allow any application to work with these files like they would work with any other text file - unaware of the additional checksums at the beginning of each line.
Since I work on a linux machine with debian wheezy (kernel 3.18.26) I want to use the LD_PRELOAD-mechanism to override the relevant file functions.
I have seen something like this with zlibc on https://zlibc.linux.lu/index.html - with an explanation of how it works ( https://zlibc.linux.lu/zlibc.html#SEC8 ).
But I dont get it. They only replace the file-opening functions. No read. No write. no fseek. Nothing. So how does it work?
Or - which functions would I have to intercept to handle every read or write operation on this file and handle them accordingly?
I didn't exactly check how it works but the reason seems to be quite simple.
Possible implementation:
zlibc open:
uncompress file you wanted to open to some temporary file
open this temporary file instead of yours
zlibc close:
Compress temporary file
Override original file
In this case you don't need to override read/write/etc because you can use original ones.
In your case you have two possible solutions:
open, that make a copy of your file with striped checksums. close that calculates checksums and override original file
read and write that are able to skip/calculate checksums.
Ad 2.
From What is the difference between read() and fread()?:
fread() is part of the C library, and provides buffered reads. It is
usually implemented by calling read() in order to fill its buffer
In this case I believe that overriding open and close will be less error prone because you can safely reuse original read, write, fread, fseek etc.