读取时文件被截断

As suggested by @UlrichEckhardt comment, it was due to read / write concurrency problem. I was trying to read a file that was being writen. I solved this by just waiting before trying to read the file again

Without example code it is impossible to give a 'fix my code' answer, but when doing file write/read sort of programming, you should follow a simple process (which, from the description, is missing one fairly critical step!)

First, write to a TEMP file (you are writing to a file, but it is important here to write to a TEMP file - otherwise, you could have race conditions....... ;);

an easy way to do that in php

$yourData = "whateverYourDataIs....";
$goodfilename = 'whateverYourGoodFileNameIsForYourData.json';
$tempfilename = 'tempfile' . time(); // MANY ways to do this (lots of SO posts on it - just get a unique name every time you write ('unique' may not be needed if you only occasionally do a write, but it is a good safety measure to avoid collisions and time() works for many programs.)
// Now, use $tempfilename in your fwrite.
$fwrite = fwrite($tempfilename,$yourData);
if ($fwrite === false) { 
    // the write failed, so do whatever 'error' function you may need
    // since it failed, there should be no file, but not a bad idea to attempt to delete it
    unlink ($tempfile);
}
else {
    // the write succeeded, so let's do a 'sanity check' on the file to make sure it is good JSON (this is a 'paranoid' check, but "better safe than sorry", right?)
    if(json_decode($tempfile)){
        // we know the file is good JSON, so now RENAME (this is really fast, so collisions are almost impossible)  NOTE: see http://php.net/manual/en/function.rename.php comments for some potential challenges and workarounds if you have trouble with rename.
        rename($tempfilename,$goodfilename);
    }
    // Now, the GOOD file will contain your new data - and those read issues are gone! (though never say 'never' - it may be possible, but very unlikely!)  
}

This may/not be your issue directly and you will have to suit this to fit your code, but as a safety factor - and a good way to avoid collisions, it should give you ~100% read success, which I believe is what you are after!)

If this doesn't help, then some direct code will be needed to provide a more complete answer.