Solving the problem of file corruption and inconsistent file sizes when downloading large files in nginx

A friend reported a problem today, saying that a 2MB file was uploaded to the webpage. When downloaded from the webpage, it was only 64KB and failed to open. After confirming that the BUG indeed existed and must appear, I embarked on the road of debugging and solving the BUG.

1. The system is nginx+php+mysql. Judging from experience, it has nothing to do with mysql, so you can ignore it.
2. After uploading the 2MB file from the PHP web page, open the file directly on the server. It can be viewed normally and is the same binary as the original file.
3. Use different browsers and different computers to repeatedly download the file from the PHP web page, and find that the downloaded files are only 64KB.
4. Change to a file with a size of only 90KB, and upload and download it from the PHP web page. There is no abnormality.
Through the above 4 points, it can be basically determined that the problem lies in nginx. At this time, open the nginx log file and find the following error log,
[crit] 21636#0: *843968 open() “/home/www/local/nginx/fastcgi_temp/0/11/0000000110” failed (13: Permission denied) while reading upstream,…..
It can be boldly guessed that because there is insufficient permission to operate the fastcgi_temp folder, normal files cannot be obtained. Therefore, after granting permissions to the folder, the problem is solved.

Looking back, what is the reason for this?

View the nginx configuration file and you can find the following paragraph:
fastcgi_connect_timeout 300;
fastcgi_send_timeout 300;
fastcgi_read_timeout 300;
**fastcgi_buffer_size 64k;
fastcgi_buffers 4 64k;**
fastcgi_busy_buffers_size 128k;
fastcgi_temp_file_write_size 128k;
Every time the download fails, the file size is always 64KB, which should be related to this. It turns out that nginx will use the buffer of the size specified by fastcgi_buffer_size to cache the content of the fastcgi stream. When the size exceeds this size, it will continue to apply for buffers with the number and size specified by fastcgi_buffers. If this size is still exceeded, the excess content will be written to a temporary file. That is to say, in this case, nginx will first use a 64K buffer to buffer the first part of the fastcgi stream. After that, it will apply for a maximum of 4*64K=256K buffer for buffering. If exceeded, a temporary file is written. Therefore, when downloading files larger than 256K, a temporary folder needs to be used for buffering, and there is no permission to operate here, which leads to this problem.

The above has introduced how to solve the problem of file corruption and inconsistent file sizes when downloading large files in nginx, including aspects of the problem. I hope it will be helpful to friends who are interested in PHP tutorials.