cookie jar updates should be atomic

[vapier]

I did this

# Create a dummy cookie jar larger than 4KiB.
$ printf '%s\tFALSE\t/\tTRUE\t2147483647\to\tfoo=bar\n' {0..200}.example.com > x

# Run curl reading & writing that cookie jar.
$ strace -e'!signal' curl https://google.com --cookie x --cookie-jar x
...
openat(AT_FDCWD, "x", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
write(4, "# Netscape HTTP Cookie File\n# ht"..., 4096) = 4096
write(4, "e.com\tFALSE\t/\tTRUE\t2147483647\to\t"..., 4096) = 4096
write(4, "LSE\t/\tTRUE\t2147483647\to\tfoo=bar\n"..., 1884) = 1884
close(4)                                = 0
...

I expected the following

other processes accessing the cookie jar should be able to rely on the file being coherent/complete. as it stands, the cookie jar is unreliable with more than one user at a time.

seems like it wouldn't be too difficult to create a tempfile using the name as a base, write to that, then do a single atomic rename.

curl/libcurl version

curl 7.66.0 (x86_64-pc-linux-gnu) libcurl/7.66.0 OpenSSL/1.1.1d zlib/1.2.11 nghttp2/1.39.2

operating system

Linux vapier 5.3.1 #1 SMP PREEMPT Fri Sep 27 00:26:44 EDT 2019 x86_64 AMD FX(tm)-4350 Quad-Core Processor AuthenticAMD GNU/Linux

[bagder]

It should indeed be possible to create the file with a temp name and then rename it. I would propose we do it by appending a suffix to the actual name.

[jay]

We could use what Mozilla Firefox uses which iirc? is .partfile or .part