Incorrect diskstat value wrap up calculations for 64-bit platforms in disk plugin

[eero-t]

• Version of collectd: v6 HEAD, v5 HEAD

Steps to reproduce

grep UINT_MAX src/disk.c

Expected output

Correct max value used in wrap up calculations.

Actual output

Both v5 (main) and v6 (collectd-6.0) HEAD give:

src/disk.c:        diff_read_sectors = 1 + read_sectors + (UINT_MAX - ds->read_sectors);
src/disk.c:        diff_write_sectors = 1 + write_sectors + (UINT_MAX - ds->write_sectors);
src/disk.c:        diff_read_ops = 1 + read_ops + (UINT_MAX - ds->read_ops);
src/disk.c:        diff_write_ops = 1 + write_ops + (UINT_MAX - ds->write_ops);
src/disk.c:        diff_read_time = 1 + read_time + (UINT_MAX - ds->read_time);
src/disk.c:        diff_write_time = 1 + write_time + (UINT_MAX - ds->write_time);
src/disk.c:        diff_io_time = 1 + io_time + (UINT_MAX - ds->io_time);

Bug description

Above variables and struct members are 64-bit (signed) derive_t values, whereas UINT_MAX is 32-bit (unsigned) value.

According to kernel documentation, values read from /proc/diskstats are unsigned long (native word size) numbers, i.e. UINT_MAX is correct only for 32-bit platforms: https://www.kernel.org/doc/Documentation/iostats.txt

(Whereas most machines today are running 64-bit Linux.)

---

Noticed while reviewing #4217.

[eero-t]

#4217 fixed the issue for v6, by using collectd counter_diff() helper. v5 still needs a fix.