lib: handle sysfs extraction failures by enabling failbit exception

[sidtronics]

ensure read failure is detected if file is empty or contains invalid data. Closes #179.

[paulmenzel]

Awesome, that you came up with a fix. Maybe elaborate in the commit message and give your battery reporting issue as an example? Also give the old and new power usage value?

[sidtronics]

Awesome, that you came up with a fix. Maybe elaborate in the commit message and give your battery reporting issue as an example? Also give the old and new power usage value?

Thanks! I’ve updated the commit message. Here are before and after screenshots:

Before:

After:

The discharge rate is now omitted because read_sysfs() fails cleanly and returns 0 when the sysfs read is invalid, instead of parsing garbage values. Since 0 < 0.0001, it's filtered out in the UI:

powertop/src/process/do_process.cpp

Lines 829 to 836 in 49045c0

	if (pw > 0.0001) {
	char buf[32];
	wprintw(win, _("The battery reports a discharge rate of %sW\n"),
	fmt_prefix(pw, buf));
	wprintw(win, _("The energy consumed was %sJ\n"),
	fmt_prefix(joules, buf));
	need_linebreak = 1;
	}

I think that's preferable to showing incorrect values.

[paulmenzel]

Awesome, that you came up with a fix. Maybe elaborate in the commit message and give your battery reporting issue as an example? Also give the old and new power usage value?

Thanks! I’ve updated the commit message. Here are before and after screenshots:

[…]

Awesome. Thank you for the great commit message, and your thorough answer. Much appreciated!

[dyfrgi]

Would it make sense to enable exceptions for the other two failure modes, eof and bad? eof would occur if the file exists but is empty, and bad if there's something like a failure in allocation or other exception caught by the istream operations.

Otherwise, LGTM.