handle return value of read() correctly

[ishiguroJSK]

I found out this node is using 100% of 1 thread, and then I found this section has the cause of that.

dynpick_driver/src/main.cpp

Lines 100 to 114 in 507cdab

	bool readCharFromSocket(const int& fdc, const int& length, char* reply){
	int len = 0;
	int c = 0;
	while ( len < length ) {
	c = read(fdc, reply+len, length-len);
	if (c >= 0) {
	len += c;
	} else {
	ROS_DEBUG("=== need to read more data ... n = %d (%d) ===", c, len);
	continue;
	}
	}
	// This could actually check if data was received and may return on timeout with false
	return true;
	}

Because this socket is opened as NONBLOCK, and read() returns "0" when there is no new received data (-1 for some error), this section is spinning so rapid unnecessarily like below (I inserted debug print).

[ INFO] [1566116808.497431092]: c=0, len=0, length=27
[ INFO] [1566116808.497445567]: c=0, len=0, length=27
[ INFO] [1566116808.497458347]: c=0, len=0, length=27
[ INFO] [1566116808.497474636]: c=27, len=27, length=27
[ INFO] [1566116808.497504326]: c=0, len=0, length=27
[ INFO] [1566116808.497519655]: c=0, len=0, length=27
[ INFO] [1566116808.497534684]: c=0, len=0, length=27
[ INFO] [1566116808.497550536]: c=0, len=0, length=27
[ INFO] [1566116808.497565303]: c=0, len=0, length=27
[ INFO] [1566116808.497578473]: c=0, len=0, length=27
[ INFO] [1566116808.497593122]: c=0, len=0, length=27
[ INFO] [1566116808.497605761]: c=0, len=0, length=27
[ INFO] [1566116808.497622420]: c=0, len=0, length=27
[ INFO] [1566116808.497635082]: c=0, len=0, length=27
[ INFO] [1566116808.497649813]: c=0, len=0, length=27
[ INFO] [1566116808.497664944]: c=0, len=0, length=27
[ INFO] [1566116808.497679948]: c=0, len=0, length=27
[ INFO] [1566116808.497694495]: c=0, len=0, length=27
[ INFO] [1566116808.497708893]: c=0, len=0, length=27
[ INFO] [1566116808.497723325]: c=0, len=0, length=27
[ INFO] [1566116808.497737710]: c=0, len=0, length=27
[ INFO] [1566116808.497751946]: c=0, len=0, length=27
[ INFO] [1566116808.497766212]: c=0, len=0, length=27
[ INFO] [1566116808.497780605]: c=0, len=0, length=27
[ INFO] [1566116808.497794989]: c=0, len=0, length=27
[ INFO] [1566116808.497809211]: c=0, len=0, length=27
[ INFO] [1566116808.497823536]: c=0, len=0, length=27
[ INFO] [1566116808.497838065]: c=0, len=0, length=27
[ INFO] [1566116808.497852595]: c=0, len=0, length=27
[ INFO] [1566116808.497865277]: c=0, len=0, length=27
[ INFO] [1566116808.497879729]: c=0, len=0, length=27
[ INFO] [1566116808.497894314]: c=0, len=0, length=27
[ INFO] [1566116808.497908938]: c=0, len=0, length=27
[ INFO] [1566116808.497923482]: c=0, len=0, length=27
[ INFO] [1566116808.497937884]: c=0, len=0, length=27
[ INFO] [1566116808.497952361]: c=0, len=0, length=27
[ INFO] [1566116808.497966956]: c=0, len=0, length=27
[ INFO] [1566116808.497981353]: c=0, len=0, length=27
[ INFO] [1566116808.497995635]: c=0, len=0, length=27
[ INFO] [1566116808.498009962]: c=0, len=0, length=27
[ INFO] [1566116808.498024313]: c=0, len=0, length=27
[ INFO] [1566116808.498038630]: c=0, len=0, length=27
[ INFO] [1566116808.498052902]: c=0, len=0, length=27
[ INFO] [1566116808.498067295]: c=0, len=0, length=27
[ INFO] [1566116808.498080885]: c=0, len=0, length=27
[ INFO] [1566116808.498095530]: c=0, len=0, length=27
[ INFO] [1566116808.498108360]: c=0, len=0, length=27
[ INFO] [1566116808.498122606]: c=0, len=0, length=27
[ INFO] [1566116808.498137254]: c=0, len=0, length=27
[ INFO] [1566116808.498151487]: c=0, len=0, length=27
[ INFO] [1566116808.498164403]: c=0, len=0, length=27
[ INFO] [1566116808.498178861]: c=0, len=0, length=27
[ INFO] [1566116808.498194524]: c=0, len=0, length=27
[ INFO] [1566116808.498210608]: c=0, len=0, length=27
[ INFO] [1566116808.498226475]: c=0, len=0, length=27
[ INFO] [1566116808.498239336]: c=0, len=0, length=27
[ INFO] [1566116808.498253562]: c=0, len=0, length=27
[ INFO] [1566116808.498271000]: c=0, len=0, length=27
[ INFO] [1566116808.498283778]: c=0, len=0, length=27
[ INFO] [1566116808.498298645]: c=0, len=0, length=27
[ INFO] [1566116808.498316287]: c=0, len=0, length=27
[ INFO] [1566116808.498331088]: c=0, len=0, length=27
[ INFO] [1566116808.498345495]: c=0, len=0, length=27
[ INFO] [1566116808.498359940]: c=0, len=0, length=27
[ INFO] [1566116808.498375037]: c=0, len=0, length=27
[ INFO] [1566116808.498389499]: c=0, len=0, length=27
[ INFO] [1566116808.498403975]: c=0, len=0, length=27
[ INFO] [1566116808.498418343]: c=0, len=0, length=27
[ INFO] [1566116808.498431096]: c=0, len=0, length=27
[ INFO] [1566116808.498445626]: c=0, len=0, length=27
[ INFO] [1566116808.498459254]: c=0, len=0, length=27
[ INFO] [1566116808.498475007]: c=27, len=27, length=27
[ INFO] [1566116808.498505095]: c=0, len=0, length=27

I think polling at 10kHz is enough to run dynpick at 1kHz.
With this PR, this node is running with 10~30% of single thread usage.
How about like this implement?

[k-okada]

@SoichiTaniguchi if you have real hardware device and free time, please check if this works.