Read serial line in chunks instead of character by character

This significantly reduces CPU load caused by this driver.

Without this change, CPU load on a powerful x86 machine with a
USB-connected GPS is 36%. The output of the strace command looks
like this:

    ...
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=999999000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=999997160})
    read(33, "$", 1)                        = 1
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=999999000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=999997160})
    read(33, "G", 1)                        = 1
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=999999000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=999997160})
    read(33, "P", 1)                        = 1
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=999999000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=999997180})
    read(33, "R", 1)                        = 1
    ...

It can be seen that characters are read from the serial line one by
one. This is due to how IOBase.readline() interacts with pySerial.

In this commit, we implement the same functionality as
IOBase.readline(), but in a way allowing to read the serial line
characters in bigger chunks. With it, CPU load decreases to 7.5% and
strace reports this:

    ...
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=926286000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=906510472})
    read(33, "$GPRMC,,V,,,,,,,,,,N*53\r\n$GPGGA,"..., 422) = 143
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=906287000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=886945652})
    read(33, "$GPRMC,,V,,,,,,,,,,N*53\r\n$GPGGA,"..., 279) = 143
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=886721000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=866882570})
    read(33, "$GPRMC,,V,,,,,,,,,,N*53\r\n$GPGGA,"..., 136) = 136
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=999996000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=999991549})
    read(33, ",,*57\r\n", 1024)             = 7
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=999827000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=985949688})
    read(33, "$GPRMC,,V,,,,,,,,,,N*53\r\n$GPGGA,"..., 1017) = 143
    pselect6(35, [33 34], [], [], {tv_sec=1, tv_nsec=985746000}, NULL) = 1 (in [33], left {tv_sec=1, tv_nsec=966235438})
    read(33, "$GPRMC,,V,,,,,,,,,,N*53\r\n$GPGGA,"..., 874) = 143
    ...

Reading from the kernel happens most of the time in chunks of 143
bytes (for our GPS) and therefore, the system call overhead is 143×
lower.

Tested on Septentrio mosaic-H GPS.

src/libnmea_navsat_driver/nodes/nmea_serial_driver.py

@@ -31,6 +31,7 @@
 # POSSIBILITY OF SUCH DAMAGE.
 
 import serial
+import io
 
 import rclpy
 
@@ -50,17 +51,22 @@ def main(args=None):
         GPS = serial.Serial(port=serial_port, baudrate=serial_baud, timeout=2)
         driver.get_logger().info("Successfully connected to {0} at {1}.".format(serial_port, serial_baud))
         try:
+            data = bytearray()
             while rclpy.ok():
-                data = GPS.readline().strip()
-                try:
-                    if isinstance(data, bytes):
-                        data = data.decode("utf-8")
-                    driver.add_sentence(data, frame_id)
-                except ValueError as e:
-                    driver.get_logger().warn(
-                        "Value error, likely due to missing fields in the NMEA message. Error was: %s. "
-                        "Please report this issue at github.com/ros-drivers/nmea_navsat_driver, including a bag file "
-                        "with the NMEA sentences that caused it." % e)
+                data.extend(GPS.read(1024)) # read at most 1024 bytes
+                lines = data.splitlines(keepends=True)
+                # process complete lines
+                for line in lines[0:-1]:
+                    line = line.decode("utf-8").rstrip()
+                    try:
+                        driver.add_sentence(line, frame_id)
+                    except ValueError as e:
+                        driver.get_logger().warn(
+                            "Value error, likely due to missing fields in the NMEA message. Error was: %s. "
+                            "Please report this issue at github.com/ros-drivers/nmea_navsat_driver, including a bag file "
+                            "with the NMEA sentences that caused it." % e)
+                # continue with the last, incomplete line
+                data = lines[-1]
 
         except Exception as e:
             driver.get_logger().error("Ros error: {0}".format(e))