refactor(anchor): buffer bytes instead of string

Resolves Riley's comments, makes it so UTF-8 characters over 1 byte aren't destroyed

src/anchor_pkg/anchor_pkg/connector.py

@@ -1,4 +1,6 @@
 from abc import ABC, abstractmethod
+from time import monotonic
+from typing import TYPE_CHECKING
 from astra_msgs.msg import VicCAN
 from std_msgs.msg import String
 from rclpy.clock import Clock
@@ -20,8 +22,11 @@ KNOWN_USBS = [
     (0x10C4, 0xEA60),  # DOIT ESP32 Devkit V1
     (0x1A86, 0x55D3),  # ESP32 S3 Development Board
 ]
+
 BAUD_RATE = 115200
 
+SERIAL_READ_TIMEOUT = 0.5  # seconds
+
 MCU_IDS = [
     "broadcast",
     "core",
@@ -96,6 +101,10 @@ class SerialConnector(Connector):
         ports = self._find_ports()
         mcu_name: str | None = None
 
+        # Serial buffering
+        self._serial_buffer: bytes = b""
+        self._last_read_time = monotonic()
+
         if serial_override:
             logger.warn(
                 f"using serial_override: `{serial_override}`! this will bypass several checks."
@@ -182,9 +191,66 @@ class SerialConnector(Connector):
 
         return None
 
+    def _try_readline(self) -> str | None:
+        """Attempts to read a full string from the MCU without blocking.
+
+        When pyserial is used with 'timeout=0', reads are performed non-blocking.
+        When used with interface.readline(), this breaks the assumption that a returned
+        string will be a completed attempt by the MCU to send a string; it may be
+        cut off between the start of the string and the newline, removing information
+        and rendering the string(s) useless; thus, to get around the downside of readline()
+        not waiting for a newline while still not blocking, this function manually
+        implements a serial input buffer and newline timeout.
+
+        If readline() returns a non-empty string, send it if it ends with a newline
+        (readline() will not read past any newline); otherwise, save the read string.
+        This buffered string should be pre-pended to the next readline() result.
+
+        If readline() does not receive a non-empty string after the last non-newline-
+        terminated readline() result within the manual timeout, send the contents of the
+        buffer as if it ended with a newline, and clear the buffer.
+
+        Returns:
+            str: A hopefully-complete string read from the MCU via the serial interface.
+        """
+        if TYPE_CHECKING:
+            assert type(self.serial_interface) == serial.Serial
+
+        # Warn on buffer timeout, as the only scenarios that would trigger this are
+        # a microcontroller output that isn't newline-terminated (bad), or the MCU is
+        # hanging (also bad).
+        if (
+            self._serial_buffer
+            and (monotonic() - self._last_read_time) > SERIAL_READ_TIMEOUT
+        ):
+            self.logger.warn(
+                f"Serial buffer timeout, last received '{self._serial_buffer}'."
+            )
+            result = self._serial_buffer
+            self._serial_buffer = b""
+            self._last_read_time = monotonic()
+            return str(result, "utf8").strip()
+
+        # No try-except here so caller catches it instead.
+        raw = self.serial_interface.readline()
+
+        # Empty or whitespace-only string
+        if not raw or not raw.strip():
+            return None
+
+        # Add to buffer or send finished buffer
+        if not (raw.endswith(b"\n") or raw.endswith(b"\r")):  # unfinished string
+            self._serial_buffer += raw
+            self._last_read_time = monotonic()
+            return None
+        else:
+            result = self._serial_buffer + raw
+            self._serial_buffer = b""
+            return str(result, "utf8").strip()
+
     def read(self) -> tuple[VicCAN | None, str | None]:
         try:
-            raw = str(self.serial_interface.readline(), "utf8")
+            raw = self._try_readline()
 
             if not raw:
                 return (None, None)