Abstract WatchLog line buffering to a separate function

Move the line buffering functionality into _readline() to improve the
readability of code. This also allows reading the file contents from
other functions, since the line buffer is now an attribute of the class.

(cherry picked from commit 67896ddde2)

bin/tests/system/isctest/log/watchlog.py

@@ -100,6 +100,36 @@ class WatchLog(abc.ABC):
         self._fd = None  # type: Optional[TextIO]
         self._path = path
         self._wait_function_called = False
+        self._linebuf = ""
+
+    def _readline(self) -> Optional[str]:
+        """
+        Wrapper around io.readline() function to handle unfinished lines.
+
+        If a line ends with newline character, it's returned immediately.
+        If a line doesn't end with a newline character, the read contents are
+        buffered until the next call of this function and None is returned
+        instead.
+        """
+        if not self._fd:
+            raise WatchLogException("file to watch isn't open")
+        read = self._fd.readline()
+        if not read.endswith("\n"):
+            self._linebuf += read
+            return None
+        read = self._linebuf + read
+        self._linebuf = ""
+        return read
+
+    def _readlines(self) -> Iterator[str]:
+        """
+        Wrapper around io.readline() which only returns finished lines.
+        """
+        while True:
+            line = self._readline()
+            if line is None:
+                return
+            yield line
 
     def wait_for_line(self, string: str, timeout: int = 10) -> None:
         """
@@ -231,24 +261,15 @@ class WatchLog(abc.ABC):
         if self._wait_function_called:
             raise WatchLogException("wait_for_*() was already called")
         self._wait_function_called = True
-        if not self._fd:
-            raise WatchLogException("No file to watch")
-        leftover = ""
         assert timeout, "Do not use this class unless you want to WAIT for something."
         deadline = time.monotonic() + timeout
         while time.monotonic() < deadline:
-            for line in self._fd.readlines():
-                if line[-1] != "\n":
-                    # Line is not completely written yet, buffer and keep on waiting
-                    leftover += line
-                else:
-                    line = leftover + line
-                    leftover = ""
-                    for string, retval in patterns.items():
-                        if isinstance(string, Pattern) and string.search(line):
-                            return retval
-                        if isinstance(string, str) and string in line:
-                            return retval
+            for line in self._readlines():
+                for string, retval in patterns.items():
+                    if isinstance(string, Pattern) and string.search(line):
+                        return retval
+                    if isinstance(string, str) and string in line:
+                        return retval
             time.sleep(0.1)
         raise TimeoutError(
             "Timeout reached watching {} for {}".format(