gdb/dmesg: print dmesg in the correct order

Now the dmesg output log in correct time order, from oldest to latest.
The NULL strings are also stripped, if the buffer is never get fully
filled.

Signed-off-by: xuxingliang <xuxingliang@xiaomi.com>

tools/gdb/nuttxgdb/dmesg.py

@@ -38,11 +38,17 @@ class Dmesg(gdb.Command):
             gdb.write("RAM log not available.\n")
             return None
 
-        rl_head = sysdev["rl_header"]
+        rl_header = sysdev["rl_header"]
         rl_bufsize = sysdev["rl_bufsize"]
 
+        offset = rl_header["rl_head"] % rl_bufsize  # Currently writing to this offset
+        tail = rl_bufsize - offset  # Total size till buffer end.
+        rl_buffer = int(rl_header["rl_buffer"].address)  # rl_buffer is a char array
+
         inf = gdb.selected_inferior()
-        buf = bytes(inf.read_memory(rl_head["rl_buffer"], rl_bufsize))
+        buf = bytes(inf.read_memory(offset + rl_buffer, tail))
+        buf = buf.lstrip(b"\0")  # Remove leading NULLs
+        buf += bytes(inf.read_memory(rl_buffer, offset))
         buf = buf.replace(
             b"\0", "␀".encode("utf-8")
         )  # NULL is valid utf-8 but not printable