The upstream patch for their bug #41486 is broken: it supposes that it
can break an input line arbitrarily, but breaking just after a backslash
is a bad idea, and even more importantly batch_readline() supposes that
it can overwrite the line's ending newline with \0.  If there isn't any
ending newline, that clobbers the first character of the rest of the line.


diff -Naur mysql-5.0.77.bad/client/my_readline.h mysql-5.0.77/client/my_readline.h
--- mysql-5.0.77.bad/client/my_readline.h	2011-03-15 15:53:04.735799369 -0400
+++ mysql-5.0.77/client/my_readline.h	2011-03-15 15:53:41.855795861 -0400
@@ -22,7 +22,8 @@
   char *end;			/* Pointer at buffer end */
   char *start_of_line,*end_of_line;
   uint bufread;			/* Number of bytes to get with each read(). */
-  uint eof;
+  char eof;			/* true after file EOF is reached */
+  char putback;			/* character to restore after line end */
   ulong max_size;
   ulong read_length;		/* Length of last read string */
 } LINE_BUFFER;
diff -Naur mysql-5.0.77.bad/client/readline.cc mysql-5.0.77/client/readline.cc
--- mysql-5.0.77.bad/client/readline.cc	2011-03-15 15:53:04.738894977 -0400
+++ mysql-5.0.77/client/readline.cc	2011-03-15 16:18:58.760831723 -0400
@@ -48,12 +48,17 @@
   ulong out_length;
   DBUG_ASSERT(truncated != NULL);
 
+  /* restore character overwritten in previous call */
+  if (line_buff->putback)
+    line_buff->start_of_line[line_buff->read_length] = line_buff->putback;
+
   if (!(pos=intern_read_line(line_buff,&out_length, truncated)))
     return 0;
   if (out_length && pos[out_length-1] == '\n')
     if (--out_length && pos[out_length-1] == '\r')  /* Remove '\n' */
       out_length--;                                 /* Remove '\r' */
   line_buff->read_length=out_length;
+  line_buff->putback = pos[out_length];
   pos[out_length]=0;
   return pos;
 }
@@ -225,7 +230,15 @@
       }
       else
         continue;
-      pos--;					/* break line here */
+      /*
+        Back up so that EOB null isn't included in the line; then back up
+        some more, if necessary, so that we don't break the incomplete line
+        immediately after a backslash.  That prevents breaking the command
+        detection logic in mysql.cc.
+       */
+      do {
+        pos--;
+      } while (*pos == '\\' && pos > buffer->end_of_line);
       *truncated= 1;
     }
     else