(tail_lines): Fix a potential (but very hard to exercise)

race condition bug.  The bug would be triggered when tailing a file
with file pointer not at beginning of file, and where the file was
truncated to have a length of less than the initial offset at just
the right moment (between the two lseek calls in this function).

diff --git a/src/tail.c b/src/tail.c
index 7d83787..f4299c9 100644 (file)
--- a/src/tail.c
+++ b/src/tail.c
@@ -1184,7 +1184,7 @@ tail_lines (const char *pretty_filename, int fd, uintmax_t n_lines,
     }
   else
     {
-      off_t start_pos;
+      off_t start_pos = -1;
       off_t end_pos;
 
       /* Use file_lines only if FD refers to a regular file for
@@ -1201,6 +1201,13 @@ tail_lines (const char *pretty_filename, int fd, uintmax_t n_lines,
        }
       else
        {
+         /* Under very unlikely circumstances, it is possible to reach
+            this point after positioning the file pointer to end of file
+            via the `lseek (...SEEK_END)' above.  In that case, reposition
+            the file pointer back to start_pos before calling pipe_lines.  */
+         if (start_pos != -1)
+           xlseek (fd, start_pos, SEEK_SET, pretty_filename);
+
          return pipe_lines (pretty_filename, fd, n_lines, read_pos);
        }
     }