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| -rw-r--r-- | src/rabbit_log_tail.erl | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/src/rabbit_log_tail.erl b/src/rabbit_log_tail.erl new file mode 100644 index 0000000000..d5764c90b9 --- /dev/null +++ b/src/rabbit_log_tail.erl @@ -0,0 +1,53 @@ +-module(rabbit_log_tail). + +-export([tail_n_lines/2]). +% -export([init_tail_stream/2, tail_send/2, tail_receive/2]). + +-define(GUESS_OFFSET, 200). + +tail_n_lines(Filename, N) -> + case file:open(Filename, [read, binary]) of + {ok, File} -> + {ok, Eof} = file:position(File, eof), + %% Eof may move. Only read up to the current one. + Result = reverse_read_n_lines(N, N, File, Eof, Eof), + file:close(File), + Result; + Error -> Error + end. + +reverse_read_n_lines(N, OffsetN, File, Position, Eof) -> + GuessPosition = offset(Position, OffsetN), + case read_lines_from_position(File, GuessPosition, Eof) of + {ok, Lines} -> + NLines = length(Lines), + case {NLines >= N, GuessPosition == 0} of + %% Take only N lines if there is more + {true, _} -> lists:nthtail(NLines - N, Lines); + %% Safe to assume that NLines is less then N + {_, true} -> Lines; + %% Adjust position + _ -> + reverse_read_n_lines(N, N - NLines + 1, File, GuessPosition, Eof) + end; + Error -> Error + end. + +read_from_position(File, GuessPosition, Eof) -> + file:pread(File, GuessPosition, max(0, Eof - GuessPosition)). + +read_lines_from_position(File, GuessPosition, Eof) -> + case read_from_position(File, GuessPosition, Eof) of + {ok, Data} -> {ok, crop_lines(Data)}; + Error -> Error + end. + +crop_lines(Data) -> + %% Drop the first line, because it's most likely partial. + case binary:split(Data, <<"\n">>, [global, trim]) of + [_|Rest] -> Rest; + [] -> [] + end. + +offset(Base, N) -> + max(0, Base - N * ?GUESS_OFFSET).
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