Functions to support getting last N log entries from the remote node via CLI.

Reads last N lines, separated by \n from the file.
This should be portable enough to windows.

Author: hairyhum

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).