Merge pull request #2047 from rabbitmq/rabbitmq_cli_log_commands

Helpers for CLI logs command

(cherry picked from commit df5952a)

Author: michaelklishin

src/rabbit_log_tail.erl

@@ -0,0 +1,95 @@
+-module(rabbit_log_tail).
+
+-export([tail_n_lines/2]).
+-export([init_tail_stream/4]).
+
+-define(GUESS_OFFSET, 200).
+
+init_tail_stream(Filename, Pid, Ref, Duration) ->
+    RPCProc = self(),
+    Reader = spawn(fun() ->
+        link(Pid),
+        case file:open(Filename, [read, binary]) of
+            {ok, File} ->
+                TimeLimit = case Duration of
+                    infinity -> infinity;
+                    _        -> erlang:system_time(second) + Duration
+                end,
+                {ok, _} = file:position(File, eof),
+                RPCProc ! {Ref, opened},
+                read_loop(File, Pid, Ref, TimeLimit);
+            {error, _} = Err ->
+                RPCProc ! {Ref, Err}
+        end
+    end),
+    receive
+        {Ref, opened} -> {ok, Ref};
+        {Ref, {error, Err}} -> {error, Err}
+    after 5000 ->
+        exit(Reader, timeout),
+        {error, timeout}
+    end.
+
+read_loop(File, Pid, Ref, TimeLimit) ->
+    case is_integer(TimeLimit) andalso erlang:system_time(second) > TimeLimit of
+        true  -> Pid ! {Ref, <<>>, finished};
+        false ->
+            case file:read(File, ?GUESS_OFFSET) of
+                {ok, Data} ->
+                    Pid ! {Ref, Data, confinue},
+                    read_loop(File, Pid, Ref, TimeLimit);
+                eof ->
+                    timer:sleep(1000),
+                    read_loop(File, Pid, Ref, TimeLimit);
+                {error, _} = Err ->
+                    Pid ! {Ref, Err, finished}
+            end
+    end.
+
+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 -> 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 -> 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} ->
+            Lines = binary:split(Data, <<"\n">>, [global, trim]),
+            case {GuessPosition, Lines} of
+                %% If position is 0 - there are no partial lines
+                {0, _}          -> {ok, Lines};
+                %% Remove first line as it can be partial
+                {_, [_ | Rest]} -> {ok, Rest};
+                {_, []}         -> {ok, []}
+            end;
+        {error, _} = Error -> Error
+    end.
+
+offset(Base, N) ->
+    max(0, Base - N * ?GUESS_OFFSET).