Github user davisp commented on a diff in the pull request:
https://github.com/apache/couchdb-couch/pull/185#discussion_r75904196
--- Diff: src/couch_file.erl ---
@@ -524,6 +528,214 @@ handle_info({'EXIT', _, Reason}, Fd) ->
{stop, Reason, Fd}.
+find_last_header(Fd, FileSize) ->
+ find_header_in_chunks(Fd, start_pos(FileSize), 1).
+
+start_pos(FileSize) when FileSize rem ?SIZE_BLOCK =:= 0 ->
+ FileSize;
+start_pos(FileSize) ->
+ %% set position to nearest block boundary past eof
+ %% if file size isn't multiple of block size
+ (FileSize div ?SIZE_BLOCK) * ?SIZE_BLOCK + ?SIZE_BLOCK.
+
+chunk_max_size() ->
+ config:get_integer("couchdb", "chunk_max_size",
?DEFAULT_CHUNK_MAX_SIZE).
+
+%% chunk size exponentially increases by iteration, up to some maximum,
+%% and should never exceed the current position in the file
+chunk_size(Pos, Multiplier) ->
+ Size = min(Multiplier * ?SIZE_BLOCK, chunk_max_size()),
+ min(Pos, Size).
+
+multiplier(Size, Multiplier) ->
+ case Size < chunk_max_size() of
+ true ->
+ Multiplier * config:get_integer(
+ "couchdb", "chunk_exponent_base",
?DEFAULT_CHUNK_EXPONENT_BASE);
+ false ->
+ Multiplier
+ end.
+
+find_header_in_chunks(_Fd, Pos, _Multiplier) when Pos < 0 ->
+ no_valid_header;
+find_header_in_chunks(Fd, Pos, Multiplier) ->
+ Size = chunk_size(Pos, Multiplier),
+ case Size > 0 of
+ false ->
+ no_valid_header;
+ true ->
+ {ok, Chunk} = file:pread(Fd, Pos - Size, Size),
+ case find_header_in_chunk(Fd, Pos, Chunk, Size - ?SIZE_BLOCK)
of
+ {ok, HeaderBin, _Offset} ->
+ %% io:format("found header at ~p multiplier ~p
chunksize ~p~n",
+ %% [Pos - Size + _Offset, Multiplier, Size]),
+ {ok, HeaderBin};
+ not_found ->
+ %% tell kernel we don't need that chunk any more
+ %% to minimize disk cache pollution
+ ok = file:advise(Fd, Pos - Size, Size, dont_need),
+ NewMultiplier = multiplier(Size, Multiplier),
+ find_header_in_chunks(Fd, Pos - Size, NewMultiplier)
+ end
+ end.
+
+-define(DEFAULT_SPAN_MAX_SIZE, ?SIZE_BLOCK * ?SIZE_BLOCK).
+-define(DEFAULT_SPAN_EXPONENT_BASE, 2).
+-define(PREFIX_SIZE, 5).
+
+system_milli_seconds() ->
+ {Mega, Sec, Micro} = os:timestamp(),
+ (Mega*1000000 + Sec)*1000 + round(Micro/1000).
+
+last_block_pos(FileSize) when FileSize rem ?SIZE_BLOCK =:= 0 ->
+ FileSize - ?SIZE_BLOCK;
+last_block_pos(FileSize) ->
+ (FileSize div ?SIZE_BLOCK) * ?SIZE_BLOCK.
+
+span_max_size() ->
+ config:get_integer("couchdb", "span_max_size", ?DEFAULT_SPAN_MAX_SIZE).
--- End diff --
Its generally easier to read code if you organize it so that major sections
for logic are grouped. Interspersing helpers like this between functions with
actual logic gets a bit distracting. Specific for these trivial functions where
the entire purpose is expressed succinctly in the function name.
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