Github user kxepal commented on a diff in the pull request:
https://github.com/apache/couchdb-couch/pull/185#discussion_r71322068
--- Diff: src/couch_file.erl ---
@@ -524,6 +525,99 @@ handle_info({'EXIT', _, Reason}, Fd) ->
{stop, Reason, Fd}.
+%% first try to read header at end, falling back to chunked reads on
failure
+find_last_header(Fd, FileSize) ->
+ ok = file:advise(Fd, 0, FileSize, dont_need), % minimize disk cache
pollution
+ Pos = (FileSize div ?SIZE_BLOCK) * ?SIZE_BLOCK,
+ case file:pread(Fd, Pos, FileSize - Pos) of
+ eof ->
+ no_valid_header;
+ {ok, Data} ->
+ try match_header(Fd, Pos, Data, 0) of
+ {ok, HeaderBin} ->
+ {ok, HeaderBin}
+ catch
+ error:{badmatch, _} ->
+ find_header_in_chunks(Fd, Pos, 1)
+ end
+ end.
+
+-define(DEFAULT_CHUNK_MAX_SIZE, ?SIZE_BLOCK).
+-define(DEFAULT_CHUNK_EXPONENT_BASE, 1).
+
+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) of
+ {ok, HeaderBin, _Offset} ->
+ %% io:format("found header at ~p multiplier ~p
chunksize ~p~n",
+ %% [Pos - Size + _Offset, Multiplier, Size]),
+ {ok, HeaderBin};
+ not_found ->
+ NewMultiplier = multiplier(Size, Multiplier),
+ find_header_in_chunks(Fd, Pos - Size, NewMultiplier)
+ end
+ end.
+
+find_header_in_chunk(_Fd, _Pos, _Chunk, Offset) when Offset < 0 ->
+ not_found;
+find_header_in_chunk(Fd, Pos, Chunk, Offset) ->
+ try match_header(Fd, Pos, Chunk, Offset) of
+ {ok, HeaderBin} ->
+ {ok, HeaderBin, Offset}
+ catch
+ error:{badmatch, _} ->
+ find_header_in_chunk(Fd, Pos, Chunk, Offset - ?SIZE_BLOCK)
+ end.
+
+read_size(Data, Offset) ->
+ min(size(Data), Offset + ?SIZE_BLOCK) - Offset.
+
+match_header(Fd, Pos, Data, Offset) ->
+ <<1, HeaderSize:32/integer-unsigned, RestBlock/binary>> =
+ binary:part(Data, Offset, read_size(Data, Offset)),
+ PrefixSize = 5, % 1 (version) + 4 (size) bytes
+ TotalSize = total_read_size(PrefixSize, HeaderSize),
+ HeaderPos = Pos - byte_size(Data) + Offset,
+ RawBin = raw_bin(Fd, HeaderPos, TotalSize, RestBlock),
+ <<Hash:16/binary, HeaderBin/binary>> =
+ iolist_to_binary(remove_block_prefixes(PrefixSize, RawBin)),
+ Hash = crypto:hash(md5, HeaderBin),
--- End diff --
Sounds like not a big deal?
```
match_header_size(Data, Offset) ->
case binary:part(Data, Offset, read_size(Data, Offset)) of
<<1, HeaderSize:32/integer-unsigned, RestBlock/binary>> ->
{HeaderSize, RestBlock};
_ ->
no_match
end.
match_header(Fd, Pos, Data, Offset) ->
case match_header_size(Data, Offset) ->
no_match -> no_match;
{HeaderSize, RestBlock} ->
PrefixSize = 5, % 1 (version) + 4 (size) bytes
TotalSize = total_read_size(PrefixSize, HeaderSize),
HeaderPos = Pos - byte_size(Data) + Offset,
RawBin = raw_bin(Fd, HeaderPos, TotalSize, RestBlock),
<<Hash:16/binary, HeaderBin/binary>> =
iolist_to_binary(remove_block_prefixes(PrefixSize, RawBin)),
case crypto:hash(md5, HeaderBin) ->
Hash -> {ok, HeaderBin};
_ -> no_match
end
end.
```
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