Github user davisp commented on a diff in the pull request:
https://github.com/apache/couchdb-couch/pull/185#discussion_r80063112
--- Diff: src/couch_file.erl ---
@@ -531,27 +537,96 @@ find_header(Fd, Block) ->
{ok, Bin} ->
{ok, Bin};
_Error ->
- find_header(Fd, Block -1)
+ find_header_pread2(Fd, Block -1)
end.
load_header(Fd, Block) ->
{ok, <<1, HeaderLen:32/integer, RestBlock/binary>>} =
file:pread(Fd, Block * ?SIZE_BLOCK, ?SIZE_BLOCK),
- TotalBytes = calculate_total_read_len(5, HeaderLen),
- case TotalBytes > byte_size(RestBlock) of
+ TotalSize = total_read_size(5, HeaderLen),
+ case TotalSize > byte_size(RestBlock) of
false ->
- <<RawBin:TotalBytes/binary, _/binary>> = RestBlock;
+ <<RawBin:TotalSize/binary, _/binary>> = RestBlock;
true ->
{ok, Missing} = file:pread(
Fd, (Block * ?SIZE_BLOCK) + 5 + byte_size(RestBlock),
- TotalBytes - byte_size(RestBlock)),
+ TotalSize - byte_size(RestBlock)),
RawBin = <<RestBlock/binary, Missing/binary>>
end,
<<Md5Sig:16/binary, HeaderBin/binary>> =
iolist_to_binary(remove_block_prefixes(5, RawBin)),
Md5Sig = couch_crypto:hash(md5, HeaderBin),
{ok, HeaderBin}.
+
+%% Read a configurable number of block locations at a time using
+%% file:pread/2. At each block location, read ?PREFIX_SIZE (5) bytes;
+%% if the first byte is <<1>>, assume it's a header block, and the
+%% next 4 bytes hold the size of the header.
+-spec find_header_pread2(file:fd(), block_id()) ->
+ {ok, binary()} | no_valid_header.
+find_header_pread2(Fd, Block) ->
+ find_header_pread2(Fd, Block, read_count()).
+
+-spec find_header_pread2(file:fd(), block_id(), non_neg_integer()) ->
+ {ok, binary()} | no_valid_header.
+find_header_pread2(_Fd, Block, _ReadCount) when Block < 0 ->
+ no_valid_header;
+find_header_pread2(Fd, Block, ReadCount) ->
+ Locations = block_locations(Block, ReadCount),
+ {ok, DataL} = file:pread(Fd, [{L, ?PREFIX_SIZE} || L <- Locations]),
+ case locate_header(Fd, header_locations(Locations, DataL)) of
--- End diff --
Or `find_newest_valid_header/2` of course.
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