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Date: Tue, 1 Dec 2009 23:34:49 +0100
Message-ID: <cadf1c690912011434m39c7b7b3jcda4cbe3c853676@mail.gmail.com>
Subject: using Erlang's debugger with couchdb
From: Filipe David Manana <fdmanana@gmail.com>
To: dev@couchdb.apache.org
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Hello,

I am having a strange situation. With Erlang's debugger , I set a
breakpoint in couch_httpd_db.erl, second clause of the function
db_attachment_req (around line 1012), and when uploading an
attachment, I get a badmatch exception. It seems to be related to the
following case statement:

data = case couch_httpd:body_length(Req) of
                    undefined ->
                        <<"">>;
                    {unknown_transfer_encoding, Unknown} ->
                        exit({unknown_transfer_encoding, Unknown});
                    chunked ->
                        fun(MaxChunkSize, ChunkFun, InitState) ->
                            couch_httpd:recv_chunked(Req, MaxChunkSize,
                                ChunkFun, InitState)
                        end;
                    0 ->
                        <<"">>;
                    Length when is_integer(Length) ->
                        fun() -> couch_httpd:recv(Req, 0) end;
                    Length ->
                        exit({length_not_integer, Length})
                    end,

A part of the stack trace:

[error] [<0.221.0>] Uncaught error in HTTP request: {error,{badmatch,34}}
[info] [<0.221.0>] Stacktrace: [{couch_httpd_db,db_attachment_req,
                 [{httpd,
                      {mochiweb_request,#Port<0.3985>,'PUT',
                          "/testedb/testdoc1/readme.txt",
                          {1,1},
                          {5,
                           {"user-agent",
                            {'User-Agent',
                                "curl/7.18.2 (i486-pc-linux-gnu)
libcurl/7.18.2 OpenSSL/0.9.8g zlib/1.2.3.3 libidn/1.10"},
                            {"host",
                             {'Host',"localhost:5984"},
                             {"accept",
                              {'Accept',"*/*"},
                              nil,
                              {"content-type",
                               {'Content-Type',"text/plain"},
                               {"content-length",
                                {'Content-Length',"34"},
                                nil,nil},
                               nil}},
                             nil},
                            nil}}},
                      "127.0.0.1",'PUT',
                      [<<"testedb">>,<<"testdoc1">>,<<"readme.txt">>],
                      {dict,6,16,16,8,80,48,

If I don't use the debugger (yet even with modules compiled using the
erlc flag +debug_info) everything goes fine, no exceptions at all.
Adding a print statement before that case statement:

io:format("~n~nBODY LENGTH: ~p~n~n", [couch_httpd:body_length(Req)]),

gives the right value to stdout (an integer).

Why do I get this strange behaviour? Is Erlang's debugger buggy?
How do you debug your code? Do you use something else ?

cheers


-- 
Filipe David Manana,
fdmanana@gmail.com
PGP key - http://pgp.mit.edu:11371/pks/lookup?op=get&search=0xC569452B

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."