Get struct fields offsets from DWARF info

[mariomac]

Instead of hardcoding the fields offsets (e.g. HTTP request path), get them at runtime from DWARF information.

Technically it is possible. E.g if you run:

objdump -Wi executable

Then you will find something like:

 <1><9bf94>: Abbrev Number: 39 (DW_TAG_structure_type)
    <9bf95>   DW_AT_name        : net/http.Request
    <9bfa6>   DW_AT_byte_size   : 248
    <9bfa8>   Unknown AT value: 2900: 25
    <9bfa9>   Unknown AT value: 2904: 0xac940
 <2><9bfb1>: Abbrev Number: 24 (DW_TAG_member)
    <9bfb2>   DW_AT_name        : Method
    <9bfb9>   DW_AT_data_member_location: 0
    <9bfba>   DW_AT_type        : <0x65741>
    <9bfbe>   Unknown AT value: 2903: 0
 <2><9bfbf>: Abbrev Number: 24 (DW_TAG_member)
    <9bfc0>   DW_AT_name        : URL
    <9bfc4>   DW_AT_data_member_location: 16
    <9bfc5>   DW_AT_type        : <0x8a4bb>
    <9bfc9>   Unknown AT value: 2903: 0
 <2><9bfca>: Abbrev Number: 24 (DW_TAG_member)
    <9bfcb>   DW_AT_name        : Proto
    <9bfd1>   DW_AT_data_member_location: 24
    <9bfd2>   DW_AT_type        : <0x65741>
    <9bfd6>   Unknown AT value: 2903: 0
 <2><9bfd7>: Abbrev Number: 24 (DW_TAG_member)
    <9bfd8>   DW_AT_name        : ProtoMajor
    <9bfe3>   DW_AT_data_member_location: 40
    <9bfe4>   DW_AT_type        : <0x65731>
    <9bfe8>   Unknown AT value: 2903: 0
 <2><9bfe9>: Abbrev Number: 24 (DW_TAG_member)
    <9bfea>   DW_AT_name        : ProtoMinor
    <9bff5>   DW_AT_data_member_location: 48
    <9bff6>   DW_AT_type        : <0x65731>
    <9bffa>   Unknown AT value: 2903: 0
 <2><9bffb>: Abbrev Number: 24 (DW_TAG_member)
    <9bffc>   DW_AT_name        : Header
    <9c003>   DW_AT_data_member_location: 56
    <9c004>   DW_AT_type        : <0x98c7d>
    <9c008>   Unknown AT value: 2903: 0
 <2><9c009>: Abbrev Number: 24 (DW_TAG_member)
    <9c00a>   DW_AT_name        : Body
    <9c00f>   DW_AT_data_member_location: 64
    <9c010>   DW_AT_type        : <0x95a2d>
    <9c014>   Unknown AT value: 2903: 0
 <2><9c015>: Abbrev Number: 24 (DW_TAG_member)
    <9c016>   DW_AT_name        : GetBody
    <9c01e>   DW_AT_data_member_location: 80
    <9c01f>   DW_AT_type        : <0x9c158>
    <9c023>   Unknown AT value: 2903: 0
 <2><9c024>: Abbrev Number: 24 (DW_TAG_member)
    <9c025>   DW_AT_name        : ContentLength
    <9c033>   DW_AT_data_member_location: 88
    <9c034>   DW_AT_type        : <0x6606b>
    <9c038>   Unknown AT value: 2903: 0
 <2><9c039>: Abbrev Number: 24 (DW_TAG_member)
    <9c03a>   DW_AT_name        : TransferEncoding
    <9c04b>   DW_AT_data_member_location: 96
    <9c04c>   DW_AT_type        : <0x69294>
    <9c050>   Unknown AT value: 2903: 0
 <2><9c051>: Abbrev Number: 24 (DW_TAG_member)
    <9c052>   DW_AT_name        : Close
    <9c058>   DW_AT_data_member_location: 120
    <9c059>   DW_AT_type        : <0x65091>
    <9c05d>   Unknown AT value: 2903: 0
 <2><9c05e>: Abbrev Number: 24 (DW_TAG_member)
    <9c05f>   DW_AT_name        : Host
    <9c064>   DW_AT_data_member_location: 128
    <9c066>   DW_AT_type        : <0x65741>
    <9c06a>   Unknown AT value: 2903: 0
 <2><9c06b>: Abbrev Number: 24 (DW_TAG_member)
    <9c06c>   DW_AT_name        : Form
    <9c071>   DW_AT_data_member_location: 144
    <9c073>   DW_AT_type        : <0x8a5a5>
    <9c077>   Unknown AT value: 2903: 0
 <2><9c078>: Abbrev Number: 24 (DW_TAG_member)
    <9c079>   DW_AT_name        : PostForm
    <9c082>   DW_AT_data_member_location: 152
    <9c084>   DW_AT_type        : <0x8a5a5>
    <9c088>   Unknown AT value: 2903: 0
 <2><9c089>: Abbrev Number: 24 (DW_TAG_member)
    <9c08a>   DW_AT_name        : MultipartForm
    <9c098>   DW_AT_data_member_location: 160
    <9c09a>   DW_AT_type        : <0x94646>
    <9c09e>   Unknown AT value: 2903: 0
 <2><9c09f>: Abbrev Number: 24 (DW_TAG_member)
    <9c0a0>   DW_AT_name        : Trailer
    <9c0a8>   DW_AT_data_member_location: 168
    <9c0aa>   DW_AT_type        : <0x98c7d>
    <9c0ae>   Unknown AT value: 2903: 0
 <2><9c0af>: Abbrev Number: 24 (DW_TAG_member)
    <9c0b0>   DW_AT_name        : RemoteAddr
    <9c0bb>   DW_AT_data_member_location: 176
    <9c0bd>   DW_AT_type        : <0x65741>
    <9c0c1>   Unknown AT value: 2903: 0
 <2><9c0c2>: Abbrev Number: 24 (DW_TAG_member)
    <9c0c3>   DW_AT_name        : RequestURI
    <9c0ce>   DW_AT_data_member_location: 192
    <9c0d0>   DW_AT_type        : <0x65741>
    <9c0d4>   Unknown AT value: 2903: 0
 <2><9c0d5>: Abbrev Number: 24 (DW_TAG_member)
    <9c0d6>   DW_AT_name        : TLS
    <9c0da>   DW_AT_data_member_location: 208
    <9c0dc>   DW_AT_type        : <0x8e6f5>
    <9c0e0>   Unknown AT value: 2903: 0
 <2><9c0e1>: Abbrev Number: 24 (DW_TAG_member)
    <9c0e2>   DW_AT_name        : Cancel
    <9c0e9>   DW_AT_data_member_location: 216
    <9c0eb>   DW_AT_type        : <0x7d89a>
    <9c0ef>   Unknown AT value: 2903: 0
 <2><9c0f0>: Abbrev Number: 24 (DW_TAG_member)
    <9c0f1>   DW_AT_name        : Response
    <9c0fa>   DW_AT_data_member_location: 224
    <9c0fc>   DW_AT_type        : <0x9c198>
    <9c100>   Unknown AT value: 2903: 0
 <2><9c101>: Abbrev Number: 24 (DW_TAG_member)
    <9c102>   DW_AT_name        : ctx
    <9c106>   DW_AT_data_member_location: 232
    <9c108>   DW_AT_type        : <0x7d950>
    <9c10c>   Unknown AT value: 2903: 0

[mariomac]

Fixed by PR #13