chunked_istream.cc

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// -*- mode: c++; c-basic-offset:4 -*-
 
// This file is part of libdap, A C++ implementation of the OPeNDAP Data
// Access Protocol.
 
// Copyright (c) 2009 OPeNDAP, Inc.
// Author: James Gallagher <jgallagher@opendap.org>
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
//
// You can contact OPeNDAP, Inc. at PO Box 112, Saunderstown, RI. 02874-0112.
//
// Portions of this code were taken verbatim from  Josuttis,
// "The C++ Standard Library," p.672
 
#include "config.h"
 
#include <arpa/inet.h>
#include <stdint.h>
 
#include <cstring>
#include <vector>
 
#include "chunked_istream.h"
#include "chunked_stream.h"
 
#include "Error.h"
 
// #define DODS_DEBUG
// #define DODS_DEBUG2
#ifdef DODS_DEBUG
#include <iostream>
#endif
 
#include "debug.h"
#include "util.h"
 
namespace libdap {
 
/*
  This code does not use a 'put back' buffer, but here's a picture of the
  d_buffer pointer, eback(), gptr() and egptr() that can be used to see how
  the I/O Stream library's streambuf class works. For the case with no
  putback, just imagine it as zero and eliminate the leftmost extension. This
  might also come in useful if the code was extended to support put back. I
  removed that feature because I don't see it being used with our chunked
  transmission protocol and it requires an extra call to memcopy() when data
  are added to the internal buffer.
 
  d_buffer  d_buffer + putBack
  |         |
  v         v
  |---------|--------------------------------------------|....
  |         |                                            |   .
  |---------|--------------------------------------------|....
            ^                         ^                   ^
            |                         |                   |
            eback()                   gptr()              egptr()
 
 */

std::streambuf::int_type chunked_inbuf::underflow() {
    DBG(cerr << "underflow..." << endl);
    DBG2(cerr << "eback(): " << (void *)eback() << ", gptr(): " << (void *)(gptr() - eback())
              << ", egptr(): " << (void *)(egptr() - eback()) << endl);
 
    // return the next character; uflow() increments the puffer pointer.
    if (gptr() < egptr())
        return traits_type::to_int_type(*gptr());
 
    // gptr() == egptr() so read more data from the underlying input source.
 
    // To read data from the chunked stream, first read the header
    uint32_t header;
    d_is.read((char *)&header, 4);
 
    // When the endian nature of the server is encoded in the chunk header, the header is
    // sent using network byte order
    header = ntohl(header);
 
    // There are two 'EOF' cases: One where the END chunk is zero bytes and one where
    // it holds data. In the latter case, bytes those will be read and moved into the
    // buffer. Once those data are consumed, we'll be back here again and this read()
    // will return EOF. See below for the other case...
    if (d_is.eof())
        return traits_type::eof();
 
    // (header & CHUNK_LITTLE_ENDIAN) --> is the sender little endian
    if (!d_set_twiddle) {
        d_twiddle_bytes = (is_host_big_endian() == (header & CHUNK_LITTLE_ENDIAN));
        d_set_twiddle = true;
    }
 
    uint32_t chunk_size = header & CHUNK_SIZE_MASK;
 
    DBG(cerr << "underflow: chunk size from header: " << chunk_size << endl);
    DBG(cerr << "underflow: chunk type from header: " << hex << (header & CHUNK_TYPE_MASK) << endl);
    DBG(cerr << "underflow: chunk byte order from header: " << hex << (header & CHUNK_BIG_ENDIAN) << endl);
 
    // Handle the case where the buffer is not big enough to hold the incoming chunk
    if (chunk_size > d_buf_size) {
        d_buf_size = chunk_size;
        m_buffer_alloc();
    }
 
    // If the END chunk has zero bytes, return EOF. See above for more information
    if (chunk_size == 0 && (header & CHUNK_TYPE_MASK) == CHUNK_END)
        return traits_type::eof();
 
    // Read the chunk's data
    d_is.read(d_buffer, chunk_size);
    DBG2(cerr << "underflow: size read: " << d_is.gcount() << ", eof: " << d_is.eof() << ", bad: " << d_is.bad()
              << endl);
    if (d_is.bad())
        return traits_type::eof();
 
    DBG2(cerr << "eback(): " << (void *)eback() << ", gptr(): " << (void *)(gptr() - eback())
              << ", egptr(): " << (void *)(egptr() - eback()) << endl);
    setg(d_buffer,               // beginning of put back area
         d_buffer,               // read position (gptr() == eback())
         d_buffer + chunk_size); // end of buffer (egptr()) chunk_size == d_is.gcount() unless there's an error
 
    DBG2(cerr << "eback(): " << (void *)eback() << ", gptr(): " << (void *)(gptr() - eback())
              << ", egptr(): " << (void *)(egptr() - eback()) << endl);
 
    switch (header & CHUNK_TYPE_MASK) {
    case CHUNK_END:
        DBG2(cerr << "Found end chunk" << endl);
        return traits_type::to_int_type(*gptr());
    case CHUNK_DATA:
        return traits_type::to_int_type(*gptr());
 
    case CHUNK_ERR:
        // this is pretty much the end of the show... Assume the buffer/chunk holds
        // the error message text.
        d_error = true;
        d_error_message = string(d_buffer, chunk_size);
        return traits_type::eof();
    default:
        d_error = true;
        d_error_message = "Failed to read known chunk header type.";
        return traits_type::eof();
    }
}

std::streamsize chunked_inbuf::xsgetn(char *s, std::streamsize num) {
    DBG(cerr << "xsgetn... num: " << num << endl);
 
    // if num is <= the chars currently in the buffer
    if (num <= (egptr() - gptr())) {
        memcpy(s, gptr(), num);
        gbump(num);
 
        return traits_type::not_eof(num);
    }
 
    // else they asked for more
    uint32_t bytes_left_to_read = num;
 
    // are there any bytes in the buffer? if so grab them first
    if (gptr() < egptr()) {
        int bytes_to_transfer = egptr() - gptr();
        memcpy(s, gptr(), bytes_to_transfer);
        gbump(bytes_to_transfer);
        s += bytes_to_transfer;
        bytes_left_to_read -= bytes_to_transfer;
    }
 
    // We need to get more bytes from the underlying stream; at this
    // point the internal buffer is empty.
 
    // read the remaining bytes to transfer, a chunk at a time,
    // and put any leftover stuff in the buffer.
 
    // note that when the code is here, gptr() == egptr(), so the
    // next call to read() will fall through the previous tests and
    // read at least one chunk here.
    bool done = false;
    while (!done) {
        // Get a chunk header
        uint32_t header;
        d_is.read((char *)&header, 4);
 
        header = ntohl(header);
 
        // There are two EOF cases: One where the END chunk is zero bytes and one where
        // it holds data. In the latter case, those will be read and moved into the
        // buffer. Once those data are consumed, we'll be back here again and this read()
        // will return EOF. See below for the other case...
        if (d_is.eof())
            return traits_type::eof();
 
        // (header & CHUNK_LITTLE_ENDIAN) --> is the sender little endian
        if (!d_set_twiddle) {
            d_twiddle_bytes = (is_host_big_endian() == (header & CHUNK_LITTLE_ENDIAN));
            d_set_twiddle = true;
        }
 
        uint32_t chunk_size = header & CHUNK_SIZE_MASK;
        DBG(cerr << "xsgetn: chunk size from header: " << chunk_size << endl);
        DBG(cerr << "xsgetn: chunk type from header: " << hex << (header & CHUNK_TYPE_MASK) << endl);
        DBG(cerr << "xsgetn: chunk byte order from header: " << hex << (header & CHUNK_BIG_ENDIAN) << endl);
 
        // handle error chunks here
        if ((header & CHUNK_TYPE_MASK) == CHUNK_ERR) {
            d_error = true;
            // Note that d_buffer is not used to avoid calling resize if it is too
            // small to hold the error message. At this point, there's not much reason
            // to optimize transport efficiency, however.
            std::vector<char> message(chunk_size);
            d_is.read(message.data(), chunk_size);
            d_error_message = string(message.data(), chunk_size);
            // leave the buffer and gptr(), ..., in a consistent state (empty)
            setg(d_buffer, d_buffer, d_buffer);
        }
        // And zero-length END chunks here.
        else if (chunk_size == 0 && (header & CHUNK_TYPE_MASK) == CHUNK_END) {
            return traits_type::not_eof(num - bytes_left_to_read);
        }
        // The next case is complicated because we read some data from the current
        // chunk into 's' an some into the internal buffer.
        else if (chunk_size > bytes_left_to_read) {
            d_is.read(s, bytes_left_to_read);
            if (d_is.bad())
                return traits_type::eof();
 
            // Now slurp up the remain part of the chunk and store it in the buffer
            uint32_t bytes_leftover = chunk_size - bytes_left_to_read;
            // expand the internal buffer if needed
            if (bytes_leftover > d_buf_size) {
                d_buf_size = chunk_size;
                m_buffer_alloc();
            }
            // read the remain stuff in to d_buffer
            d_is.read(d_buffer, bytes_leftover);
            if (d_is.bad())
                return traits_type::eof();
 
            setg(d_buffer,                                     // beginning of put back area
                 d_buffer,                                     // read position (gptr() == eback())
                 d_buffer + bytes_leftover /*d_is.gcount()*/); // end of buffer (egptr())
 
            bytes_left_to_read = 0 /* -= d_is.gcount()*/;
        } else {
            // expand the internal buffer if needed
            if (chunk_size > d_buf_size) {
                d_buf_size = chunk_size;
                m_buffer_alloc();
            }
            // If we get a chunk that's zero bytes, Don't call read()
            // to save the kernel context switch overhead.
            if (chunk_size > 0) {
                d_is.read(s, chunk_size);
                if (d_is.bad())
                    return traits_type::eof();
                bytes_left_to_read -= chunk_size /*d_is.gcount()*/;
                s += chunk_size;
            }
        }
 
        switch (header & CHUNK_TYPE_MASK) {
        case CHUNK_END:
            DBG(cerr << "Found end chunk" << endl);
            // in this case bytes_left_to_read can be > 0 because we ran out of data
            // before reading all the requested bytes. The next read() call will return
            // eof; this call returns the number of bytes read and transferred to 's'.
            done = true;
            break;
 
        case CHUNK_DATA:
            done = bytes_left_to_read == 0;
            break;
 
        case CHUNK_ERR:
            // this is pretty much the end of the show... The error message has
            // already been read above
            return traits_type::eof();
 
        default:
            d_error = true;
            d_error_message = "Failed to read known chunk header type.";
            return traits_type::eof();
        }
    }
 
    return traits_type::not_eof(num - bytes_left_to_read);
}

std::streambuf::int_type chunked_inbuf::read_next_chunk() {
    // To read data from the chunked stream, first read the header
    uint32_t header;
    d_is.read((char *)&header, 4);
 
    header = ntohl(header);
 
    // There are two 'EOF' cases: One where the END chunk is zero bytes and one where
    // it holds data. In the latter case, bytes those will be read and moved into the
    // buffer. Once those data are consumed, we'll be back here again and this read()
    // will return EOF. See below for the other case...
    if (d_is.eof())
        return traits_type::eof();
 
    // (header & CHUNK_LITTLE_ENDIAN) --> is the sender little endian
    if (!d_set_twiddle) {
        d_twiddle_bytes = (is_host_big_endian() == (header & CHUNK_LITTLE_ENDIAN));
        d_set_twiddle = true;
    }
 
    uint32_t chunk_size = header & CHUNK_SIZE_MASK;
 
    DBG(cerr << "read_next_chunk: chunk size from header: " << chunk_size << endl);
    DBG(cerr << "read_next_chunk: chunk type from header: " << hex << (header & CHUNK_TYPE_MASK) << endl);
    DBG(cerr << "read_next_chunk: chunk byte order from header: " << hex << (header & CHUNK_BIG_ENDIAN) << endl);
 
    // Handle the case where the buffer is not big enough to hold the incoming chunk
    if (chunk_size > d_buf_size) {
        d_buf_size = chunk_size;
        m_buffer_alloc();
    }
 
    // If the END chunk has zero bytes, return EOF. See above for more information
    if (chunk_size == 0 && (header & CHUNK_TYPE_MASK) == CHUNK_END)
        return traits_type::eof();
 
    // Read the chunk's data
    d_is.read(d_buffer, chunk_size);
    DBG2(cerr << "read_next_chunk: size read: " << d_is.gcount() << ", eof: " << d_is.eof() << ", bad: " << d_is.bad()
              << endl);
    if (d_is.bad())
        return traits_type::eof();
 
    DBG2(cerr << "eback(): " << (void *)eback() << ", gptr(): " << (void *)(gptr() - eback())
              << ", egptr(): " << (void *)(egptr() - eback()) << endl);
    setg(d_buffer,               // beginning of put back area
         d_buffer,               // read position (gptr() == eback())
         d_buffer + chunk_size); // end of buffer (egptr()) chunk_size == d_is.gcount() unless there's an error
 
    DBG2(cerr << "eback(): " << (void *)eback() << ", gptr(): " << (void *)(gptr() - eback())
              << ", egptr(): " << (void *)(egptr() - eback()) << endl);
 
    switch (header & CHUNK_TYPE_MASK) {
    case CHUNK_END:
        DBG(cerr << "Found end chunk" << endl);
        return traits_type::not_eof(chunk_size);
 
    case CHUNK_DATA:
        return traits_type::not_eof(chunk_size);
 
    case CHUNK_ERR:
        // this is pretty much the end of the show... Assume the buffer/chunk holds
        // the error message text.
        d_error = true;
        d_error_message = string(d_buffer, chunk_size);
        return traits_type::eof();
 
    default:
        d_error = true;
        d_error_message = "Failed to read known chunk header type.";
        return traits_type::eof();
    }
}
 
} // namespace libdap