source: vbox/trunk/src/libs/liblzma-5.4.1/common/file_info.c

///////////////////////////////////////////////////////////////////////////////
//
/// \file       file_info.c
/// \brief      Decode .xz file information into a lzma_index structure
//
//  Author:     Lasse Collin
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////

#include "index_decoder.h"


typedef struct {
        enum {
                SEQ_MAGIC_BYTES,
                SEQ_PADDING_SEEK,
                SEQ_PADDING_DECODE,
                SEQ_FOOTER,
                SEQ_INDEX_INIT,
                SEQ_INDEX_DECODE,
                SEQ_HEADER_DECODE,
                SEQ_HEADER_COMPARE,
        } sequence;

        /// Absolute position of in[*in_pos] in the file. All code that
        /// modifies *in_pos also updates this. seek_to_pos() needs this
        /// to determine if we need to request the application to seek for
        /// us or if we can do the seeking internally by adjusting *in_pos.
        uint64_t file_cur_pos;

        /// This refers to absolute positions of interesting parts of the
        /// input file. Sometimes it points to the *beginning* of a specific
        /// field and sometimes to the *end* of a field. The current target
        /// position at each moment is explained in the comments.
        uint64_t file_target_pos;

        /// Size of the .xz file (from the application).
        uint64_t file_size;

        /// Index decoder
        lzma_next_coder index_decoder;

        /// Number of bytes remaining in the Index field that is currently
        /// being decoded.
        lzma_vli index_remaining;

        /// The Index decoder will store the decoded Index in this pointer.
        lzma_index *this_index;

        /// Amount of Stream Padding in the current Stream.
        lzma_vli stream_padding;

        /// The final combined index is collected here.
        lzma_index *combined_index;

        /// Pointer from the application where to store the index information
        /// after successful decoding.
        lzma_index **dest_index;

        /// Pointer to lzma_stream.seek_pos to be used when returning
        /// LZMA_SEEK_NEEDED. This is set by seek_to_pos() when needed.
        uint64_t *external_seek_pos;

        /// Memory usage limit
        uint64_t memlimit;

        /// Stream Flags from the very beginning of the file.
        lzma_stream_flags first_header_flags;

        /// Stream Flags from Stream Header of the current Stream.
        lzma_stream_flags header_flags;

        /// Stream Flags from Stream Footer of the current Stream.
        lzma_stream_flags footer_flags;

        size_t temp_pos;
        size_t temp_size;
        uint8_t temp[8192];

} lzma_file_info_coder;


/// Copies data from in[*in_pos] into coder->temp until
/// coder->temp_pos == coder->temp_size. This also keeps coder->file_cur_pos
/// in sync with *in_pos. Returns true if more input is needed.
static bool
fill_temp(lzma_file_info_coder *coder, const uint8_t *restrict in,
                size_t *restrict in_pos, size_t in_size)
{
        coder->file_cur_pos += lzma_bufcpy(in, in_pos, in_size,
                        coder->temp, &coder->temp_pos, coder->temp_size);
        return coder->temp_pos < coder->temp_size;
}


/// Seeks to the absolute file position specified by target_pos.
/// This tries to do the seeking by only modifying *in_pos, if possible.
/// The main benefit of this is that if one passes the whole file at once
/// to lzma_code(), the decoder will never need to return LZMA_SEEK_NEEDED
/// as all the seeking can be done by adjusting *in_pos in this function.
///
/// Returns true if an external seek is needed and the caller must return
/// LZMA_SEEK_NEEDED.
static bool
seek_to_pos(lzma_file_info_coder *coder, uint64_t target_pos,
                size_t in_start, size_t *in_pos, size_t in_size)
{
        // The input buffer doesn't extend beyond the end of the file.
        // This has been checked by file_info_decode() already.
        assert(coder->file_size - coder->file_cur_pos >= in_size - *in_pos);

        const uint64_t pos_min = coder->file_cur_pos - (*in_pos - in_start);
        const uint64_t pos_max = coder->file_cur_pos + (in_size - *in_pos);

        bool external_seek_needed;

        if (target_pos >= pos_min && target_pos <= pos_max) {
                // The requested position is available in the current input
                // buffer or right after it. That is, in a corner case we
                // end up setting *in_pos == in_size and thus will immediately
                // need new input bytes from the application.
                *in_pos += (size_t)(target_pos - coder->file_cur_pos);
                external_seek_needed = false;
        } else {
                // Ask the application to seek the input file.
                *coder->external_seek_pos = target_pos;
                external_seek_needed = true;

                // Mark the whole input buffer as used. This way
                // lzma_stream.total_in will have a better estimate
                // of the amount of data read. It still won't be perfect
                // as the value will depend on the input buffer size that
                // the application uses, but it should be good enough for
                // those few who want an estimate.
                *in_pos = in_size;
        }

        // After seeking (internal or external) the current position
        // will match the requested target position.
        coder->file_cur_pos = target_pos;

        return external_seek_needed;
}


/// The caller sets coder->file_target_pos so that it points to the *end*
/// of the desired file position. This function then determines how far
/// backwards from that position we can seek. After seeking fill_temp()
/// can be used to read data into coder->temp. When fill_temp() has finished,
/// coder->temp[coder->temp_size] will match coder->file_target_pos.
///
/// This also validates that coder->target_file_pos is sane in sense that
/// we aren't trying to seek too far backwards (too close or beyond the
/// beginning of the file).
static lzma_ret
reverse_seek(lzma_file_info_coder *coder,
                size_t in_start, size_t *in_pos, size_t in_size)
{
        // Check that there is enough data before the target position
        // to contain at least Stream Header and Stream Footer. If there
        // isn't, the file cannot be valid.
        if (coder->file_target_pos < 2 * LZMA_STREAM_HEADER_SIZE)
                return LZMA_DATA_ERROR;

        coder->temp_pos = 0;

        // The Stream Header at the very beginning of the file gets handled
        // specially in SEQ_MAGIC_BYTES and thus we will never need to seek
        // there. By not seeking to the first LZMA_STREAM_HEADER_SIZE bytes
        // we avoid a useless external seek after SEQ_MAGIC_BYTES if the
        // application uses an extremely small input buffer and the input
        // file is very small.
        if (coder->file_target_pos - LZMA_STREAM_HEADER_SIZE
                        < sizeof(coder->temp))
                coder->temp_size = (size_t)(coder->file_target_pos
                                - LZMA_STREAM_HEADER_SIZE);
        else
                coder->temp_size = sizeof(coder->temp);

        // The above if-statements guarantee this. This is important because
        // the Stream Header/Footer decoders assume that there's at least
        // LZMA_STREAM_HEADER_SIZE bytes in coder->temp.
        assert(coder->temp_size >= LZMA_STREAM_HEADER_SIZE);

        if (seek_to_pos(coder, coder->file_target_pos - coder->temp_size,
                        in_start, in_pos, in_size))
                return LZMA_SEEK_NEEDED;

        return LZMA_OK;
}


/// Gets the number of zero-bytes at the end of the buffer.
static size_t
get_padding_size(const uint8_t *buf, size_t buf_size)
{
        size_t padding = 0;
        while (buf_size > 0 && buf[--buf_size] == 0x00)
                ++padding;

        return padding;
}


/// With the Stream Header at the very beginning of the file, LZMA_FORMAT_ERROR
/// is used to tell the application that Magic Bytes didn't match. In other
/// Stream Header/Footer fields (in the middle/end of the file) it could be
/// a bit confusing to return LZMA_FORMAT_ERROR as we already know that there
/// is a valid Stream Header at the beginning of the file. For those cases
/// this function is used to convert LZMA_FORMAT_ERROR to LZMA_DATA_ERROR.
static lzma_ret
hide_format_error(lzma_ret ret)
{
        if (ret == LZMA_FORMAT_ERROR)
                ret = LZMA_DATA_ERROR;

        return ret;
}


/// Calls the Index decoder and updates coder->index_remaining.
/// This is a separate function because the input can be either directly
/// from the application or from coder->temp.
static lzma_ret
decode_index(lzma_file_info_coder *coder, const lzma_allocator *allocator,
                const uint8_t *restrict in, size_t *restrict in_pos,
                size_t in_size, bool update_file_cur_pos)
{
        const size_t in_start = *in_pos;

        const lzma_ret ret = coder->index_decoder.code(
                        coder->index_decoder.coder,
                        allocator, in, in_pos, in_size,
                        NULL, NULL, 0, LZMA_RUN);

        coder->index_remaining -= *in_pos - in_start;

        if (update_file_cur_pos)
                coder->file_cur_pos += *in_pos - in_start;

        return ret;
}


static lzma_ret
file_info_decode(void *coder_ptr, const lzma_allocator *allocator,
                const uint8_t *restrict in, size_t *restrict in_pos,
                size_t in_size,
                uint8_t *restrict out lzma_attribute((__unused__)),
                size_t *restrict out_pos lzma_attribute((__unused__)),
                size_t out_size lzma_attribute((__unused__)),
                lzma_action action lzma_attribute((__unused__)))
{
        lzma_file_info_coder *coder = coder_ptr;
        const size_t in_start = *in_pos;

        // If the caller provides input past the end of the file, trim
        // the extra bytes from the buffer so that we won't read too far.
        assert(coder->file_size >= coder->file_cur_pos);
        if (coder->file_size - coder->file_cur_pos < in_size - in_start)
                in_size = in_start
                        + (size_t)(coder->file_size - coder->file_cur_pos);

        while (true)
        switch (coder->sequence) {
        case SEQ_MAGIC_BYTES:
                // Decode the Stream Header at the beginning of the file
                // first to check if the Magic Bytes match. The flags
                // are stored in coder->first_header_flags so that we
                // don't need to seek to it again.
                //
                // Check that the file is big enough to contain at least
                // Stream Header.
                if (coder->file_size < LZMA_STREAM_HEADER_SIZE)
                        return LZMA_FORMAT_ERROR;

                // Read the Stream Header field into coder->temp.
                if (fill_temp(coder, in, in_pos, in_size))
                        return LZMA_OK;

                // This is the only Stream Header/Footer decoding where we
                // want to return LZMA_FORMAT_ERROR if the Magic Bytes don't
                // match. Elsewhere it will be converted to LZMA_DATA_ERROR.
                return_if_error(lzma_stream_header_decode(
                                &coder->first_header_flags, coder->temp));

                // Now that we know that the Magic Bytes match, check the
                // file size. It's better to do this here after checking the
                // Magic Bytes since this way we can give LZMA_FORMAT_ERROR
                // instead of LZMA_DATA_ERROR when the Magic Bytes don't
                // match in a file that is too big or isn't a multiple of
                // four bytes.
                if (coder->file_size > LZMA_VLI_MAX || (coder->file_size & 3))
                        return LZMA_DATA_ERROR;

                // Start looking for Stream Padding and Stream Footer
                // at the end of the file.
                coder->file_target_pos = coder->file_size;

        // Fall through

        case SEQ_PADDING_SEEK:
                coder->sequence = SEQ_PADDING_DECODE;
                return_if_error(reverse_seek(
                                coder, in_start, in_pos, in_size));

        // Fall through

        case SEQ_PADDING_DECODE: {
                // Copy to coder->temp first. This keeps the code simpler if
                // the application only provides input a few bytes at a time.
                if (fill_temp(coder, in, in_pos, in_size))
                        return LZMA_OK;

                // Scan the buffer backwards to get the size of the
                // Stream Padding field (if any).
                const size_t new_padding = get_padding_size(
                                coder->temp, coder->temp_size);
                coder->stream_padding += new_padding;

                // Set the target position to the beginning of Stream Padding
                // that has been observed so far. If all Stream Padding has
                // been seen, then the target position will be at the end
                // of the Stream Footer field.
                coder->file_target_pos -= new_padding;

                if (new_padding == coder->temp_size) {
                        // The whole buffer was padding. Seek backwards in
                        // the file to get more input.
                        coder->sequence = SEQ_PADDING_SEEK;
                        break;
                }

                // Size of Stream Padding must be a multiple of 4 bytes.
                if (coder->stream_padding & 3)
                        return LZMA_DATA_ERROR;

                coder->sequence = SEQ_FOOTER;

                // Calculate the amount of non-padding data in coder->temp.
                coder->temp_size -= new_padding;
                coder->temp_pos = coder->temp_size;

                // We can avoid an external seek if the whole Stream Footer
                // is already in coder->temp. In that case SEQ_FOOTER won't
                // read more input and will find the Stream Footer from
                // coder->temp[coder->temp_size - LZMA_STREAM_HEADER_SIZE].
                //
                // Otherwise we will need to seek. The seeking is done so
                // that Stream Footer wil be at the end of coder->temp.
                // This way it's likely that we also get a complete Index
                // field into coder->temp without needing a separate seek
                // for that (unless the Index field is big).
                if (coder->temp_size < LZMA_STREAM_HEADER_SIZE)
                        return_if_error(reverse_seek(
                                        coder, in_start, in_pos, in_size));
        }

        // Fall through

        case SEQ_FOOTER:
                // Copy the Stream Footer field into coder->temp.
                // If Stream Footer was already available in coder->temp
                // in SEQ_PADDING_DECODE, then this does nothing.
                if (fill_temp(coder, in, in_pos, in_size))
                        return LZMA_OK;

                // Make coder->file_target_pos and coder->temp_size point
                // to the beginning of Stream Footer and thus to the end
                // of the Index field. coder->temp_pos will be updated
                // a bit later.
                coder->file_target_pos -= LZMA_STREAM_HEADER_SIZE;
                coder->temp_size -= LZMA_STREAM_HEADER_SIZE;

                // Decode Stream Footer.
                return_if_error(hide_format_error(lzma_stream_footer_decode(
                                &coder->footer_flags,
                                coder->temp + coder->temp_size)));

                // Check that we won't seek past the beginning of the file.
                //
                // LZMA_STREAM_HEADER_SIZE is added because there must be
                // space for Stream Header too even though we won't seek
                // there before decoding the Index field.
                //
                // There's no risk of integer overflow here because
                // Backward Size cannot be greater than 2^34.
                if (coder->file_target_pos < coder->footer_flags.backward_size
                                + LZMA_STREAM_HEADER_SIZE)
                        return LZMA_DATA_ERROR;

                // Set the target position to the beginning of the Index field.
                coder->file_target_pos -= coder->footer_flags.backward_size;
                coder->sequence = SEQ_INDEX_INIT;

                // We can avoid an external seek if the whole Index field is
                // already available in coder->temp.
                if (coder->temp_size >= coder->footer_flags.backward_size) {
                        // Set coder->temp_pos to point to the beginning
                        // of the Index.
                        coder->temp_pos = coder->temp_size
                                        - coder->footer_flags.backward_size;
                } else {
                        // These are set to zero to indicate that there's no
                        // useful data (Index or anything else) in coder->temp.
                        coder->temp_pos = 0;
                        coder->temp_size = 0;

                        // Seek to the beginning of the Index field.
                        if (seek_to_pos(coder, coder->file_target_pos,
                                        in_start, in_pos, in_size))
                                return LZMA_SEEK_NEEDED;
                }

        // Fall through

        case SEQ_INDEX_INIT: {
                // Calculate the amount of memory already used by the earlier
                // Indexes so that we know how big memory limit to pass to
                // the Index decoder.
                //
                // NOTE: When there are multiple Streams, the separate
                // lzma_index structures can use more RAM (as measured by
                // lzma_index_memused()) than the final combined lzma_index.
                // Thus memlimit may need to be slightly higher than the final
                // calculated memory usage will be. This is perhaps a bit
                // confusing to the application, but I think it shouldn't
                // cause problems in practice.
                uint64_t memused = 0;
                if (coder->combined_index != NULL) {
                        memused = lzma_index_memused(coder->combined_index);
                        assert(memused <= coder->memlimit);
                        if (memused > coder->memlimit) // Extra sanity check
                                return LZMA_PROG_ERROR;
                }

                // Initialize the Index decoder.
                return_if_error(lzma_index_decoder_init(
                                &coder->index_decoder, allocator,
                                &coder->this_index,
                                coder->memlimit - memused));

                coder->index_remaining = coder->footer_flags.backward_size;
                coder->sequence = SEQ_INDEX_DECODE;
        }

        // Fall through

        case SEQ_INDEX_DECODE: {
                // Decode (a part of) the Index. If the whole Index is already
                // in coder->temp, read it from there. Otherwise read from
                // in[*in_pos] onwards. Note that index_decode() updates
                // coder->index_remaining and optionally coder->file_cur_pos.
                lzma_ret ret;
                if (coder->temp_size != 0) {
                        assert(coder->temp_size - coder->temp_pos
                                        == coder->index_remaining);
                        ret = decode_index(coder, allocator, coder->temp,
                                        &coder->temp_pos, coder->temp_size,
                                        false);
                } else {
                        // Don't give the decoder more input than the known
                        // remaining size of the Index field.
                        size_t in_stop = in_size;
                        if (in_size - *in_pos > coder->index_remaining)
                                in_stop = *in_pos
                                        + (size_t)(coder->index_remaining);

                        ret = decode_index(coder, allocator,
                                        in, in_pos, in_stop, true);
                }

                switch (ret) {
                case LZMA_OK:
                        // If the Index docoder asks for more input when we
                        // have already given it as much input as Backward Size
                        // indicated, the file is invalid.
                        if (coder->index_remaining == 0)
                                return LZMA_DATA_ERROR;

                        // We cannot get here if we were reading Index from
                        // coder->temp because when reading from coder->temp
                        // we give the Index decoder exactly
                        // coder->index_remaining bytes of input.
                        assert(coder->temp_size == 0);

                        return LZMA_OK;

                case LZMA_STREAM_END:
                        // If the decoding seems to be successful, check also
                        // that the Index decoder consumed as much input as
                        // indicated by the Backward Size field.
                        if (coder->index_remaining != 0)
                                return LZMA_DATA_ERROR;

                        break;

                default:
                        return ret;
                }

                // Calculate how much the Index tells us to seek backwards
                // (relative to the beginning of the Index): Total size of
                // all Blocks plus the size of the Stream Header field.
                // No integer overflow here because lzma_index_total_size()
                // cannot return a value greater than LZMA_VLI_MAX.
                const uint64_t seek_amount
                                = lzma_index_total_size(coder->this_index)
                                        + LZMA_STREAM_HEADER_SIZE;

                // Check that Index is sane in sense that seek_amount won't
                // make us seek past the beginning of the file when locating
                // the Stream Header.
                //
                // coder->file_target_pos still points to the beginning of
                // the Index field.
                if (coder->file_target_pos < seek_amount)
                        return LZMA_DATA_ERROR;

                // Set the target to the beginning of Stream Header.
                coder->file_target_pos -= seek_amount;

                if (coder->file_target_pos == 0) {
                        // We would seek to the beginning of the file, but
                        // since we already decoded that Stream Header in
                        // SEQ_MAGIC_BYTES, we can use the cached value from
                        // coder->first_header_flags to avoid the seek.
                        coder->header_flags = coder->first_header_flags;
                        coder->sequence = SEQ_HEADER_COMPARE;
                        break;
                }

                coder->sequence = SEQ_HEADER_DECODE;

                // Make coder->file_target_pos point to the end of
                // the Stream Header field.
                coder->file_target_pos += LZMA_STREAM_HEADER_SIZE;

                // If coder->temp_size is non-zero, it points to the end
                // of the Index field. Then the beginning of the Index
                // field is at coder->temp[coder->temp_size
                // - coder->footer_flags.backward_size].
                assert(coder->temp_size == 0 || coder->temp_size
                                >= coder->footer_flags.backward_size);

                // If coder->temp contained the whole Index, see if it has
                // enough data to contain also the Stream Header. If so,
                // we avoid an external seek.
                //
                // NOTE: This can happen only with small .xz files and only
                // for the non-first Stream as the Stream Flags of the first
                // Stream are cached and already handled a few lines above.
                // So this isn't as useful as the other seek-avoidance cases.
                if (coder->temp_size != 0 && coder->temp_size
                                - coder->footer_flags.backward_size
                                >= seek_amount) {
                        // Make temp_pos and temp_size point to the *end* of
                        // Stream Header so that SEQ_HEADER_DECODE will find
                        // the start of Stream Header from coder->temp[
                        // coder->temp_size - LZMA_STREAM_HEADER_SIZE].
                        coder->temp_pos = coder->temp_size
                                        - coder->footer_flags.backward_size
                                        - seek_amount
                                        + LZMA_STREAM_HEADER_SIZE;
                        coder->temp_size = coder->temp_pos;
                } else {
                        // Seek so that Stream Header will be at the end of
                        // coder->temp. With typical multi-Stream files we
                        // will usually also get the Stream Footer and Index
                        // of the *previous* Stream in coder->temp and thus
                        // won't need a separate seek for them.
                        return_if_error(reverse_seek(coder,
                                        in_start, in_pos, in_size));
                }
        }

        // Fall through

        case SEQ_HEADER_DECODE:
                // Copy the Stream Header field into coder->temp.
                // If Stream Header was already available in coder->temp
                // in SEQ_INDEX_DECODE, then this does nothing.
                if (fill_temp(coder, in, in_pos, in_size))
                        return LZMA_OK;

                // Make all these point to the beginning of Stream Header.
                coder->file_target_pos -= LZMA_STREAM_HEADER_SIZE;
                coder->temp_size -= LZMA_STREAM_HEADER_SIZE;
                coder->temp_pos = coder->temp_size;

                // Decode the Stream Header.
                return_if_error(hide_format_error(lzma_stream_header_decode(
                                &coder->header_flags,
                                coder->temp + coder->temp_size)));

                coder->sequence = SEQ_HEADER_COMPARE;

        // Fall through

        case SEQ_HEADER_COMPARE:
                // Compare Stream Header against Stream Footer. They must
                // match.
                return_if_error(lzma_stream_flags_compare(
                                &coder->header_flags, &coder->footer_flags));

                // Store the decoded Stream Flags into the Index. Use the
                // Footer Flags because it contains Backward Size, although
                // it shouldn't matter in practice.
                if (lzma_index_stream_flags(coder->this_index,
                                &coder->footer_flags) != LZMA_OK)
                        return LZMA_PROG_ERROR;

                // Store also the size of the Stream Padding field. It is
                // needed to calculate the offsets of the Streams correctly.
                if (lzma_index_stream_padding(coder->this_index,
                                coder->stream_padding) != LZMA_OK)
                        return LZMA_PROG_ERROR;

                // Reset it so that it's ready for the next Stream.
                coder->stream_padding = 0;

                // Append the earlier decoded Indexes after this_index.
                if (coder->combined_index != NULL)
                        return_if_error(lzma_index_cat(coder->this_index,
                                        coder->combined_index, allocator));

                coder->combined_index = coder->this_index;
                coder->this_index = NULL;

                // If the whole file was decoded, tell the caller that we
                // are finished.
                if (coder->file_target_pos == 0) {
                        // The combined index must indicate the same file
                        // size as was told to us at initialization.
                        assert(lzma_index_file_size(coder->combined_index)
                                        == coder->file_size);

                        // Make the combined index available to
                        // the application.
                        *coder->dest_index = coder->combined_index;
                        coder->combined_index = NULL;

                        // Mark the input buffer as used since we may have
                        // done internal seeking and thus don't know how
                        // many input bytes were actually used. This way
                        // lzma_stream.total_in gets a slightly better
                        // estimate of the amount of input used.
                        *in_pos = in_size;
                        return LZMA_STREAM_END;
                }

                // We didn't hit the beginning of the file yet, so continue
                // reading backwards in the file. If we have unprocessed
                // data in coder->temp, use it before requesting more data
                // from the application.
                //
                // coder->file_target_pos, coder->temp_size, and
                // coder->temp_pos all point to the beginning of Stream Header
                // and thus the end of the previous Stream in the file.
                coder->sequence = coder->temp_size > 0
                                ? SEQ_PADDING_DECODE : SEQ_PADDING_SEEK;
                break;

        default:
                assert(0);
                return LZMA_PROG_ERROR;
        }
}


static lzma_ret
file_info_decoder_memconfig(void *coder_ptr, uint64_t *memusage,
                uint64_t *old_memlimit, uint64_t new_memlimit)
{
        lzma_file_info_coder *coder = coder_ptr;

        // The memory usage calculation comes from three things:
        //
        // (1) The Indexes that have already been decoded and processed into
        //     coder->combined_index.
        //
        // (2) The latest Index in coder->this_index that has been decoded but
        //     not yet put into coder->combined_index.
        //
        // (3) The latest Index that we have started decoding but haven't
        //     finished and thus isn't available in coder->this_index yet.
        //     Memory usage and limit information needs to be communicated
        //     from/to coder->index_decoder.
        //
        // Care has to be taken to not do both (2) and (3) when calculating
        // the memory usage.
        uint64_t combined_index_memusage = 0;
        uint64_t this_index_memusage = 0;

        // (1) If we have already successfully decoded one or more Indexes,
        // get their memory usage.
        if (coder->combined_index != NULL)
                combined_index_memusage = lzma_index_memused(
                                coder->combined_index);

        // Choose between (2), (3), or neither.
        if (coder->this_index != NULL) {
                // (2) The latest Index is available. Use its memory usage.
                this_index_memusage = lzma_index_memused(coder->this_index);

        } else if (coder->sequence == SEQ_INDEX_DECODE) {
                // (3) The Index decoder is activate and hasn't yet stored
                // the new index in coder->this_index. Get the memory usage
                // information from the Index decoder.
                //
                // NOTE: If the Index decoder doesn't yet know how much memory
                // it will eventually need, it will return a tiny value here.
                uint64_t dummy;
                if (coder->index_decoder.memconfig(coder->index_decoder.coder,
                                        &this_index_memusage, &dummy, 0)
                                != LZMA_OK) {
                        assert(0);
                        return LZMA_PROG_ERROR;
                }
        }

        // Now we know the total memory usage/requirement. If we had neither
        // old Indexes nor a new Index, this will be zero which isn't
        // acceptable as lzma_memusage() has to return non-zero on success
        // and even with an empty .xz file we will end up with a lzma_index
        // that takes some memory.
        *memusage = combined_index_memusage + this_index_memusage;
        if (*memusage == 0)
                *memusage = lzma_index_memusage(1, 0);

        *old_memlimit = coder->memlimit;

        // If requested, set a new memory usage limit.
        if (new_memlimit != 0) {
                if (new_memlimit < *memusage)
                        return LZMA_MEMLIMIT_ERROR;

                // In the condition (3) we need to tell the Index decoder
                // its new memory usage limit.
                if (coder->this_index == NULL
                                && coder->sequence == SEQ_INDEX_DECODE) {
                        const uint64_t idec_new_memlimit = new_memlimit
                                        - combined_index_memusage;

                        assert(this_index_memusage > 0);
                        assert(idec_new_memlimit > 0);

                        uint64_t dummy1;
                        uint64_t dummy2;

                        if (coder->index_decoder.memconfig(
                                        coder->index_decoder.coder,
                                        &dummy1, &dummy2, idec_new_memlimit)
                                        != LZMA_OK) {
                                assert(0);
                                return LZMA_PROG_ERROR;
                        }
                }

                coder->memlimit = new_memlimit;
        }

        return LZMA_OK;
}


static void
file_info_decoder_end(void *coder_ptr, const lzma_allocator *allocator)
{
        lzma_file_info_coder *coder = coder_ptr;

        lzma_next_end(&coder->index_decoder, allocator);
        lzma_index_end(coder->this_index, allocator);
        lzma_index_end(coder->combined_index, allocator);

        lzma_free(coder, allocator);
        return;
}


static lzma_ret
lzma_file_info_decoder_init(lzma_next_coder *next,
                const lzma_allocator *allocator, uint64_t *seek_pos,
                lzma_index **dest_index,
                uint64_t memlimit, uint64_t file_size)
{
        lzma_next_coder_init(&lzma_file_info_decoder_init, next, allocator);

        if (dest_index == NULL)
                return LZMA_PROG_ERROR;

        lzma_file_info_coder *coder = next->coder;
        if (coder == NULL) {
                coder = lzma_alloc(sizeof(lzma_file_info_coder), allocator);
                if (coder == NULL)
                        return LZMA_MEM_ERROR;

                next->coder = coder;
                next->code = &file_info_decode;
                next->end = &file_info_decoder_end;
                next->memconfig = &file_info_decoder_memconfig;

                coder->index_decoder = LZMA_NEXT_CODER_INIT;
                coder->this_index = NULL;
                coder->combined_index = NULL;
        }

        coder->sequence = SEQ_MAGIC_BYTES;
        coder->file_cur_pos = 0;
        coder->file_target_pos = 0;
        coder->file_size = file_size;

        lzma_index_end(coder->this_index, allocator);
        coder->this_index = NULL;

        lzma_index_end(coder->combined_index, allocator);
        coder->combined_index = NULL;

        coder->stream_padding = 0;

        coder->dest_index = dest_index;
        coder->external_seek_pos = seek_pos;

        // If memlimit is 0, make it 1 to ensure that lzma_memlimit_get()
        // won't return 0 (which would indicate an error).
        coder->memlimit = my_max(1, memlimit);

        // Prepare these for reading the first Stream Header into coder->temp.
        coder->temp_pos = 0;
        coder->temp_size = LZMA_STREAM_HEADER_SIZE;

        return LZMA_OK;
}


extern LZMA_API(lzma_ret)
lzma_file_info_decoder(lzma_stream *strm, lzma_index **dest_index,
                uint64_t memlimit, uint64_t file_size)
{
        lzma_next_strm_init(lzma_file_info_decoder_init, strm, &strm->seek_pos,
                        dest_index, memlimit, file_size);

        // We allow LZMA_FINISH in addition to LZMA_RUN for convenience.
        // lzma_code() is able to handle the LZMA_FINISH + LZMA_SEEK_NEEDED
        // combination in a sane way. Applications still need to be careful
        // if they use LZMA_FINISH so that they remember to reset it back
        // to LZMA_RUN after seeking if needed.
        strm->internal->supported_actions[LZMA_RUN] = true;
        strm->internal->supported_actions[LZMA_FINISH] = true;

        return LZMA_OK;
}