[wimlib] / src / resource.c

/*
 * resource.c
 *
 * Read uncompressed and compressed metadata and file resources from a WIM file.
 */

/*
 * Copyright (C) 2012, 2013 Eric Biggers
 *
 * This file is part of wimlib, a library for working with WIM files.
 *
 * wimlib is free software; you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free Software
 * Foundation; either version 3 of the License, or (at your option) any later
 * version.
 *
 * wimlib 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * wimlib; if not, see http://www.gnu.org/licenses/.
 */

#ifdef HAVE_CONFIG_H
#  include "config.h"
#endif

#include "wimlib.h"
#include "wimlib/buffer_io.h"
#include "wimlib/dentry.h"
#include "wimlib/error.h"
#include "wimlib/file_io.h"
#include "wimlib/lookup_table.h"
#include "wimlib/resource.h"
#include "wimlib/sha1.h"

#ifdef __WIN32__
/* for read_win32_file_prefix(), read_win32_encrypted_file_prefix() */
#  include "wimlib/win32.h"
#endif

#ifdef WITH_NTFS_3G
/* for read_ntfs_file_prefix() */
#  include "wimlib/ntfs_3g.h"
#endif

#ifdef HAVE_ALLOCA_H
#  include <alloca.h>
#endif
#include <errno.h>
#include <fcntl.h>
#include <stdarg.h>
#include <stdlib.h>
#include <unistd.h>

/*
 * Reads all or part of a compressed WIM resource.
 *
 * Returns zero on success, nonzero on failure.
 */
static int
read_compressed_resource(int in_fd,
                         u64 resource_compressed_size,
                         u64 resource_uncompressed_size,
                         u64 resource_offset,
                         int resource_ctype,
                         u64 len,
                         u64 offset,
                         consume_data_callback_t cb,
                         void *ctx_or_buf)
{
        int ret;

        /* Trivial case */
        if (len == 0)
                return 0;

        int (*decompress)(const void *, unsigned, void *, unsigned);
        /* Set the appropriate decompress function. */
        if (resource_ctype == WIMLIB_COMPRESSION_TYPE_LZX)
                decompress = wimlib_lzx_decompress;
        else
                decompress = wimlib_xpress_decompress;

        /* The structure of a compressed resource consists of a table of chunk
         * offsets followed by the chunks themselves.  Each chunk consists of
         * compressed data, and there is one chunk for each WIM_CHUNK_SIZE =
         * 32768 bytes of the uncompressed file, with the last chunk having any
         * remaining bytes.
         *
         * The chunk offsets are measured relative to the end of the chunk
         * table.  The first chunk is omitted from the table in the WIM file
         * because its offset is implicitly given by the fact that it directly
         * follows the chunk table and therefore must have an offset of 0.
         */

        /* Calculate how many chunks the resource consists of in its entirety.
         * */
        u64 num_chunks = (resource_uncompressed_size + WIM_CHUNK_SIZE - 1) /
                                                                WIM_CHUNK_SIZE;
        /* As mentioned, the first chunk has no entry in the chunk table. */
        u64 num_chunk_entries = num_chunks - 1;


        /* The index of the chunk that the read starts at. */
        u64 start_chunk = offset / WIM_CHUNK_SIZE;
        /* The byte offset at which the read starts, within the start chunk. */
        u64 start_chunk_offset = offset % WIM_CHUNK_SIZE;

        /* The index of the chunk that contains the last byte of the read. */
        u64 end_chunk   = (offset + len - 1) / WIM_CHUNK_SIZE;
        /* The byte offset of the last byte of the read, within the end chunk */
        u64 end_chunk_offset = (offset + len - 1) % WIM_CHUNK_SIZE;

        /* Number of chunks that are actually needed to read the requested part
         * of the file. */
        u64 num_needed_chunks = end_chunk - start_chunk + 1;

        /* If the end chunk is not the last chunk, an extra chunk entry is
         * needed because we need to know the offset of the chunk after the last
         * chunk read to figure out the size of the last read chunk. */
        if (end_chunk != num_chunks - 1)
                num_needed_chunks++;

        /* Allocate the chunk table.  It will only contain offsets for the
         * chunks that are actually needed for this read. */
        u64 *chunk_offsets;
        bool chunk_offsets_malloced;
        if (num_needed_chunks < 1000) {
                chunk_offsets = alloca(num_needed_chunks * sizeof(u64));
                chunk_offsets_malloced = false;
        } else {
                chunk_offsets = malloc(num_needed_chunks * sizeof(u64));
                if (!chunk_offsets) {
                        ERROR("Failed to allocate chunk table "
                              "with %"PRIu64" entries", num_needed_chunks);
                        return WIMLIB_ERR_NOMEM;
                }
                chunk_offsets_malloced = true;
        }

        /* Set the implicit offset of the first chunk if it is included in the
         * needed chunks.
         *
         * Note: M$'s documentation includes a picture that shows the first
         * chunk starting right after the chunk entry table, labeled as offset
         * 0x10.  However, in the actual file format, the offset is measured
         * from the end of the chunk entry table, so the first chunk has an
         * offset of 0. */
        if (start_chunk == 0)
                chunk_offsets[0] = 0;

        /* According to M$'s documentation, if the uncompressed size of
         * the file is greater than 4 GB, the chunk entries are 8-byte
         * integers.  Otherwise, they are 4-byte integers. */
        u64 chunk_entry_size = (resource_uncompressed_size >= (u64)1 << 32) ?
                                                                        8 : 4;

        /* Size of the full chunk table in the WIM file. */
        u64 chunk_table_size = chunk_entry_size * num_chunk_entries;

        /* Read the needed chunk offsets from the table in the WIM file. */

        /* Index, in the WIM file, of the first needed entry in the
         * chunk table. */
        u64 start_table_idx = (start_chunk == 0) ? 0 : start_chunk - 1;

        /* Number of entries we need to actually read from the chunk
         * table (excludes the implicit first chunk). */
        u64 num_needed_chunk_entries = (start_chunk == 0) ?
                                num_needed_chunks - 1 : num_needed_chunks;

        /* Skip over unneeded chunk table entries. */
        u64 file_offset_of_needed_chunk_entries = resource_offset +
                                start_table_idx * chunk_entry_size;

        /* Number of bytes we need to read from the chunk table. */
        size_t size = num_needed_chunk_entries * chunk_entry_size;

        /* Read the raw data into the end of the chunk_offsets array to
         * avoid allocating another array. */
        void *chunk_tab_buf = (void*)&chunk_offsets[num_needed_chunks] - size;

        if (full_pread(in_fd, chunk_tab_buf, size,
                       file_offset_of_needed_chunk_entries) != size)
                goto read_error;

        /* Now fill in chunk_offsets from the entries we have read in
         * chunk_tab_buf. */

        u64 *chunk_tab_p = chunk_offsets;
        if (start_chunk == 0)
                chunk_tab_p++;

        if (chunk_entry_size == 4) {
                u32 *entries = (u32*)chunk_tab_buf;
                while (num_needed_chunk_entries--)
                        *chunk_tab_p++ = le32_to_cpu(*entries++);
        } else {
                u64 *entries = (u64*)chunk_tab_buf;
                while (num_needed_chunk_entries--)
                        *chunk_tab_p++ = le64_to_cpu(*entries++);
        }

        /* Done with the chunk table now.  We must now seek to the first chunk
         * that is needed for the read. */

        u64 cur_read_offset = resource_offset + chunk_table_size + chunk_offsets[0];

        /* Pointer to current position in the output buffer for uncompressed
         * data.  Alternatively, if using a callback function, we repeatedly
         * fill a temporary buffer to feed data into the callback function.  */
        u8 *out_p;
        if (cb)
                out_p = alloca(WIM_CHUNK_SIZE);
        else
                out_p = ctx_or_buf;

        /* Buffer for compressed data.  While most compressed chunks will have a
         * size much less than WIM_CHUNK_SIZE, WIM_CHUNK_SIZE - 1 is the maximum
         * size in the worst-case.  This assumption is valid only if chunks that
         * happen to compress to more than the uncompressed size (i.e. a
         * sequence of random bytes) are always stored uncompressed. But this seems
         * to be the case in M$'s WIM files, even though it is undocumented. */
        void *compressed_buf = alloca(WIM_CHUNK_SIZE - 1);

        /* Decompress all the chunks. */
        for (u64 i = start_chunk; i <= end_chunk; i++) {

                /* Calculate the sizes of the compressed chunk and of the
                 * uncompressed chunk. */
                unsigned compressed_chunk_size;
                unsigned uncompressed_chunk_size;
                if (i != num_chunks - 1) {
                        /* All the chunks except the last one in the resource
                         * expand to WIM_CHUNK_SIZE uncompressed, and the amount
                         * of compressed data for the chunk is given by the
                         * difference of offsets in the chunk offset table. */
                        compressed_chunk_size = chunk_offsets[i + 1 - start_chunk] -
                                                chunk_offsets[i - start_chunk];
                        uncompressed_chunk_size = WIM_CHUNK_SIZE;
                } else {
                        /* The last compressed chunk consists of the remaining
                         * bytes in the file resource, and the last uncompressed
                         * chunk has size equal to however many bytes are left-
                         * that is, the remainder of the uncompressed size when
                         * divided by WIM_CHUNK_SIZE.
                         *
                         * Note that the resource_compressed_size includes the
                         * chunk table, so the size of it must be subtracted. */
                        compressed_chunk_size = resource_compressed_size -
                                                chunk_table_size -
                                                chunk_offsets[i - start_chunk];

                        uncompressed_chunk_size = resource_uncompressed_size %
                                                                WIM_CHUNK_SIZE;

                        /* If the remainder is 0, the last chunk actually
                         * uncompresses to a full WIM_CHUNK_SIZE bytes. */
                        if (uncompressed_chunk_size == 0)
                                uncompressed_chunk_size = WIM_CHUNK_SIZE;
                }

                /* Figure out how much of this chunk we actually need to read */
                u64 start_offset;
                if (i == start_chunk)
                        start_offset = start_chunk_offset;
                else
                        start_offset = 0;
                u64 end_offset;
                if (i == end_chunk)
                        end_offset = end_chunk_offset;
                else
                        end_offset = WIM_CHUNK_SIZE - 1;

                unsigned partial_chunk_size = end_offset + 1 - start_offset;
                bool is_partial_chunk = (partial_chunk_size != uncompressed_chunk_size);

                /* This is undocumented, but chunks can be uncompressed.  This
                 * appears to always be the case when the compressed chunk size
                 * is equal to the uncompressed chunk size. */
                if (compressed_chunk_size == uncompressed_chunk_size) {
                        /* Uncompressed chunk */
                        if (full_pread(in_fd,
                                       cb ? out_p + start_offset : out_p,
                                       partial_chunk_size,
                                       cur_read_offset + start_offset) != partial_chunk_size)
                        {
                                goto read_error;
                        }
                } else {
                        /* Compressed chunk */

                        /* Read the compressed data into compressed_buf. */
                        if (full_pread(in_fd,
                                       compressed_buf,
                                       compressed_chunk_size,
                                       cur_read_offset) != compressed_chunk_size)
                        {
                                goto read_error;
                        }

                        /* For partial chunks and when writing directly to a
                         * buffer, we must buffer the uncompressed data because
                         * we don't need all of it. */
                        if (is_partial_chunk && !cb) {
                                u8 uncompressed_buf[uncompressed_chunk_size];

                                ret = decompress(compressed_buf,
                                                 compressed_chunk_size,
                                                 uncompressed_buf,
                                                 uncompressed_chunk_size);
                                if (ret) {
                                        ret = WIMLIB_ERR_DECOMPRESSION;
                                        goto out;
                                }
                                memcpy(out_p, uncompressed_buf + start_offset,
                                       partial_chunk_size);
                        } else {
                                ret = decompress(compressed_buf,
                                                 compressed_chunk_size,
                                                 out_p,
                                                 uncompressed_chunk_size);
                                if (ret) {
                                        ret = WIMLIB_ERR_DECOMPRESSION;
                                        goto out;
                                }
                        }
                }
                if (cb) {
                        /* Feed the data to the callback function */
                        ret = cb(out_p + start_offset,
                                 partial_chunk_size, ctx_or_buf);
                        if (ret)
                                goto out;
                } else {
                        /* No callback function provided; we are writing
                         * directly to a buffer.  Advance the pointer into this
                         * buffer by the number of uncompressed bytes that were
                         * written.  */
                        out_p += partial_chunk_size;
                }
                cur_read_offset += compressed_chunk_size;
        }

        ret = 0;
out:
        if (chunk_offsets_malloced)
                FREE(chunk_offsets);
        return ret;

read_error:
        ERROR_WITH_ERRNO("Error reading compressed file resource");
        ret = WIMLIB_ERR_READ;
        goto out;
}

/* Reads the contents of a struct resource_entry, as represented in the on-disk
 * format, from the memory pointed to by @p, and fills in the fields of @entry.
 * A pointer to the byte after the memory read at @p is returned. */
const void *
get_resource_entry(const void *p, struct resource_entry *entry)
{
        u64 size;
        u8 flags;

        p = get_u56(p, &size);
        p = get_u8(p, &flags);
        entry->size = size;
        entry->flags = flags;

        /* offset and original_size are truncated to 62 bits to avoid possible
         * overflows, when converting to a signed 64-bit integer (off_t) or when
         * adding size or original_size.  This is okay since no one would ever
         * actually have a WIM bigger than 4611686018427387903 bytes... */
        p = get_u64(p, &entry->offset);
        if (entry->offset & 0xc000000000000000ULL) {
                WARNING("Truncating offset in resource entry");
                entry->offset &= 0x3fffffffffffffffULL;
        }
        p = get_u64(p, &entry->original_size);
        if (entry->original_size & 0xc000000000000000ULL) {
                WARNING("Truncating original_size in resource entry");
                entry->original_size &= 0x3fffffffffffffffULL;
        }
        return p;
}

/* Copies the struct resource_entry @entry to the memory pointed to by @p in the
 * on-disk format.  A pointer to the byte after the memory written at @p is
 * returned. */
void *
put_resource_entry(void *p, const struct resource_entry *entry)
{
        p = put_u56(p, entry->size);
        p = put_u8(p, entry->flags);
        p = put_u64(p, entry->offset);
        p = put_u64(p, entry->original_size);
        return p;
}

static int
read_partial_wim_resource(const struct wim_lookup_table_entry *lte,
                          u64 size,
                          consume_data_callback_t cb,
                          void *ctx_or_buf,
                          int flags,
                          u64 offset)
{
        WIMStruct *wim;
        int in_fd;
        int ret;

        wimlib_assert(lte->resource_location == RESOURCE_IN_WIM);

        wim = lte->wim;
        in_fd = wim->in_fd;

        if (lte->resource_entry.flags & WIM_RESHDR_FLAG_COMPRESSED &&
            !(flags & WIMLIB_RESOURCE_FLAG_RAW))
        {
                ret = read_compressed_resource(in_fd,
                                               lte->resource_entry.size,
                                               lte->resource_entry.original_size,
                                               lte->resource_entry.offset,
                                               wimlib_get_compression_type(wim),
                                               size,
                                               offset,
                                               cb,
                                               ctx_or_buf);
        } else {
                offset += lte->resource_entry.offset;
                if (cb) {
                        /* Send data to callback function */
                        u8 buf[min(WIM_CHUNK_SIZE, size)];
                        while (size) {
                                size_t bytes_to_read = min(WIM_CHUNK_SIZE, size);
                                size_t bytes_read = full_pread(in_fd, buf,
                                                               bytes_to_read, offset);
                                if (bytes_read != bytes_to_read)
                                        goto read_error;
                                ret = cb(buf, bytes_read, ctx_or_buf);
                                if (ret)
                                        goto out;
                                size -= bytes_read;
                                offset += bytes_read;
                        }
                } else {
                        /* Send data directly to a buffer */
                        if (full_pread(in_fd, ctx_or_buf, size, offset) != size)
                                goto read_error;
                }
                ret = 0;
        }
        goto out;
read_error:
        ERROR_WITH_ERRNO("Error reading data from WIM");
        ret = WIMLIB_ERR_READ;
out:
        if (ret) {
                if (errno == 0)
                        errno = EIO;
        }
        return ret;
}


int
read_partial_wim_resource_into_buf(const struct wim_lookup_table_entry *lte,
                                   size_t size, u64 offset, void *buf)
{
        return read_partial_wim_resource(lte, size, NULL, buf, 0, offset);
}

static int
read_wim_resource_prefix(const struct wim_lookup_table_entry *lte,
                         u64 size,
                         consume_data_callback_t cb,
                         void *ctx_or_buf,
                         int flags)
{
        return read_partial_wim_resource(lte, size, cb, ctx_or_buf, flags, 0);
}


#ifndef __WIN32__
static int
read_file_on_disk_prefix(const struct wim_lookup_table_entry *lte,
                         u64 size,
                         consume_data_callback_t cb,
                         void *ctx_or_buf,
                         int _ignored_flags)
{
        const tchar *filename = lte->file_on_disk;
        int ret;
        int fd;
        size_t bytes_read;

        fd = open(filename, O_RDONLY);
        if (fd < 0) {
                ERROR_WITH_ERRNO("Can't open \"%"TS"\"", filename);
                return WIMLIB_ERR_OPEN;
        }
        if (cb) {
                /* Send data to callback function */
                u8 buf[min(WIM_CHUNK_SIZE, size)];
                size_t bytes_to_read;
                while (size) {
                        bytes_to_read = min(WIM_CHUNK_SIZE, size);
                        bytes_read = full_read(fd, buf, bytes_to_read);
                        if (bytes_read != bytes_to_read)
                                goto read_error;
                        ret = cb(buf, bytes_read, ctx_or_buf);
                        if (ret)
                                goto out_close;
                        size -= bytes_read;
                }
        } else {
                /* Send data directly to a buffer */
                bytes_read = full_read(fd, ctx_or_buf, size);
                if (bytes_read != size)
                        goto read_error;
        }
        ret = 0;
        goto out_close;
read_error:
        ERROR_WITH_ERRNO("Error reading \"%"TS"\"", filename);
        ret = WIMLIB_ERR_READ;
out_close:
        close(fd);
        return ret;
}
#endif /* !__WIN32__ */

static int
read_buffer_prefix(const struct wim_lookup_table_entry *lte,
                   u64 size, consume_data_callback_t cb,
                   void *ctx_or_buf, int _ignored_flags)
{
        const void *inbuf = lte->attached_buffer;
        int ret;

        if (cb) {
                while (size) {
                        size_t chunk_size = min(WIM_CHUNK_SIZE, size);
                        ret = cb(inbuf, chunk_size, ctx_or_buf);
                        if (ret)
                                return ret;
                        size -= chunk_size;
                        inbuf += chunk_size;
                }
        } else {
                memcpy(ctx_or_buf, inbuf, size);
        }
        return 0;
}

typedef int (*read_resource_prefix_handler_t)(const struct wim_lookup_table_entry *lte,
                                              u64 size,
                                              consume_data_callback_t cb,
                                              void *ctx_or_buf,
                                              int flags);

/*
 * Read the first @size bytes from a generic "resource", which may be located in
 * the WIM (compressed or uncompressed), in an external file, or directly in an
 * in-memory buffer.
 *
 * Feed the data either to a callback function (cb != NULL, passing it
 * ctx_or_buf), or write it directly into a buffer (cb == NULL, ctx_or_buf
 * specifies the buffer, which must have room for @size bytes).
 *
 * When using a callback function, it is called with chunks up to 32768 bytes in
 * size until the resource is exhausted.
 *
 * If the resource is located in a WIM file, @flags can be:
 *   * WIMLIB_RESOURCE_FLAG_RAW if the raw compressed data is to be supplied
 *     instead of the uncompressed data.
 * Otherwise, the @flags are ignored.
 */
int
read_resource_prefix(const struct wim_lookup_table_entry *lte,
                     u64 size, consume_data_callback_t cb, void *ctx_or_buf,
                     int flags)
{
        static const read_resource_prefix_handler_t handlers[] = {
                [RESOURCE_IN_WIM]             = read_wim_resource_prefix,
        #ifndef __WIN32__
                [RESOURCE_IN_FILE_ON_DISK]    = read_file_on_disk_prefix,
        #endif
                [RESOURCE_IN_ATTACHED_BUFFER] = read_buffer_prefix,
        #ifdef WITH_FUSE
                [RESOURCE_IN_STAGING_FILE]    = read_file_on_disk_prefix,
        #endif
        #ifdef WITH_NTFS_3G
                [RESOURCE_IN_NTFS_VOLUME]     = read_ntfs_file_prefix,
        #endif
        #ifdef __WIN32__
                [RESOURCE_WIN32]              = read_win32_file_prefix,
                [RESOURCE_WIN32_ENCRYPTED]    = read_win32_encrypted_file_prefix,
        #endif
        };
        wimlib_assert(lte->resource_location < ARRAY_LEN(handlers)
                      && handlers[lte->resource_location] != NULL);
        return handlers[lte->resource_location](lte, size, cb, ctx_or_buf, flags);
}

int
read_full_resource_into_buf(const struct wim_lookup_table_entry *lte,
                            void *buf)
{
        return read_resource_prefix(lte, wim_resource_size(lte), NULL, buf, 0);
}

struct extract_ctx {
        SHA_CTX sha_ctx;
        consume_data_callback_t extract_chunk;
        void *extract_chunk_arg;
};

static int
extract_chunk_sha1_wrapper(const void *chunk, size_t chunk_size,
                           void *_ctx)
{
        struct extract_ctx *ctx = _ctx;

        sha1_update(&ctx->sha_ctx, chunk, chunk_size);
        return ctx->extract_chunk(chunk, chunk_size, ctx->extract_chunk_arg);
}

/* Extracts the first @size bytes of a WIM resource to somewhere.  In the
 * process, the SHA1 message digest of the resource is checked if the full
 * resource is being extracted.
 *
 * @extract_chunk is a function that is called to extract each chunk of the
 * resource. */
int
extract_wim_resource(const struct wim_lookup_table_entry *lte,
                     u64 size,
                     consume_data_callback_t extract_chunk,
                     void *extract_chunk_arg)
{
        int ret;
        if (size == wim_resource_size(lte)) {
                /* Do SHA1 */
                struct extract_ctx ctx;
                ctx.extract_chunk = extract_chunk;
                ctx.extract_chunk_arg = extract_chunk_arg;
                sha1_init(&ctx.sha_ctx);
                ret = read_resource_prefix(lte, size,
                                           extract_chunk_sha1_wrapper,
                                           &ctx, 0);
                if (ret == 0) {
                        u8 hash[SHA1_HASH_SIZE];
                        sha1_final(hash, &ctx.sha_ctx);
                        if (!hashes_equal(hash, lte->hash)) {
                        #ifdef ENABLE_ERROR_MESSAGES
                                ERROR("Invalid SHA1 message digest "
                                      "on the following WIM resource:");
                                print_lookup_table_entry(lte, stderr);
                                if (lte->resource_location == RESOURCE_IN_WIM)
                                        ERROR("The WIM file appears to be corrupt!");
                        #endif
                                ret = WIMLIB_ERR_INVALID_RESOURCE_HASH;
                        }
                }
        } else {
                /* Don't do SHA1 */
                ret = read_resource_prefix(lte, size, extract_chunk,
                                           extract_chunk_arg, 0);
        }
        return ret;
}

static int
extract_wim_chunk_to_fd(const void *buf, size_t len, void *_fd_p)
{
        int fd = *(int*)_fd_p;
        ssize_t ret = full_write(fd, buf, len);
        if (ret < len) {
                ERROR_WITH_ERRNO("Error writing to file descriptor");
                return WIMLIB_ERR_WRITE;
        } else {
                return 0;
        }
}

int
extract_wim_resource_to_fd(const struct wim_lookup_table_entry *lte,
                           int fd, u64 size)
{
        return extract_wim_resource(lte, size, extract_wim_chunk_to_fd, &fd);
}


static int
sha1_chunk(const void *buf, size_t len, void *ctx)
{
        sha1_update(ctx, buf, len);
        return 0;
}

/* Calculate the SHA1 message digest of a stream. */
int
sha1_resource(struct wim_lookup_table_entry *lte)
{
        int ret;
        SHA_CTX sha_ctx;

        sha1_init(&sha_ctx);
        ret = read_resource_prefix(lte, wim_resource_size(lte),
                                   sha1_chunk, &sha_ctx, 0);
        if (ret == 0)
                sha1_final(lte->hash, &sha_ctx);
        return ret;
}

/*
 * Copies the file resource specified by the lookup table entry @lte from the
 * input WIM to the output WIM that has its FILE * given by
 * ((WIMStruct*)wim)->out_fp.
 *
 * The output_resource_entry, out_refcnt, and part_number fields of @lte are
 * updated.
 *
 * (This function is confusing and should be refactored somehow.)
 */
int
copy_resource(struct wim_lookup_table_entry *lte, void *wim)
{
        WIMStruct *w = wim;
        int ret;

        ret = write_wim_resource(lte, w->out_fd,
                                 wim_resource_compression_type(lte),
                                 &lte->output_resource_entry, 0);
        if (ret == 0) {
                lte->out_refcnt = lte->refcnt;
                lte->part_number = w->hdr.part_number;
        }
        return ret;
}