tar.c

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#include "c.h"
#include "pgtar.h"
#include <sys/stat.h>

/*
 * Utility routine to print possibly larger than 32 bit integers in a
 * portable fashion.  Filled with zeros.
 */
static void
print_val(char *s, uint64 val, unsigned int base, size_t len)
{
    int         i;

    for (i = len; i > 0; i--)
    {
        int         digit = val % base;

        s[i - 1] = '0' + digit;
        val = val / base;
    }
}


/*
 * Calculate the tar checksum for a header. The header is assumed to always
 * be 512 bytes, per the tar standard.
 */
int
tarChecksum(char *header)
{
    int         i,
                sum;

    /*
     * Per POSIX, the checksum is the simple sum of all bytes in the header,
     * treating the bytes as unsigned, and treating the checksum field (at
     * offset 148) as though it contained 8 spaces.
     */
    sum = 8 * ' ';              /* presumed value for checksum field */
    for (i = 0; i < 512; i++)
        if (i < 148 || i >= 156)
            sum += 0xFF & header[i];
    return sum;
}


/*
 * Fill in the buffer pointed to by h with a tar format header. This buffer
 * must always have space for 512 characters, which is a requirement by
 * the tar format.
 */
void
tarCreateHeader(char *h, const char *filename, const char *linktarget,
                size_t size, mode_t mode, uid_t uid, gid_t gid, time_t mtime)
{
    /*
     * Note: most of the fields in a tar header are not supposed to be
     * null-terminated.  We use sprintf, which will write a null after the
     * required bytes; that null goes into the first byte of the next field.
     * This is okay as long as we fill the fields in order.
     */
    memset(h, 0, 512);          /* assume tar header size */

    /* Name 100 */
    sprintf(&h[0], "%.99s", filename);
    if (linktarget != NULL || S_ISDIR(mode))
    {
        /*
         * We only support symbolic links to directories, and this is
         * indicated in the tar format by adding a slash at the end of the
         * name, the same as for regular directories.
         */
        int         flen = strlen(filename);

        flen = Min(flen, 99);
        h[flen] = '/';
        h[flen + 1] = '\0';
    }

    /* Mode 8 */
    sprintf(&h[100], "%07o ", (int) mode);

    /* User ID 8 */
    sprintf(&h[108], "%07o ", uid);

    /* Group 8 */
    sprintf(&h[116], "%07o ", gid);

    /* File size 12 - 11 digits, 1 space; use print_val for 64 bit support */
    if (linktarget != NULL || S_ISDIR(mode))
        /* Symbolic link or directory has size zero */
        print_val(&h[124], 0, 8, 11);
    else
        print_val(&h[124], size, 8, 11);
    sprintf(&h[135], " ");

    /* Mod Time 12 */
    sprintf(&h[136], "%011o ", (int) mtime);

    /* Checksum 8 cannot be calculated until we've filled all other fields */

    if (linktarget != NULL)
    {
        /* Type - Symbolic link */
        sprintf(&h[156], "2");
        /* Link Name 100 */
        sprintf(&h[157], "%.99s", linktarget);
    }
    else if (S_ISDIR(mode))
        /* Type - directory */
        sprintf(&h[156], "5");
    else
        /* Type - regular file */
        sprintf(&h[156], "0");

    /* Magic 6 */
    sprintf(&h[257], "ustar");

    /* Version 2 */
    sprintf(&h[263], "00");

    /* User 32 */
    /* XXX: Do we need to care about setting correct username? */
    sprintf(&h[265], "%.31s", "postgres");

    /* Group 32 */
    /* XXX: Do we need to care about setting correct group name? */
    sprintf(&h[297], "%.31s", "postgres");

    /* Major Dev 8 */
    sprintf(&h[329], "%07o ", 0);

    /* Minor Dev 8 */
    sprintf(&h[337], "%07o ", 0);

    /* Prefix 155 - not used, leave as nulls */

    /*
     * We mustn't overwrite the next field while inserting the checksum.
     * Fortunately, the checksum can't exceed 6 octal digits, so we just write
     * 6 digits, a space, and a null, which is legal per POSIX.
     */
    sprintf(&h[148], "%06o ", tarChecksum(h));
}