/*

 * gzlog.c

 * Copyright (C) 2004, 2008 Mark Adler, all rights reserved

 * For conditions of distribution and use, see copyright notice in gzlog.h

 * version 2.0, 25 Apr 2008

 */

/*

   gzlog provides a mechanism for frequently appending short strings to a gzip

   file that is efficient both in execution time and compression ratio.  The

   strategy is to write the short strings in an uncompressed form to the end of

   the gzip file, only compressing when the amount of uncompressed data has

   reached a given threshold.

   gzlog also provides protection against interruptions in the process due to

   system crashes.  The status of the operation is recorded in an extra field

   in the gzip file, and is only updated once the gzip file is brought to a

   valid state.  The last data to be appended or compressed is saved in an

   auxiliary file, so that if the operation is interrupted, it can be completed

   the next time an append operation is attempted.

   gzlog maintains another auxiliary file with the last 32K of data from the

   compressed portion, which is preloaded for the compression of the subsequent

   data.  This minimizes the impact to the compression ratio of appending.

 */

/*

   Operations Concept:

   Files (log name "foo"):

   foo.gz -- gzip file with the complete log

   foo.add -- last message to append or last data to compress

   foo.dict -- dictionary of the last 32K of data for next compression

   foo.temp -- temporary dictionary file for compression after this one

   foo.lock -- lock file for reading and writing the other files

   foo.repairs -- log file for log file recovery operations (not compressed)

   gzip file structure:

   - fixed-length (no file name) header with extra field (see below)

   - compressed data ending initially with empty stored block

   - uncompressed data filling out originally empty stored block and

     subsequent stored blocks as needed (16K max each)

   - gzip trailer

   - no junk at end (no other gzip streams)

   When appending data, the information in the first three items above plus the

   foo.add file are sufficient to recover an interrupted append operation.  The

   extra field has the necessary information to restore the start of the last

   stored block and determine where to append the data in the foo.add file, as

   well as the crc and length of the gzip data before the append operation.

   The foo.add file is created before the gzip file is marked for append, and

   deleted after the gzip file is marked as complete.  So if the append

   operation is interrupted, the data to add will still be there.  If due to

   some external force, the foo.add file gets deleted between when the append

   operation was interrupted and when recovery is attempted, the gzip file will

   still be restored, but without the appended data.

   When compressing data, the information in the first two items above plus the

   foo.add file are sufficient to recover an interrupted compress operation.

   The extra field has the necessary information to find the end of the

   compressed data, and contains both the crc and length of just the compressed

   data and of the complete set of data including the contents of the foo.add

   file.

   Again, the foo.add file is maintained during the compress operation in case

   of an interruption.  If in the unlikely event the foo.add file with the data

   to be compressed is missing due to some external force, a gzip file with

   just the previous compressed data will be reconstructed.  In this case, all

   of the data that was to be compressed is lost (approximately one megabyte).

   This will not occur if all that happened was an interruption of the compress

   operation.

   The third state that is marked is the replacement of the old dictionary with

   the new dictionary after a compress operation.  Once compression is

   complete, the gzip file is marked as being in the replace state.  This

   completes the gzip file, so an interrupt after being so marked does not

   result in recompression.  Then the dictionary file is replaced, and the gzip

   file is marked as completed.  This state prevents the possibility of

   restarting compression with the wrong dictionary file.

   All three operations are wrapped by a lock/unlock procedure.  In order to

   gain exclusive access to the log files, first a foo.lock file must be

   exclusively created.  When all operations are complete, the lock is

   released by deleting the foo.lock file.  If when attempting to create the

   lock file, it already exists and the modify time of the lock file is more

   than five minutes old (set by the PATIENCE define below), then the old

   lock file is considered stale and deleted, and the exclusive creation of

   the lock file is retried.  To assure that there are no false assessments

   of the staleness of the lock file, the operations periodically touch the

   lock file to update the modified date.

   Following is the definition of the extra field with all of the information

   required to enable the above append and compress operations and their

   recovery if interrupted.  Multi-byte values are stored little endian

   (consistent with the gzip format).  File pointers are eight bytes long.

   The crc's and lengths for the gzip trailer are four bytes long.  (Note that

   the length at the end of a gzip file is used for error checking only, and

   for large files is actually the length modulo 2^32.)  The stored block

   length is two bytes long.  The gzip extra field two-byte identification is

   "ap" for append.  It is assumed that writing the extra field to the file is

   an "atomic" operation.  That is, either all of the extra field is written

   to the file, or none of it is, if the operation is interrupted right at the

   point of updating the extra field.  This is a reasonable assumption, since

   the extra field is within the first 52 bytes of the file, which is smaller

   than any expected block size for a mass storage device (usually 512 bytes or

   larger).

   Extra field (35 bytes):

   - Pointer to first stored block length -- this points to the two-byte length

     of the first stored block, which is followed by the two-byte, one's

     complement of that length.  The stored block length is preceded by the

     three-bit header of the stored block, which is the actual start of the

     stored block in the deflate format.  See the bit offset field below.

   - Pointer to the last stored block length.  This is the same as above, but

     for the last stored block of the uncompressed data in the gzip file.

     Initially this is the same as the first stored block length pointer.

     When the stored block gets to 16K (see the MAX_STORE define), then a new

     stored block as added, at which point the last stored block length pointer

     is different from the first stored block length pointer.  When they are

     different, the first bit of the last stored block header is eight bits, or

     one byte back from the block length.

   - Compressed data crc and length.  This is the crc and length of the data

     that is in the compressed portion of the deflate stream.  These are used

     only in the event that the foo.add file containing the data to compress is

     lost after a compress operation is interrupted.

   - Total data crc and length.  This is the crc and length of all of the data

     stored in the gzip file, compressed and uncompressed.  It is used to

     reconstruct the gzip trailer when compressing, as well as when recovering

     interrupted operations.

   - Final stored block length.  This is used to quickly find where to append,

     and allows the restoration of the original final stored block state when

     an append operation is interrupted.

   - First stored block start as the number of bits back from the final stored

     block first length byte.  This value is in the range of 3..10, and is

     stored as the low three bits of the final byte of the extra field after

     subtracting three (0..7).  This allows the last-block bit of the stored

     block header to be updated when a new stored block is added, for the case

     when the first stored block and the last stored block are the same.  (When

     they are different, the numbers of bits back is known to be eight.)  This

     also allows for new compressed data to be appended to the old compressed

     data in the compress operation, overwriting the previous first stored

     block, or for the compressed data to be terminated and a valid gzip file

     reconstructed on the off chance that a compression operation was

     interrupted and the data to compress in the foo.add file was deleted.

   - The operation in process.  This is the next two bits in the last byte (the

     bits under the mask 0x18).  The are interpreted as 0: nothing in process,

     1: append in process, 2: compress in process, 3: replace in process.

   - The top three bits of the last byte in the extra field are reserved and

     are currently set to zero.

   Main procedure:

   - Exclusively create the foo.lock file using the O_CREAT and O_EXCL modes of

     the system open() call.  If the modify time of an existing lock file is

     more than PATIENCE seconds old, then the lock file is deleted and the

     exclusive create is retried.

   - Load the extra field from the foo.gz file, and see if an operation was in

     progress but not completed.  If so, apply the recovery procedure below.

   - Perform the append procedure with the provided data.

   - If the uncompressed data in the foo.gz file is 1MB or more, apply the

     compress procedure.

   - Delete the foo.lock file.

   Append procedure:

   - Put what to append in the foo.add file so that the operation can be

     restarted if this procedure is interrupted.

   - Mark the foo.gz extra field with the append operation in progress.

   + Restore the original last-block bit and stored block length of the last

     stored block from the information in the extra field, in case a previous

     append operation was interrupted.

   - Append the provided data to the last stored block, creating new stored

     blocks as needed and updating the stored blocks last-block bits and

     lengths.

   - Update the crc and length with the new data, and write the gzip trailer.

   - Write over the extra field (with a single write operation) with the new

     pointers, lengths, and crc's, and mark the gzip file as not in process.

     Though there is still a foo.add file, it will be ignored since nothing

     is in process.  If a foo.add file is leftover from a previously

     completed operation, it is truncated when writing new data to it.

   - Delete the foo.add file.

   Compress and replace procedures:

   - Read all of the uncompressed data in the stored blocks in foo.gz and write

     it to foo.add.  Also write foo.temp with the last 32K of that data to

     provide a dictionary for the next invocation of this procedure.

   - Rewrite the extra field marking foo.gz with a compression in process.

   * If there is no data provided to compress (due to a missing foo.add file

     when recovering), reconstruct and truncate the foo.gz file to contain

     only the previous compressed data and proceed to the step after the next

     one.  Otherwise ...

   - Compress the data with the dictionary in foo.dict, and write to the

     foo.gz file starting at the bit immediately following the last previously

     compressed block.  If there is no foo.dict, proceed anyway with the

     compression at slightly reduced efficiency.  (For the foo.dict file to be

     missing requires some external failure beyond simply the interruption of

     a compress operation.)  During this process, the foo.lock file is

     periodically touched to assure that that file is not considered stale by

     another process before we're done.  The deflation is terminated with a

     non-last empty static block (10 bits long), that is then located and

     written over by a last-bit-set empty stored block.

   - Append the crc and length of the data in the gzip file (previously

     calculated during the append operations).

   - Write over the extra field with the updated stored block offsets, bits

     back, crc's, and lengths, and mark foo.gz as in process for a replacement

     of the dictionary.

   @ Delete the foo.add file.

   - Replace foo.dict with foo.temp.

   - Write over the extra field, marking foo.gz as complete.

   Recovery procedure:

   - If not a replace recovery, read in the foo.add file, and provide that data

     to the appropriate recovery below.  If there is no foo.add file, provide

     a zero data length to the recovery.  In that case, the append recovery

     restores the foo.gz to the previous compressed + uncompressed data state.

     For the the compress recovery, a missing foo.add file results in foo.gz

     being restored to the previous compressed-only data state.

   - Append recovery:

     - Pick up append at + step above

   - Compress recovery:

     - Pick up compress at * step above

   - Replace recovery:

     - Pick up compress at @ step above

   - Log the repair with a date stamp in foo.repairs

 */

#include <sys types.h=""></sys>

#include <stdio.h>      /* rename, fopen, fprintf, fclose */</stdio.h>

#include <stdlib.h>     /* malloc, free */</stdlib.h>

#include <string.h>     /* strlen, strrchr, strcpy, strncpy, strcmp */</string.h>

#include <fcntl.h>      /* open */</fcntl.h>

#include <unistd.h>     /* lseek, read, write, close, unlink, sleep, */</unistd.h>

                        /* ftruncate, fsync */

#include <errno.h>      /* errno */</errno.h>

#include <time.h>       /* time, ctime */</time.h>

#include <sys stat.h="">   /* stat */</sys>

#include <sys time.h="">   /* utimes */</sys>

#include "zlib.h"       /* crc32 */

#include "gzlog.h"      /* header for external access */

#define local static

typedef unsigned int uint;

typedef unsigned long ulong;

/* Macro for debugging to deterministically force recovery operations */

#ifdef DEBUG

    #include <setjmp.h>         /* longjmp */</setjmp.h>

    jmp_buf gzlog_jump;         /* where to go back to */

    int gzlog_bail = 0;         /* which point to bail at (1..8) */

    int gzlog_count = -1;       /* number of times through to wait */

#   define BAIL(n) do { if (n == gzlog_bail && gzlog_count-- == 0) \

                            longjmp(gzlog_jump, gzlog_bail); } while (0)

#else

#   define BAIL(n)

#endif

/* how old the lock file can be in seconds before considering it stale */

#define PATIENCE 300

/* maximum stored block size in Kbytes -- must be in 1..63 */

#define MAX_STORE 16

/* number of stored Kbytes to trigger compression (must be >= 32 to allow

   dictionary construction, and <= 204 * MAX_STORE, in order for >> 10 to

   discard the stored block headers contribution of five bytes each) */

#define TRIGGER 1024

/* size of a deflate dictionary (this cannot be changed) */

#define DICT 32768U

/* values for the operation (2 bits) */

#define NO_OP 0

#define APPEND_OP 1

#define COMPRESS_OP 2

#define REPLACE_OP 3

/* macros to extract little-endian integers from an unsigned byte buffer */

#define PULL2(p) ((p)[0]+((uint)((p)[1])<<8))

#define PULL4(p) (PULL2(p)+((ulong)PULL2(p+2)<<16))

#define PULL8(p) (PULL4(p)+((off_t)PULL4(p+4)<<32))

/* macros to store integers into a byte buffer in little-endian order */

#define PUT2(p,a) do {(p)[0]=a;(p)[1]=(a)>>8;} while(0)

#define PUT4(p,a) do {PUT2(p,a);PUT2(p+2,a>>16);} while(0)

#define PUT8(p,a) do {PUT4(p,a);PUT4(p+4,a>>32);} while(0)

/* internal structure for log information */

#define LOGID "\106\035\172"    /* should be three non-zero characters */

struct log {

    char id[4];     /* contains LOGID to detect inadvertent overwrites */

    int fd;         /* file descriptor for .gz file, opened read/write */

    char *path;     /* allocated path, e.g. "/var/log/foo" or "foo" */

    char *end;      /* end of path, for appending suffices such as ".gz" */

    off_t first;    /* offset of first stored block first length byte */

    int back;       /* location of first block id in bits back from first */

    uint stored;    /* bytes currently in last stored block */

    off_t last;     /* offset of last stored block first length byte */

    ulong ccrc;     /* crc of compressed data */

    ulong clen;     /* length (modulo 2^32) of compressed data */

    ulong tcrc;     /* crc of total data */

    ulong tlen;     /* length (modulo 2^32) of total data */

    time_t lock;    /* last modify time of our lock file */

};

/* gzip header for gzlog */

local unsigned char log_gzhead[] = {

    0x1f, 0x8b,                 /* magic gzip id */

    8,                          /* compression method is deflate */

    4,                          /* there is an extra field (no file name) */

    0, 0, 0, 0,                 /* no modification time provided */

    0, 0xff,                    /* no extra flags, no OS specified */

    39, 0, 'a', 'p', 35, 0      /* extra field with "ap" subfield */

                                /* 35 is EXTRA, 39 is EXTRA + 4 */

};

#define HEAD sizeof(log_gzhead)     /* should be 16 */

/* initial gzip extra field content (52 == HEAD + EXTRA + 1) */

local unsigned char log_gzext[] = {

    52, 0, 0, 0, 0, 0, 0, 0,    /* offset of first stored block length */

    52, 0, 0, 0, 0, 0, 0, 0,    /* offset of last stored block length */

    0, 0, 0, 0, 0, 0, 0, 0,     /* compressed data crc and length */

    0, 0, 0, 0, 0, 0, 0, 0,     /* total data crc and length */

    0, 0,                       /* final stored block data length */

    5                           /* op is NO_OP, last bit 8 bits back */

};

#define EXTRA sizeof(log_gzext)     /* should be 35 */

/* initial gzip data and trailer */

local unsigned char log_gzbody[] = {

    1, 0, 0, 0xff, 0xff,        /* empty stored block (last) */

    0, 0, 0, 0,                 /* crc */

    0, 0, 0, 0                  /* uncompressed length */

};

#define BODY sizeof(log_gzbody)

/* Exclusively create foo.lock in order to negotiate exclusive access to the

   foo.* files.  If the modify time of an existing lock file is greater than

   PATIENCE seconds in the past, then consider the lock file to have been

   abandoned, delete it, and try the exclusive create again.  Save the lock

   file modify time for verification of ownership.  Return 0 on success, or -1

   on failure, usually due to an access restriction or invalid path.  Note that

   if stat() or unlink() fails, it may be due to another process noticing the

   abandoned lock file a smidge sooner and deleting it, so those are not

   flagged as an error. */

local int log_lock(struct log *log)

{

    int fd;

    struct stat st;

    strcpy(log->end, ".lock");

    while ((fd = open(log->path, O_CREAT | O_EXCL, 0644)) < 0) {

        if (errno != EEXIST)

            return -1;

        if (stat(log->path, &st) == 0 && time(NULL) - st.st_mtime > PATIENCE) {

            unlink(log->path);

            continue;

        }

        sleep(2);       /* relinquish the CPU for two seconds while waiting */

    }

    close(fd);

    if (stat(log->path, &st) == 0)

        log->lock = st.st_mtime;

    return 0;

}

/* Update the modify time of the lock file to now, in order to prevent another

   task from thinking that the lock is stale.  Save the lock file modify time

   for verification of ownership. */

local void log_touch(struct log *log)

{

    struct stat st;

    strcpy(log->end, ".lock");

    utimes(log->path, NULL);

    if (stat(log->path, &st) == 0)

        log->lock = st.st_mtime;

}

/* Check the log file modify time against what is expected.  Return true if

   this is not our lock.  If it is our lock, touch it to keep it. */

local int log_check(struct log *log)

{

    struct stat st;

    strcpy(log->end, ".lock");

    if (stat(log->path, &st) || st.st_mtime != log->lock)

        return 1;

    log_touch(log);

    return 0;

}

/* Unlock a previously acquired lock, but only if it's ours. */

local void log_unlock(struct log *log)

{

    if (log_check(log))

        return;

    strcpy(log->end, ".lock");

    unlink(log->path);

    log->lock = 0;

}

/* Check the gzip header and read in the extra field, filling in the values in

   the log structure.  Return op on success or -1 if the gzip header was not as

   expected.  op is the current operation in progress last written to the extra

   field.  This assumes that the gzip file has already been opened, with the

   file descriptor log->fd. */

local int log_head(struct log *log)

{

    int op;

    unsigned char buf[HEAD + EXTRA];

    if (lseek(log->fd, 0, SEEK_SET) < 0 ||

        read(log->fd, buf, HEAD + EXTRA) != HEAD + EXTRA ||

        memcmp(buf, log_gzhead, HEAD)) {

        return -1;

    }

    log->first = PULL8(buf + HEAD);

    log->last = PULL8(buf + HEAD + 8);

    log->ccrc = PULL4(buf + HEAD + 16);

    log->clen = PULL4(buf + HEAD + 20);

    log->tcrc = PULL4(buf + HEAD + 24);

    log->tlen = PULL4(buf + HEAD + 28);

    log->stored = PULL2(buf + HEAD + 32);

    log->back = 3 + (buf[HEAD + 34] & 7);

    op = (buf[HEAD + 34] >> 3) & 3;

    return op;

}

/* Write over the extra field contents, marking the operation as op.  Use fsync

   to assure that the device is written to, and in the requested order.  This

   operation, and only this operation, is assumed to be atomic in order to

   assure that the log is recoverable in the event of an interruption at any

   point in the process.  Return -1 if the write to foo.gz failed. */

local int log_mark(struct log *log, int op)

{

    int ret;

    unsigned char ext[EXTRA];

    PUT8(ext, log->first);

    PUT8(ext + 8, log->last);

    PUT4(ext + 16, log->ccrc);

    PUT4(ext + 20, log->clen);

    PUT4(ext + 24, log->tcrc);

    PUT4(ext + 28, log->tlen);

    PUT2(ext + 32, log->stored);

    ext[34] = log->back - 3 + (op << 3);

    fsync(log->fd);

    ret = lseek(log->fd, HEAD, SEEK_SET) < 0 ||

          write(log->fd, ext, EXTRA) != EXTRA ? -1 : 0;

    fsync(log->fd);

    return ret;

}

/* Rewrite the last block header bits and subsequent zero bits to get to a byte

   boundary, setting the last block bit if last is true, and then write the

   remainder of the stored block header (length and one's complement).  Leave

   the file pointer after the end of the last stored block data.  Return -1 if

   there is a read or write failure on the foo.gz file */

local int log_last(struct log *log, int last)

{

    int back, len, mask;

    unsigned char buf[6];

    /* determine the locations of the bytes and bits to modify */

    back = log->last == log->first ? log->back : 8;

    len = back > 8 ? 2 : 1;                 /* bytes back from log->last */

    mask = 0x80 >> ((back - 1) & 7);        /* mask for block last-bit */

    /* get the byte to modify (one or two back) into buf[0] -- don't need to

       read the byte if the last-bit is eight bits back, since in that case

       the entire byte will be modified */

    buf[0] = 0;

    if (back != 8 && (lseek(log->fd, log->last - len, SEEK_SET) < 0 ||

                      read(log->fd, buf, 1) != 1))

        return -1;

    /* change the last-bit of the last stored block as requested -- note

       that all bits above the last-bit are set to zero, per the type bits

       of a stored block being 00 and per the convention that the bits to

       bring the stream to a byte boundary are also zeros */

    buf[1] = 0;

    buf[2 - len] = (*buf & (mask - 1)) + (last ? mask : 0);

    /* write the modified stored block header and lengths, move the file

       pointer to after the last stored block data */

    PUT2(buf + 2, log->stored);

    PUT2(buf + 4, log->stored ^ 0xffff);

    return lseek(log->fd, log->last - len, SEEK_SET) < 0 ||

           write(log->fd, buf + 2 - len, len + 4) != len + 4 ||

           lseek(log->fd, log->stored, SEEK_CUR) < 0 ? -1 : 0;

}

/* Append len bytes from data to the locked and open log file.  len may be zero

   if recovering and no .add file was found.  In that case, the previous state

   of the foo.gz file is restored.  The data is appended uncompressed in

   deflate stored blocks.  Return -1 if there was an error reading or writing

   the foo.gz file. */

local int log_append(struct log *log, unsigned char *data, size_t len)

{

    uint put;

    off_t end;

    unsigned char buf[8];

    /* set the last block last-bit and length, in case recovering an

       interrupted append, then position the file pointer to append to the

       block */

    if (log_last(log, 1))

        return -1;

    /* append, adding stored blocks and updating the offset of the last stored

       block as needed, and update the total crc and length */

    while (len) {

        /* append as much as we can to the last block */

        put = (MAX_STORE << 10) - log->stored;

        if (put > len)

            put = (uint)len;

        if (put) {

            if (write(log->fd, data, put) != put)

                return -1;

            BAIL(1);

            log->tcrc = crc32(log->tcrc, data, put);

            log->tlen += put;

            log->stored += put;

            data += put;

            len -= put;

        }

        /* if we need to, add a new empty stored block */

        if (len) {

            /* mark current block as not last */

            if (log_last(log, 0))

                return -1;

            /* point to new, empty stored block */

            log->last += 4 + log->stored + 1;

            log->stored = 0;

        }

        /* mark last block as last, update its length */

        if (log_last(log, 1))

            return -1;

        BAIL(2);

    }

    /* write the new crc and length trailer, and truncate just in case (could

       be recovering from partial append with a missing foo.add file) */

    PUT4(buf, log->tcrc);

    PUT4(buf + 4, log->tlen);

    if (write(log->fd, buf, 8) != 8 ||

        (end = lseek(log->fd, 0, SEEK_CUR)) < 0 || ftruncate(log->fd, end))

        return -1;

    /* write the extra field, marking the log file as done, delete .add file */

    if (log_mark(log, NO_OP))

        return -1;

    strcpy(log->end, ".add");

    unlink(log->path);          /* ignore error, since may not exist */

    return 0;

}

/* Replace the foo.dict file with the foo.temp file.  Also delete the foo.add

   file, since the compress operation may have been interrupted before that was

   done.  Returns 1 if memory could not be allocated, or -1 if reading or

   writing foo.gz fails, or if the rename fails for some reason other than

   foo.temp not existing.  foo.temp not existing is a permitted error, since

   the replace operation may have been interrupted after the rename is done,

   but before foo.gz is marked as complete. */

local int log_replace(struct log *log)

{

    int ret;

    char *dest;

    /* delete foo.add file */

    strcpy(log->end, ".add");

    unlink(log->path);         /* ignore error, since may not exist */

    BAIL(3);

    /* rename foo.name to foo.dict, replacing foo.dict if it exists */

    strcpy(log->end, ".dict");

    dest = malloc(strlen(log->path) + 1);

    if (dest == NULL)

        return -2;

    strcpy(dest, log->path);

    strcpy(log->end, ".temp");

    ret = rename(log->path, dest);

    free(dest);

    if (ret && errno != ENOENT)

        return -1;

    BAIL(4);

    /* mark the foo.gz file as done */

    return log_mark(log, NO_OP);

}

/* Compress the len bytes at data and append the compressed data to the

   foo.gz deflate data immediately after the previous compressed data.  This

   overwrites the previous uncompressed data, which was stored in foo.add

   and is the data provided in data[0..len-1].  If this operation is

   interrupted, it picks up at the start of this routine, with the foo.add

   file read in again.  If there is no data to compress (len == 0), then we

   simply terminate the foo.gz file after the previously compressed data,

   appending a final empty stored block and the gzip trailer.  Return -1 if

   reading or writing the log.gz file failed, or -2 if there was a memory

   allocation failure. */

local int log_compress(struct log *log, unsigned char *data, size_t len)

{

    int fd;

    uint got, max;

    ssize_t dict;

    off_t end;

    z_stream strm;

    unsigned char buf[DICT];

    /* compress and append compressed data */

    if (len) {

        /* set up for deflate, allocating memory */

        strm.zalloc = Z_NULL;

        strm.zfree = Z_NULL;

        strm.opaque = Z_NULL;

        if (deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED, -15, 8,

                         Z_DEFAULT_STRATEGY) != Z_OK)

            return -2;

        /* read in dictionary (last 32K of data that was compressed) */

        strcpy(log->end, ".dict");

        fd = open(log->path, O_RDONLY, 0);

        if (fd >= 0) {

            dict = read(fd, buf, DICT);

            close(fd);

            if (dict < 0) {

                deflateEnd(&strm);

                return -1;

            }

            if (dict)

                deflateSetDictionary(&strm, buf, (uint)dict);

        }

        log_touch(log);

        /* prime deflate with last bits of previous block, position write

           pointer to write those bits and overwrite what follows */

        if (lseek(log->fd, log->first - (log->back > 8 ? 2 : 1),

                SEEK_SET) < 0 ||

            read(log->fd, buf, 1) != 1 || lseek(log->fd, -1, SEEK_CUR) < 0) {

            deflateEnd(&strm);

            return -1;

        }

        deflatePrime(&strm, (8 - log->back) & 7, *buf);

        /* compress, finishing with a partial non-last empty static block */

        strm.next_in = data;

        max = (((uint)0 - 1) >> 1) + 1; /* in case int smaller than size_t */

        do {

            strm.avail_in = len > max ? max : (uint)len;

            len -= strm.avail_in;

            do {

                strm.avail_out = DICT;

                strm.next_out = buf;

                deflate(&strm, len ? Z_NO_FLUSH : Z_PARTIAL_FLUSH);

                got = DICT - strm.avail_out;

                if (got && write(log->fd, buf, got) != got) {

                    deflateEnd(&strm);

                    return -1;

                }

                log_touch(log);

            } while (strm.avail_out == 0);

        } while (len);

        deflateEnd(&strm);

        BAIL(5);

        /* find start of empty static block -- scanning backwards the first one

           bit is the second bit of the block, if the last byte is zero, then

           we know the byte before that has a one in the top bit, since an

           empty static block is ten bits long */

        if ((log->first = lseek(log->fd, -1, SEEK_CUR)) < 0 ||

            read(log->fd, buf, 1) != 1)

            return -1;

        log->first++;

        if (*buf) {

            log->back = 1;

            while ((*buf & ((uint)1 << (8 - log->back++))) == 0)

                ;       /* guaranteed to terminate, since *buf != 0 */

        }

        else

            log->back = 10;

        /* update compressed crc and length */

        log->ccrc = log->tcrc;

        log->clen = log->tlen;

    }

    else {

        /* no data to compress -- fix up existing gzip stream */

        log->tcrc = log->ccrc;

        log->tlen = log->clen;

    }

    /* complete and truncate gzip stream */

    log->last = log->first;

    log->stored = 0;

    PUT4(buf, log->tcrc);

    PUT4(buf + 4, log->tlen);

    if (log_last(log, 1) || write(log->fd, buf, 8) != 8 ||

        (end = lseek(log->fd, 0, SEEK_CUR)) < 0 || ftruncate(log->fd, end))

        return -1;

    BAIL(6);

    /* mark as being in the replace operation */

    if (log_mark(log, REPLACE_OP))

        return -1;

    /* execute the replace operation and mark the file as done */

    return log_replace(log);

}

/* log a repair record to the .repairs file */

local void log_log(struct log *log, int op, char *record)

{

    time_t now;

    FILE *rec;

    now = time(NULL);

    strcpy(log->end, ".repairs");

    rec = fopen(log->path, "a");

    if (rec == NULL)

        return;

    fprintf(rec, "%.24s %s recovery: %s\n", ctime(&now), op == APPEND_OP ?

            "append" : (op == COMPRESS_OP ? "compress" : "replace"), record);

    fclose(rec);

    return;

}

/* Recover the interrupted operation op.  First read foo.add for recovering an

   append or compress operation.  Return -1 if there was an error reading or

   writing foo.gz or reading an existing foo.add, or -2 if there was a memory

   allocation failure. */

local int log_recover(struct log *log, int op)

{

    int fd, ret = 0;

    unsigned char *data = NULL;

    size_t len = 0;

    struct stat st;

    /* log recovery */

    log_log(log, op, "start");

    /* load foo.add file if expected and present */

    if (op == APPEND_OP || op == COMPRESS_OP) {

        strcpy(log->end, ".add");

        if (stat(log->path, &st) == 0 && st.st_size) {

            len = (size_t)(st.st_size);

            if (len != st.st_size || (data = malloc(st.st_size)) == NULL) {

                log_log(log, op, "allocation failure");

                return -2;

            }

            if ((fd = open(log->path, O_RDONLY, 0)) < 0) {

                log_log(log, op, ".add file read failure");

                return -1;

            }

            ret = read(fd, data, len) != len;

            close(fd);

            if (ret) {

                log_log(log, op, ".add file read failure");

                return -1;

            }

            log_log(log, op, "loaded .add file");

        }

        else

            log_log(log, op, "missing .add file!");

    }

    /* recover the interrupted operation */

    switch (op) {

    case APPEND_OP:

        ret = log_append(log, data, len);

        break;

    case COMPRESS_OP:

        ret = log_compress(log, data, len);

        break;

    case REPLACE_OP:

        ret = log_replace(log);

    }

    /* log status */

    log_log(log, op, ret ? "failure" : "complete");

    /* clean up */

    if (data != NULL)

        free(data);

    return ret;

}

/* Close the foo.gz file (if open) and release the lock. */

local void log_close(struct log *log)

{

    if (log->fd >= 0)

        close(log->fd);

    log->fd = -1;

    log_unlock(log);

}

/* Open foo.gz, verify the header, and load the extra field contents, after

   first creating the foo.lock file to gain exclusive access to the foo.*

   files.  If foo.gz does not exist or is empty, then write the initial header,

   extra, and body content of an empty foo.gz log file.  If there is an error

   creating the lock file due to access restrictions, or an error reading or

   writing the foo.gz file, or if the foo.gz file is not a proper log file for

   this object (e.g. not a gzip file or does not contain the expected extra

   field), then return true.  If there is an error, the lock is released.

   Otherwise, the lock is left in place. */

local int log_open(struct log *log)

{

    int op;

    /* release open file resource if left over -- can occur if lock lost

       between gzlog_open() and gzlog_write() */

    if (log->fd >= 0)

        close(log->fd);

    log->fd = -1;

    /* negotiate exclusive access */

    if (log_lock(log) < 0)

        return -1;

    /* open the log file, foo.gz */

    strcpy(log->end, ".gz");

    log->fd = open(log->path, O_RDWR | O_CREAT, 0644);

    if (log->fd < 0) {

        log_close(log);

        return -1;

    }

    /* if new, initialize foo.gz with an empty log, delete old dictionary */

    if (lseek(log->fd, 0, SEEK_END) == 0) {

        if (write(log->fd, log_gzhead, HEAD) != HEAD ||

            write(log->fd, log_gzext, EXTRA) != EXTRA ||

            write(log->fd, log_gzbody, BODY) != BODY) {

            log_close(log);

            return -1;

        }

        strcpy(log->end, ".dict");

        unlink(log->path);

    }

    /* verify log file and load extra field information */

    if ((op = log_head(log)) < 0) {

        log_close(log);

        return -1;

    }

    /* check for interrupted process and if so, recover */

    if (op != NO_OP && log_recover(log, op)) {

        log_close(log);

        return -1;

    }

    /* touch the lock file to prevent another process from grabbing it */

    log_touch(log);

    return 0;

}

/* See gzlog.h for the description of the external methods below */

gzlog *gzlog_open(char *path)

{

    size_t n;

    struct log *log;

    /* check arguments */

    if (path == NULL || *path == 0)

        return NULL;

    /* allocate and initialize log structure */

    log = malloc(sizeof(struct log));

    if (log == NULL)

        return NULL;

    strcpy(log->id, LOGID);

    log->fd = -1;