freelist.c

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 /* 
   Unix SMB/CIFS implementation.

   trivial database library

   Copyright (C) Andrew Tridgell              1999-2005
   Copyright (C) Paul `Rusty' Russell		   2000
   Copyright (C) Jeremy Allison			   2000-2003
   
     ** NOTE! The following LGPL license applies to the tdb
     ** library. This does NOT imply that all of Samba is released
     ** under the LGPL
   
   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2 of the License, or (at your option) any later version.

   This library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

#include "tdb_private.h"

/* read a freelist record and check for simple errors */
int rec_free_read(struct tdb_context *tdb, tdb_off_t off, struct list_struct *rec)
{
	if (tdb->methods->tdb_read(tdb, off, rec, sizeof(*rec),DOCONV()) == -1)
		return -1;

	if (rec->magic == TDB_MAGIC) {
		/* this happens when a app is showdown while deleting a record - we should
		   not completely fail when this happens */
		TDB_LOG((tdb, TDB_DEBUG_WARNING, "rec_free_read non-free magic 0x%x at offset=%d - fixing\n", 
			 rec->magic, off));
		rec->magic = TDB_FREE_MAGIC;
		if (tdb->methods->tdb_write(tdb, off, rec, sizeof(*rec)) == -1)
			return -1;
	}

	if (rec->magic != TDB_FREE_MAGIC) {
		/* Ensure ecode is set for log fn. */
		tdb->ecode = TDB_ERR_CORRUPT;
		TDB_LOG((tdb, TDB_DEBUG_WARNING, "rec_free_read bad magic 0x%x at offset=%d\n", 
			   rec->magic, off));
		return TDB_ERRCODE(TDB_ERR_CORRUPT, -1);
	}
	if (tdb->methods->tdb_oob(tdb, rec->next+sizeof(*rec), 0) != 0)
		return -1;
	return 0;
}



/* Remove an element from the freelist.  Must have alloc lock. */
static int remove_from_freelist(struct tdb_context *tdb, tdb_off_t off, tdb_off_t next)
{
	tdb_off_t last_ptr, i;

	/* read in the freelist top */
	last_ptr = FREELIST_TOP;
	while (tdb_ofs_read(tdb, last_ptr, &i) != -1 && i != 0) {
		if (i == off) {
			/* We've found it! */
			return tdb_ofs_write(tdb, last_ptr, &next);
		}
		/* Follow chain (next offset is at start of record) */
		last_ptr = i;
	}
	TDB_LOG((tdb, TDB_DEBUG_FATAL,"remove_from_freelist: not on list at off=%d\n", off));
	return TDB_ERRCODE(TDB_ERR_CORRUPT, -1);
}


/* update a record tailer (must hold allocation lock) */
static int update_tailer(struct tdb_context *tdb, tdb_off_t offset,
			 const struct list_struct *rec)
{
	tdb_off_t totalsize;

	/* Offset of tailer from record header */
	totalsize = sizeof(*rec) + rec->rec_len;
	return tdb_ofs_write(tdb, offset + totalsize - sizeof(tdb_off_t),
			 &totalsize);
}

/* Add an element into the freelist. Merge adjacent records if
   neccessary. */
int tdb_free(struct tdb_context *tdb, tdb_off_t offset, struct list_struct *rec)
{
	tdb_off_t right, left;

	/* Allocation and tailer lock */
	if (tdb_lock(tdb, -1, F_WRLCK) != 0)
		return -1;

	/* set an initial tailer, so if we fail we don't leave a bogus record */
	if (update_tailer(tdb, offset, rec) != 0) {
		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: update_tailer failed!\n"));
		goto fail;
	}

	/* Look right first (I'm an Australian, dammit) */
	right = offset + sizeof(*rec) + rec->rec_len;
	if (right + sizeof(*rec) <= tdb->map_size) {
		struct list_struct r;

		if (tdb->methods->tdb_read(tdb, right, &r, sizeof(r), DOCONV()) == -1) {
			TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: right read failed at %u\n", right));
			goto left;
		}

		/* If it's free, expand to include it. */
		if (r.magic == TDB_FREE_MAGIC) {
			if (remove_from_freelist(tdb, right, r.next) == -1) {
				TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: right free failed at %u\n", right));
				goto left;
			}
			rec->rec_len += sizeof(r) + r.rec_len;
		}
	}

left:
	/* Look left */
	left = offset - sizeof(tdb_off_t);
	if (left > TDB_DATA_START(tdb->header.hash_size)) {
		struct list_struct l;
		tdb_off_t leftsize;
		
		/* Read in tailer and jump back to header */
		if (tdb_ofs_read(tdb, left, &leftsize) == -1) {
			TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: left offset read failed at %u\n", left));
			goto update;
		}

		/* it could be uninitialised data */
		if (leftsize == 0 || leftsize == TDB_PAD_U32) {
			goto update;
		}

		left = offset - leftsize;

		/* Now read in record */
		if (tdb->methods->tdb_read(tdb, left, &l, sizeof(l), DOCONV()) == -1) {
			TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: left read failed at %u (%u)\n", left, leftsize));
			goto update;
		}

		/* If it's free, expand to include it. */
		if (l.magic == TDB_FREE_MAGIC) {
			if (remove_from_freelist(tdb, left, l.next) == -1) {
				TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: left free failed at %u\n", left));
				goto update;
			} else {
				offset = left;
				rec->rec_len += leftsize;
			}
		}
	}

update:
	if (update_tailer(tdb, offset, rec) == -1) {
		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free: update_tailer failed at %u\n", offset));
		goto fail;
	}

	/* Now, prepend to free list */
	rec->magic = TDB_FREE_MAGIC;

	if (tdb_ofs_read(tdb, FREELIST_TOP, &rec->next) == -1 ||
	    tdb_rec_write(tdb, offset, rec) == -1 ||
	    tdb_ofs_write(tdb, FREELIST_TOP, &offset) == -1) {
		TDB_LOG((tdb, TDB_DEBUG_FATAL, "tdb_free record write failed at offset=%d\n", offset));
		goto fail;
	}

	/* And we're done. */
	tdb_unlock(tdb, -1, F_WRLCK);
	return 0;

 fail:
	tdb_unlock(tdb, -1, F_WRLCK);
	return -1;
}


/* 
   the core of tdb_allocate - called when we have decided which
   free list entry to use
 */
static tdb_off_t tdb_allocate_ofs(struct tdb_context *tdb, tdb_len_t length, tdb_off_t rec_ptr,
				struct list_struct *rec, tdb_off_t last_ptr)
{
	struct list_struct newrec;
	tdb_off_t newrec_ptr;

	memset(&newrec, '\0', sizeof(newrec));

	/* found it - now possibly split it up  */
	if (rec->rec_len > length + MIN_REC_SIZE) {
		/* Length of left piece */
		length = TDB_ALIGN(length, TDB_ALIGNMENT);
		
		/* Right piece to go on free list */
		newrec.rec_len = rec->rec_len - (sizeof(*rec) + length);
		newrec_ptr = rec_ptr + sizeof(*rec) + length;
		
		/* And left record is shortened */
		rec->rec_len = length;
	} else {
		newrec_ptr = 0;
	}
	
	/* Remove allocated record from the free list */
	if (tdb_ofs_write(tdb, last_ptr, &rec->next) == -1) {
		return 0;
	}
	
	/* Update header: do this before we drop alloc
	   lock, otherwise tdb_free() might try to
	   merge with us, thinking we're free.
	   (Thanks Jeremy Allison). */
	rec->magic = TDB_MAGIC;
	if (tdb_rec_write(tdb, rec_ptr, rec) == -1) {
		return 0;
	}
	
	/* Did we create new block? */
	if (newrec_ptr) {
		/* Update allocated record tailer (we
		   shortened it). */
		if (update_tailer(tdb, rec_ptr, rec) == -1) {
			return 0;
		}
		
		/* Free new record */
		if (tdb_free(tdb, newrec_ptr, &newrec) == -1) {
			return 0;
		}
	}
	
	/* all done - return the new record offset */
	return rec_ptr;
}

/* allocate some space from the free list. The offset returned points
   to a unconnected list_struct within the database with room for at
   least length bytes of total data

   0 is returned if the space could not be allocated
 */
tdb_off_t tdb_allocate(struct tdb_context *tdb, tdb_len_t length, struct list_struct *rec)
{
	tdb_off_t rec_ptr, last_ptr, newrec_ptr;
	struct {
		tdb_off_t rec_ptr, last_ptr;
		tdb_len_t rec_len;
	} bestfit;

	if (tdb_lock(tdb, -1, F_WRLCK) == -1)
		return 0;

	/* Extra bytes required for tailer */
	length += sizeof(tdb_off_t);

 again:
	last_ptr = FREELIST_TOP;

	/* read in the freelist top */
	if (tdb_ofs_read(tdb, FREELIST_TOP, &rec_ptr) == -1)
		goto fail;

	bestfit.rec_ptr = 0;
	bestfit.last_ptr = 0;
	bestfit.rec_len = 0;

	/* 
	   this is a best fit allocation strategy. Originally we used
	   a first fit strategy, but it suffered from massive fragmentation
	   issues when faced with a slowly increasing record size.
	 */
	while (rec_ptr) {
		if (rec_free_read(tdb, rec_ptr, rec) == -1) {
			goto fail;
		}

		if (rec->rec_len >= length) {
			if (bestfit.rec_ptr == 0 ||
			    rec->rec_len < bestfit.rec_len) {
				bestfit.rec_len = rec->rec_len;
				bestfit.rec_ptr = rec_ptr;
				bestfit.last_ptr = last_ptr;
				/* consider a fit to be good enough if
				   we aren't wasting more than half
				   the space */
				if (bestfit.rec_len < 2*length) {
					break;
				}
			}
		}

		/* move to the next record */
		last_ptr = rec_ptr;
		rec_ptr = rec->next;
	}

	if (bestfit.rec_ptr != 0) {
		if (rec_free_read(tdb, bestfit.rec_ptr, rec) == -1) {
			goto fail;
		}

		newrec_ptr = tdb_allocate_ofs(tdb, length, bestfit.rec_ptr, rec, bestfit.last_ptr);
		tdb_unlock(tdb, -1, F_WRLCK);
		return newrec_ptr;
	}

	/* we didn't find enough space. See if we can expand the
	   database and if we can then try again */
	if (tdb_expand(tdb, length + sizeof(*rec)) == 0)
		goto again;
 fail:
	tdb_unlock(tdb, -1, F_WRLCK);
	return 0;
}