annotate mercurial/mpatch.c @ 47072:4c041c71ec01

revlog: introduce an explicit tracking of what the revlog is about

Since the dawn of time, people have been forced to rely to lossy introspection
of the index filename to determine what the purpose and role of the revlog they
encounter is. This is hacky, error prone, inflexible, abstraction-leaky,
<insert-your-own-complaints-here>.

In f63299ee7e4d Raphaël introduced a new attribute to track this information:
`revlog_kind`. However it is initialized in an odd place and various instances
end up not having it set. In addition is only tracking some of the information
we end up having to introspect in various pieces of code.

So we add a new attribute that holds more data and is more strictly enforced.
This work is done in collaboration with Raphaël.

The `revlog_kind` one will be removed/adapted in the next changeset. We expect
to be able to clean up various existing piece of code and to simplify coming
work around the newer revlog format.

Differential Revision: https://phab.mercurial-scm.org/D10352

/*
 mpatch.c - efficient binary patching for Mercurial

 This implements a patch algorithm that's O(m + nlog n) where m is the
 size of the output and n is the number of patches.

 Given a list of binary patches, it unpacks each into a hunk list,
 then combines the hunk lists with a treewise recursion to form a
 single hunk list. This hunk list is then applied to the original
 text.

 The text (or binary) fragments are copied directly from their source
 Python objects into a preallocated output string to avoid the
 allocation of intermediate Python objects. Working memory is about 2x
 the total number of hunks.

 Copyright 2005, 2006 Olivia Mackall <[e-mail]>

 This software may be used and distributed according to the terms
 of the GNU General Public License, incorporated herein by reference.
*/

#include <limits.h>
#include <stdlib.h>
#include <string.h>

#include "bitmanipulation.h"
#include "compat.h"
#include "mpatch.h"

/* VC9 doesn't include bool and lacks stdbool.h based on cext/util.h */
#if defined(_MSC_VER) || __STDC_VERSION__ < 199901L
#define true 1
#define false 0
typedef unsigned char bool;
#else
#include <stdbool.h>
#endif

static struct mpatch_flist *lalloc(ssize_t size)
{
	struct mpatch_flist *a = NULL;

	if (size < 1) {
		size = 1;
	}

	a = (struct mpatch_flist *)malloc(sizeof(struct mpatch_flist));
	if (a) {
		a->base = (struct mpatch_frag *)malloc(
		    sizeof(struct mpatch_frag) * size);
		if (a->base) {
			a->head = a->tail = a->base;
			return a;
		}
		free(a);
	}
	return NULL;
}

void mpatch_lfree(struct mpatch_flist *a)
{
	if (a) {
		free(a->base);
		free(a);
	}
}

static ssize_t lsize(struct mpatch_flist *a)
{
	return a->tail - a->head;
}

/* add helper to add src and *dest iff it won't overflow */
static inline bool safeadd(int src, int *dest)
{
	if ((src > 0) == (*dest > 0)) {
		if (*dest > 0) {
			if (src > (INT_MAX - *dest)) {
				return false;
			}
		} else {
			if (src < (INT_MIN - *dest)) {
				return false;
			}
		}
	}
	*dest += src;
	return true;
}

/* subtract src from dest and store result in dest */
static inline bool safesub(int src, int *dest)
{
	if (((src > 0) && (*dest < INT_MIN + src)) ||
	    ((src < 0) && (*dest > INT_MAX + src))) {
		return false;
	}
	*dest -= src;
	return true;
}

/* move hunks in source that are less cut to dest, compensating
   for changes in offset. the last hunk may be split if necessary.
*/
static int gather(struct mpatch_flist *dest, struct mpatch_flist *src, int cut,
                  int offset)
{
	struct mpatch_frag *d = dest->tail, *s = src->head;
	int postend, c, l;

	while (s != src->tail) {
		int soffset = s->start;
		if (!safeadd(offset, &soffset)) {
			break; /* add would overflow, oh well */
		}
		if (soffset >= cut) {
			break; /* we've gone far enough */
		}

		postend = offset;
		if (!safeadd(s->start, &postend) ||
		    !safeadd(s->len, &postend)) {
			break;
		}
		if (postend <= cut) {
			/* save this hunk */
			int tmp = s->start;
			if (!safesub(s->end, &tmp)) {
				break;
			}
			if (!safeadd(s->len, &tmp)) {
				break;
			}
			if (!safeadd(tmp, &offset)) {
				break; /* add would overflow, oh well */
			}
			*d++ = *s++;
		} else {
			/* break up this hunk */
			c = cut;
			if (!safesub(offset, &c)) {
				break;
			}
			if (s->end < c) {
				c = s->end;
			}
			l = cut - offset - s->start;
			if (s->len < l) {
				l = s->len;
			}

			offset += s->start + l - c;

			d->start = s->start;
			d->end = c;
			d->len = l;
			d->data = s->data;
			d++;
			s->start = c;
			s->len = s->len - l;
			s->data = s->data + l;

			break;
		}
	}

	dest->tail = d;
	src->head = s;
	return offset;
}

/* like gather, but with no output list */
static int discard(struct mpatch_flist *src, int cut, int offset)
{
	struct mpatch_frag *s = src->head;
	int postend, c, l;

	while (s != src->tail) {
		int cmpcut = s->start;
		if (!safeadd(offset, &cmpcut)) {
			break;
		}
		if (cmpcut >= cut) {
			break;
		}

		postend = offset;
		if (!safeadd(s->start, &postend)) {
			break;
		}
		if (!safeadd(s->len, &postend)) {
			break;
		}
		if (postend <= cut) {
			/* do the subtraction first to avoid UB integer overflow
			 */
			int tmp = s->start;
			if (!safesub(s->end, &tmp)) {
				break;
			}
			if (!safeadd(s->len, &tmp)) {
				break;
			}
			if (!safeadd(tmp, &offset)) {
				break;
			}
			s++;
		} else {
			c = cut;
			if (!safesub(offset, &c)) {
				break;
			}
			if (s->end < c) {
				c = s->end;
			}
			l = cut - offset - s->start;
			if (s->len < l) {
				l = s->len;
			}

			offset += s->start + l - c;
			s->start = c;
			s->len = s->len - l;
			s->data = s->data + l;

			break;
		}
	}

	src->head = s;
	return offset;
}

/* combine hunk lists a and b, while adjusting b for offset changes in a/
   this deletes a and b and returns the resultant list. */
static struct mpatch_flist *combine(struct mpatch_flist *a,
                                    struct mpatch_flist *b)
{
	struct mpatch_flist *c = NULL;
	struct mpatch_frag *bh, *ct;
	int offset = 0, post;

	if (a && b) {
		c = lalloc((lsize(a) + lsize(b)) * 2);
	}

	if (c) {

		for (bh = b->head; bh != b->tail; bh++) {
			/* save old hunks */
			offset = gather(c, a, bh->start, offset);

			/* discard replaced hunks */
			post = discard(a, bh->end, offset);

			/* insert new hunk */
			ct = c->tail;
			ct->start = bh->start;
			ct->end = bh->end;
			if (!safesub(offset, &(ct->start)) ||
			    !safesub(post, &(ct->end))) {
				/* It was already possible to exit
				 * this function with a return value
				 * of NULL before the safesub()s were
				 * added, so this should be fine. */
				mpatch_lfree(c);
				c = NULL;
				goto done;
			}
			ct->len = bh->len;
			ct->data = bh->data;
			c->tail++;
			offset = post;
		}

		/* hold on to tail from a */
		memcpy(c->tail, a->head, sizeof(struct mpatch_frag) * lsize(a));
		c->tail += lsize(a);
	}
done:
	mpatch_lfree(a);
	mpatch_lfree(b);
	return c;
}

/* decode a binary patch into a hunk list */
int mpatch_decode(const char *bin, ssize_t len, struct mpatch_flist **res)
{
	struct mpatch_flist *l;
	struct mpatch_frag *lt;
	int pos = 0;

	/* assume worst case size, we won't have many of these lists */
	l = lalloc(len / 12 + 1);
	if (!l) {
		return MPATCH_ERR_NO_MEM;
	}

	lt = l->tail;

	/* We check against len-11 to ensure we have at least 12 bytes
	   left in the patch so we can read our three be32s out of it. */
	while (pos >= 0 && pos < (len - 11)) {
		lt->start = getbe32(bin + pos);
		lt->end = getbe32(bin + pos + 4);
		lt->len = getbe32(bin + pos + 8);
		if (lt->start < 0 || lt->start > lt->end || lt->len < 0) {
			break; /* sanity check */
		}
		if (!safeadd(12, &pos)) {
			break;
		}
		lt->data = bin + pos;
		if (!safeadd(lt->len, &pos)) {
			break;
		}
		lt++;
	}

	if (pos != len) {
		mpatch_lfree(l);
		return MPATCH_ERR_CANNOT_BE_DECODED;
	}

	l->tail = lt;
	*res = l;
	return 0;
}

/* calculate the size of resultant text */
ssize_t mpatch_calcsize(ssize_t len, struct mpatch_flist *l)
{
	ssize_t outlen = 0, last = 0;
	struct mpatch_frag *f = l->head;

	while (f != l->tail) {
		if (f->start < last || f->end > len) {
			return MPATCH_ERR_INVALID_PATCH;
		}
		outlen += f->start - last;
		last = f->end;
		outlen += f->len;
		f++;
	}

	outlen += len - last;
	return outlen;
}

int mpatch_apply(char *buf, const char *orig, ssize_t len,
                 struct mpatch_flist *l)
{
	struct mpatch_frag *f = l->head;
	int last = 0;
	char *p = buf;

	while (f != l->tail) {
		if (f->start < last || f->start > len || f->end > len ||
		    last < 0) {
			return MPATCH_ERR_INVALID_PATCH;
		}
		memcpy(p, orig + last, f->start - last);
		p += f->start - last;
		memcpy(p, f->data, f->len);
		last = f->end;
		p += f->len;
		f++;
	}
	if (last < 0) {
		return MPATCH_ERR_INVALID_PATCH;
	}
	memcpy(p, orig + last, len - last);
	return 0;
}

/* recursively generate a patch of all bins between start and end */
struct mpatch_flist *
mpatch_fold(void *bins, struct mpatch_flist *(*get_next_item)(void *, ssize_t),
            ssize_t start, ssize_t end)
{
	ssize_t len;

	if (start + 1 == end) {
		/* trivial case, output a decoded list */
		return get_next_item(bins, start);
	}

	/* divide and conquer, memory management is elsewhere */
	len = (end - start) / 2;
	return combine(mpatch_fold(bins, get_next_item, start, start + len),
	               mpatch_fold(bins, get_next_item, start + len, end));
}