annotate mercurial/mpatch.c @ 50411:841b13e6e84c

rust-changelog: introducing an intermediate `ChangelogEntry`

Before this change, client code needing to extract, e.g, the Node ID and the
description from a changeset had no other choice than calling both
`entry_for_rev()` and `data_for_rev()`. This duplicates some (limited) computation, and
more importantly imposes bad hygiene for client code: at some point of developement,
the client code would have to pass over both entry and data in its internal layers,
which at some point of development would raise the question whether they are consistent.

We introduce the intermediate `ChangelogEntry` from which both conversion to the generic
`RevlogEntry` and extraction of `ChangelogRevisionData` are possible.
It might grow some convenience methods in the future.

We keep the `data_for_rev()` method of `Changelog` for compatibility, pointing users at the more
powerful alternative.

/*
 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));
}