view mercurial/manifest.c @ 26623:5a95fe44121d

clonebundles: support for seeding clones from pre-generated bundles

Cloning can be an expensive operation for servers because the server
generates a bundle from existing repository data at request time. For
a large repository like mozilla-central, this consumes 4+ minutes
of CPU time on the server. It also results in significant network
utilization. Multiplied by hundreds or even thousands of clients and
the ensuing load can result in difficulties scaling the Mercurial server.

Despite generation of bundles being deterministic until the next
changeset is added, the generation of bundles to service a clone request
is not cached. Each clone thus performs redundant work. This is
wasteful.

This patch introduces the "clonebundles" extension and related
client-side functionality to help alleviate this deficiency. The
client-side feature is behind an experimental flag and is not enabled by
default.

It works as follows:

1) Server operator generates a bundle and makes it available on a
   server (likely HTTP).
2) Server operator defines the URL of a bundle file in a
   .hg/clonebundles.manifest file.
3) Client `hg clone`ing sees the server is advertising bundle URLs.
4) Client fetches and applies the advertised bundle.
5) Client performs equivalent of `hg pull` to fetch changes made since
   the bundle was created.

Essentially, the server performs the expensive work of generating a
bundle once and all subsequent clones fetch a static file from
somewhere. Scaling static file serving is a much more manageable
problem than scaling a Python application like Mercurial. Assuming your
repository grows less than 1% per day, the end result is 99+% of CPU
and network load from clones is eliminated, allowing Mercurial servers
to scale more easily. Serving static files also means data can be
transferred to clients as fast as they can consume it, rather than as
fast as servers can generate it. This makes clones faster.

Mozilla has implemented similar functionality of this patch on
hg.mozilla.org using a custom extension. We are hosting bundle files in
Amazon S3 and CloudFront (a CDN) and have successfully offloaded
>1 TB/day in data transfer from hg.mozilla.org, freeing up significant
bandwidth and CPU resources. The positive impact has been stellar and
I believe it has proved its value to be included in Mercurial core. I
feel it is important for the client-side support to be enabled in core
by default because it means that clients will get faster, more reliable
clones and will enable server operators to reduce load without
requiring any client-side configuration changes (assuming clients are
up to date, of course).

The scope of this feature is narrowly and specifically tailored to
cloning, despite "serve pulls from pre-generated bundles" being a valid
and useful feature. I would eventually like for Mercurial servers to
support transferring *all* repository data via statically hosted files.
You could imagine a server that siphons all pushed data to bundle files
and instructs clients to apply a stream of bundles to reconstruct all
repository data. This feature, while useful and powerful, is
significantly more work to implement because it requires the server
component have awareness of discovery and a mapping of which changesets
are in which files. Full, clone bundles, by contrast, are much simpler.

The wire protocol command is named "clonebundles" instead of something
more generic like "staticbundles" to leave the door open for a new, more
powerful and more generic server-side component with minimal backwards
compatibility implications. The name "bundleclone" is used by Mozilla's
extension and would cause problems since there are subtle differences
in Mozilla's extension.

Mozilla's experience with this idea has taught us that some form of
"content negotiation" is required. Not all clients will support all
bundle formats or even URLs (advanced TLS requirements, etc). To ensure
the highest uptake possible, a server needs to advertise multiple
versions of bundles and clients need to be able to choose the most
appropriate from that list one. The "attributes" in each
server-advertised entry facilitate this filtering and sorting. Their
use will become apparent in subsequent patches.

Initial inspiration and credit for the idea of cloning from static files
belongs to Augie Fackler and his "lookaside clone" extension proof of
concept.

/*
 * manifest.c - manifest type that does on-demand parsing.
 *
 * Copyright 2015, Google Inc.
 *
 * This software may be used and distributed according to the terms of
 * the GNU General Public License, incorporated herein by reference.
 */
#include <Python.h>

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

#include "util.h"

#define DEFAULT_LINES 100000

typedef struct {
	char *start;
	Py_ssize_t len; /* length of line including terminal newline */
	char hash_suffix;
	bool from_malloc;
	bool deleted;
} line;

typedef struct {
	PyObject_HEAD
	PyObject *pydata;
	line *lines;
	int numlines; /* number of line entries */
	int livelines; /* number of non-deleted lines */
	int maxlines; /* allocated number of lines */
	bool dirty;
} lazymanifest;

#define MANIFEST_OOM -1
#define MANIFEST_NOT_SORTED -2
#define MANIFEST_MALFORMED -3

/* defined in parsers.c */
PyObject *unhexlify(const char *str, int len);

/* get the length of the path for a line */
static size_t pathlen(line *l) {
	return strlen(l->start);
}

/* get the node value of a single line */
static PyObject *nodeof(line *l) {
	char *s = l->start;
	ssize_t llen = pathlen(l);
	PyObject *hash = unhexlify(s + llen + 1, 40);
	if (!hash) {
		return NULL;
	}
	if (l->hash_suffix != '\0') {
		char newhash[21];
		memcpy(newhash, PyString_AsString(hash), 20);
		Py_DECREF(hash);
		newhash[20] = l->hash_suffix;
		hash = PyString_FromStringAndSize(newhash, 21);
	}
	return hash;
}

/* get the node hash and flags of a line as a tuple */
static PyObject *hashflags(line *l)
{
	char *s = l->start;
	size_t plen = pathlen(l);
	PyObject *hash = nodeof(l);

	/* 40 for hash, 1 for null byte, 1 for newline */
	size_t hplen = plen + 42;
	Py_ssize_t flen = l->len - hplen;
	PyObject *flags;
	PyObject *tup;

	if (!hash)
		return NULL;
	flags = PyString_FromStringAndSize(s + hplen - 1, flen);
	if (!flags) {
		Py_DECREF(hash);
		return NULL;
	}
	tup = PyTuple_Pack(2, hash, flags);
	Py_DECREF(flags);
	Py_DECREF(hash);
	return tup;
}

/* if we're about to run out of space in the line index, add more */
static bool realloc_if_full(lazymanifest *self)
{
	if (self->numlines == self->maxlines) {
		self->maxlines *= 2;
		self->lines = realloc(self->lines, self->maxlines * sizeof(line));
	}
	return !!self->lines;
}

/*
 * Find the line boundaries in the manifest that 'data' points to and store
 * information about each line in 'self'.
 */
static int find_lines(lazymanifest *self, char *data, Py_ssize_t len)
{
	char *prev = NULL;
	while (len > 0) {
		line *l;
		char *next = memchr(data, '\n', len);
		if (!next) {
			return MANIFEST_MALFORMED;
		}
		next++; /* advance past newline */
		if (!realloc_if_full(self)) {
			return MANIFEST_OOM; /* no memory */
		}
		if (prev && strcmp(prev, data) > -1) {
			/* This data isn't sorted, so we have to abort. */
			return MANIFEST_NOT_SORTED;
		}
		l = self->lines + ((self->numlines)++);
		l->start = data;
		l->len = next - data;
		l->hash_suffix = '\0';
		l->from_malloc = false;
		l->deleted = false;
		len = len - l->len;
		prev = data;
		data = next;
	}
	self->livelines = self->numlines;
	return 0;
}

static int lazymanifest_init(lazymanifest *self, PyObject *args)
{
	char *data;
	Py_ssize_t len;
	int err, ret;
	PyObject *pydata;
	if (!PyArg_ParseTuple(args, "S", &pydata)) {
		return -1;
	}
	err = PyString_AsStringAndSize(pydata, &data, &len);

	self->dirty = false;
	if (err == -1)
		return -1;
	self->pydata = pydata;
	Py_INCREF(self->pydata);
	Py_BEGIN_ALLOW_THREADS
	self->lines = malloc(DEFAULT_LINES * sizeof(line));
	self->maxlines = DEFAULT_LINES;
	self->numlines = 0;
	if (!self->lines)
		ret = MANIFEST_OOM;
	else
		ret = find_lines(self, data, len);
	Py_END_ALLOW_THREADS
	switch (ret) {
	case 0:
		break;
	case MANIFEST_OOM:
		PyErr_NoMemory();
		break;
	case MANIFEST_NOT_SORTED:
		PyErr_Format(PyExc_ValueError,
			     "Manifest lines not in sorted order.");
		break;
	case MANIFEST_MALFORMED:
		PyErr_Format(PyExc_ValueError,
			     "Manifest did not end in a newline.");
		break;
	default:
		PyErr_Format(PyExc_ValueError,
			     "Unknown problem parsing manifest.");
	}
	return ret == 0 ? 0 : -1;
}

static void lazymanifest_dealloc(lazymanifest *self)
{
	/* free any extra lines we had to allocate */
	int i;
	for (i = 0; i < self->numlines; i++) {
		if (self->lines[i].from_malloc) {
			free(self->lines[i].start);
		}
	}
	if (self->lines) {
		free(self->lines);
		self->lines = NULL;
	}
	if (self->pydata) {
		Py_DECREF(self->pydata);
		self->pydata = NULL;
	}
	PyObject_Del(self);
}

/* iteration support */

typedef struct {
	PyObject_HEAD lazymanifest *m;
	Py_ssize_t pos;
} lmIter;

static void lmiter_dealloc(PyObject *o)
{
	lmIter *self = (lmIter *)o;
	Py_DECREF(self->m);
	PyObject_Del(self);
}

static line *lmiter_nextline(lmIter *self)
{
	do {
		self->pos++;
		if (self->pos >= self->m->numlines) {
			return NULL;
		}
		/* skip over deleted manifest entries */
	} while (self->m->lines[self->pos].deleted);
	return self->m->lines + self->pos;
}

static PyObject *lmiter_iterentriesnext(PyObject *o)
{
	size_t pl;
	line *l;
	Py_ssize_t consumed;
	PyObject *ret = NULL, *path = NULL, *hash = NULL, *flags = NULL;
	l = lmiter_nextline((lmIter *)o);
	if (!l) {
		goto done;
	}
	pl = pathlen(l);
	path = PyString_FromStringAndSize(l->start, pl);
	hash = nodeof(l);
	consumed = pl + 41;
	flags = PyString_FromStringAndSize(l->start + consumed,
									   l->len - consumed - 1);
	if (!path || !hash || !flags) {
		goto done;
	}
	ret = PyTuple_Pack(3, path, hash, flags);
done:
	Py_XDECREF(path);
	Py_XDECREF(hash);
	Py_XDECREF(flags);
	return ret;
}

static PyTypeObject lazymanifestEntriesIterator = {
	PyObject_HEAD_INIT(NULL)
	0,                               /*ob_size */
	"parsers.lazymanifest.entriesiterator", /*tp_name */
	sizeof(lmIter),                  /*tp_basicsize */
	0,                               /*tp_itemsize */
	lmiter_dealloc,                  /*tp_dealloc */
	0,                               /*tp_print */
	0,                               /*tp_getattr */
	0,                               /*tp_setattr */
	0,                               /*tp_compare */
	0,                               /*tp_repr */
	0,                               /*tp_as_number */
	0,                               /*tp_as_sequence */
	0,                               /*tp_as_mapping */
	0,                               /*tp_hash */
	0,                               /*tp_call */
	0,                               /*tp_str */
	0,                               /*tp_getattro */
	0,                               /*tp_setattro */
	0,                               /*tp_as_buffer */
	/* tp_flags: Py_TPFLAGS_HAVE_ITER tells python to
	   use tp_iter and tp_iternext fields. */
	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_ITER,
	"Iterator for 3-tuples in a lazymanifest.",  /* tp_doc */
	0,                               /* tp_traverse */
	0,                               /* tp_clear */
	0,                               /* tp_richcompare */
	0,                               /* tp_weaklistoffset */
	PyObject_SelfIter,               /* tp_iter: __iter__() method */
	lmiter_iterentriesnext,          /* tp_iternext: next() method */
};

static PyObject *lmiter_iterkeysnext(PyObject *o)
{
	size_t pl;
	line *l = lmiter_nextline((lmIter *)o);
	if (!l) {
		return NULL;
	}
	pl = pathlen(l);
	return PyString_FromStringAndSize(l->start, pl);
}

static PyTypeObject lazymanifestKeysIterator = {
	PyObject_HEAD_INIT(NULL)
	0,                               /*ob_size */
	"parsers.lazymanifest.keysiterator", /*tp_name */
	sizeof(lmIter),                  /*tp_basicsize */
	0,                               /*tp_itemsize */
	lmiter_dealloc,                  /*tp_dealloc */
	0,                               /*tp_print */
	0,                               /*tp_getattr */
	0,                               /*tp_setattr */
	0,                               /*tp_compare */
	0,                               /*tp_repr */
	0,                               /*tp_as_number */
	0,                               /*tp_as_sequence */
	0,                               /*tp_as_mapping */
	0,                               /*tp_hash */
	0,                               /*tp_call */
	0,                               /*tp_str */
	0,                               /*tp_getattro */
	0,                               /*tp_setattro */
	0,                               /*tp_as_buffer */
	/* tp_flags: Py_TPFLAGS_HAVE_ITER tells python to
	   use tp_iter and tp_iternext fields. */
	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_ITER,
	"Keys iterator for a lazymanifest.",  /* tp_doc */
	0,                               /* tp_traverse */
	0,                               /* tp_clear */
	0,                               /* tp_richcompare */
	0,                               /* tp_weaklistoffset */
	PyObject_SelfIter,               /* tp_iter: __iter__() method */
	lmiter_iterkeysnext,             /* tp_iternext: next() method */
};

static lazymanifest *lazymanifest_copy(lazymanifest *self);

static PyObject *lazymanifest_getentriesiter(lazymanifest *self)
{
	lmIter *i = NULL;
	lazymanifest *t = lazymanifest_copy(self);
	if (!t) {
		PyErr_NoMemory();
		return NULL;
	}
	i = PyObject_New(lmIter, &lazymanifestEntriesIterator);
	if (i) {
		i->m = t;
		i->pos = -1;
	} else {
		Py_DECREF(t);
		PyErr_NoMemory();
	}
	return (PyObject *)i;
}

static PyObject *lazymanifest_getkeysiter(lazymanifest *self)
{
	lmIter *i = NULL;
	lazymanifest *t = lazymanifest_copy(self);
	if (!t) {
		PyErr_NoMemory();
		return NULL;
	}
	i = PyObject_New(lmIter, &lazymanifestKeysIterator);
	if (i) {
		i->m = t;
		i->pos = -1;
	} else {
		Py_DECREF(t);
		PyErr_NoMemory();
	}
	return (PyObject *)i;
}

/* __getitem__ and __setitem__ support */

static Py_ssize_t lazymanifest_size(lazymanifest *self)
{
	return self->livelines;
}

static int linecmp(const void *left, const void *right)
{
	return strcmp(((const line *)left)->start,
		      ((const line *)right)->start);
}

static PyObject *lazymanifest_getitem(lazymanifest *self, PyObject *key)
{
	line needle;
	line *hit;
	if (!PyString_Check(key)) {
		PyErr_Format(PyExc_TypeError,
			     "getitem: manifest keys must be a string.");
		return NULL;
	}
	needle.start = PyString_AsString(key);
	hit = bsearch(&needle, self->lines, self->numlines, sizeof(line),
		      &linecmp);
	if (!hit || hit->deleted) {
		PyErr_Format(PyExc_KeyError, "No such manifest entry.");
		return NULL;
	}
	return hashflags(hit);
}

static int lazymanifest_delitem(lazymanifest *self, PyObject *key)
{
	line needle;
	line *hit;
	if (!PyString_Check(key)) {
		PyErr_Format(PyExc_TypeError,
			     "delitem: manifest keys must be a string.");
		return -1;
	}
	needle.start = PyString_AsString(key);
	hit = bsearch(&needle, self->lines, self->numlines, sizeof(line),
		      &linecmp);
	if (!hit || hit->deleted) {
		PyErr_Format(PyExc_KeyError,
			     "Tried to delete nonexistent manifest entry.");
		return -1;
	}
	self->dirty = true;
	hit->deleted = true;
	self->livelines--;
	return 0;
}

/* Do a binary search for the insertion point for new, creating the
 * new entry if needed. */
static int internalsetitem(lazymanifest *self, line *new) {
	int start = 0, end = self->numlines;
	while (start < end) {
		int pos = start + (end - start) / 2;
		int c = linecmp(new, self->lines + pos);
		if (c < 0)
			end = pos;
		else if (c > 0)
			start = pos + 1;
		else {
			if (self->lines[pos].deleted)
				self->livelines++;
			if (self->lines[pos].from_malloc)
				free(self->lines[pos].start);
			start = pos;
			goto finish;
		}
	}
	/* being here means we need to do an insert */
	if (!realloc_if_full(self)) {
		PyErr_NoMemory();
		return -1;
	}
	memmove(self->lines + start + 1, self->lines + start,
		(self->numlines - start) * sizeof(line));
	self->numlines++;
	self->livelines++;
finish:
	self->lines[start] = *new;
	self->dirty = true;
	return 0;
}

static int lazymanifest_setitem(
	lazymanifest *self, PyObject *key, PyObject *value)
{
	char *path;
	Py_ssize_t plen;
	PyObject *pyhash;
	Py_ssize_t hlen;
	char *hash;
	PyObject *pyflags;
	char *flags;
	Py_ssize_t flen;
	size_t dlen;
	char *dest;
	int i;
	line new;
	if (!PyString_Check(key)) {
		PyErr_Format(PyExc_TypeError,
			     "setitem: manifest keys must be a string.");
		return -1;
	}
	if (!value) {
		return lazymanifest_delitem(self, key);
	}
	if (!PyTuple_Check(value) || PyTuple_Size(value) != 2) {
		PyErr_Format(PyExc_TypeError,
			     "Manifest values must be a tuple of (node, flags).");
		return -1;
	}
	if (PyString_AsStringAndSize(key, &path, &plen) == -1) {
		return -1;
	}

	pyhash = PyTuple_GetItem(value, 0);
	if (!PyString_Check(pyhash)) {
		PyErr_Format(PyExc_TypeError,
			     "node must be a 20-byte string");
		return -1;
	}
	hlen = PyString_Size(pyhash);
	/* Some parts of the codebase try and set 21 or 22
	 * byte "hash" values in order to perturb things for
	 * status. We have to preserve at least the 21st
	 * byte. Sigh. If there's a 22nd byte, we drop it on
	 * the floor, which works fine.
	 */
	if (hlen != 20 && hlen != 21 && hlen != 22) {
		PyErr_Format(PyExc_TypeError,
			     "node must be a 20-byte string");
		return -1;
	}
	hash = PyString_AsString(pyhash);

	pyflags = PyTuple_GetItem(value, 1);
	if (!PyString_Check(pyflags) || PyString_Size(pyflags) > 1) {
		PyErr_Format(PyExc_TypeError,
			     "flags must a 0 or 1 byte string");
		return -1;
	}
	if (PyString_AsStringAndSize(pyflags, &flags, &flen) == -1) {
		return -1;
	}
	/* one null byte and one newline */
	dlen = plen + 41 + flen + 1;
	dest = malloc(dlen);
	if (!dest) {
		PyErr_NoMemory();
		return -1;
	}
	memcpy(dest, path, plen + 1);
	for (i = 0; i < 20; i++) {
		/* Cast to unsigned, so it will not get sign-extended when promoted
		 * to int (as is done when passing to a variadic function)
		 */
		sprintf(dest + plen + 1 + (i * 2), "%02x", (unsigned char)hash[i]);
	}
	memcpy(dest + plen + 41, flags, flen);
	dest[plen + 41 + flen] = '\n';
	new.start = dest;
	new.len = dlen;
	new.hash_suffix = '\0';
	if (hlen > 20) {
		new.hash_suffix = hash[20];
	}
	new.from_malloc = true;     /* is `start` a pointer we allocated? */
	new.deleted = false;        /* is this entry deleted? */
	if (internalsetitem(self, &new)) {
		return -1;
	}
	return 0;
}

static PyMappingMethods lazymanifest_mapping_methods = {
	(lenfunc)lazymanifest_size,             /* mp_length */
	(binaryfunc)lazymanifest_getitem,       /* mp_subscript */
	(objobjargproc)lazymanifest_setitem,    /* mp_ass_subscript */
};

/* sequence methods (important or __contains__ builds an iterator */

static int lazymanifest_contains(lazymanifest *self, PyObject *key)
{
	line needle;
	line *hit;
	if (!PyString_Check(key)) {
		/* Our keys are always strings, so if the contains
		 * check is for a non-string, just return false. */
		return 0;
	}
	needle.start = PyString_AsString(key);
	hit = bsearch(&needle, self->lines, self->numlines, sizeof(line),
		      &linecmp);
	if (!hit || hit->deleted) {
		return 0;
	}
	return 1;
}

static PySequenceMethods lazymanifest_seq_meths = {
	(lenfunc)lazymanifest_size, /* sq_length */
	0, /* sq_concat */
	0, /* sq_repeat */
	0, /* sq_item */
	0, /* sq_slice */
	0, /* sq_ass_item */
	0, /* sq_ass_slice */
	(objobjproc)lazymanifest_contains, /* sq_contains */
	0, /* sq_inplace_concat */
	0, /* sq_inplace_repeat */
};


/* Other methods (copy, diff, etc) */
static PyTypeObject lazymanifestType;

/* If the manifest has changes, build the new manifest text and reindex it. */
static int compact(lazymanifest *self) {
	int i;
	ssize_t need = 0;
	char *data;
	line *src, *dst;
	PyObject *pydata;
	if (!self->dirty)
		return 0;
	for (i = 0; i < self->numlines; i++) {
		if (!self->lines[i].deleted) {
			need += self->lines[i].len;
		}
	}
	pydata = PyString_FromStringAndSize(NULL, need);
	if (!pydata)
		return -1;
	data = PyString_AsString(pydata);
	if (!data) {
		return -1;
	}
	src = self->lines;
	dst = self->lines;
	for (i = 0; i < self->numlines; i++, src++) {
		char *tofree = NULL;
		if (src->from_malloc) {
			tofree = src->start;
		}
		if (!src->deleted) {
			memcpy(data, src->start, src->len);
			*dst = *src;
			dst->start = data;
			dst->from_malloc = false;
			data += dst->len;
			dst++;
		}
		free(tofree);
	}
	Py_DECREF(self->pydata);
	self->pydata = pydata;
	self->numlines = self->livelines;
	self->dirty = false;
	return 0;
}

static PyObject *lazymanifest_text(lazymanifest *self)
{
	if (compact(self) != 0) {
		PyErr_NoMemory();
		return NULL;
	}
	Py_INCREF(self->pydata);
	return self->pydata;
}

static lazymanifest *lazymanifest_copy(lazymanifest *self)
{
	lazymanifest *copy = NULL;
	if (compact(self) != 0) {
		goto nomem;
	}
	copy = PyObject_New(lazymanifest, &lazymanifestType);
	if (!copy) {
		goto nomem;
	}
	copy->numlines = self->numlines;
	copy->livelines = self->livelines;
	copy->dirty = false;
	copy->lines = malloc(self->maxlines *sizeof(line));
	if (!copy->lines) {
		goto nomem;
	}
	memcpy(copy->lines, self->lines, self->numlines * sizeof(line));
	copy->maxlines = self->maxlines;
	copy->pydata = self->pydata;
	Py_INCREF(copy->pydata);
	return copy;
nomem:
	PyErr_NoMemory();
	Py_XDECREF(copy);
	return NULL;
}

static lazymanifest *lazymanifest_filtercopy(
	lazymanifest *self, PyObject *matchfn)
{
	lazymanifest *copy = NULL;
	int i;
	if (!PyCallable_Check(matchfn)) {
		PyErr_SetString(PyExc_TypeError, "matchfn must be callable");
		return NULL;
	}
	/* compact ourselves first to avoid double-frees later when we
	 * compact tmp so that it doesn't have random pointers to our
	 * underlying from_malloc-data (self->pydata is safe) */
	if (compact(self) != 0) {
		goto nomem;
	}
	copy = PyObject_New(lazymanifest, &lazymanifestType);
	copy->dirty = true;
	copy->lines = malloc(self->maxlines * sizeof(line));
	if (!copy->lines) {
		goto nomem;
	}
	copy->maxlines = self->maxlines;
	copy->numlines = 0;
	copy->pydata = self->pydata;
	Py_INCREF(self->pydata);
	for (i = 0; i < self->numlines; i++) {
		PyObject *arg = PyString_FromString(self->lines[i].start);
		PyObject *arglist = PyTuple_Pack(1, arg);
		PyObject *result = PyObject_CallObject(matchfn, arglist);
		Py_DECREF(arglist);
		Py_DECREF(arg);
		/* if the callback raised an exception, just let it
		 * through and give up */
		if (!result) {
			free(copy->lines);
			Py_DECREF(self->pydata);
			return NULL;
		}
		if (PyObject_IsTrue(result)) {
			assert(!(self->lines[i].from_malloc));
			copy->lines[copy->numlines++] = self->lines[i];
		}
		Py_DECREF(result);
	}
	copy->livelines = copy->numlines;
	return copy;
nomem:
	PyErr_NoMemory();
	Py_XDECREF(copy);
	return NULL;
}

static PyObject *lazymanifest_diff(lazymanifest *self, PyObject *args)
{
	lazymanifest *other;
	PyObject *pyclean = NULL;
	bool listclean;
	PyObject *emptyTup = NULL, *ret = NULL;
	PyObject *es;
	int sneedle = 0, oneedle = 0;
	if (!PyArg_ParseTuple(args, "O!|O", &lazymanifestType, &other, &pyclean)) {
		return NULL;
	}
	listclean = (!pyclean) ? false : PyObject_IsTrue(pyclean);
	es = PyString_FromString("");
	if (!es) {
		goto nomem;
	}
	emptyTup = PyTuple_Pack(2, Py_None, es);
	Py_DECREF(es);
	if (!emptyTup) {
		goto nomem;
	}
	ret = PyDict_New();
	if (!ret) {
		goto nomem;
	}
	while (sneedle != self->numlines || oneedle != other->numlines) {
		line *left = self->lines + sneedle;
		line *right = other->lines + oneedle;
		int result;
		PyObject *key;
		PyObject *outer;
		/* If we're looking at a deleted entry and it's not
		 * the end of the manifest, just skip it. */
		if (left->deleted && sneedle < self->numlines) {
			sneedle++;
			continue;
		}
		if (right->deleted && oneedle < other->numlines) {
			oneedle++;
			continue;
		}
		/* if we're at the end of either manifest, then we
		 * know the remaining items are adds so we can skip
		 * the strcmp. */
		if (sneedle == self->numlines) {
			result = 1;
		} else if (oneedle == other->numlines) {
			result = -1;
		} else {
			result = linecmp(left, right);
		}
		key = result <= 0 ?
			PyString_FromString(left->start) :
			PyString_FromString(right->start);
		if (!key)
			goto nomem;
		if (result < 0) {
			PyObject *l = hashflags(left);
			if (!l) {
				goto nomem;
			}
			outer = PyTuple_Pack(2, l, emptyTup);
			Py_DECREF(l);
			if (!outer) {
				goto nomem;
			}
			PyDict_SetItem(ret, key, outer);
			Py_DECREF(outer);
			sneedle++;
		} else if (result > 0) {
			PyObject *r = hashflags(right);
			if (!r) {
				goto nomem;
			}
			outer = PyTuple_Pack(2, emptyTup, r);
			Py_DECREF(r);
			if (!outer) {
				goto nomem;
			}
			PyDict_SetItem(ret, key, outer);
			Py_DECREF(outer);
			oneedle++;
		} else {
			/* file exists in both manifests */
			if (left->len != right->len
			    || memcmp(left->start, right->start, left->len)
			    || left->hash_suffix != right->hash_suffix) {
				PyObject *l = hashflags(left);
				PyObject *r;
				if (!l) {
					goto nomem;
				}
				r = hashflags(right);
				if (!r) {
					Py_DECREF(l);
					goto nomem;
				}
				outer = PyTuple_Pack(2, l, r);
				Py_DECREF(l);
				Py_DECREF(r);
				if (!outer) {
					goto nomem;
				}
				PyDict_SetItem(ret, key, outer);
				Py_DECREF(outer);
			} else if (listclean) {
				PyDict_SetItem(ret, key, Py_None);
			}
			sneedle++;
			oneedle++;
		}
		Py_DECREF(key);
	}
	Py_DECREF(emptyTup);
	return ret;
nomem:
	PyErr_NoMemory();
	Py_XDECREF(ret);
	Py_XDECREF(emptyTup);
	return NULL;
}

static PyMethodDef lazymanifest_methods[] = {
	{"iterkeys", (PyCFunction)lazymanifest_getkeysiter, METH_NOARGS,
	 "Iterate over file names in this lazymanifest."},
	{"iterentries", (PyCFunction)lazymanifest_getentriesiter, METH_NOARGS,
	 "Iterate over (path, nodeid, flags) typles in this lazymanifest."},
	{"copy", (PyCFunction)lazymanifest_copy, METH_NOARGS,
	 "Make a copy of this lazymanifest."},
	{"filtercopy", (PyCFunction)lazymanifest_filtercopy, METH_O,
	 "Make a copy of this manifest filtered by matchfn."},
	{"diff", (PyCFunction)lazymanifest_diff, METH_VARARGS,
	 "Compare this lazymanifest to another one."},
	{"text", (PyCFunction)lazymanifest_text, METH_NOARGS,
	 "Encode this manifest to text."},
	{NULL},
};

static PyTypeObject lazymanifestType = {
	PyObject_HEAD_INIT(NULL)
	0,                                                /* ob_size */
	"parsers.lazymanifest",                           /* tp_name */
	sizeof(lazymanifest),                             /* tp_basicsize */
	0,                                                /* tp_itemsize */
	(destructor)lazymanifest_dealloc,                 /* tp_dealloc */
	0,                                                /* tp_print */
	0,                                                /* tp_getattr */
	0,                                                /* tp_setattr */
	0,                                                /* tp_compare */
	0,                                                /* tp_repr */
	0,                                                /* tp_as_number */
	&lazymanifest_seq_meths,                          /* tp_as_sequence */
	&lazymanifest_mapping_methods,                    /* tp_as_mapping */
	0,                                                /* tp_hash */
	0,                                                /* tp_call */
	0,                                                /* tp_str */
	0,                                                /* tp_getattro */
	0,                                                /* tp_setattro */
	0,                                                /* tp_as_buffer */
	Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_SEQUENCE_IN, /* tp_flags */
	"TODO(augie)",                                    /* tp_doc */
	0,                                                /* tp_traverse */
	0,                                                /* tp_clear */
	0,                                                /* tp_richcompare */
	0,                                             /* tp_weaklistoffset */
	(getiterfunc)lazymanifest_getkeysiter,                /* tp_iter */
	0,                                                /* tp_iternext */
	lazymanifest_methods,                             /* tp_methods */
	0,                                                /* tp_members */
	0,                                                /* tp_getset */
	0,                                                /* tp_base */
	0,                                                /* tp_dict */
	0,                                                /* tp_descr_get */
	0,                                                /* tp_descr_set */
	0,                                                /* tp_dictoffset */
	(initproc)lazymanifest_init,                      /* tp_init */
	0,                                                /* tp_alloc */
};

void manifest_module_init(PyObject * mod)
{
	lazymanifestType.tp_new = PyType_GenericNew;
	if (PyType_Ready(&lazymanifestType) < 0)
		return;
	Py_INCREF(&lazymanifestType);

	PyModule_AddObject(mod, "lazymanifest",
			   (PyObject *)&lazymanifestType);
}