Modularize commit-walker

This turns the extern functions to be provided by the backend into a
struct of pointers, renames the functions to be more
namespace-friendly, and updates http-fetch to this interface. It
removes the unused include from http-push.c. It makes git-http-fetch a
builtin (with the implementation a separate file, accessible
directly).

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 1035 insertions, 0 deletions

diff --git a/http-walker.c b/http-walker.c
new file mode 100644
index 000000000..444aebf52
--- /dev/null
+++ b/http-walker.c
@@ -0,0 +1,1035 @@
+#include "cache.h"
+#include "commit.h"
+#include "pack.h"
+#include "walker.h"
+#include "http.h"
+
+#define PREV_BUF_SIZE 4096
+#define RANGE_HEADER_SIZE 30
+
+struct alt_base
+{
+	char *base;
+	int got_indices;
+	struct packed_git *packs;
+	struct alt_base *next;
+};
+
+enum object_request_state {
+	WAITING,
+	ABORTED,
+	ACTIVE,
+	COMPLETE,
+};
+
+struct object_request
+{
+	struct walker *walker;
+	unsigned char sha1[20];
+	struct alt_base *repo;
+	char *url;
+	char filename[PATH_MAX];
+	char tmpfile[PATH_MAX];
+	int local;
+	enum object_request_state state;
+	CURLcode curl_result;
+	char errorstr[CURL_ERROR_SIZE];
+	long http_code;
+	unsigned char real_sha1[20];
+	SHA_CTX c;
+	z_stream stream;
+	int zret;
+	int rename;
+	struct active_request_slot *slot;
+	struct object_request *next;
+};
+
+struct alternates_request {
+	struct walker *walker;
+	const char *base;
+	char *url;
+	struct buffer *buffer;
+	struct active_request_slot *slot;
+	int http_specific;
+};
+
+struct walker_data {
+	const char *url;
+	int got_alternates;
+	struct alt_base *alt;
+	struct curl_slist *no_pragma_header;
+};
+
+static struct object_request *object_queue_head;
+
+static size_t fwrite_sha1_file(void *ptr, size_t eltsize, size_t nmemb,
+			       void *data)
+{
+	unsigned char expn[4096];
+	size_t size = eltsize * nmemb;
+	int posn = 0;
+	struct object_request *obj_req = (struct object_request *)data;
+	do {
+		ssize_t retval = xwrite(obj_req->local,
+				       (char *) ptr + posn, size - posn);
+		if (retval < 0)
+			return posn;
+		posn += retval;
+	} while (posn < size);
+
+	obj_req->stream.avail_in = size;
+	obj_req->stream.next_in = ptr;
+	do {
+		obj_req->stream.next_out = expn;
+		obj_req->stream.avail_out = sizeof(expn);
+		obj_req->zret = inflate(&obj_req->stream, Z_SYNC_FLUSH);
+		SHA1_Update(&obj_req->c, expn,
+			    sizeof(expn) - obj_req->stream.avail_out);
+	} while (obj_req->stream.avail_in && obj_req->zret == Z_OK);
+	data_received++;
+	return size;
+}
+
+static int missing__target(int code, int result)
+{
+	return	/* file:// URL -- do we ever use one??? */
+		(result == CURLE_FILE_COULDNT_READ_FILE) ||
+		/* http:// and https:// URL */
+		(code == 404 && result == CURLE_HTTP_RETURNED_ERROR) ||
+		/* ftp:// URL */
+		(code == 550 && result == CURLE_FTP_COULDNT_RETR_FILE)
+		;
+}
+
+#define missing_target(a) missing__target((a)->http_code, (a)->curl_result)
+
+static void fetch_alternates(struct walker *walker, const char *base);
+
+static void process_object_response(void *callback_data);
+
+static void start_object_request(struct walker *walker,
+				 struct object_request *obj_req)
+{
+	char *hex = sha1_to_hex(obj_req->sha1);
+	char prevfile[PATH_MAX];
+	char *url;
+	char *posn;
+	int prevlocal;
+	unsigned char prev_buf[PREV_BUF_SIZE];
+	ssize_t prev_read = 0;
+	long prev_posn = 0;
+	char range[RANGE_HEADER_SIZE];
+	struct curl_slist *range_header = NULL;
+	struct active_request_slot *slot;
+	struct walker_data *data = walker->data;
+
+	snprintf(prevfile, sizeof(prevfile), "%s.prev", obj_req->filename);
+	unlink(prevfile);
+	rename(obj_req->tmpfile, prevfile);
+	unlink(obj_req->tmpfile);
+
+	if (obj_req->local != -1)
+		error("fd leakage in start: %d", obj_req->local);
+	obj_req->local = open(obj_req->tmpfile,
+			      O_WRONLY | O_CREAT | O_EXCL, 0666);
+	/* This could have failed due to the "lazy directory creation";
+	 * try to mkdir the last path component.
+	 */
+	if (obj_req->local < 0 && errno == ENOENT) {
+		char *dir = strrchr(obj_req->tmpfile, '/');
+		if (dir) {
+			*dir = 0;
+			mkdir(obj_req->tmpfile, 0777);
+			*dir = '/';
+		}
+		obj_req->local = open(obj_req->tmpfile,
+				      O_WRONLY | O_CREAT | O_EXCL, 0666);
+	}
+
+	if (obj_req->local < 0) {
+		obj_req->state = ABORTED;
+		error("Couldn't create temporary file %s for %s: %s",
+		      obj_req->tmpfile, obj_req->filename, strerror(errno));
+		return;
+	}
+
+	memset(&obj_req->stream, 0, sizeof(obj_req->stream));
+
+	inflateInit(&obj_req->stream);
+
+	SHA1_Init(&obj_req->c);
+
+	url = xmalloc(strlen(obj_req->repo->base) + 51);
+	obj_req->url = xmalloc(strlen(obj_req->repo->base) + 51);
+	strcpy(url, obj_req->repo->base);
+	posn = url + strlen(obj_req->repo->base);
+	strcpy(posn, "/objects/");
+	posn += 9;
+	memcpy(posn, hex, 2);
+	posn += 2;
+	*(posn++) = '/';
+	strcpy(posn, hex + 2);
+	strcpy(obj_req->url, url);
+
+	/* If a previous temp file is present, process what was already
+	   fetched. */
+	prevlocal = open(prevfile, O_RDONLY);
+	if (prevlocal != -1) {
+		do {
+			prev_read = xread(prevlocal, prev_buf, PREV_BUF_SIZE);
+			if (prev_read>0) {
+				if (fwrite_sha1_file(prev_buf,
+						     1,
+						     prev_read,
+						     obj_req) == prev_read) {
+					prev_posn += prev_read;
+				} else {
+					prev_read = -1;
+				}
+			}
+		} while (prev_read > 0);
+		close(prevlocal);
+	}
+	unlink(prevfile);
+
+	/* Reset inflate/SHA1 if there was an error reading the previous temp
+	   file; also rewind to the beginning of the local file. */
+	if (prev_read == -1) {
+		memset(&obj_req->stream, 0, sizeof(obj_req->stream));
+		inflateInit(&obj_req->stream);
+		SHA1_Init(&obj_req->c);
+		if (prev_posn>0) {
+			prev_posn = 0;
+			lseek(obj_req->local, 0, SEEK_SET);
+			ftruncate(obj_req->local, 0);
+		}
+	}
+
+	slot = get_active_slot();
+	slot->callback_func = process_object_response;
+	slot->callback_data = obj_req;
+	obj_req->slot = slot;
+
+	curl_easy_setopt(slot->curl, CURLOPT_FILE, obj_req);
+	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite_sha1_file);
+	curl_easy_setopt(slot->curl, CURLOPT_ERRORBUFFER, obj_req->errorstr);
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, data->no_pragma_header);
+
+	/* If we have successfully processed data from a previous fetch
+	   attempt, only fetch the data we don't already have. */
+	if (prev_posn>0) {
+		if (walker->get_verbosely)
+			fprintf(stderr,
+				"Resuming fetch of object %s at byte %ld\n",
+				hex, prev_posn);
+		sprintf(range, "Range: bytes=%ld-", prev_posn);
+		range_header = curl_slist_append(range_header, range);
+		curl_easy_setopt(slot->curl,
+				 CURLOPT_HTTPHEADER, range_header);
+	}
+
+	/* Try to get the request started, abort the request on error */
+	obj_req->state = ACTIVE;
+	if (!start_active_slot(slot)) {
+		obj_req->state = ABORTED;
+		obj_req->slot = NULL;
+		close(obj_req->local); obj_req->local = -1;
+		free(obj_req->url);
+		return;
+	}
+}
+
+static void finish_object_request(struct object_request *obj_req)
+{
+	struct stat st;
+
+	fchmod(obj_req->local, 0444);
+	close(obj_req->local); obj_req->local = -1;
+
+	if (obj_req->http_code == 416) {
+		fprintf(stderr, "Warning: requested range invalid; we may already have all the data.\n");
+	} else if (obj_req->curl_result != CURLE_OK) {
+		if (stat(obj_req->tmpfile, &st) == 0)
+			if (st.st_size == 0)
+				unlink(obj_req->tmpfile);
+		return;
+	}
+
+	inflateEnd(&obj_req->stream);
+	SHA1_Final(obj_req->real_sha1, &obj_req->c);
+	if (obj_req->zret != Z_STREAM_END) {
+		unlink(obj_req->tmpfile);
+		return;
+	}
+	if (hashcmp(obj_req->sha1, obj_req->real_sha1)) {
+		unlink(obj_req->tmpfile);
+		return;
+	}
+	obj_req->rename =
+		move_temp_to_file(obj_req->tmpfile, obj_req->filename);
+
+	if (obj_req->rename == 0)
+		walker_say(obj_req->walker, "got %s\n", sha1_to_hex(obj_req->sha1));
+}
+
+static void process_object_response(void *callback_data)
+{
+	struct object_request *obj_req =
+		(struct object_request *)callback_data;
+	struct walker *walker = obj_req->walker;
+	struct walker_data *data = walker->data;
+	struct alt_base *alt = data->alt;
+
+	obj_req->curl_result = obj_req->slot->curl_result;
+	obj_req->http_code = obj_req->slot->http_code;
+	obj_req->slot = NULL;
+	obj_req->state = COMPLETE;
+
+	/* Use alternates if necessary */
+	if (missing_target(obj_req)) {
+		fetch_alternates(walker, alt->base);
+		if (obj_req->repo->next != NULL) {
+			obj_req->repo =
+				obj_req->repo->next;
+			close(obj_req->local);
+			obj_req->local = -1;
+			start_object_request(walker, obj_req);
+			return;
+		}
+	}
+
+	finish_object_request(obj_req);
+}
+
+static void release_object_request(struct object_request *obj_req)
+{
+	struct object_request *entry = object_queue_head;
+
+	if (obj_req->local != -1)
+		error("fd leakage in release: %d", obj_req->local);
+	if (obj_req == object_queue_head) {
+		object_queue_head = obj_req->next;
+	} else {
+		while (entry->next != NULL && entry->next != obj_req)
+			entry = entry->next;
+		if (entry->next == obj_req)
+			entry->next = entry->next->next;
+	}
+
+	free(obj_req->url);
+	free(obj_req);
+}
+
+#ifdef USE_CURL_MULTI
+static int fill_active_slot(struct walker *walker)
+{
+	struct object_request *obj_req;
+
+	for (obj_req = object_queue_head; obj_req; obj_req = obj_req->next) {
+		if (obj_req->state == WAITING) {
+			if (has_sha1_file(obj_req->sha1))
+				obj_req->state = COMPLETE;
+			else {
+				start_object_request(walker, obj_req);
+				return 1;
+			}
+		}
+	}
+	return 0;
+}
+#endif
+
+static void prefetch(struct walker *walker, unsigned char *sha1)
+{
+	struct object_request *newreq;
+	struct object_request *tail;
+	struct walker_data *data = walker->data;
+	char *filename = sha1_file_name(sha1);
+
+	newreq = xmalloc(sizeof(*newreq));
+	newreq->walker = walker;
+	hashcpy(newreq->sha1, sha1);
+	newreq->repo = data->alt;
+	newreq->url = NULL;
+	newreq->local = -1;
+	newreq->state = WAITING;
+	snprintf(newreq->filename, sizeof(newreq->filename), "%s", filename);
+	snprintf(newreq->tmpfile, sizeof(newreq->tmpfile),
+		 "%s.temp", filename);
+	newreq->slot = NULL;
+	newreq->next = NULL;
+
+	if (object_queue_head == NULL) {
+		object_queue_head = newreq;
+	} else {
+		tail = object_queue_head;
+		while (tail->next != NULL) {
+			tail = tail->next;
+		}
+		tail->next = newreq;
+	}
+
+#ifdef USE_CURL_MULTI
+	fill_active_slots();
+	step_active_slots();
+#endif
+}
+
+static int fetch_index(struct walker *walker, struct alt_base *repo, unsigned char *sha1)
+{
+	char *hex = sha1_to_hex(sha1);
+	char *filename;
+	char *url;
+	char tmpfile[PATH_MAX];
+	long prev_posn = 0;
+	char range[RANGE_HEADER_SIZE];
+	struct curl_slist *range_header = NULL;
+	struct walker_data *data = walker->data;
+
+	FILE *indexfile;
+	struct active_request_slot *slot;
+	struct slot_results results;
+
+	if (has_pack_index(sha1))
+		return 0;
+
+	if (walker->get_verbosely)
+		fprintf(stderr, "Getting index for pack %s\n", hex);
+
+	url = xmalloc(strlen(repo->base) + 64);
+	sprintf(url, "%s/objects/pack/pack-%s.idx", repo->base, hex);
+
+	filename = sha1_pack_index_name(sha1);
+	snprintf(tmpfile, sizeof(tmpfile), "%s.temp", filename);
+	indexfile = fopen(tmpfile, "a");
+	if (!indexfile)
+		return error("Unable to open local file %s for pack index",
+			     filename);
+
+	slot = get_active_slot();
+	slot->results = &results;
+	curl_easy_setopt(slot->curl, CURLOPT_FILE, indexfile);
+	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite);
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, data->no_pragma_header);
+	slot->local = indexfile;
+
+	/* If there is data present from a previous transfer attempt,
+	   resume where it left off */
+	prev_posn = ftell(indexfile);
+	if (prev_posn>0) {
+		if (walker->get_verbosely)
+			fprintf(stderr,
+				"Resuming fetch of index for pack %s at byte %ld\n",
+				hex, prev_posn);
+		sprintf(range, "Range: bytes=%ld-", prev_posn);
+		range_header = curl_slist_append(range_header, range);
+		curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, range_header);
+	}
+
+	if (start_active_slot(slot)) {
+		run_active_slot(slot);
+		if (results.curl_result != CURLE_OK) {
+			fclose(indexfile);
+			return error("Unable to get pack index %s\n%s", url,
+				     curl_errorstr);
+		}
+	} else {
+		fclose(indexfile);
+		return error("Unable to start request");
+	}
+
+	fclose(indexfile);
+
+	return move_temp_to_file(tmpfile, filename);
+}
+
+static int setup_index(struct walker *walker, struct alt_base *repo, unsigned char *sha1)
+{
+	struct packed_git *new_pack;
+	if (has_pack_file(sha1))
+		return 0; /* don't list this as something we can get */
+
+	if (fetch_index(walker, repo, sha1))
+		return -1;
+
+	new_pack = parse_pack_index(sha1);
+	new_pack->next = repo->packs;
+	repo->packs = new_pack;
+	return 0;
+}
+
+static void process_alternates_response(void *callback_data)
+{
+	struct alternates_request *alt_req =
+		(struct alternates_request *)callback_data;
+	struct walker *walker = alt_req->walker;
+	struct walker_data *cdata = walker->data;
+	struct active_request_slot *slot = alt_req->slot;
+	struct alt_base *tail = cdata->alt;
+	const char *base = alt_req->base;
+	static const char null_byte = '\0';
+	char *data;
+	int i = 0;
+
+	if (alt_req->http_specific) {
+		if (slot->curl_result != CURLE_OK ||
+		    !alt_req->buffer->posn) {
+
+			/* Try reusing the slot to get non-http alternates */
+			alt_req->http_specific = 0;
+			sprintf(alt_req->url, "%s/objects/info/alternates",
+				base);
+			curl_easy_setopt(slot->curl, CURLOPT_URL,
+					 alt_req->url);
+			active_requests++;
+			slot->in_use = 1;
+			if (slot->finished != NULL)
+				(*slot->finished) = 0;
+			if (!start_active_slot(slot)) {
+				cdata->got_alternates = -1;
+				slot->in_use = 0;
+				if (slot->finished != NULL)
+					(*slot->finished) = 1;
+			}
+			return;
+		}
+	} else if (slot->curl_result != CURLE_OK) {
+		if (!missing_target(slot)) {
+			cdata->got_alternates = -1;
+			return;
+		}
+	}
+
+	fwrite_buffer(&null_byte, 1, 1, alt_req->buffer);
+	alt_req->buffer->posn--;
+	data = alt_req->buffer->buffer;
+
+	while (i < alt_req->buffer->posn) {
+		int posn = i;
+		while (posn < alt_req->buffer->posn && data[posn] != '\n')
+			posn++;
+		if (data[posn] == '\n') {
+			int okay = 0;
+			int serverlen = 0;
+			struct alt_base *newalt;
+			char *target = NULL;
+			if (data[i] == '/') {
+				/* This counts
+				 * http://git.host/pub/scm/linux.git/
+				 * -----------here^
+				 * so memcpy(dst, base, serverlen) will
+				 * copy up to "...git.host".
+				 */
+				const char *colon_ss = strstr(base,"://");
+				if (colon_ss) {
+					serverlen = (strchr(colon_ss + 3, '/')
+						     - base);
+					okay = 1;
+				}
+			} else if (!memcmp(data + i, "../", 3)) {
+				/* Relative URL; chop the corresponding
+				 * number of subpath from base (and ../
+				 * from data), and concatenate the result.
+				 *
+				 * The code first drops ../ from data, and
+				 * then drops one ../ from data and one path
+				 * from base.  IOW, one extra ../ is dropped
+				 * from data than path is dropped from base.
+				 *
+				 * This is not wrong.  The alternate in
+				 *     http://git.host/pub/scm/linux.git/
+				 * to borrow from
+				 *     http://git.host/pub/scm/linus.git/
+				 * is ../../linus.git/objects/.  You need
+				 * two ../../ to borrow from your direct
+				 * neighbour.
+				 */
+				i += 3;
+				serverlen = strlen(base);
+				while (i + 2 < posn &&
+				       !memcmp(data + i, "../", 3)) {
+					do {
+						serverlen--;
+					} while (serverlen &&
+						 base[serverlen - 1] != '/');
+					i += 3;
+				}
+				/* If the server got removed, give up. */
+				okay = strchr(base, ':') - base + 3 <
+					serverlen;
+			} else if (alt_req->http_specific) {
+				char *colon = strchr(data + i, ':');
+				char *slash = strchr(data + i, '/');
+				if (colon && slash && colon < data + posn &&
+				    slash < data + posn && colon < slash) {
+					okay = 1;
+				}
+			}
+			/* skip "objects\n" at end */
+			if (okay) {
+				target = xmalloc(serverlen + posn - i - 6);
+				memcpy(target, base, serverlen);
+				memcpy(target + serverlen, data + i,
+				       posn - i - 7);
+				target[serverlen + posn - i - 7] = 0;
+				if (walker->get_verbosely)
+					fprintf(stderr,
+						"Also look at %s\n", target);
+				newalt = xmalloc(sizeof(*newalt));
+				newalt->next = NULL;
+				newalt->base = target;
+				newalt->got_indices = 0;
+				newalt->packs = NULL;
+
+				while (tail->next != NULL)
+					tail = tail->next;
+				tail->next = newalt;
+			}
+		}
+		i = posn + 1;
+	}
+
+	cdata->got_alternates = 1;
+}
+
+static void fetch_alternates(struct walker *walker, const char *base)
+{
+	struct buffer buffer;
+	char *url;
+	char *data;
+	struct active_request_slot *slot;
+	struct alternates_request alt_req;
+	struct walker_data *cdata = walker->data;
+
+	/* If another request has already started fetching alternates,
+	   wait for them to arrive and return to processing this request's
+	   curl message */
+#ifdef USE_CURL_MULTI
+	while (cdata->got_alternates == 0) {
+		step_active_slots();
+	}
+#endif
+
+	/* Nothing to do if they've already been fetched */
+	if (cdata->got_alternates == 1)
+		return;
+
+	/* Start the fetch */
+	cdata->got_alternates = 0;
+
+	data = xmalloc(4096);
+	buffer.size = 4096;
+	buffer.posn = 0;
+	buffer.buffer = data;
+
+	if (walker->get_verbosely)
+		fprintf(stderr, "Getting alternates list for %s\n", base);
+
+	url = xmalloc(strlen(base) + 31);
+	sprintf(url, "%s/objects/info/http-alternates", base);
+
+	/* Use a callback to process the result, since another request
+	   may fail and need to have alternates loaded before continuing */
+	slot = get_active_slot();
+	slot->callback_func = process_alternates_response;
+	alt_req.walker = walker;
+	slot->callback_data = &alt_req;
+
+	curl_easy_setopt(slot->curl, CURLOPT_FILE, &buffer);
+	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite_buffer);
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+
+	alt_req.base = base;
+	alt_req.url = url;
+	alt_req.buffer = &buffer;
+	alt_req.http_specific = 1;
+	alt_req.slot = slot;
+
+	if (start_active_slot(slot))
+		run_active_slot(slot);
+	else
+		cdata->got_alternates = -1;
+
+	free(data);
+	free(url);
+}
+
+static int fetch_indices(struct walker *walker, struct alt_base *repo)
+{
+	unsigned char sha1[20];
+	char *url;
+	struct buffer buffer;
+	char *data;
+	int i = 0;
+
+	struct active_request_slot *slot;
+	struct slot_results results;
+
+	if (repo->got_indices)
+		return 0;
+
+	data = xmalloc(4096);
+	buffer.size = 4096;
+	buffer.posn = 0;
+	buffer.buffer = data;
+
+	if (walker->get_verbosely)
+		fprintf(stderr, "Getting pack list for %s\n", repo->base);
+
+	url = xmalloc(strlen(repo->base) + 21);
+	sprintf(url, "%s/objects/info/packs", repo->base);
+
+	slot = get_active_slot();
+	slot->results = &results;
+	curl_easy_setopt(slot->curl, CURLOPT_FILE, &buffer);
+	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite_buffer);
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, NULL);
+	if (start_active_slot(slot)) {
+		run_active_slot(slot);
+		if (results.curl_result != CURLE_OK) {
+			if (missing_target(&results)) {
+				repo->got_indices = 1;
+				free(buffer.buffer);
+				return 0;
+			} else {
+				repo->got_indices = 0;
+				free(buffer.buffer);
+				return error("%s", curl_errorstr);
+			}
+		}
+	} else {
+		repo->got_indices = 0;
+		free(buffer.buffer);
+		return error("Unable to start request");
+	}
+
+	data = buffer.buffer;
+	while (i < buffer.posn) {
+		switch (data[i]) {
+		case 'P':
+			i++;
+			if (i + 52 <= buffer.posn &&
+			    !prefixcmp(data + i, " pack-") &&
+			    !prefixcmp(data + i + 46, ".pack\n")) {
+				get_sha1_hex(data + i + 6, sha1);
+				setup_index(walker, repo, sha1);
+				i += 51;
+				break;
+			}
+		default:
+			while (i < buffer.posn && data[i] != '\n')
+				i++;
+		}
+		i++;
+	}
+
+	free(buffer.buffer);
+	repo->got_indices = 1;
+	return 0;
+}
+
+static int fetch_pack(struct walker *walker, struct alt_base *repo, unsigned char *sha1)
+{
+	char *url;
+	struct packed_git *target;
+	struct packed_git **lst;
+	FILE *packfile;
+	char *filename;
+	char tmpfile[PATH_MAX];
+	int ret;
+	long prev_posn = 0;
+	char range[RANGE_HEADER_SIZE];
+	struct curl_slist *range_header = NULL;
+	struct walker_data *data = walker->data;
+
+	struct active_request_slot *slot;
+	struct slot_results results;
+
+	if (fetch_indices(walker, repo))
+		return -1;
+	target = find_sha1_pack(sha1, repo->packs);
+	if (!target)
+		return -1;
+
+	if (walker->get_verbosely) {
+		fprintf(stderr, "Getting pack %s\n",
+			sha1_to_hex(target->sha1));
+		fprintf(stderr, " which contains %s\n",
+			sha1_to_hex(sha1));
+	}
+
+	url = xmalloc(strlen(repo->base) + 65);
+	sprintf(url, "%s/objects/pack/pack-%s.pack",
+		repo->base, sha1_to_hex(target->sha1));
+
+	filename = sha1_pack_name(target->sha1);
+	snprintf(tmpfile, sizeof(tmpfile), "%s.temp", filename);
+	packfile = fopen(tmpfile, "a");
+	if (!packfile)
+		return error("Unable to open local file %s for pack",
+			     filename);
+
+	slot = get_active_slot();
+	slot->results = &results;
+	curl_easy_setopt(slot->curl, CURLOPT_FILE, packfile);
+	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite);
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, data->no_pragma_header);
+	slot->local = packfile;
+
+	/* If there is data present from a previous transfer attempt,
+	   resume where it left off */
+	prev_posn = ftell(packfile);
+	if (prev_posn>0) {
+		if (walker->get_verbosely)
+			fprintf(stderr,
+				"Resuming fetch of pack %s at byte %ld\n",
+				sha1_to_hex(target->sha1), prev_posn);
+		sprintf(range, "Range: bytes=%ld-", prev_posn);
+		range_header = curl_slist_append(range_header, range);
+		curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, range_header);
+	}
+
+	if (start_active_slot(slot)) {
+		run_active_slot(slot);
+		if (results.curl_result != CURLE_OK) {
+			fclose(packfile);
+			return error("Unable to get pack file %s\n%s", url,
+				     curl_errorstr);
+		}
+	} else {
+		fclose(packfile);
+		return error("Unable to start request");
+	}
+
+	target->pack_size = ftell(packfile);
+	fclose(packfile);
+
+	ret = move_temp_to_file(tmpfile, filename);
+	if (ret)
+		return ret;
+
+	lst = &repo->packs;
+	while (*lst != target)
+		lst = &((*lst)->next);
+	*lst = (*lst)->next;
+
+	if (verify_pack(target, 0))
+		return -1;
+	install_packed_git(target);
+
+	return 0;
+}
+
+static void abort_object_request(struct object_request *obj_req)
+{
+	if (obj_req->local >= 0) {
+		close(obj_req->local);
+		obj_req->local = -1;
+	}
+	unlink(obj_req->tmpfile);
+	if (obj_req->slot) {
+		release_active_slot(obj_req->slot);
+		obj_req->slot = NULL;
+	}
+	release_object_request(obj_req);
+}
+
+static int fetch_object(struct walker *walker, struct alt_base *repo, unsigned char *sha1)
+{
+	char *hex = sha1_to_hex(sha1);
+	int ret = 0;
+	struct object_request *obj_req = object_queue_head;
+
+	while (obj_req != NULL && hashcmp(obj_req->sha1, sha1))
+		obj_req = obj_req->next;
+	if (obj_req == NULL)
+		return error("Couldn't find request for %s in the queue", hex);
+
+	if (has_sha1_file(obj_req->sha1)) {
+		abort_object_request(obj_req);
+		return 0;
+	}
+
+#ifdef USE_CURL_MULTI
+	while (obj_req->state == WAITING) {
+		step_active_slots();
+	}
+#else
+	start_object_request(walker, obj_req);
+#endif
+
+	while (obj_req->state == ACTIVE) {
+		run_active_slot(obj_req->slot);
+	}
+	if (obj_req->local != -1) {
+		close(obj_req->local); obj_req->local = -1;
+	}
+
+	if (obj_req->state == ABORTED) {
+		ret = error("Request for %s aborted", hex);
+	} else if (obj_req->curl_result != CURLE_OK &&
+		   obj_req->http_code != 416) {
+		if (missing_target(obj_req))
+			ret = -1; /* Be silent, it is probably in a pack. */
+		else
+			ret = error("%s (curl_result = %d, http_code = %ld, sha1 = %s)",
+				    obj_req->errorstr, obj_req->curl_result,
+				    obj_req->http_code, hex);
+	} else if (obj_req->zret != Z_STREAM_END) {
+		walker->corrupt_object_found++;
+		ret = error("File %s (%s) corrupt", hex, obj_req->url);
+	} else if (hashcmp(obj_req->sha1, obj_req->real_sha1)) {
+		ret = error("File %s has bad hash", hex);
+	} else if (obj_req->rename < 0) {
+		ret = error("unable to write sha1 filename %s",
+			    obj_req->filename);
+	}
+
+	release_object_request(obj_req);
+	return ret;
+}
+
+static int fetch(struct walker *walker, unsigned char *sha1)
+{
+	struct walker_data *data = walker->data;
+	struct alt_base *altbase = data->alt;
+
+	if (!fetch_object(walker, altbase, sha1))
+		return 0;
+	while (altbase) {
+		if (!fetch_pack(walker, altbase, sha1))
+			return 0;
+		fetch_alternates(walker, data->alt->base);
+		altbase = altbase->next;
+	}
+	return error("Unable to find %s under %s", sha1_to_hex(sha1),
+		     data->alt->base);
+}
+
+static inline int needs_quote(int ch)
+{
+	if (((ch >= 'A') && (ch <= 'Z'))
+			|| ((ch >= 'a') && (ch <= 'z'))
+			|| ((ch >= '0') && (ch <= '9'))
+			|| (ch == '/')
+			|| (ch == '-')
+			|| (ch == '.'))
+		return 0;
+	return 1;
+}
+
+static inline int hex(int v)
+{
+	if (v < 10) return '0' + v;
+	else return 'A' + v - 10;
+}
+
+static char *quote_ref_url(const char *base, const char *ref)
+{
+	const char *cp;
+	char *dp, *qref;
+	int len, baselen, ch;
+
+	baselen = strlen(base);
+	len = baselen + 7; /* "/refs/" + NUL */
+	for (cp = ref; (ch = *cp) != 0; cp++, len++)
+		if (needs_quote(ch))
+			len += 2; /* extra two hex plus replacement % */
+	qref = xmalloc(len);
+	memcpy(qref, base, baselen);
+	memcpy(qref + baselen, "/refs/", 6);
+	for (cp = ref, dp = qref + baselen + 6; (ch = *cp) != 0; cp++) {
+		if (needs_quote(ch)) {
+			*dp++ = '%';
+			*dp++ = hex((ch >> 4) & 0xF);
+			*dp++ = hex(ch & 0xF);
+		}
+		else
+			*dp++ = ch;
+	}
+	*dp = 0;
+
+	return qref;
+}
+
+static int fetch_ref(struct walker *walker, char *ref, unsigned char *sha1)
+{
+        char *url;
+        char hex[42];
+        struct buffer buffer;
+	struct walker_data *data = walker->data;
+	const char *base = data->alt->base;
+	struct active_request_slot *slot;
+	struct slot_results results;
+        buffer.size = 41;
+        buffer.posn = 0;
+        buffer.buffer = hex;
+        hex[41] = '\0';
+
+	url = quote_ref_url(base, ref);
+	slot = get_active_slot();
+	slot->results = &results;
+	curl_easy_setopt(slot->curl, CURLOPT_FILE, &buffer);
+	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite_buffer);
+	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, NULL);
+	curl_easy_setopt(slot->curl, CURLOPT_URL, url);
+	if (start_active_slot(slot)) {
+		run_active_slot(slot);
+		if (results.curl_result != CURLE_OK)
+			return error("Couldn't get %s for %s\n%s",
+				     url, ref, curl_errorstr);
+	} else {
+		return error("Unable to start request");
+	}
+
+        hex[40] = '\0';
+        get_sha1_hex(hex, sha1);
+        return 0;
+}
+
+static void cleanup(struct walker *walker)
+{
+	struct walker_data *data = walker->data;
+	http_cleanup();
+
+	curl_slist_free_all(data->no_pragma_header);
+}
+
+struct walker *get_http_walker(const char *url)
+{
+	char *s;
+	struct walker_data *data = xmalloc(sizeof(struct walker_data));
+	struct walker *walker = xmalloc(sizeof(struct walker));
+
+	http_init();
+
+	data->no_pragma_header = curl_slist_append(NULL, "Pragma:");
+
+	data->alt = xmalloc(sizeof(*data->alt));
+	data->alt->base = xmalloc(strlen(url) + 1);
+	strcpy(data->alt->base, url);
+	for (s = data->alt->base + strlen(data->alt->base) - 1; *s == '/'; --s)
+		*s = 0;
+
+	data->alt->got_indices = 0;
+	data->alt->packs = NULL;
+	data->alt->next = NULL;
+	data->got_alternates = -1;
+
+	walker->corrupt_object_found = 0;
+	walker->fetch = fetch;
+	walker->fetch_ref = fetch_ref;
+	walker->prefetch = prefetch;
+	walker->cleanup = cleanup;
+	walker->data = data;
+
+#ifdef USE_CURL_MULTI
+	add_fill_function(walker, (int (*)(void *)) fill_active_slot);
+#endif
+
+	return walker;
+}