Merge branch 'ds/bundle-uri'

Preliminary code refactoring around transport and bundle code.

* ds/bundle-uri:
  bundle.h: make "fd" version of read_bundle_header() public
  remote: allow relative_url() to return an absolute url
  remote: move relative_url()
  http: make http_get_file() external
  fetch-pack: move --keep=* option filling to a function
  fetch-pack: add a deref_without_lazy_fetch_extended()
  dir API: add a generalized path_match_flags() function
  connect.c: refactor sending of agent & object-format

17 files changed, 321 insertions, 191 deletions

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index 1a8e5d0621..3aed3a4e48 100644
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -72,135 +72,6 @@ static char *get_default_remote(void)
 	return repo_get_default_remote(the_repository);
 }
 
-static int starts_with_dot_slash(const char *str)
-{
-	return str[0] == '.' && is_dir_sep(str[1]);
-}
-
-static int starts_with_dot_dot_slash(const char *str)
-{
-	return str[0] == '.' && str[1] == '.' && is_dir_sep(str[2]);
-}
-
-/*
- * Returns 1 if it was the last chop before ':'.
- */
-static int chop_last_dir(char **remoteurl, int is_relative)
-{
-	char *rfind = find_last_dir_sep(*remoteurl);
-	if (rfind) {
-		*rfind = '\0';
-		return 0;
-	}
-
-	rfind = strrchr(*remoteurl, ':');
-	if (rfind) {
-		*rfind = '\0';
-		return 1;
-	}
-
-	if (is_relative || !strcmp(".", *remoteurl))
-		die(_("cannot strip one component off url '%s'"),
-			*remoteurl);
-
-	free(*remoteurl);
-	*remoteurl = xstrdup(".");
-	return 0;
-}
-
-/*
- * The `url` argument is the URL that navigates to the submodule origin
- * repo. When relative, this URL is relative to the superproject origin
- * URL repo. The `up_path` argument, if specified, is the relative
- * path that navigates from the submodule working tree to the superproject
- * working tree. Returns the origin URL of the submodule.
- *
- * Return either an absolute URL or filesystem path (if the superproject
- * origin URL is an absolute URL or filesystem path, respectively) or a
- * relative file system path (if the superproject origin URL is a relative
- * file system path).
- *
- * When the output is a relative file system path, the path is either
- * relative to the submodule working tree, if up_path is specified, or to
- * the superproject working tree otherwise.
- *
- * NEEDSWORK: This works incorrectly on the domain and protocol part.
- * remote_url      url              outcome          expectation
- * http://a.com/b  ../c             http://a.com/c   as is
- * http://a.com/b/ ../c             http://a.com/c   same as previous line, but
- *                                                   ignore trailing slash in url
- * http://a.com/b  ../../c          http://c         error out
- * http://a.com/b  ../../../c       http:/c          error out
- * http://a.com/b  ../../../../c    http:c           error out
- * http://a.com/b  ../../../../../c    .:c           error out
- * NEEDSWORK: Given how chop_last_dir() works, this function is broken
- * when a local part has a colon in its path component, too.
- */
-static char *relative_url(const char *remote_url,
-				const char *url,
-				const char *up_path)
-{
-	int is_relative = 0;
-	int colonsep = 0;
-	char *out;
-	char *remoteurl = xstrdup(remote_url);
-	struct strbuf sb = STRBUF_INIT;
-	size_t len = strlen(remoteurl);
-
-	if (is_dir_sep(remoteurl[len-1]))
-		remoteurl[len-1] = '\0';
-
-	if (!url_is_local_not_ssh(remoteurl) || is_absolute_path(remoteurl))
-		is_relative = 0;
-	else {
-		is_relative = 1;
-		/*
-		 * Prepend a './' to ensure all relative
-		 * remoteurls start with './' or '../'
-		 */
-		if (!starts_with_dot_slash(remoteurl) &&
-		    !starts_with_dot_dot_slash(remoteurl)) {
-			strbuf_reset(&sb);
-			strbuf_addf(&sb, "./%s", remoteurl);
-			free(remoteurl);
-			remoteurl = strbuf_detach(&sb, NULL);
-		}
-	}
-	/*
-	 * When the url starts with '../', remove that and the
-	 * last directory in remoteurl.
-	 */
-	while (url) {
-		if (starts_with_dot_dot_slash(url)) {
-			url += 3;
-			colonsep |= chop_last_dir(&remoteurl, is_relative);
-		} else if (starts_with_dot_slash(url))
-			url += 2;
-		else
-			break;
-	}
-	strbuf_reset(&sb);
-	strbuf_addf(&sb, "%s%s%s", remoteurl, colonsep ? ":" : "/", url);
-	if (ends_with(url, "/"))
-		strbuf_setlen(&sb, sb.len - 1);
-	free(remoteurl);
-
-	if (starts_with_dot_slash(sb.buf))
-		out = xstrdup(sb.buf + 2);
-	else
-		out = xstrdup(sb.buf);
-
-	if (!up_path || !is_relative) {
-		strbuf_release(&sb);
-		return out;
-	}
-
-	strbuf_reset(&sb);
-	strbuf_addf(&sb, "%s%s", up_path, out);
-	free(out);
-	return strbuf_detach(&sb, NULL);
-}
-
 static char *resolve_relative_url(const char *rel_url, const char *up_path, int quiet)
 {
 	char *remoteurl, *resolved_url;
@@ -592,6 +463,18 @@ static int module_foreach(int argc, const char **argv, const char *prefix)
 	return 0;
 }
 
+static int starts_with_dot_slash(const char *const path)
+{
+	return path_match_flags(path, PATH_MATCH_STARTS_WITH_DOT_SLASH |
+				PATH_MATCH_XPLATFORM);
+}
+
+static int starts_with_dot_dot_slash(const char *const path)
+{
+	return path_match_flags(path, PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH |
+				PATH_MATCH_XPLATFORM);
+}
+
 struct init_cb {
 	const char *prefix;
 	const char *superprefix;
diff --git a/bundle.c b/bundle.c
index d50cfb5aa7..5fa41a52f1 100644
--- a/bundle.c
+++ b/bundle.c
@@ -66,8 +66,8 @@ static int parse_bundle_signature(struct bundle_header *header, const char *line
 	return -1;
 }
 
-static int parse_bundle_header(int fd, struct bundle_header *header,
-			       const char *report_path)
+int read_bundle_header_fd(int fd, struct bundle_header *header,
+			  const char *report_path)
 {
 	struct strbuf buf = STRBUF_INIT;
 	int status = 0;
@@ -143,7 +143,7 @@ int read_bundle_header(const char *path, struct bundle_header *header)
 
 	if (fd < 0)
 		return error(_("could not open '%s'"), path);
-	return parse_bundle_header(fd, header, path);
+	return read_bundle_header_fd(fd, header, path);
 }
 
 int is_bundle(const char *path, int quiet)
@@ -153,7 +153,7 @@ int is_bundle(const char *path, int quiet)
 
 	if (fd < 0)
 		return 0;
-	fd = parse_bundle_header(fd, &header, quiet ? NULL : path);
+	fd = read_bundle_header_fd(fd, &header, quiet ? NULL : path);
 	if (fd >= 0)
 		close(fd);
 	bundle_header_release(&header);
diff --git a/bundle.h b/bundle.h
index 7fef2108f4..0c052f5496 100644
--- a/bundle.h
+++ b/bundle.h
@@ -24,6 +24,8 @@ void bundle_header_release(struct bundle_header *header);
 
 int is_bundle(const char *path, int quiet);
 int read_bundle_header(const char *path, struct bundle_header *header);
+int read_bundle_header_fd(int fd, struct bundle_header *header,
+			  const char *report_path);
 int create_bundle(struct repository *r, const char *path,
 		  int argc, const char **argv, struct strvec *pack_options,
 		  int version);
diff --git a/compat/mingw.c b/compat/mingw.c
index 5772692a0a..2607de93af 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -2830,7 +2830,7 @@ not_a_reserved_name:
 			}
 
 			c = path[i];
-			if (c && c != '.' && c != ':' && c != '/' && c != '\\')
+			if (c && c != '.' && c != ':' && !is_xplatform_dir_sep(c))
 				goto not_a_reserved_name;
 
 			/* contains reserved name */
diff --git a/compat/win32/path-utils.h b/compat/win32/path-utils.h
index bba2b64408..65fa3b9263 100644
--- a/compat/win32/path-utils.h
+++ b/compat/win32/path-utils.h
@@ -6,11 +6,7 @@ int win32_has_dos_drive_prefix(const char *path);
 
 int win32_skip_dos_drive_prefix(char **path);
 #define skip_dos_drive_prefix win32_skip_dos_drive_prefix
-static inline int win32_is_dir_sep(int c)
-{
-	return c == '/' || c == '\\';
-}
-#define is_dir_sep win32_is_dir_sep
+#define is_dir_sep is_xplatform_dir_sep
 static inline char *win32_find_last_dir_sep(const char *path)
 {
 	char *ret = NULL;
diff --git a/connect.c b/connect.c
index afc79a6236..e6d0b1d34b 100644
--- a/connect.c
+++ b/connect.c
@@ -473,6 +473,24 @@ void check_stateless_delimiter(int stateless_rpc,
 		die("%s", error);
 }
 
+static void send_capabilities(int fd_out, struct packet_reader *reader)
+{
+	const char *hash_name;
+
+	if (server_supports_v2("agent", 0))
+		packet_write_fmt(fd_out, "agent=%s", git_user_agent_sanitized());
+
+	if (server_feature_v2("object-format", &hash_name)) {
+		int hash_algo = hash_algo_by_name(hash_name);
+		if (hash_algo == GIT_HASH_UNKNOWN)
+			die(_("unknown object format '%s' specified by server"), hash_name);
+		reader->hash_algo = &hash_algos[hash_algo];
+		packet_write_fmt(fd_out, "object-format=%s", reader->hash_algo->name);
+	} else {
+		reader->hash_algo = &hash_algos[GIT_HASH_SHA1];
+	}
+}
+
 struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 			     struct ref **list, int for_push,
 			     struct transport_ls_refs_options *transport_options,
@@ -480,7 +498,6 @@ struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 			     int stateless_rpc)
 {
 	int i;
-	const char *hash_name;
 	struct strvec *ref_prefixes = transport_options ?
 		&transport_options->ref_prefixes : NULL;
 	const char **unborn_head_target = transport_options ?
@@ -490,18 +507,8 @@ struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 	if (server_supports_v2("ls-refs", 1))
 		packet_write_fmt(fd_out, "command=ls-refs\n");
 
-	if (server_supports_v2("agent", 0))
-		packet_write_fmt(fd_out, "agent=%s", git_user_agent_sanitized());
-
-	if (server_feature_v2("object-format", &hash_name)) {
-		int hash_algo = hash_algo_by_name(hash_name);
-		if (hash_algo == GIT_HASH_UNKNOWN)
-			die(_("unknown object format '%s' specified by server"), hash_name);
-		reader->hash_algo = &hash_algos[hash_algo];
-		packet_write_fmt(fd_out, "object-format=%s", reader->hash_algo->name);
-	} else {
-		reader->hash_algo = &hash_algos[GIT_HASH_SHA1];
-	}
+	/* Send capabilities */
+	send_capabilities(fd_out, reader);
 
 	if (server_options && server_options->nr &&
 	    server_supports_v2("server-option", 1))
diff --git a/dir.c b/dir.c
index 3633c0852e..6ca2ef5f04 100644
--- a/dir.c
+++ b/dir.c
@@ -3955,3 +3955,32 @@ void relocate_gitdir(const char *path, const char *old_git_dir, const char *new_
 
 	connect_work_tree_and_git_dir(path, new_git_dir, 0);
 }
+
+int path_match_flags(const char *const str, const enum path_match_flags flags)
+{
+	const char *p = str;
+
+	if (flags & PATH_MATCH_NATIVE &&
+	    flags & PATH_MATCH_XPLATFORM)
+		BUG("path_match_flags() must get one match kind, not multiple!");
+	else if (!(flags & PATH_MATCH_KINDS_MASK))
+		BUG("path_match_flags() must get at least one match kind!");
+
+	if (flags & PATH_MATCH_STARTS_WITH_DOT_SLASH &&
+	    flags & PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH)
+		BUG("path_match_flags() must get one platform kind, not multiple!");
+	else if (!(flags & PATH_MATCH_PLATFORM_MASK))
+		BUG("path_match_flags() must get at least one platform kind!");
+
+	if (*p++ != '.')
+		return 0;
+	if (flags & PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH &&
+	    *p++ != '.')
+		return 0;
+
+	if (flags & PATH_MATCH_NATIVE)
+		return is_dir_sep(*p);
+	else if (flags & PATH_MATCH_XPLATFORM)
+		return is_xplatform_dir_sep(*p);
+	BUG("unreachable");
+}
diff --git a/dir.h b/dir.h
index 8e02dfb505..7bc862030c 100644
--- a/dir.h
+++ b/dir.h
@@ -578,4 +578,67 @@ void connect_work_tree_and_git_dir(const char *work_tree,
 void relocate_gitdir(const char *path,
 		     const char *old_git_dir,
 		     const char *new_git_dir);
+
+/**
+ * The "enum path_matches_kind" determines how path_match_flags() will
+ * behave. The flags come in sets, and one (and only one) must be
+ * provided out of each "set":
+ *
+ * PATH_MATCH_NATIVE:
+ *	Path separator is is_dir_sep()
+ * PATH_MATCH_XPLATFORM:
+ *	Path separator is is_xplatform_dir_sep()
+ *
+ * Do we use is_dir_sep() to check for a directory separator
+ * (*_NATIVE), or do we always check for '/' or '\' (*_XPLATFORM). The
+ * "*_NATIVE" version on Windows is the same as "*_XPLATFORM",
+ * everywhere else "*_NATIVE" means "only /".
+ *
+ * PATH_MATCH_STARTS_WITH_DOT_SLASH:
+ *	Match a path starting with "./"
+ * PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH:
+ *	Match a path starting with "../"
+ *
+ * The "/" in the above is adjusted based on the "*_NATIVE" and
+ * "*_XPLATFORM" flags.
+ */
+enum path_match_flags {
+	PATH_MATCH_NATIVE = 1 << 0,
+	PATH_MATCH_XPLATFORM = 1 << 1,
+	PATH_MATCH_STARTS_WITH_DOT_SLASH = 1 << 2,
+	PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH = 1 << 3,
+};
+#define PATH_MATCH_KINDS_MASK (PATH_MATCH_STARTS_WITH_DOT_SLASH | \
+	PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH)
+#define PATH_MATCH_PLATFORM_MASK (PATH_MATCH_NATIVE | PATH_MATCH_XPLATFORM)
+
+/**
+ * path_match_flags() checks if a given "path" matches a given "enum
+ * path_match_flags" criteria.
+ */
+int path_match_flags(const char *const path, const enum path_match_flags f);
+
+/**
+ * starts_with_dot_slash_native(): convenience wrapper for
+ * path_match_flags() with PATH_MATCH_STARTS_WITH_DOT_SLASH and
+ * PATH_MATCH_NATIVE.
+ */
+static inline int starts_with_dot_slash_native(const char *const path)
+{
+	const enum path_match_flags what = PATH_MATCH_STARTS_WITH_DOT_SLASH;
+
+	return path_match_flags(path, what | PATH_MATCH_NATIVE);
+}
+
+/**
+ * starts_with_dot_slash_native(): convenience wrapper for
+ * path_match_flags() with PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH and
+ * PATH_MATCH_NATIVE.
+ */
+static inline int starts_with_dot_dot_slash_native(const char *const path)
+{
+	const enum path_match_flags what = PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH;
+
+	return path_match_flags(path, what | PATH_MATCH_NATIVE);
+}
 #endif
diff --git a/fetch-pack.c b/fetch-pack.c
index 6d0d271259..cb6647d657 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -115,11 +115,12 @@ static void for_each_cached_alternate(struct fetch_negotiator *negotiator,
 		cb(negotiator, cache.items[i]);
 }
 
-static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
-					       int mark_tags_complete)
+static struct commit *deref_without_lazy_fetch_extended(const struct object_id *oid,
+							int mark_tags_complete,
+							enum object_type *type,
+							unsigned int oi_flags)
 {
-	enum object_type type;
-	struct object_info info = { .typep = &type };
+	struct object_info info = { .typep = type };
 	struct commit *commit;
 
 	commit = lookup_commit_in_graph(the_repository, oid);
@@ -128,9 +129,9 @@ static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
 
 	while (1) {
 		if (oid_object_info_extended(the_repository, oid, &info,
-					     OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK))
+					     oi_flags))
 			return NULL;
-		if (type == OBJ_TAG) {
+		if (*type == OBJ_TAG) {
 			struct tag *tag = (struct tag *)
 				parse_object(the_repository, oid);
 
@@ -144,7 +145,7 @@ static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
 		}
 	}
 
-	if (type == OBJ_COMMIT) {
+	if (*type == OBJ_COMMIT) {
 		struct commit *commit = lookup_commit(the_repository, oid);
 		if (!commit || repo_parse_commit(the_repository, commit))
 			return NULL;
@@ -154,6 +155,16 @@ static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
 	return NULL;
 }
 
+
+static struct commit *deref_without_lazy_fetch(const struct object_id *oid,
+					       int mark_tags_complete)
+{
+	enum object_type type;
+	unsigned flags = OBJECT_INFO_SKIP_FETCH_OBJECT | OBJECT_INFO_QUICK;
+	return deref_without_lazy_fetch_extended(oid, mark_tags_complete,
+						 &type, flags);
+}
+
 static int rev_list_insert_ref(struct fetch_negotiator *negotiator,
 			       const struct object_id *oid)
 {
@@ -836,6 +847,16 @@ static void parse_gitmodules_oids(int fd, struct oidset *gitmodules_oids)
 	} while (1);
 }
 
+static void add_index_pack_keep_option(struct strvec *args)
+{
+	char hostname[HOST_NAME_MAX + 1];
+
+	if (xgethostname(hostname, sizeof(hostname)))
+		xsnprintf(hostname, sizeof(hostname), "localhost");
+	strvec_pushf(args, "--keep=fetch-pack %"PRIuMAX " on %s",
+		     (uintmax_t)getpid(), hostname);
+}
+
 /*
  * If packfile URIs were provided, pass a non-NULL pointer to index_pack_args.
  * The strings to pass as the --index-pack-arg arguments to http-fetch will be
@@ -905,14 +926,8 @@ static int get_pack(struct fetch_pack_args *args,
 			strvec_push(&cmd.args, "-v");
 		if (args->use_thin_pack)
 			strvec_push(&cmd.args, "--fix-thin");
-		if ((do_keep || index_pack_args) && (args->lock_pack || unpack_limit)) {
-			char hostname[HOST_NAME_MAX + 1];
-			if (xgethostname(hostname, sizeof(hostname)))
-				xsnprintf(hostname, sizeof(hostname), "localhost");
-			strvec_pushf(&cmd.args,
-				     "--keep=fetch-pack %"PRIuMAX " on %s",
-				     (uintmax_t)getpid(), hostname);
-		}
+		if ((do_keep || index_pack_args) && (args->lock_pack || unpack_limit))
+			add_index_pack_keep_option(&cmd.args);
 		if (!index_pack_args && args->check_self_contained_and_connected)
 			strvec_push(&cmd.args, "--check-self-contained-and-connected");
 		else
diff --git a/fsck.c b/fsck.c
index 3ec500d707..dd4822ba1b 100644
--- a/fsck.c
+++ b/fsck.c
@@ -975,27 +975,16 @@ done:
 	return ret;
 }
 
-/*
- * Like builtin/submodule--helper.c's starts_with_dot_slash, but without
- * relying on the platform-dependent is_dir_sep helper.
- *
- * This is for use in checking whether a submodule URL is interpreted as
- * relative to the current directory on any platform, since \ is a
- * directory separator on Windows but not on other platforms.
- */
-static int starts_with_dot_slash(const char *str)
+static int starts_with_dot_slash(const char *const path)
 {
-	return str[0] == '.' && (str[1] == '/' || str[1] == '\\');
+	return path_match_flags(path, PATH_MATCH_STARTS_WITH_DOT_SLASH |
+				PATH_MATCH_XPLATFORM);
 }
 
-/*
- * Like starts_with_dot_slash, this is a variant of submodule--helper's
- * helper of the same name with the twist that it accepts backslash as a
- * directory separator even on non-Windows platforms.
- */
-static int starts_with_dot_dot_slash(const char *str)
+static int starts_with_dot_dot_slash(const char *const path)
 {
-	return str[0] == '.' && starts_with_dot_slash(str + 1);
+	return path_match_flags(path, PATH_MATCH_STARTS_WITH_DOT_DOT_SLASH |
+				PATH_MATCH_XPLATFORM);
 }
 
 static int submodule_url_is_relative(const char *url)
diff --git a/git-compat-util.h b/git-compat-util.h
index ddc3fa2a4e..d9457b9aae 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -236,6 +236,12 @@
 #include <sys/sysctl.h>
 #endif
 
+/* Used by compat/win32/path-utils.h, and more */
+static inline int is_xplatform_dir_sep(int c)
+{
+	return c == '/' || c == '\\';
+}
+
 #if defined(__CYGWIN__)
 #include "compat/win32/path-utils.h"
 #endif
@@ -416,11 +422,11 @@ static inline int git_skip_dos_drive_prefix(char **path)
 #define skip_dos_drive_prefix git_skip_dos_drive_prefix
 #endif
 
-#ifndef is_dir_sep
 static inline int git_is_dir_sep(int c)
 {
 	return c == '/';
 }
+#ifndef is_dir_sep
 #define is_dir_sep git_is_dir_sep
 #endif
 
diff --git a/http.c b/http.c
index 68573b6da5..11c6f69fac 100644
--- a/http.c
+++ b/http.c
@@ -1989,8 +1989,8 @@ int http_get_strbuf(const char *url,
  * If a previous interrupted download is detected (i.e. a previous temporary
  * file is still around) the download is resumed.
  */
-static int http_get_file(const char *url, const char *filename,
-			 struct http_get_options *options)
+int http_get_file(const char *url, const char *filename,
+		  struct http_get_options *options)
 {
 	int ret;
 	struct strbuf tmpfile = STRBUF_INIT;
diff --git a/http.h b/http.h
index df1590e53a..ba303cfb37 100644
--- a/http.h
+++ b/http.h
@@ -163,6 +163,15 @@ struct http_get_options {
  */
 int http_get_strbuf(const char *url, struct strbuf *result, struct http_get_options *options);
 
+/*
+ * Downloads a URL and stores the result in the given file.
+ *
+ * If a previous interrupted download is detected (i.e. a previous temporary
+ * file is still around) the download is resumed.
+ */
+int http_get_file(const char *url, const char *filename,
+		  struct http_get_options *options);
+
 int http_fetch_ref(const char *base, struct ref *ref);
 
 /* Helpers for fetching packs */
diff --git a/path.c b/path.c
index 3236122aad..a3cfcd8a6e 100644
--- a/path.c
+++ b/path.c
@@ -1413,7 +1413,7 @@ int is_ntfs_dotgit(const char *name)
 
 	for (;;) {
 		c = *(name++);
-		if (!c || c == '\\' || c == '/' || c == ':')
+		if (!c || is_xplatform_dir_sep(c) || c == ':')
 			return 1;
 		if (c != '.' && c != ' ')
 			return 0;
diff --git a/remote.c b/remote.c
index 930fdc9c2f..e98148ac22 100644
--- a/remote.c
+++ b/remote.c
@@ -14,6 +14,7 @@
 #include "strvec.h"
 #include "commit-reach.h"
 #include "advice.h"
+#include "connect.h"
 
 enum map_direction { FROM_SRC, FROM_DST };
 
@@ -2729,3 +2730,101 @@ void remote_state_clear(struct remote_state *remote_state)
 	hashmap_clear_and_free(&remote_state->remotes_hash, struct remote, ent);
 	hashmap_clear_and_free(&remote_state->branches_hash, struct remote, ent);
 }
+
+/*
+ * Returns 1 if it was the last chop before ':'.
+ */
+static int chop_last_dir(char **remoteurl, int is_relative)
+{
+	char *rfind = find_last_dir_sep(*remoteurl);
+	if (rfind) {
+		*rfind = '\0';
+		return 0;
+	}
+
+	rfind = strrchr(*remoteurl, ':');
+	if (rfind) {
+		*rfind = '\0';
+		return 1;
+	}
+
+	if (is_relative || !strcmp(".", *remoteurl))
+		die(_("cannot strip one component off url '%s'"),
+			*remoteurl);
+
+	free(*remoteurl);
+	*remoteurl = xstrdup(".");
+	return 0;
+}
+
+char *relative_url(const char *remote_url, const char *url,
+		   const char *up_path)
+{
+	int is_relative = 0;
+	int colonsep = 0;
+	char *out;
+	char *remoteurl;
+	struct strbuf sb = STRBUF_INIT;
+	size_t len;
+
+	if (!url_is_local_not_ssh(url) || is_absolute_path(url))
+		return xstrdup(url);
+
+	len = strlen(remote_url);
+	if (!len)
+		BUG("invalid empty remote_url");
+
+	remoteurl = xstrdup(remote_url);
+	if (is_dir_sep(remoteurl[len-1]))
+		remoteurl[len-1] = '\0';
+
+	if (!url_is_local_not_ssh(remoteurl) || is_absolute_path(remoteurl))
+		is_relative = 0;
+	else {
+		is_relative = 1;
+		/*
+		 * Prepend a './' to ensure all relative
+		 * remoteurls start with './' or '../'
+		 */
+		if (!starts_with_dot_slash_native(remoteurl) &&
+		    !starts_with_dot_dot_slash_native(remoteurl)) {
+			strbuf_reset(&sb);
+			strbuf_addf(&sb, "./%s", remoteurl);
+			free(remoteurl);
+			remoteurl = strbuf_detach(&sb, NULL);
+		}
+	}
+	/*
+	 * When the url starts with '../', remove that and the
+	 * last directory in remoteurl.
+	 */
+	while (url) {
+		if (starts_with_dot_dot_slash_native(url)) {
+			url += 3;
+			colonsep |= chop_last_dir(&remoteurl, is_relative);
+		} else if (starts_with_dot_slash_native(url))
+			url += 2;
+		else
+			break;
+	}
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%s%s%s", remoteurl, colonsep ? ":" : "/", url);
+	if (ends_with(url, "/"))
+		strbuf_setlen(&sb, sb.len - 1);
+	free(remoteurl);
+
+	if (starts_with_dot_slash_native(sb.buf))
+		out = xstrdup(sb.buf + 2);
+	else
+		out = xstrdup(sb.buf);
+
+	if (!up_path || !is_relative) {
+		strbuf_release(&sb);
+		return out;
+	}
+
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%s%s", up_path, out);
+	free(out);
+	return strbuf_detach(&sb, NULL);
+}
diff --git a/remote.h b/remote.h
index 4a1209ae2c..dd4402436f 100644
--- a/remote.h
+++ b/remote.h
@@ -409,4 +409,36 @@ int parseopt_push_cas_option(const struct option *, const char *arg, int unset);
 int is_empty_cas(const struct push_cas_option *);
 void apply_push_cas(struct push_cas_option *, struct remote *, struct ref *);
 
+/*
+ * The `url` argument is the URL that navigates to the submodule origin
+ * repo. When relative, this URL is relative to the superproject origin
+ * URL repo. The `up_path` argument, if specified, is the relative
+ * path that navigates from the submodule working tree to the superproject
+ * working tree. Returns the origin URL of the submodule.
+ *
+ * Return either an absolute URL or filesystem path (if the superproject
+ * origin URL is an absolute URL or filesystem path, respectively) or a
+ * relative file system path (if the superproject origin URL is a relative
+ * file system path).
+ *
+ * When the output is a relative file system path, the path is either
+ * relative to the submodule working tree, if up_path is specified, or to
+ * the superproject working tree otherwise.
+ *
+ * NEEDSWORK: This works incorrectly on the domain and protocol part.
+ * remote_url      url              outcome          expectation
+ * http://a.com/b  ../c             http://a.com/c   as is
+ * http://a.com/b/ ../c             http://a.com/c   same as previous line, but
+ *                                                   ignore trailing slash in url
+ * http://a.com/b  ../../c          http://c         error out
+ * http://a.com/b  ../../../c       http:/c          error out
+ * http://a.com/b  ../../../../c    http:c           error out
+ * http://a.com/b  ../../../../../c    .:c           error out
+ * http://a.com/b  http://d.org/e   http://d.org/e   as is
+ * NEEDSWORK: Given how chop_last_dir() works, this function is broken
+ * when a local part has a colon in its path component, too.
+ */
+char *relative_url(const char *remote_url, const char *url,
+		   const char *up_path);
+
 #endif
diff --git a/submodule-config.c b/submodule-config.c
index 29668b0620..ce3beaf5d4 100644
--- a/submodule-config.c
+++ b/submodule-config.c
@@ -204,17 +204,17 @@ int check_submodule_name(const char *name)
 		return -1;
 
 	/*
-	 * Look for '..' as a path component. Check both '/' and '\\' as
+	 * Look for '..' as a path component. Check is_xplatform_dir_sep() as
 	 * separators rather than is_dir_sep(), because we want the name rules
 	 * to be consistent across platforms.
 	 */
 	goto in_component; /* always start inside component */
 	while (*name) {
 		char c = *name++;
-		if (c == '/' || c == '\\') {
+		if (is_xplatform_dir_sep(c)) {
 in_component:
 			if (name[0] == '.' && name[1] == '.' &&
-			    (!name[2] || name[2] == '/' || name[2] == '\\'))
+			    (!name[2] || is_xplatform_dir_sep(name[2])))
 				return -1;
 		}
 	}