add uploadarchive.allowUnreachable option

In commit ee27ca4, we started restricting remote git-archive
invocations to only accessing reachable commits. This
matches what upload-pack allows, but does restrict some
useful cases (e.g., HEAD:foo). We loosened this in 0f544ee,
which allows `foo:bar` as long as `foo` is a ref tip.
However, that still doesn't allow many useful things, like:

  1. Commits accessible from a ref, like `foo^:bar`, which
     are reachable

  2. Arbitrary sha1s, even if they are reachable.

We can do a full object-reachability check for these cases,
but it can be quite expensive if the client has sent us the
sha1 of a tree; we have to visit every sub-tree of every
commit in the worst case.

Let's instead give site admins an escape hatch, in case they
prefer the more liberal behavior.  For many sites, the full
object database is public anyway (e.g., if you allow dumb
walker access), or the site admin may simply decide the
security/convenience tradeoff is not worth it.

This patch adds a new config option to disable the
restrictions added in ee27ca4. It defaults to off, meaning
there is no change in behavior by default.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2291,6 +2291,13 @@ transfer.unpackLimit::
 	not set, the value of this variable is used instead.
 	The default value is 100.
 
+uploadarchive.allowUnreachable::
+	If true, allow clients to use `git archive --remote` to request
+	any tree, whether reachable from the ref tips or not. See the
+	discussion in the `SECURITY` section of
+	linkgit:git-upload-archive[1] for more details. Defaults to
+	`false`.
+
 uploadpack.hiderefs::
 	String(s) `upload-pack` uses to decide which refs to omit
 	from its initial advertisement.  Use more than one

Documentation/git-upload-archive.txt

@@ -46,6 +46,12 @@ implications. These rules are subject to change in future versions of
 git, and the server accessed by `git archive --remote` may or may not
 follow these exact rules.
 
+If the config option `uploadArchive.allowUnreachable` is true, these
+rules are ignored, and clients may use arbitrary sha1 expressions.
+This is useful if you do not care about the privacy of unreachable
+objects, or if your object database is already publicly available for
+access via non-smart-http.
+
 OPTIONS
 -------
 <directory>::

archive.c

@@ -17,6 +17,7 @@ static char const * const archive_usage[] = {
 static const struct archiver **archivers;
 static int nr_archivers;
 static int alloc_archivers;
+static int remote_allow_unreachable;
 
 void register_archiver(struct archiver *ar)
 {
@@ -257,7 +258,7 @@ static void parse_treeish_arg(const char **argv,
 	unsigned char sha1[20];
 
 	/* Remotes are only allowed to fetch actual refs */
-	if (remote) {
+	if (remote && !remote_allow_unreachable) {
 		char *ref = NULL;
 		const char *colon = strchr(name, ':');
 		int refnamelen = colon ? colon - name : strlen(name);
@@ -401,6 +402,14 @@ static int parse_archive_args(int argc, const char **argv,
 	return argc;
 }
 
+static int git_default_archive_config(const char *var, const char *value,
+				      void *cb)
+{
+	if (!strcmp(var, "uploadarchive.allowunreachable"))
+		remote_allow_unreachable = git_config_bool(var, value);
+	return git_default_config(var, value, cb);
+}
+
 int write_archive(int argc, const char **argv, const char *prefix,
 		  int setup_prefix, const char *name_hint, int remote)
 {
@@ -411,7 +420,7 @@ int write_archive(int argc, const char **argv, const char *prefix,
 	if (setup_prefix && prefix == NULL)
 		prefix = setup_git_directory_gently(&nongit);
 
-	git_config(git_default_config, NULL);
+	git_config(git_default_archive_config, NULL);
 	init_tar_archiver();
 	init_zip_archiver();
 

t/t5000-tar-tree.sh

@@ -213,6 +213,15 @@ test_expect_success 'clients cannot access unreachable commits' '
 	test_must_fail git archive --remote=. $sha1 >remote.tar
 '
 
+test_expect_success 'upload-archive can allow unreachable commits' '
+	test_commit unreachable1 &&
+	sha1=`git rev-parse HEAD` &&
+	git reset --hard HEAD^ &&
+	git archive $sha1 >remote.tar &&
+	test_config uploadarchive.allowUnreachable true &&
+	git archive --remote=. $sha1 >remote.tar
+'
+
 test_expect_success 'setup tar filters' '
 	git config tar.tar.foo.command "tr ab ba" &&
 	git config tar.bar.command "tr ab ba" &&