http: limit redirection depth

By default, libcurl will follow circular http redirects
forever. Let's put a cap on this so that somebody who can
trigger an automated fetch of an arbitrary repository (e.g.,
for CI) cannot convince git to loop infinitely.

The value chosen is 20, which is the same default that
Firefox uses.

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

http.c

@@ -352,6 +352,7 @@ static CURL *get_curl_handle(void)
 	}
 
 	curl_easy_setopt(result, CURLOPT_FOLLOWLOCATION, 1);
+	curl_easy_setopt(result, CURLOPT_MAXREDIRS, 20);
 #if LIBCURL_VERSION_NUM >= 0x071301
 	curl_easy_setopt(result, CURLOPT_POSTREDIR, CURL_REDIR_POST_ALL);
 #elif LIBCURL_VERSION_NUM >= 0x071101

t/lib-httpd/apache.conf

@@ -121,6 +121,9 @@ RewriteRule ^/smart-redir-auth/(.*)$ /auth/smart/$1 [R=301]
 RewriteRule ^/smart-redir-limited/(.*)/info/refs$ /smart/$1/info/refs [R=301]
 RewriteRule ^/ftp-redir/(.*)$ ftp://localhost:1000/$1 [R=302]
 
+RewriteRule ^/loop-redir/x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-(.*) /$1 [R=302]
+RewriteRule ^/loop-redir/(.*)$ /loop-redir/x-$1 [R=302]
+
 <IfDefine SSL>
 LoadModule ssl_module modules/mod_ssl.so
 

t/t5812-proto-disable-http.sh

@@ -25,5 +25,9 @@ test_expect_success 'curl redirects respect whitelist' '
 	}
 '
 
+test_expect_success 'curl limits redirects' '
+	test_must_fail git clone "$HTTPD_URL/loop-redir/smart/repo.git"
+'
+
 stop_httpd
 test_done