[SVN-4514] HTTP Header "Cache-Control" should be omitted on HEAD URLs

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: trunk
• Fix Version/s: 1.10.0
• Component/s: mod_dav_svn
• Assignee: Ivan Zhakov
• Reporter: Subversion Importer
• Created: 10/Jul/14 15:36
• Updated: 27/Oct/18 16:28
• Resolved: 21/Nov/15 19:36

Description

When accessing a Subversion repository via http:// URL using a web browser, mod_dav_svn returns a "Cache-Control: max-age=604800" header (only on files, not on directories).

This makes the web browser cache the file content for up to a week.

This makes a lot of sense when a fixed revision was requested, but URLs that imply HEAD, no Cache-Control should be sent. (Note that a Last-Modified header is always sent as well, which should be good enough AFAICT.)

Example:

▶ wget -Sq -O /dev/null http://svn.apache.org/repos/asf/subversion/trunk/subversion/mod_dav_svn/repos.c
  HTTP/1.1 200 OK
  Date: Thu, 10 Jul 2014 15:16:28 GMT
  Server: Apache/2.2.25 (Unix) DAV/2 mod_wsgi/3.1 Python/2.7.3 SVN/1.8.1 mod_ssl/2.2.25 OpenSSL/1.0.1h
  Last-Modified: Mon, 23 Jun 2014 20:50:11 GMT
  ETag: "1604933//subversion/trunk/subversion/mod_dav_svn/repos.c"
  Cache-Control: max-age=604800
  Accept-Ranges: bytes
  Content-Length: 164114
  Vary: Accept-Encoding
  Keep-Alive: timeout=15, max=1000
  Connection: Keep-Alive
  Content-Type: text/plain; charset=ISO-8859-1

(Cache-Control is sent but should not be, as no revision was requested.)

URLs like these should still return a Cache-Control header, as they contain an explicit revision:

http://svn.apache.org/repos/asf/subversion/trunk/subversion/mod_dav_svn/repos.c?p=1000000

http://svn.apache.org/repos/asf/subversion/trunk/subversion/mod_dav_svn/repos.c?r=1000000

URLs that point at directories never include a Cache-Control header. IMHO directories with explicit revisions should also have a Cache-Control header.

The main point of this issue however: do not confuse web browsers & force users to hard-reload a file URL of HEAD to see recent modifications.

The header is conditionally hardcoded in subversion/mod_dav_svn/repos.c around line 3050 as:

  /* As version resources don't change, encourage caching. */
  if ((resource->type == DAV_RESOURCE_TYPE_REGULAR
       && resource->versioned && !resource->collection)
      || resource->type == DAV_RESOURCE_TYPE_VERSION)
    /* Cache resource for one week (specified in seconds). */
    apr_table_setn(r->headers_out, "Cache-Control", "max-age=604800");

In pseudo code, I think this should become:

  if ((...) && an_explicit_revision_number_was_passed_as_GET_param)
    apr_table_setn(r->headers_out, "Cache-Control", "max-age=604800");

The GET params are 'r' and 'p'. It seems they don't accept "HEAD", so it might be enough to include the Cache-Control header iff a revision param is non-empty (not having looked through the code too closely yet).

Original issue reported by neels