[core] open fd when appending file to cq (fixes #2655)

http_chunk_append_file() opens fd when appending file to chunkqueue.
Defers calculation of content length until response is finished.

This reduces race conditions pertaining to stat() and then (later)
open(), when the result of the stat() was used for Content-Length
or to generate chunked headers.

Note: this does not change how lighttpd handles files that are modified
in-place by another process after having been opened by lighttpd --
don't do that.  This *does* improve handling of files that are
frequently modified via a temporary file and then atomically renamed
into place.

mod_fastcgi has been modified to use http_chunk_append_file_range() with
X-Sendfile2 and will open the target file multiple times if there are
multiple ranges.

Note: (future todo) not implemented for chunk.[ch] interfaces used by
range requests in mod_staticfile or by mod_ssi.  Those uses could lead
to too many open fds.  For mod_staticfile, limits should be put in place
for max number of ranges accepted by mod_staticfile.  For mod_ssi,
limits would need to be placed on the maximum number of includes, and
the primary SSI file split across lots of SSI directives should either
copy the pieces or perhaps chunk.h could be extended to allow for an
open fd to be shared across multiple chunks.  Doing either of these
would improve the performance of SSI since they would replace many file
opens on the pieces of the SSI file around the SSI directives.

x-ref:
  "Serving a file that is getting updated can cause an empty response or incorrect content-length error"
  https://redmine.lighttpd.net/issues/2655

github:
Closes #49

1 files changed, 1 insertions, 0 deletions

diff --git a/src/stat_cache.h b/src/stat_cache.h
index 5c22e49a..72c61f4d 100644
--- a/src/stat_cache.h
+++ b/src/stat_cache.h
@@ -9,6 +9,7 @@ void stat_cache_free(stat_cache *fc);
 
 handler_t stat_cache_get_entry(server *srv, connection *con, buffer *name, stat_cache_entry **fce);
 handler_t stat_cache_handle_fdevent(server *srv, void *_fce, int revent);
+int stat_cache_open_rdonly_fstat (server *srv, connection *con, buffer *name, struct stat *st);
 
 int stat_cache_trigger_cleanup(server *srv);
 #endif