This flag appeared in Linux 4.5 and is useful for avoiding thundering herd
problem.
The current Linux kernel implementation walks the list of exclusive waiters,
and queues an event to each epfd, until it finds the first waiter that has
threads blocked on it via epoll_wait().
It was introduced in Linux 2.6.39, glibc 2.14 and allows to obtain
file descriptors without actually opening files. Thus made it possible
to traverse path with openat() syscalls without the need to have read
permissions for path components. It is effectively emulates O_SEARCH
which is missing on Linux.
O_PATH is used in combination with O_RDONLY. The last one is ignored
if O_PATH is used, but it allows nginx to not fail when it was built on
modern system (i.e. glibc 2.14+) and run with a kernel older than 2.6.39.
Then O_PATH is unknown to the kernel and ignored, while O_RDONLY is used.
Sadly, fstat() is not working with O_PATH descriptors till Linux 3.6.
As a workaround we fallback to fstatat() with the AT_EMPTY_PATH flag
that was introduced at the same time as O_PATH.
Catch up with new Linux version numbering scheme as announced at [1] and
suppress unrecognized versions to actually use default 0.
[1] https://lkml.org/lkml/2011/5/29/204
*) Feature: the ngx_http_addition_filter_module.
*) Feature: the "proxy_pass" and "fastcgi_pass" directives may be used
inside the "if" block.
*) Feature: the "proxy_ignore_client_abort" and
"fastcgi_ignore_client_abort" directives.
*) Feature: the "$request_completion" variable.
*) Feature: the ngx_http_perl_module supports the $r->request_method
and $r->remote_addr.
*) Feature: the ngx_http_ssi_module supports the "elif" command.
*) Bugfix: the "\/" string in the expression of the "if" command of the
ngx_http_ssi_module was treated incorrectly.
*) Bugfix: in the regular expressions in the "if" command of the
ngx_http_ssi_module.
*) Bugfix: if the relative path was specified in the
"client_body_temp_path", "proxy_temp_path", "fastcgi_temp_path", and
"perl_modules" directives, then the directory was used relatively to
a current path but not to a server prefix.
*) Change: now on Linux configure checks the presence of epoll and
sendfile64() in kernel.
*) Feature: the "map" directive supports domain names in the
".domain.tld" form.
*) Bugfix: the timeouts were not used in SSL handshake; the bug had
appeared in 0.2.4.
*) Bugfix: in the HTTPS protocol in the "proxy_pass" directive.
*) Bugfix: when the HTTPS protocol was used in the "proxy_pass"
directive the port 80 was used by default.
*) Feature: the IMAP/POP3 proxy supports STARTTLS and STLS.
*) Bugfix: the IMAP/POP3 proxy did not work with the select, poll, and
/dev/poll methods.
*) Bugfix: in SSI handling.
*) Bugfix: now Solaris sendfilev() is not used to transfer the client
request body to FastCGI-server via the unix domain socket.
*) Bugfix: the "auth_basic" directive did not disable the
authorization; the bug had appeared in 0.3.11.
*) Change: the "valid_referers" directive and the "$invalid_referer"
variable were moved to the new ngx_http_referer_module from the
ngx_http_rewrite_module.
*) Change: the "$apache_bytes_sent" variable name was changed to
"$body_bytes_sent".
*) Feature: the "$sent_http_..." variables.
*) Feature: the "if" directive supports the "=" and "!=" operations.
*) Feature: the "proxy_pass" directive supports the HTTPS protocol.
*) Feature: the "proxy_set_body" directive.
*) Feature: the "post_action" directive.
*) Feature: the ngx_http_empty_gif_module.
*) Feature: the "worker_cpu_affinity" directive for Linux.
*) Bugfix: the "rewrite" directive did not unescape URI part in
redirect, now it is unescaped except the %00-%25 and %7F-%FF
characters.
*) Bugfix: nginx could not be built by the icc 9.0 compiler.
*) Bugfix: if the SSI was enabled for zero size static file, then the
chunked response was encoded incorrectly.
*) Change: the "bl" and "af" parameters of the "listen" directive was
renamed to the "backlog" and "accept_filter".
*) Feature: the "rcvbuf" and "sndbuf" parameters of the "listen"
directive.
*) Change: the "$msec" log parameter does not require now the
additional the gettimeofday() system call.
*) Feature: the -t switch now tests the "listen" directives.
*) Bugfix: if the invalid address was specified in the "listen"
directive, then after the -HUP signal nginx left an open socket in
the CLOSED state.
*) Bugfix: the mime type may be incorrectly set to default value for
index file with variable in the name; the bug had appeared in 0.3.0.
*) Feature: the "timer_resolution" directive.
*) Feature: the millisecond "$upstream_response_time" log parameter.
*) Bugfix: a temporary file with client request body now is removed
just after the response header was transferred to a client.
*) Bugfix: OpenSSL 0.9.6 compatibility.
*) Bugfix: the SSL certificate and key file paths could not be relative.
*) Bugfix: the "ssl_prefer_server_ciphers" directive did not work in
the ngx_imap_ssl_module.
*) Bugfix: the "ssl_protocols" directive allowed to specify the single
protocol only.
*) Bugfix: the arguments were omitted in the redirects, issued by the
"rewrite" directive; the bug had appeared in 0.1.29.
*) Feature: the "if" directive supports the captures in regular
expressions.
*) Feature: the "set" directive supports the variables and the captures
of regular expressions.
*) Feature: the "X-Accel-Redirect" response header line is supported in
proxy and FastCGI mode.
*) Feature: the ngx_http_ssi_module supports "include virtual" command.
*) Feature: the ngx_http_ssi_module supports the condition command like
'if expr="$NAME"' and "else" and "endif" commands. Only one nested
level is supported.
*) Feature: the ngx_http_ssi_module supports the DATE_LOCAL and
DATE_GMT variables and "config timefmt" command.
*) Feature: the "ssi_ignore_recycled_buffers" directive.
*) Bugfix: the "echo" command did not show the default value for the
empty QUERY_STRING variable.
*) Change: the ngx_http_proxy_module was rewritten.
*) Feature: the "proxy_redirect", "proxy_pass_request_headers",
"proxy_pass_request_body", and "proxy_method" directives.
*) Feature: the "proxy_set_header" directive. The "proxy_x_var" was
canceled and must be replaced with the proxy_set_header directive.
*) Change: the "proxy_preserve_host" is canceled and must be replaced
with the "proxy_set_header Host $host" and the "proxy_redirect off"
directives, the "proxy_set_header Host $host:$proxy_port" directive
and the appropriate proxy_redirect directives.
*) Change: the "proxy_set_x_real_ip" is canceled and must be replaced
with the "proxy_set_header X-Real-IP $remote_addr" directive.
*) Change: the "proxy_add_x_forwarded_for" is canceled and must be
replaced with
the "proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for"
directive.
*) Change: the "proxy_set_x_url" is canceled and must be replaced with
the "proxy_set_header X-URL http://$host:$server_port$request_uri"
directive.
*) Feature: the "fastcgi_param" directive.
*) Change: the "fastcgi_root", "fastcgi_set_var" and "fastcgi_params"
directive are canceled and must be replaced with the fastcgi_param
directives.
*) Feature: the "index" directive can use the variables.
*) Feature: the "index" directive can be used at http and server levels.
*) Change: the last index only in the "index" directive can be absolute.
*) Feature: the "rewrite" directive can use the variables.
*) Feature: the "internal" directive.
*) Feature: the CONTENT_LENGTH, CONTENT_TYPE, REMOTE_PORT, SERVER_ADDR,
SERVER_PORT, SERVER_PROTOCOL, DOCUMENT_ROOT, SERVER_NAME,
REQUEST_METHOD, REQUEST_URI, and REMOTE_USER variables.
*) Change: nginx now passes the invalid lines in a client request
headers or a backend response header.
*) Bugfix: if the backend did not transfer response for a long time and
the "send_timeout" was less than "proxy_read_timeout", then nginx
returned the 408 response.
*) Bugfix: the segmentation fault was occurred if the backend sent an
invalid line in response header; the bug had appeared in 0.1.26.
*) Bugfix: the segmentation fault may occurred in FastCGI fault
tolerance configuration.
*) Bugfix: the "expires" directive did not remove the previous
"Expires" and "Cache-Control" headers.
*) Bugfix: nginx did not take into account trailing dot in "Host"
header line.
*) Bugfix: the ngx_http_auth_module did not work under Linux.
*) Bugfix: the rewrite directive worked incorrectly, if the arguments
were in a request.
*) Bugfix: nginx could not be built on MacOS X.
*) Change: the invalid client header lines are now ignored and logged
at the info level.
*) Change: the server name is also logged in error log.
*) Feature: the ngx_http_auth_basic_module module and the auth_basic
and auth_basic_user_file directives.
*) Bugfix: the ngx_http_stub_status_module showed incorrect statistics
if "rtsig" method was used or if several worker process ran on SMP.
*) Bugfix: nginx could not be built by the icc compiler on Linux or if
the zlib-1.2.x library was building from sources.
*) Bugfix: nginx could not be built on NetBSD 2.0.
*) Feature: the server_names_hash and server_names_hash_threshold
directives.
*) Bugfix: the *.domain.tld names in the "server_name" directive did
not work.
*) Bugfix: the %request_length log parameter logged the incorrect
length.
*) Feature: the worker_priority directive.
*) Change: both tcp_nopush and tcp_nodelay directives affect the
transferred response.
*) Bugfix: nginx did not call initgroups().
Thanks to Andrew Sitnikov and Andrei Nigmatulin.
*) Change: now the ngx_http_autoindex_module shows the file size in the
bytes.
*) Bugfix: the ngx_http_autoindex_module returned the 500 error if the
broken symlink was in a directory.
*) Bugfix: the files bigger than 4G could not be transferred using
sendfile.
*) Bugfix: if the backend was resolved to several backends and there
was an error while the response waiting then process may got caught
in an endless loop.
*) Bugfix: the worker process may exit with the "unknown cycle" message
when the /dev/poll method was used.
*) Bugfix: "close() channel failed" errors.
*) Bugfix: the autodetection of the "nobody" and "nogroup" groups.
*) Bugfix: the send_lowat directive did not work on Linux.
*) Bugfix: the segmentation fault occurred if there was no events
section in configuration.
*) Bugfix: nginx could not be built on OpenBSD.
*) Bugfix: the double slashes in "://" in the URI were converted to
":/".
*) Bugfix: the proxied request was sent without arguments if the
request contains "//", "/./", "/../" or "%XX".
*) Bugfix: the large compressed responses may be transferred not
completely.
*) Bugfix: the files bigger than 2G was not transferred on Linux that
does not support sendfile64().
*) Bugfix: while the build configuration on Linux the
--with-poll_module parameter was required; the bug had appeared in
0.1.8.
*) Bugfix: in the ngx_http_autoindex_module if the long file names were
in the listing.
*) Feature: the "^~" modifier in the location directive.
*) Feature: the proxy_max_temp_file_size directive.
*) Bugfix: on Solaris and Linux there may be too many "recvmsg()
returned not enough data" alerts.
*) Bugfix: there were the "writev() failed (22: Invalid argument)"
errors on Solaris in proxy mode without sendfile. On other platforms
that do not support sendfile at all the process got caught in an
endless loop.
*) Bugfix: segmentation fault on Solaris in proxy mode and using
sendfile.
*) Bugfix: segmentation fault on Solaris.
*) Bugfix: on-line upgrade did not work on Linux.
*) Bugfix: the ngx_http_autoindex_module module did not escape the
spaces, the quotes, and the percent signs in the directory listing.
*) Change: the decrease of the copy operations.
*) Feature: the userid_p3p directive.
*) Feature: the --user=USER, --group=GROUP, and --with-ld-opt=OPTIONS
options in configure.
*) Feature: the server_name directive supports *.domain.tld.
*) Bugfix: the portability improvements.
*) Bugfix: if configuration file was set in command line, the
reconfiguration was impossible; the bug had appeared in 0.1.1.
*) Bugfix: proxy module may get caught in an endless loop when sendfile
is not used.
*) Bugfix: with sendfile the response was not recoded according to the
charset module directives; the bug had appeared in 0.1.1.
*) Bugfix: very seldom bug in the kqueue processing.
*) Bugfix: the gzip module compressed the proxied responses that was
already compressed.
