Without this change, aio used with HTTP/2 can result in connection hang,
as observed with "aio threads; aio_write on;" and proxying (ticket #2248).

The problem is that HTTP/2 updates buffers outside of the output filters
(notably, marks them as sent), and then posts a write event to call
output filters.  If a filter does not call the next one for some reason
(for example, because of an AIO operation in progress), this might
result in a state when the owner of a buffer already called
ngx_chain_update_chains() and can reuse the buffer, while the same buffer
is still sitting in the busy chain of some other filter.

In the particular case a buffer was sitting in output chain's ctx->busy,
and was reused by event pipe.  Output chain's ctx->busy was permanently
blocked by it, and this resulted in connection hang.

Fix is to change ngx_chain_update_chains() to skip buffers from other
modules unconditionally, without trying to wait for these buffers to
become empty.

The "sendfile_max_chunk" directive is important to prevent worker
monopolization by fast connections.  The 2m value implies maximum 200ms
delay with 100 Mbps links, 20ms delay with 1 Gbps links, and 2ms on
10 Gbps links.  It also seems to be a good value for disks.

Previously, connections to upstream servers used sendfile() if it was
enabled, but never honored sendfile_max_chunk.  This might result
in worker monopolization for a long time if large request bodies
are allowed.

On Linux starting with 2.6.16, sendfile() silently limits all operations
to MAX_RW_COUNT, defined as (INT_MAX & PAGE_MASK).  This incorrectly
triggered the interrupt check, and resulted in 0-sized writev() on the
next loop iteration.

Fix is to make sure the limit is always checked, so we will return from
the loop if the limit is already reached even if number of bytes sent is
not exactly equal to the number of bytes we've tried to send.

Previously, it was checked that sendfile_max_chunk was enabled and
almost whole sendfile_max_chunk was sent (see e67ef50c3176), to avoid
delaying connections where sendfile_max_chunk wasn't reached (for example,
when sending responses smaller than sendfile_max_chunk).  Now we instead
check if there are unsent data, and the connection is still ready for writing.
Additionally we also check c->write->delayed to ignore connections already
delayed by limit_rate.

This approach is believed to be more robust, and correctly handles
not only sendfile_max_chunk, but also internal limits of c->send_chain(),
such as sendfile() maximum supported length (ticket #1870).

Previously, 1 millisecond delay was used instead.  In certain edge cases
this might result in noticeable performance degradation though, notably on
Linux with typical CONFIG_HZ=250 (so 1ms delay becomes 4ms),
sendfile_max_chunk 2m, and link speed above 2.5 Gbps.

Using posted next events removes the artificial delay and makes processing
fast in all cases.

The directive enables including all frames from start time to the most recent
key frame in the result.  Those frames are removed from presentation timeline
using mp4 edit lists.

Edit lists are currently supported by popular players and browsers such as
Chrome, Safari, QuickTime and ffmpeg.  Among those not supporting them properly
is Firefox[1].

Based on a patch by Tracey Jaquith, Internet Archive.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1735300

The function updates the duration field of mdhd atom.  Previously it was
updated in ngx_http_mp4_read_mdhd_atom().  The change makes it possible to
alter track duration as a result of processing track frames.

Hash initialization ignores elements with key.data set to NULL.
Nevertheless, the initial hash bucket size check didn't skip them,
resulting in unnecessary restrictions on, for example, variables with
long names and with the NGX_HTTP_VARIABLE_NOHASH flag.

Fix is to update the initial hash bucket size check to skip elements
with key.data set to NULL, similarly to how it is done in other parts
of the code.