avformat: make avformat_network_init() explicitly optional

It was sort of optional before - if you didn't call it, networking was
initialized on demand, and an ugly warning was logged. Also, the doxygen
comments threatened that it would be made strictly required one day.

Make it explicitly optional. I would prefer to deprecate it fully, but
there might still be legitimate reasons to use this. But the average
user won't need it.

This is needed only for two reasons: to initialize TLS libraries like
OpenSSL and GnuTLS, and winsock.

OpenSSL and GnuTLS were already silently initialized on demand if the
global network init function was not called. They also have various
thread-safety acrobatics, which make concurrent initialization within
libavformat safe. In addition, the libraries are moving towards making
their global init functions safe, which removes all need for central
global init. In particular, GnuTLS 3.5.16 and OpenSSL 1.1.0g have been
found to have safe init functions. In all cases, they use internal
reference counters to avoid that the global uninit functions interfere
with concurrent uses of the library by other API users who called global
init.

winsock should be thread-safe as well, and maintains an internal
reference counter as well.

Since we still support ancient TLS libraries, which do not have this
fixed, and since it's unknown whether winsock and GnuTLS
reinitialization is costly in any way, don't deprecate the libavformat
functions yet.

1 files changed, 13 insertions, 6 deletions

diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index 8556f80372..b0387214c5 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -1998,17 +1998,24 @@ void av_register_input_format(AVInputFormat *format);
 void av_register_output_format(AVOutputFormat *format);
 
 /**
- * Do global initialization of network components. This is optional,
- * but recommended, since it avoids the overhead of implicitly
- * doing the setup for each session.
+ * Do global initialization of network libraries. This is optional,
+ * and not recommended anymore.
  *
- * Calling this function will become mandatory if using network
- * protocols at some major version bump.
+ * This functions only exists to work around thread-safety issues
+ * with older GnuTLS or OpenSSL libraries. If libavformat is linked
+ * to newer versions of those libraries, or if you do not use them,
+ * calling this function is unnecessary. Otherwise, you need to call
+ * this function before any other threads using them are started.
+ *
+ * This function will be deprecated once support for older GnuTLS and
+ * OpenSSL libraries is removed, and this function has no purpose
+ * anymore.
  */
 int avformat_network_init(void);
 
 /**
- * Undo the initialization done by avformat_network_init.
+ * Undo the initialization done by avformat_network_init. Call it only
+ * once for each time you called avformat_network_init.
  */
 int avformat_network_deinit(void);