Merge pull request #3985 from libgit2/cmn/threading-update

Update THREADING for OpenSSL 1.1

THREADING.md

@@ -62,29 +62,34 @@ general case still affects you if you use ssh.
 General Case
 ------------
 
-By default we use libcurl, which has its own ![recommendations for
-thread safety](http://curl.haxx.se/libcurl/c/libcurl-tutorial.html#Multi-threading).
-
-If libcurl was not found or was disabled, libgit2 uses OpenSSL to be
-able to use HTTPS as a transport. This library is made to be
-thread-implementation agnostic, and the users of the library must set
-which locking function it should use. This means that libgit2 cannot
-know what to set as the user of libgit2 may use OpenSSL independently
-and the locking settings must survive libgit2 shutting down.
-
-Even if libgit2 doesn't use OpenSSL directly, OpenSSL can still be used
-by libssh2 depending on the configuration.  If OpenSSL is used both by
-libgit2 and libssh2, you only need to set up threading for OpenSSL once.
-
-libgit2 does provide a last-resort convenience function
+If it's available, by default we use libcurl to provide HTTP tunneling support,
+which may be linked against a number of cryptographic libraries and has its
+own
+[recommendations for thread safety](https://curl.haxx.se/libcurl/c/threadsafe.html).
+
+If there are no alternative TLS implementations (currently only
+SecureTransport), libgit2 uses OpenSSL in order to use HTTPS as a transport.
+OpenSSL is thread-safe starting at version 1.1.0. If your copy of libgit2 is
+linked against that version, you do not need to take any further steps.
+
+Older versions of OpenSSL are made to be thread-implementation agnostic, and the
+users of the library must set which locking function it should use. libgit2
+cannot know what to set as the user of libgit2 may also be using OpenSSL independently and
+the locking settings must then live outside the lifetime of libgit2.
+
+Even if libgit2 doesn't use OpenSSL directly, OpenSSL can still be used by
+libssh2 or libcurl depending on the configuration. If OpenSSL is used by
+more than one library, you only need to set up threading for OpenSSL once.
+
+If libgit2 is linked against OpenSSL, it provides a last-resort convenience function
 `git_openssl_set_locking()` (available in `sys/openssl.h`) to use the
-platform-native mutex mechanisms to perform the locking, which you may
-rely on if you do not want to use OpenSSL outside of libgit2, or you
-know that libgit2 will outlive the rest of the operations. It is not
+platform-native mutex mechanisms to perform the locking, which you can use
+if you do not want to use OpenSSL outside of libgit2, or you
+know that libgit2 will outlive the rest of the operations. It is then not
 safe to use OpenSSL multi-threaded after libgit2's shutdown function
 has been called.  Note `git_openssl_set_locking()` only works if
 libgit2 uses OpenSSL directly - if OpenSSL is only used as a dependency
-of libssh2 as described above, `git_openssl_set_locking()` is a no-op.
+of libssh2 or libcurl as described above, `git_openssl_set_locking()` is a no-op.
 
 If your programming language offers a package/bindings for OpenSSL,
 you should very strongly prefer to use that in order to set up
@@ -96,9 +101,6 @@ See the
 on threading for more details, and http://trac.libssh2.org/wiki/MultiThreading
 for a specific example of providing the threading callbacks.
 
-Be also aware that libgit2 does not always link against OpenSSL
-if there are alternatives provided by the system.
-
 libssh2 may be linked against OpenSSL or libgcrypt. If it uses OpenSSL,
 see the above paragraphs. If it uses libgcrypt, then you need to
 set up its locking before using it multi-threaded. libgit2 has no