openssl: make compatible with libressl

LibreSSL uses a mostly OpenSSL 1.0.1 compatible API, with the
alarmingly notable exception of the version API. When the library is
loaded on macOS with LibreSSL the verison check introduced in #42 the
gem was incompatible with LibreSSL.

This change may not be ideally forward-compatible if APIs change and
may need a new strategy eventually, but is not noticeably weaker than
the version based strategy. It is notably functional on both macOS
with LibreSSL 2.8.3, as well as remaining compatible with OpenSSL
1.1.1f on Ubuntu.

Author: raggi

.github/workflows/ruby.yml

@@ -19,8 +19,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        # os: [ubuntu-latest, macos-latest]
-        os: [ubuntu-latest]
+        os: [ubuntu-latest, macos-latest]
         ruby-version: ['2.6', '2.7', '3.0']
 
     steps:

lib/bitcoin/ffi/openssl.rb

@@ -11,6 +11,7 @@ module OpenSSL_EC # rubocop:disable Naming/ClassAndModuleCamelCase
       ffi_lib 'libeay32', 'ssleay32'
     else
       ffi_lib [
+        FFI::CURRENT_PROCESS,
         'libssl.so.1.1.0', 'libssl.so.1.1',
         'libssl.so.1.0.0', 'libssl.so.10',
         'ssl'
@@ -21,10 +22,6 @@ module OpenSSL_EC # rubocop:disable Naming/ClassAndModuleCamelCase
     POINT_CONVERSION_COMPRESSED = 2
     POINT_CONVERSION_UNCOMPRESSED = 4
 
-    # OpenSSL 1.1.0 version as a numerical version value as defined in:
-    # https://www.openssl.org/docs/man1.1.0/man3/OpenSSL_version.html
-    VERSION_1_1_0_NUM = 0x10100000
-
     # OpenSSL 1.1.0 engine constants, taken from:
     # https://github.com/openssl/openssl/blob/2be8c56a39b0ec2ec5af6ceaf729df154d784a43/include/openssl/crypto.h
     OPENSSL_INIT_ENGINE_RDRAND = 0x00000200
@@ -52,21 +49,10 @@ module OpenSSL_EC # rubocop:disable Naming/ClassAndModuleCamelCase
       attach_function :SSLeay, [], :long
     end
 
-    # Returns the version of SSL present.
-    #
-    # @return [Integer] version number as an integer.
-    def self.version
-      if self.respond_to?(:OpenSSL_version_num)
-        OpenSSL_version_num()
-      else
-        SSLeay()
-      end
-    end
-
-    if version >= VERSION_1_1_0_NUM
+    begin
       # Initialization procedure for the library was changed in OpenSSL 1.1.0
       attach_function :OPENSSL_init_ssl, [:uint64, :pointer], :int
-    else
+    rescue FFI::NotFoundError
       attach_function :SSL_library_init, [], :int
       attach_function :ERR_load_crypto_strings, [], :void
       attach_function :SSL_load_error_strings, [], :void
@@ -391,7 +377,7 @@ def self.init_ffi_ssl
       @ssl_loaded ||= false
       return if @ssl_loaded
 
-      if version >= VERSION_1_1_0_NUM
+      if self.method_defined?(:OPENSSL_init_ssl)
         OPENSSL_init_ssl(
           OPENSSL_INIT_LOAD_SSL_STRINGS | OPENSSL_INIT_ENGINE_ALL_BUILTIN,
           nil