Add ssl_library preset parameter

This allows querying the SSL implementation used on the server side.
It's analogous to using PQsslAttribute(conn, "library") in libpq.
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>

doc/src/sgml/config.sgml

@@ -8401,6 +8401,22 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-ssl-library" xreflabel="ssl_library">
+      <term><varname>ssl_library</varname> (<type>string</type>)
+      <indexterm>
+       <primary><varname>ssl_library</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Reports the name of the SSL library that this PostgreSQL server was
+        built with (even if SSL is not currently configured or in use on this
+        instance), for example <literal>OpenSSL</literal>, or an empty string
+        if none.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-wal-block-size" xreflabel="wal_block_size">
       <term><varname>wal_block_size</varname> (<type>integer</type>)
       <indexterm>

src/backend/libpq/be-secure.c

@@ -38,6 +38,7 @@
 #include "storage/proc.h"
 
 
+char	   *ssl_library;
 char	   *ssl_cert_file;
 char	   *ssl_key_file;
 char	   *ssl_ca_file;

src/backend/utils/misc/guc.c

@@ -3723,6 +3723,21 @@ static struct config_string ConfigureNamesString[] =
 		check_canonical_path, NULL, NULL
 	},
 
+	{
+		{"ssl_library", PGC_INTERNAL, PRESET_OPTIONS,
+			gettext_noop("Name of the SSL library."),
+			NULL,
+			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
+		},
+		&ssl_library,
+#ifdef USE_SSL
+		"OpenSSL",
+#else
+		"",
+#endif
+		NULL, NULL, NULL
+	},
+
 	{
 		{"ssl_cert_file", PGC_SIGHUP, CONN_AUTH_SSL,
 			gettext_noop("Location of the SSL server certificate file."),

src/include/libpq/libpq.h

@@ -75,6 +75,7 @@ extern int	pq_putbytes(const char *s, size_t len);
 /*
  * prototypes for functions in be-secure.c
  */
+extern char *ssl_library;
 extern char *ssl_cert_file;
 extern char *ssl_key_file;
 extern char *ssl_ca_file;

src/test/ssl/t/001_ssltests.pl

@@ -8,7 +8,7 @@ use File::Copy;
 
 if ($ENV{with_openssl} eq 'yes')
 {
-	plan tests => 64;
+	plan tests => 65;
 }
 else
 {
@@ -49,6 +49,11 @@ $node->init;
 $ENV{PGHOST} = $node->host;
 $ENV{PGPORT} = $node->port;
 $node->start;
+
+# Run this before we lock down access below.
+my $result = $node->safe_psql('postgres', "SHOW ssl_library");
+is($result, 'OpenSSL', 'ssl_library parameter');
+
 configure_test_server_for_ssl($node, $SERVERHOSTADDR, 'trust');
 
 note "testing password-protected keys";