gopher: Implement secure gopher protocol.

This commit introduces a "gophers" handler inside the gopher protocol if
USE_SSL is defined. This protocol is no different than the usual gopher
prococol, with the added TLS encapsulation upon connecting. The protocol
has been adopted in the gopher community, and many people have enabled
TLS in their gopher daemons like geomyidae(8), and clients, like clic(1)
and hurl(1).

I have not implemented test units for this protocol because my knowledge
of Perl is sub-par. However, for someone more knowledgeable it might be
fairly trivial, because the same test that tests the plain gopher
protocol can be used for "gophers" just by adding a TLS listener.

Signed-off-by: parazyd <parazyd@dyne.org>

Closes #6208

configure.ac

@@ -5111,6 +5111,9 @@ if test "x$CURL_DISABLE_TFTP" != "x1"; then
 fi
 if test "x$CURL_DISABLE_GOPHER" != "x1"; then
   SUPPORT_PROTOCOLS="$SUPPORT_PROTOCOLS GOPHER"
+  if test "x$SSL_ENABLED" = "x1"; then
+    SUPPORT_PROTOCOLS="$SUPPORT_PROTOCOLS GOPHERS"
+  fi
 fi
 if test "x$CURL_DISABLE_MQTT" != "x1"; then
   SUPPORT_PROTOCOLS="$SUPPORT_PROTOCOLS MQTT"

lib/gopher.c

@@ -33,6 +33,7 @@
 #include "gopher.h"
 #include "select.h"
 #include "strdup.h"
+#include "vtls/vtls.h"
 #include "url.h"
 #include "escape.h"
 #include "warnless.h"
@@ -46,6 +47,10 @@
  */
 
 static CURLcode gopher_do(struct connectdata *conn, bool *done);
+#ifdef USE_SSL
+static CURLcode gopher_connect(struct connectdata *conn, bool *done);
+static CURLcode gopher_connecting(struct connectdata *conn, bool *done);
+#endif
 
 /*
  * Gopher protocol handler.
@@ -75,6 +80,46 @@ const struct Curl_handler Curl_handler_gopher = {
   PROTOPT_NONE                          /* flags */
 };
 
+#ifdef USE_SSL
+const struct Curl_handler Curl_handler_gophers = {
+  "GOPHERS",                            /* scheme */
+  ZERO_NULL,                            /* setup_connection */
+  gopher_do,                            /* do_it */
+  ZERO_NULL,                            /* done */
+  ZERO_NULL,                            /* do_more */
+  gopher_connect,                       /* connect_it */
+  gopher_connecting,                    /* connecting */
+  ZERO_NULL,                            /* doing */
+  ZERO_NULL,                            /* proto_getsock */
+  ZERO_NULL,                            /* doing_getsock */
+  ZERO_NULL,                            /* domore_getsock */
+  ZERO_NULL,                            /* perform_getsock */
+  ZERO_NULL,                            /* disconnect */
+  ZERO_NULL,                            /* readwrite */
+  ZERO_NULL,                            /* connection_check */
+  PORT_GOPHER,                          /* defport */
+  CURLPROTO_GOPHER,                     /* protocol */
+  CURLPROTO_GOPHER,                     /* family */
+  PROTOPT_SSL                           /* flags */
+};
+
+static CURLcode gopher_connect(struct connectdata *conn, bool *done)
+{
+  (void)conn;
+  (void)done;
+  return CURLE_OK;
+}
+
+static CURLcode gopher_connecting(struct connectdata *conn, bool *done)
+{
+  CURLcode result = Curl_ssl_connect(conn, FIRSTSOCKET);
+  if(result)
+    connclose(conn, "Failed TLS connection");
+  *done = TRUE;
+  return result;
+}
+#endif
+
 static CURLcode gopher_do(struct connectdata *conn, bool *done)
 {
   CURLcode result = CURLE_OK;
@@ -127,6 +172,11 @@ static CURLcode gopher_do(struct connectdata *conn, bool *done)
   k = curlx_uztosz(len);
 
   for(;;) {
+    /* Break out of the loop if the selector is empty because OpenSSL and/or
+       LibreSSL fail with errno 0 if this is the case. */
+    if(strlen(sel) < 1)
+      break;
+
     result = Curl_write(conn, sockfd, sel, k, &amount);
     if(!result) { /* Which may not have written it all! */
       result = Curl_client_write(conn, CLIENTWRITE_HEADER, sel, amount);

lib/gopher.h

@@ -24,6 +24,9 @@
 
 #ifndef CURL_DISABLE_GOPHER
 extern const struct Curl_handler Curl_handler_gopher;
+#ifdef USE_SSL
+extern const struct Curl_handler Curl_handler_gophers;
+#endif
 #endif
 
 #endif /* HEADER_CURL_GOPHER_H */

lib/url.c

@@ -251,6 +251,9 @@ static const struct Curl_handler * const protocols[] = {
 
 #ifndef CURL_DISABLE_GOPHER
   &Curl_handler_gopher,
+#ifdef USE_SSL
+  &Curl_handler_gophers,
+#endif
 #endif
 
 #ifdef USE_LIBRTMP

lib/version.c

@@ -274,6 +274,9 @@ static const char * const protocols[] = {
 #ifndef CURL_DISABLE_GOPHER
   "gopher",
 #endif
+#if defined(USE_SSL) && !defined(CURL_DISABLE_GOPHER)
+  "gophers",
+#endif
 #ifndef CURL_DISABLE_HTTP
   "http",
 #endif