vtls: introduce curl_global_sslset()

Let's add a compile time safe API to select an SSL backend. This
function needs to be called *before* curl_global_init(), and can be
called only once.

Side note: we do not explicitly test that it is called before
curl_global_init(), but we do verify that it is not called multiple times
(even implicitly).

If SSL is used before the function was called, it will use whatever the
CURL_SSL_BACKEND environment variable says (or default to the first
available SSL backend), and if a subsequent call to
curl_global_sslset() disagrees with the previous choice, it will fail
with CURLSSLSET_TOO_LATE.

The function also accepts an "avail" parameter to point to a (read-only)
NULL-terminated list of available backends. This comes in real handy if
an application wants to let the user choose between whatever SSL backends
the currently available libcurl has to offer: simply call

	curl_global_sslset(-1, NULL, &avail);

which will return CURLSSLSET_UNKNOWN_BACKEND and populate the avail
variable to point to the relevant information to present to the user.

Just like with the HTTP/2 push functions, we have to add the function
declaration of curl_global_sslset() function to the header file
*multi.h* because VMS and OS/400 require a stable order of functions
declared in include/curl/*.h (where the header files are sorted
alphabetically). This looks a bit funny, but it cannot be helped.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

docs/libcurl/symbols-in-versions

@@ -690,6 +690,9 @@ CURLSSLBACKEND_SCHANNEL         7.34.0
 CURLSSLBACKEND_WOLFSSL          7.49.0
 CURLSSLOPT_ALLOW_BEAST          7.25.0
 CURLSSLOPT_NO_REVOKE            7.44.0
+CURLSSLSET_OK                   7.56.0
+CURLSSLSET_UNKNOWN_BACKEND      7.56.0
+CURLSSLSET_TOO_LATE             7.56.0
 CURLUSESSL_ALL                  7.17.0
 CURLUSESSL_CONTROL              7.17.0
 CURLUSESSL_NONE                 7.17.0

include/curl/curl.h

@@ -2540,11 +2540,6 @@ CURL_EXTERN CURLcode curl_easy_pause(CURL *handle, int bitmask);
 #define CURLPAUSE_ALL       (CURLPAUSE_RECV|CURLPAUSE_SEND)
 #define CURLPAUSE_CONT      (CURLPAUSE_RECV_CONT|CURLPAUSE_SEND_CONT)
 
-typedef struct {
-  curl_sslbackend id;
-  const char *name;
-} curl_ssl_backend;
-
 #ifdef  __cplusplus
 }
 #endif

include/curl/multi.h

@@ -432,6 +432,50 @@ typedef int (*curl_push_callback)(CURL *parent,
                                   struct curl_pushheaders *headers,
                                   void *userp);
 
+/*
+ * NAME curl_global_sslset()
+ *
+ * DESCRIPTION
+ *
+ * When built with multiple SSL backends, curl_global_sslset() allows to
+ * choose one. This function can only be called once, and it must be called
+ * *before* curl_global_init().
+ *
+ * The backend can be identified by the id (e.g. CURLSSLBACKEND_OPENSSL). The
+ * backend can also be specified via the name parameter (passing -1 as id).
+ * If both id and name are specified, the name will be ignored. If neither id
+ * nor name are specified, the function will fail with
+ * CURLSSLSET_UNKNOWN_BACKEND and set the "avail" pointer to the
+ * NULL-terminated list of available backends.
+ *
+ * Upon success, the function returns CURLSSLSET_OK.
+ *
+ * If the specified SSL backend is not available, the function returns
+ * CURLSSLSET_UNKNOWN_BACKEND and sets the "avail" pointer to a NULL-terminated
+ * list of available SSL backends.
+ *
+ * The SSL backend can be set only once. If it has already been set, a
+ * subsequent attempt to change it will result in a CURLSSLSET_TOO_LATE.
+ */
+
+typedef struct {
+  curl_sslbackend id;
+  const char *name;
+} curl_ssl_backend;
+
+typedef enum {
+  CURLSSLSET_OK = 0,
+  CURLSSLSET_UNKNOWN_BACKEND,
+  CURLSSLSET_TOO_LATE
+} CURLsslset;
+
+CURL_EXTERN CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
+                                          const curl_ssl_backend ***avail);
+
+#ifdef  __cplusplus
+}
+#endif
+
 #ifdef __cplusplus
 } /* end of extern "C" */
 #endif

lib/vtls/vtls.c

@@ -131,13 +131,13 @@ void Curl_free_primary_ssl_config(struct ssl_primary_config* sslc)
 }
 
 #ifdef USE_SSL
-static int multissl_init(void);
+static int multissl_init(const struct Curl_ssl *backend);
 #endif
 
 int Curl_ssl_backend(void)
 {
 #ifdef USE_SSL
-  multissl_init();
+  multissl_init(NULL);
   return Curl_ssl->info.id;
 #else
   return (int)CURLSSLBACKEND_NONE;
@@ -1056,21 +1056,21 @@ CURLcode Curl_none_md5sum(unsigned char *input, size_t inputlen,
 
 static int Curl_multissl_init(void)
 {
-  if(multissl_init())
+  if(multissl_init(NULL))
     return 1;
   return Curl_ssl->init();
 }
 
 static size_t Curl_multissl_version(char *buffer, size_t size)
 {
-  if(multissl_init())
+  if(multissl_init(NULL))
     return 0;
   return Curl_ssl->version(buffer, size);
 }
 
 static CURLcode Curl_multissl_connect(struct connectdata *conn, int sockindex)
 {
-  if(multissl_init())
+  if(multissl_init(NULL))
     return CURLE_FAILED_INIT;
   return Curl_ssl->connect(conn, sockindex);
 }
@@ -1078,7 +1078,7 @@ static CURLcode Curl_multissl_connect(struct connectdata *conn, int sockindex)
 static CURLcode Curl_multissl_connect_nonblocking(struct connectdata *conn,
                                                   int sockindex, bool *done)
 {
-  if(multissl_init())
+  if(multissl_init(NULL))
     return CURLE_FAILED_INIT;
   return Curl_ssl->connect_nonblocking(conn, sockindex, done);
 }
@@ -1086,14 +1086,14 @@ static CURLcode Curl_multissl_connect_nonblocking(struct connectdata *conn,
 static void *Curl_multissl_get_internals(struct ssl_connect_data *connssl,
                                          CURLINFO info)
 {
-  if(multissl_init())
+  if(multissl_init(NULL))
     return NULL;
   return Curl_ssl->get_internals(connssl, info);
 }
 
 static void Curl_multissl_close(struct connectdata *conn, int sockindex)
 {
-  if(multissl_init())
+  if(multissl_init(NULL))
     return;
   Curl_ssl->close(conn, sockindex);
 }
@@ -1167,7 +1167,7 @@ static const struct Curl_ssl *available_backends[] = {
   NULL
 };
 
-static int multissl_init(void)
+static int multissl_init(const struct Curl_ssl *backend)
 {
   const char *env;
   int i;
@@ -1175,6 +1175,11 @@ static int multissl_init(void)
   if(Curl_ssl != &Curl_ssl_multi)
     return 1;
 
+  if(backend) {
+    Curl_ssl = backend;
+    return 0;
+  }
+
   if(!available_backends[0])
     return 1;
 
@@ -1191,4 +1196,24 @@ static int multissl_init(void)
   return 0;
 }
 
+CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
+                              const curl_ssl_backend ***avail)
+{
+  int i;
+
+  if(Curl_ssl != &Curl_ssl_multi)
+    return id == Curl_ssl->info.id ? CURLSSLSET_OK : CURLSSLSET_TOO_LATE;
+
+  for(i = 0; available_backends[i]; i++)
+    if(available_backends[i]->info.id == id ||
+       (name && !strcmp(available_backends[i]->info.name, name))) {
+      multissl_init(available_backends[i]);
+      return CURLSSLSET_OK;
+    }
+
+  if(avail)
+    *avail = (const curl_ssl_backend **)&available_backends;
+  return CURLSSLSET_UNKNOWN_BACKEND;
+}
+
 #endif /* USE_SSL */

tests/data/test1135

@@ -89,6 +89,7 @@ CURL_EXTERN CURLMcode curl_multi_setopt(CURLM *multi_handle,
 CURL_EXTERN CURLMcode curl_multi_assign(CURLM *multi_handle,
 CURL_EXTERN char *curl_pushheader_bynum(struct curl_pushheaders *h,
 CURL_EXTERN char *curl_pushheader_byname(struct curl_pushheaders *h,
+CURL_EXTERN CURLsslset curl_global_sslset(curl_sslbackend id, const char *name,
 </stdout>
 </verify>