Make it possible to set SSL verify mode

If no SSL certificates are provided, many Redis clients default to
disabling SSL peer verification. Previously it was a bit cumbersome to
configure this because the client would either have to reimplement
`redisCreateSSLContext()` or reach into the internals to set the
OpenSSL verify mode.

We can improve the SSL API by introducing a
`redisCreateSSLContextWithOptions()` call that takes into structured
parameters for SSL initialization. This structure contains a verify
mode that is used to set the OpenSSL verify mode.

Relates to #646

hiredis_ssl.h

@@ -61,6 +61,27 @@ typedef enum {
     REDIS_SSL_CTX_OS_CERT_ADD_FAILED            /* Failed to add CA certificates obtained from system to the SSL context */
 } redisSSLContextError;
 
+/* Constants that mirror OpenSSL's verify modes. By default,
+ * REDIS_SSL_VERIFY_PEER is used with redisCreateSSLContext().
+ * Some Redis clients disable peer verification if there are no
+ * certificates specified.
+ */
+#define REDIS_SSL_VERIFY_NONE 0x00
+#define REDIS_SSL_VERIFY_PEER 0x01
+#define REDIS_SSL_VERIFY_FAIL_IF_NO_PEER_CERT 0x02
+#define REDIS_SSL_VERIFY_CLIENT_ONCE 0x04
+#define REDIS_SSL_VERIFY_POST_HANDSHAKE 0x08
+
+/* Options to create an OpenSSL context. */
+typedef struct {
+    const char *cacert_filename;
+    const char *capath;
+    const char *cert_filename;
+    const char *private_key_filename;
+    const char *server_name;
+    int verify_mode;
+} redisSSLOptions;
+
 /**
  * Return the error message corresponding with the specified error code.
  */
@@ -101,6 +122,18 @@ redisSSLContext *redisCreateSSLContext(const char *cacert_filename, const char *
         const char *cert_filename, const char *private_key_filename,
         const char *server_name, redisSSLContextError *error);
 
+/**
+  * Helper function to initialize an OpenSSL context that can be used
+  * to initiate SSL connections. This is a more extensible version of redisCreateSSLContext().
+  *
+  * options contains a structure of SSL options to use.
+  *
+  * If error is non-null, it will be populated in case the context creation fails
+  * (returning a NULL).
+*/
+redisSSLContext *redisCreateSSLContextWithOptions(redisSSLOptions *options,
+        redisSSLContextError *error);
+
 /**
  * Free a previously created OpenSSL context.
  */

ssl.c

@@ -219,6 +219,25 @@ redisSSLContext *redisCreateSSLContext(const char *cacert_filename, const char *
         const char *cert_filename, const char *private_key_filename,
         const char *server_name, redisSSLContextError *error)
 {
+    redisSSLOptions options = {
+        .cacert_filename = cacert_filename,
+        .capath = capath,
+        .cert_filename = cert_filename,
+        .private_key_filename = private_key_filename,
+        .server_name = server_name,
+        .verify_mode = REDIS_SSL_VERIFY_PEER,
+    };
+
+    return redisCreateSSLContextWithOptions(&options, error);
+}
+
+redisSSLContext *redisCreateSSLContextWithOptions(redisSSLOptions *options, redisSSLContextError *error) {
+    const char *cacert_filename = options->cacert_filename;
+    const char *capath = options->capath;
+    const char *cert_filename = options->cert_filename;
+    const char *private_key_filename = options->private_key_filename;
+    const char *server_name = options->server_name;
+
 #ifdef _WIN32
     HCERTSTORE win_store = NULL;
     PCCERT_CONTEXT win_ctx = NULL;
@@ -235,7 +254,7 @@ redisSSLContext *redisCreateSSLContext(const char *cacert_filename, const char *
     }
 
     SSL_CTX_set_options(ctx->ssl_ctx, SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3);
-    SSL_CTX_set_verify(ctx->ssl_ctx, SSL_VERIFY_PEER, NULL);
+    SSL_CTX_set_verify(ctx->ssl_ctx, options->verify_mode, NULL);
 
     if ((cert_filename != NULL && private_key_filename == NULL) ||
             (private_key_filename != NULL && cert_filename == NULL)) {