ssl: temporary lock string buffer while reading

Similarly to SSLSocket#syswrite, the blocking SSLSocket#sysread allows
context switches. We must prevent other threads from modifying the
string buffer.

We can use rb_str_locktmp() and rb_str_unlocktmp() to temporarily
prohibit modification of the string.

ext/openssl/ossl_ssl.c

@@ -1848,26 +1848,36 @@ ossl_ssl_read_internal(int argc, VALUE *argv, VALUE self, int nonblock)
     io = rb_attr_get(self, id_i_io);
     GetOpenFile(io, fptr);
     if (ssl_started(ssl)) {
-	for (;;){
+        rb_str_locktmp(str);
+        for (;;) {
 	    nread = SSL_read(ssl, RSTRING_PTR(str), ilen);
 	    switch(ssl_get_error(ssl, nread)){
 	    case SSL_ERROR_NONE:
+                rb_str_unlocktmp(str);
 		goto end;
 	    case SSL_ERROR_ZERO_RETURN:
+                rb_str_unlocktmp(str);
 		if (no_exception_p(opts)) { return Qnil; }
 		rb_eof_error();
 	    case SSL_ERROR_WANT_WRITE:
-		if (no_exception_p(opts)) { return sym_wait_writable; }
-                write_would_block(nonblock);
+                if (nonblock) {
+                    rb_str_unlocktmp(str);
+                    if (no_exception_p(opts)) { return sym_wait_writable; }
+                    write_would_block(nonblock);
+                }
                 rb_io_wait_writable(fptr->fd);
                 continue;
 	    case SSL_ERROR_WANT_READ:
-		if (no_exception_p(opts)) { return sym_wait_readable; }
-                read_would_block(nonblock);
+                if (nonblock) {
+                    rb_str_unlocktmp(str);
+                    if (no_exception_p(opts)) { return sym_wait_readable; }
+                    read_would_block(nonblock);
+                }
                 rb_io_wait_readable(fptr->fd);
 		continue;
 	    case SSL_ERROR_SYSCALL:
 		if (!ERR_peek_error()) {
+                    rb_str_unlocktmp(str);
 		    if (errno)
 			rb_sys_fail(0);
 		    else {
@@ -1884,6 +1894,7 @@ ossl_ssl_read_internal(int argc, VALUE *argv, VALUE self, int nonblock)
 		}
                 /* fall through */
 	    default:
+                rb_str_unlocktmp(str);
 		ossl_raise(eSSLError, "SSL_read");
 	    }
         }