buffer: Add ssh_buffer_allocate() function

Signed-off-by: Aris Adamantiadis <aris@0xbadc0de.be>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/buffer.h

@@ -50,6 +50,7 @@ int ssh_buffer_add_u64(ssh_buffer buffer, uint64_t data);
 
 int ssh_buffer_validate_length(struct ssh_buffer_struct *buffer, size_t len);
 
+void *ssh_buffer_allocate(struct ssh_buffer_struct *buffer, uint32_t len);
 int ssh_buffer_pack_va(struct ssh_buffer_struct *buffer,
                        const char *format,
                        int argc,

src/buffer.c

@@ -248,6 +248,44 @@ int ssh_buffer_add_data(struct ssh_buffer_struct *buffer, const void *data, uint
   return 0;
 }
 
+/**
+ * @internal
+ *
+ * @brief Allocate space for data at the tail of a buffer.
+ *
+ * @param[in]  buffer   The buffer to add the data.
+ *
+ * @param[in]  len      The length of the data to add.
+ *
+ * @return              Pointer on the allocated space
+ *                      NULL on error.
+ */
+void *ssh_buffer_allocate(struct ssh_buffer_struct *buffer, uint32_t len)
+{
+    void *ptr;
+    buffer_verify(buffer);
+
+    if (buffer->used + len < len) {
+        return NULL;
+    }
+
+    if (buffer->allocated < (buffer->used + len)) {
+        if (buffer->pos > 0) {
+            buffer_shift(buffer);
+        }
+
+        if (realloc_buffer(buffer, buffer->used + len) < 0) {
+            return NULL;
+        }
+    }
+
+    ptr = buffer->data + buffer->used;
+    buffer->used+=len;
+    buffer_verify(buffer);
+
+    return ptr;
+}
+
 /**
  * @internal
  *