Fix prepend bug

include/libssh/buffer.h

@@ -23,7 +23,13 @@
 #define BUFFER_H_
 
 #include "libssh/libssh.h"
-/* Describes a buffer state */
+/*
+ * Describes a buffer state
+ * [XXXXXXXXXXXXDATA PAYLOAD       XXXXXXXXXXXXXXXXXXXXXXXX]
+ * ^            ^                  ^                       ^]
+ * \_data points\_pos points here  \_used points here |    /
+ *   here                                          Allocated
+ */
 struct ssh_buffer_struct {
     char *data;
     uint32_t used;

src/buffer.c

@@ -293,14 +293,24 @@ int buffer_add_u8(struct ssh_buffer_struct *buffer,uint8_t data){
 int buffer_prepend_data(struct ssh_buffer_struct *buffer, const void *data,
     uint32_t len) {
   buffer_verify(buffer);
-  if (buffer->allocated < (buffer->used + len)) {
-    if (realloc_buffer(buffer, buffer->used + len) < 0) {
+
+  if(len <= buffer->pos){
+    /* It's possible to insert data between begin and pos */
+    memcpy(buffer->data + (buffer->pos - len), data, len);
+    buffer->pos -= len;
+    buffer_verify(buffer);
+    return 0;
+  }
+  /* pos isn't high enough */
+  if (buffer->allocated < (buffer->used - buffer->pos + len)) {
+    if (realloc_buffer(buffer, buffer->used - buffer->pos + len) < 0) {
       return -1;
     }
   }
-  memmove(buffer->data + len, buffer->data, buffer->used);
+  memmove(buffer->data + len, buffer->data + buffer->pos, buffer->used - buffer->pos);
   memcpy(buffer->data, data, len);
-  buffer->used += len;
+  buffer->used += len - buffer->pos;
+  buffer->pos = 0;
   buffer_verify(buffer);
   return 0;
 }

tests/unittests/torture_buffer.c

@@ -70,6 +70,14 @@ START_TEST (torture_buffer_prepend)
   buffer_prepend_data(buffer,"aris",4);
   ck_assert_int_eq(buffer_get_rest_len(buffer),9);
   ck_assert_int_eq(memcmp(buffer_get_rest(buffer), "arisbcdef", 9), 0);
+  /* same thing but we add 5 bytes now */
+  buffer_get_u32(buffer,&v);
+  ck_assert_int_eq(buffer_get_rest_len(buffer),5);
+  ck_assert_int_eq(memcmp(buffer_get_rest(buffer), "bcdef", 5), 0);
+  buffer_prepend_data(buffer,"12345",5);
+  ck_assert_int_eq(buffer_get_rest_len(buffer),10);
+  ck_assert_int_eq(memcmp(buffer_get_rest(buffer), "12345bcdef", 10), 0);
+
 }
 END_TEST