Do the right thing with sites that use absolute links

On some sites, the link to each subfolder is an absolute link rather
than a relative one. To accommodate this, convert the links from
absolute to relative before storing them in the link table.

src/link.c

@@ -27,6 +27,7 @@ int ROOT_LINK_OFFSET = 0;
  * effectively gives LinkTable generation priority over file transfer.
  */
 static pthread_mutex_t link_lock;
+static void make_link_relative(const char *page_url, char *link_url);
 
 /**
  * \brief create a new Link
@@ -382,7 +383,8 @@ static int linknames_equal(char *linkname, const char *linkname_new)
  * Shamelessly copied and pasted from:
  * https://github.com/google/gumbo-parser/blob/master/examples/find_links.cc
  */
-static void HTML_to_LinkTable(GumboNode *node, LinkTable *linktbl)
+static void HTML_to_LinkTable(const char *url, GumboNode *node,
+ LinkTable *linktbl)
 {
  if (node->type != GUMBO_NODE_ELEMENT) {
  return;
@@ -391,31 +393,33 @@ static void HTML_to_LinkTable(GumboNode *node, LinkTable *linktbl)
  if (node->v.element.tag == GUMBO_TAG_A &&
  (href =
  gumbo_get_attribute(&node->v.element.attributes, "href"))) {
+ char *link_url = href->value;
+ make_link_relative(url, link_url);
  /*
  * if it is valid, copy the link onto the heap
  */
- LinkType type = linkname_to_LinkType(href->value);
+ LinkType type = linkname_to_LinkType(link_url);
  /*
  * We also check if the link being added is the same as the last link.
  * This is to prevent duplicated link, if an Apache server has the
  * IconsAreLinks option.
  */
- size_t comp_len = strnlen(href->value, MAX_FILENAME_LEN);
+ size_t comp_len = strnlen(link_url, MAX_FILENAME_LEN);
  if (type == LINK_DIR) {
  comp_len--;
  }
  if (((type == LINK_DIR) || (type == LINK_UNINITIALISED_FILE)) &&
  !linknames_equal(linktbl->links[linktbl->num - 1]->linkname,
- href->value)) {
- LinkTable_add(linktbl, Link_new(href->value, type));
+ link_url)) {
+ LinkTable_add(linktbl, Link_new(link_url, type));
  }
  }
  /*
  * Note the recursive call, lol.
  */
  GumboVector *children = &node->v.element.children;
  for (size_t i = 0; i < children->length; ++i) {
- HTML_to_LinkTable((GumboNode *) children->data[i], linktbl);
+ HTML_to_LinkTable(url, (GumboNode *) children->data[i], linktbl);
  }
  return;
 }
@@ -568,7 +572,7 @@ LinkTable *LinkTable_new(const char *url)
  * Otherwise parsed the received data
  */
  GumboOutput *output = gumbo_parse(ts.data);
- HTML_to_LinkTable(output->root, linktbl);
+ HTML_to_LinkTable(url, output->root, linktbl);
  gumbo_destroy_output(&kGumboDefaultOptions, output);
  FREE(ts.data);
 
@@ -1058,3 +1062,46 @@ long path_download(const char *path, char *output_buf, size_t req_size,
 
  return Link_download(link, output_buf, req_size, offset);
 }
+
+static void make_link_relative(const char *page_url, char *link_url) 
+{
+ /*
+ Some servers make the links to subdirectories absolute, but our code
+ expects them to be relative, so change the contents of link_url as
+ needed to accommodate that.
+ */
+ if (link_url[0] != '/') {
+ /* Already relative, nothing to do here! */
+ return;
+ }
+
+ /* Find the slash after the host name. */
+ int slashes_left_to_find = 3;
+ while (*page_url) {
+ if (*page_url == '/' && ! --slashes_left_to_find)
+ break;
+ /* N.B. This is here, rather than doing `while (*page_url++)`, because
+ when we're done we want the pointer to point at the final slash. */
+ page_url++;
+ }
+ if (slashes_left_to_find)
+ /* Well, that's odd. Let's return rather than trying to dig ourselves
+ deeper into whatever hole we're in. */
+ return;
+ /* The page URL is no longer the full page_url, it's just the part after
+ the host name.
+ /* The link URL should start with the page URL. */
+ if (strstr(link_url, page_url) != link_url)
+ return;
+ int skip_len = strlen(page_url);
+ if (page_url[skip_len-1] != '/') {
+ if (page_url[skip_len] != '/')
+ /* Um, I'm not sure what to do here, so give up. */
+ return;
+ skip_len++;
+ }
+ /* Move the part of the link URL after the parent page's pat to
+ the beginning of the link URL string, discarding what came
+ before it. */
+ memmove(link_url, link_url + skip_len, strlen(link_url) - skip_len + 1);
+}