Extract text from singlefile.html when indexing

singlefile.html contains a lot of large strings in the form of `data:`
URLs, which can be unnecessarily stored in full-text indices. Also,
large chunks of JavaScript shouldn't be indexed, either, as they pollute
search results for searches about JS functions, etc.

This commit takes a blanket approach of parsing singlefile.html as it is
read and only outputting text and selected textual attributes (like
`alt`) for indexing.

archivebox/search/utils.py

@@ -1,13 +1,103 @@
+from html.parser import HTMLParser
+import io
+
 from django.db.models import QuerySet
 
 from archivebox.util import enforce_types
 from archivebox.config import ANSI
 
+BLOCK_SIZE = 32768
+
 def log_index_started(url):
     print('{green}[*] Indexing url: {} in the search index {reset}'.format(url, **ANSI))
     print( )
 
-def get_file_result_content(res, extra_path, use_pwd=False):
+
+class HTMLTextExtractor(HTMLParser):
+
+    TEXT_ATTRS = ["alt", "cite", "href", "label", "list", "placeholder", "title", "value"]
+    NOTEXT_TAGS = ["script", "style", "template"]
+    NOTEXT_HREF = ["data:", "javascript:", "#"]
+
+    def __init__(self):
+        super().__init__()
+
+        self.output = io.StringIO()
+        self._tag_stack = []
+
+    def _is_text_attr(self, name, value):
+        if not isinstance(value, str):
+            return False
+        if name == "href" and any(map(lambda p: value.startswith(p), self.NOTEXT_HREF)):
+            return False
+
+        if name in self.TEXT_ATTRS:
+            return True
+
+        return False
+
+    def _parent_tag(self):
+        try:
+            return self._tag_stack[-1]
+        except IndexError:
+            return None
+
+    def _in_notext_tag(self):
+        return any([t in self._tag_stack for t in self.NOTEXT_TAGS])
+
+    def handle_starttag(self, tag, attrs):
+        self._tag_stack.append(tag)
+
+        # Don't write out attribute values if any ancestor
+        # is in NOTEXT_TAGS
+        if self._in_notext_tag():
+            return
+
+        for name, value in attrs:
+            if self._is_text_attr(name, value):
+                self.output.write(value.strip())
+                self.output.write(" ")
+
+    def handle_endtag(self, tag):
+        orig_stack = self._tag_stack.copy()
+        try:
+            # Keep popping tags until we find the nearest
+            # ancestor matching this end tag
+            while tag != self._tag_stack.pop():
+                pass
+        except IndexError:
+            # Got to the top of the stack, but somehow missed
+            # this end tag -- maybe malformed markup -- restore the
+            # stack
+            self._tag_stack = orig_stack
+
+    def handle_data(self, data):
+        # Don't output text data if any ancestor is in NOTEXT_TAGS
+        if self._in_notext_tag():
+            return
+        if stripped := data.strip():
+            self.output.write(stripped)
+            self.output.write(" ")
+
+    def __str__(self):
+        return self.output.getvalue()
+
+
+def _read_all(file: io.TextIOBase) -> str:
+    return file.read()
+
+
+def _extract_html_text(file: io.TextIOBase) -> str:
+    extractor = HTMLTextExtractor()
+    while (block := file.read(BLOCK_SIZE)):
+        extractor.feed(block)
+    else:
+        extractor.close()
+
+    return str(extractor)
+
+
+def get_file_result_content(res, extra_path, use_pwd=False, *, filter=_read_all):
     if use_pwd: 
         fpath = f'{res.pwd}/{res.output}'
     else:
@@ -16,8 +106,8 @@ def get_file_result_content(res, extra_path, use_pwd=False):
     if extra_path:
         fpath = f'{fpath}/{extra_path}'
 
-    with open(fpath, 'r', encoding='utf-8') as file:
-        data = file.read()
+    with open(fpath, 'r', encoding='utf-8', errors='replace') as file:
+        data = filter(file)
     if data:
         return [data]
     return []
@@ -38,7 +128,7 @@ def get_indexable_content(results: QuerySet):
     if method == 'readability':
         return get_file_result_content(res, 'content.txt', use_pwd=True)
     elif method == 'singlefile':
-        return get_file_result_content(res, '', use_pwd=True)
+        return get_file_result_content(res, '', use_pwd=True, filter=_extract_html_text)
     elif method == 'dom':
         return get_file_result_content(res, '', use_pwd=True)
     elif method == 'wget':