From 9b6ceee4906175b468f5d621494b21e342c7f7ae Mon Sep 17 00:00:00 2001
From: Michael Manfre <mike@manfre.net>
Date: Mon, 16 Jan 2023 13:59:46 -0500
Subject: [PATCH] Apply Mastodon style link text shortening (#426)

---
 core/html.py            | 39 +++++++++++++++++++++++++++++++++++++--
 static/css/style.css    |  4 ++++
 tests/core/test_html.py | 17 +++++++++++++++++
 3 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/core/html.py b/core/html.py
index 849d26f..24d8b9f 100644
--- a/core/html.py
+++ b/core/html.py
@@ -2,6 +2,7 @@ import re
 from functools import partial
 
 import bleach
+import bleach.callbacks
 from bleach.html5lib_shim import Filter
 from bleach.linkifier import LinkifyFilter
 from django.utils.safestring import mark_safe
@@ -90,6 +91,35 @@ def allow_a(tag: str, name: str, value: str):
     return False
 
 
+def shorten_link_text(attrs, new=False):
+    """
+    Applies Mastodon's link shortening behavior where URL text links are
+    shortened by removing the scheme and only showing the first 30 chars.
+
+    Orig:
+        <a>https://social.example.com/a-long/path/2023/01/16/that-should-be-shortened</a>
+
+    Becomes:
+        <a>social.example.com/a-long/path</a>
+
+    """
+    text = attrs.get("_text")
+    if not text:
+        text = attrs.get((None, "href"))
+    if text and "://" in text and len(text) > 30:
+        attrs[(None, "class")] = " ".join(
+            filter(None, [attrs.pop((None, "class"), ""), "ellipsis"])
+        )
+        # Add the full URL in to title for easier user inspection
+        attrs[(None, "title")] = attrs.get((None, "href"))
+        attrs["_text"] = text.split("://", 1)[-1][:30]
+
+    return attrs
+
+
+linkify_callbacks = [bleach.callbacks.nofollow, shorten_link_text]
+
+
 def sanitize_html(post_html: str) -> str:
     """
     Only allows a, br, p and span tags, and class attributes.
@@ -100,7 +130,10 @@ def sanitize_html(post_html: str) -> str:
             "a": allow_a,
             "p": ["class"],
         },
-        filters=[partial(LinkifyFilter, url_re=url_regex), MastodonStrictTagFilter],
+        filters=[
+            partial(LinkifyFilter, url_re=url_regex, callbacks=linkify_callbacks),
+            MastodonStrictTagFilter,
+        ],
         strip=True,
     )
     return mark_safe(cleaner.clean(post_html))
@@ -113,7 +146,9 @@ def strip_html(post_html: str, *, linkify: bool = True) -> str:
     cleaner = bleach.Cleaner(
         tags=[],
         strip=True,
-        filters=[partial(LinkifyFilter, url_re=url_regex)] if linkify else [],
+        filters=[partial(LinkifyFilter, url_re=url_regex, callbacks=linkify_callbacks)]
+        if linkify
+        else [],
     )
     return mark_safe(cleaner.clean(post_html))
 
diff --git a/static/css/style.css b/static/css/style.css
index 6138b31..45b69ed 100644
--- a/static/css/style.css
+++ b/static/css/style.css
@@ -392,6 +392,10 @@ img.emoji {
     height: 0.8em;
 }
 
+.ellipsis::after {
+    content: "…";
+}
+
 /* Generic markdown styling and sections */
 
 .no-sidebar section {
diff --git a/tests/core/test_html.py b/tests/core/test_html.py
index 3926093..bc63920 100644
--- a/tests/core/test_html.py
+++ b/tests/core/test_html.py
@@ -37,6 +37,23 @@ def test_sanitize_post():
     )
 
 
+def test_shorten_url():
+    full_url = (
+        "https://social.example.com/a-long/path/2023/01/16/that-should-be-shortened"
+    )
+    assert (
+        sanitize_html(f"<p>{full_url}</p>")
+        == f'<p><a href="{full_url}" rel="nofollow" class="ellipsis" title="{full_url}">social.example.com/a-long/path</a></p>'
+    )
+
+    assert (
+        sanitize_html(
+            f'<p><a href="{full_url}">This is a long link text, but cannot be shortened as a URL</a></p>'
+        )
+        == f'<p><a href="{full_url}" rel="nofollow">This is a long link text, but cannot be shortened as a URL</a></p>'
+    )
+
+
 @pytest.mark.django_db
 def test_link_preservation():
     """