Escape auto-linked GitHub usernames more precisely

More precisely match how GitHub auto-links usernames when escaping
usernames to suppress pings.

Perhaps most noticeably, this now keeps punctuation outside of the
escaped username.

Author: thaliaarchi

homu/main.py

@@ -41,6 +41,31 @@
 
 VARIABLES_RE = re.compile(r'\${([a-zA-Z_]+)}')
 
+# Pattern for matching an auto-linked GitHub username.
+#
+# This is the behavior used by GitHub when detecting usernames, as deduced from
+# the username constraints for new accounts and testing in their Markdown
+# renderer:
+# - Usernames are auto-linked with an @ prefix.
+# - Usernames can only contain alphanumeric characters or hyphens and must be
+#   between 1 and 39 characters (inclusive): /@[A-Za-z0-9\-]{1,39}/
+# - Usernames cannot start with hyphen: /@[A-Za-z0-9][A-Za-z0-9\-]{,38}/
+# - A username preceded by an alphanumeric character or underscore is not
+#   auto-linked: /(?<![A-Za-z0-9_])/
+# - A username followed by an underscore or slash is not auto-linked:
+#   /(?![A-Za-z0-9\-_\/])/
+#
+# Although usernames cannot contain consecutive hyphens or end with a hyphen,
+# GitHub still auto-links such patterns.
+#
+# The logic for username auto-linking appears to not be open source.
+# github-markup (https://github.com/github/markup), the markup renderer used by
+# GitHub, does not contain this logic, nor do its dependencies. Their README
+# says this is part of their "special sauce".
+#
+# This pattern does not handle Markdown code blocks.
+GITHUB_USERNAME_RE = re.compile(r'(?<![A-Za-z0-9_])(@[A-Za-z0-9][A-Za-z0-9\-]{,38})(?![A-Za-z0-9\-_/])')
+
 IGNORE_BLOCK_START = '<!-- homu-ignore:start -->'
 IGNORE_BLOCK_END = '<!-- homu-ignore:end -->'
 IGNORE_BLOCK_RE = re.compile(
@@ -56,7 +81,7 @@
 # Replace @mention with `@mention` to suppress pings in merge commits.
 # Note: Don't replace non-mentions like "[email protected]".
 def suppress_pings(text):
-    return re.sub(r'\B(@\S+)', r'`\g<1>`', text)  # noqa
+    return GITHUB_USERNAME_RE.sub(r'`\g<1>`', text)
 
 
 # Replace any text between IGNORE_BLOCK_START and IGNORE_BLOCK_END

homu/tests/test_pr_body.py

@@ -7,16 +7,58 @@
 
 
 def test_suppress_pings_in_PR_body():
+    # This behavior can be verified by pasting the text into a Markdown editor
+    # on Github and checking which usernames are auto-linked.
+
     body = (
-        "r? @matklad\n"         # should escape
-        "@bors r+\n"            # shouldn't
-        "[email protected]"      # shouldn't
+        # Should escape:
+        "r? @matklad\n"
+        "@bors r+\n"
+        "@a\n"  # Minimum length
+        "@abcdefghijklmnopqrstuvwxyzabcdefghijklm\n"  # Maximum length
+        "@user. @user, @user; @user? @user!\n"
+        "@user@user\n"  # Only the first is auto-linked
+        "@user/@user/@user\n"  # Only the last is auto-linked
+        "@@user\n"
+        "/@user\n"
+        "-@user\n"
+        "@user--name\n"  # Auto-linked, despite being an invalid username
+        "@user-\n"  # Auto-linked, despite being an invalid username
+        "`@user`\n"  # Code block handling is not implemented
+
+        # Shouldn't escape:
+        "[email protected]\n"
+        "@abcdefghijklmnopqrstuvwxyzabcdefghijklmo\n"  # Over maximum length
+        "text@user\n"
+        "@-\n"
+        "@-user\n"
+        "@user/\n"
+        "@user_\n"
+        "_@user\n"
     )
 
     expect = (
         "r? `@matklad`\n"
         "`@bors` r+\n"
-        "[email protected]"
+        "`@a`\n"
+        "`@abcdefghijklmnopqrstuvwxyzabcdefghijklm`\n"
+        "`@user`. `@user`, `@user`; `@user`? `@user`!\n"
+        "`@user`@user\n"
+        "@user/@user/`@user`\n"
+        "@`@user`\n"
+        "/`@user`\n"
+        "-`@user`\n"
+        "`@user--name`\n"
+        "`@user-`\n"
+        "``@user``\n"
+        "[email protected]\n"
+        "@abcdefghijklmnopqrstuvwxyzabcdefghijklmo\n"
+        "text@user\n"
+        "@-\n"
+        "@-user\n"
+        "@user/\n"
+        "@user_\n"
+        "_@user\n"
     )
 
     assert suppress_pings(body) == expect