fix(detect): honor ignore files saved with a UTF-8 BOM (#2163)

.gitignore/.graphifyignore/info-exclude read with encoding=utf-8 kept a
leading BOM (U+FEFF) on the first line, so the first pattern (e.g. *.log
or .fable-wt/) silently matched nothing and a BOM'd full-line comment
became a bogus pattern. git strips a single leading BOM; switching the
two ignore read sites to utf-8-sig matches git exactly (strips at most
one, file-start only).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

graphify/detect.py

@@ -970,7 +970,7 @@ def _load_dir_own_ignore(d: Path, *, gitignore: bool = True) -> list[tuple[Path,
     for fname in ((".gitignore", ".graphifyignore") if gitignore else (".graphifyignore",)):
         ignore_file = d / fname
         if ignore_file.exists():
-            for raw in ignore_file.read_text(encoding="utf-8", errors="ignore").splitlines():
+            for raw in ignore_file.read_text(encoding="utf-8-sig", errors="ignore").splitlines():
                 line = _parse_gitignore_line(raw)
                 if line:
                     patterns.append((d, line))
@@ -1012,7 +1012,7 @@ def _load_graphifyignore(root: Path, *, gitignore: bool = True) -> list[tuple[Pa
     # re-include still override it (#1810).
     info_exclude = _git_info_exclude(ceiling) if gitignore else None
     if info_exclude is not None:
-        for raw in info_exclude.read_text(encoding="utf-8", errors="ignore").splitlines():
+        for raw in info_exclude.read_text(encoding="utf-8-sig", errors="ignore").splitlines():
             line = _parse_gitignore_line(raw)
             if line:
                 patterns.append((ceiling, line))

tests/test_detect.py

@@ -136,6 +136,92 @@ def test_graphifyignore_comments_ignored(tmp_path):
     assert any("other.py" in f for f in result["files"]["code"])
 
 
+def test_graphifyignore_utf8_bom_first_pattern_honored(tmp_path):
+    """A UTF-8 BOM at the start of .graphifyignore must not corrupt the first
+    pattern (#2163): git strips a single leading BOM, so `*.log` on line 1
+    must still exclude app.log."""
+    (tmp_path / ".graphifyignore").write_bytes(b"\xef\xbb\xbf*.log\nbuild/\n")
+    build = tmp_path / "build"
+    build.mkdir()
+    (build / "lib.py").write_text("x = 1")
+    (tmp_path / "app.log").write_text("log line")
+    (tmp_path / "main.py").write_text("print('hi')")
+
+    result = detect(tmp_path)
+    all_files = [f for files in result["files"].values() for f in files]
+    assert not any("app.log" in f for f in all_files), "BOM'd first pattern was dropped"
+    assert not any("build" in f for f in all_files)
+    assert any("main.py" in f for f in all_files)
+    assert result["graphifyignore_patterns"] == 2
+
+
+def test_gitignore_utf8_bom_matches_git(tmp_path):
+    """A BOM'd .gitignore first pattern must match, exactly like git (#2163)."""
+    (tmp_path / ".gitignore").write_bytes(b"\xef\xbb\xbf*.log\n")
+    (tmp_path / "app.log").write_text("log line")
+    (tmp_path / "main.py").write_text("print('hi')")
+
+    result = detect(tmp_path)
+    all_files = [f for files in result["files"].values() for f in files]
+    assert not any("app.log" in f for f in all_files)
+    assert any("main.py" in f for f in all_files)
+
+
+def test_graphifyignore_bom_only_file(tmp_path):
+    """A .graphifyignore containing only a BOM yields zero patterns, not one
+    bogus U+FEFF pattern (#2163)."""
+    (tmp_path / ".graphifyignore").write_bytes(b"\xef\xbb\xbf")
+    (tmp_path / "main.py").write_text("x = 1")
+
+    result = detect(tmp_path)
+    assert result["graphifyignore_patterns"] == 0
+    assert any("main.py" in f for f in result["files"]["code"])
+
+
+def test_graphifyignore_bom_then_comment(tmp_path):
+    """A BOM followed by a comment must still parse as a comment, not become
+    a `\\ufeff# comment` pattern (#2163)."""
+    (tmp_path / ".graphifyignore").write_bytes(b"\xef\xbb\xbf# comment\nmain.py\n")
+    (tmp_path / "main.py").write_text("x = 1")
+    (tmp_path / "other.py").write_text("x = 2")
+
+    result = detect(tmp_path)
+    assert not any("main.py" in f for f in result["files"]["code"])
+    assert any("other.py" in f for f in result["files"]["code"])
+    assert result["graphifyignore_patterns"] == 1, "BOM'd comment became a pattern"
+
+
+def test_nested_gitignore_utf8_bom(tmp_path):
+    """A BOM'd .gitignore below the scan root (loaded live during the walk,
+    #1206 path) must also have its first pattern honored (#2163)."""
+    sub = tmp_path / "sub"
+    sub.mkdir()
+    (sub / ".gitignore").write_bytes(b"\xef\xbb\xbf*.log\n")
+    (sub / "app.log").write_text("log line")
+    (sub / "keep.py").write_text("x = 1")
+
+    result = detect(tmp_path)
+    all_files = [f for files in result["files"].values() for f in files]
+    assert not any("app.log" in f for f in all_files)
+    assert any("keep.py" in f for f in all_files)
+
+
+def test_git_info_exclude_utf8_bom(tmp_path):
+    """A BOM at the start of $GIT_DIR/info/exclude must not corrupt the first
+    pattern either (#2163) — second read site in _load_graphifyignore."""
+    (tmp_path / ".git" / "info").mkdir(parents=True)
+    (tmp_path / ".git" / "info" / "exclude").write_bytes(b"\xef\xbb\xbfsecrets/\n")
+    secrets = tmp_path / "secrets"
+    secrets.mkdir()
+    (secrets / "x.py").write_text("token = 'x'")
+    (tmp_path / "real.py").write_text("def real(): pass")
+
+    result = detect(tmp_path)
+    all_files = [f for files in result["files"].values() for f in files]
+    assert not any("secrets" in f for f in all_files), "BOM'd info/exclude pattern was dropped"
+    assert any("real.py" in f for f in all_files)
+
+
 def test_detect_follows_symlinked_directory(tmp_path):
     real_dir = tmp_path / "real_lib"
     real_dir.mkdir()