Sanitize upon saving

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/prosemirror/sanitize.go

@@ -0,0 +1,96 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package prosemirror
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// ValidateDocumentContentJSON returns nil if s is empty or whitespace-only.
+// Otherwise s must be valid ProseMirror JSON whose root node has type "doc".
+func ValidateDocumentContentJSON(s string) error {
+	if strings.TrimSpace(s) == "" {
+		return nil
+	}
+	_, err := parseDocRoot(s)
+	return err
+}
+
+func parseDocRoot(s string) (Node, error) {
+	n, err := Parse(s)
+	if err != nil {
+		return Node{}, fmt.Errorf("cannot parse document content as ProseMirror JSON: %w", err)
+	}
+	if n.Type != NodeDoc {
+		return Node{}, fmt.Errorf("document content root must be type %q", NodeDoc)
+	}
+	return n, nil
+}
+
+// SanitizeDocumentJSON parses a ProseMirror/Tiptap JSON document, replaces
+// unsafe link mark href values using the same rules as RenderHTML, and
+// re-serializes the document. Whitespace-only input is returned unchanged.
+// Non-empty content must be valid JSON whose root node has type "doc".
+func SanitizeDocumentJSON(s string) (string, error) {
+	if strings.TrimSpace(s) == "" {
+		return s, nil
+	}
+
+	n, err := parseDocRoot(s)
+	if err != nil {
+		return "", err
+	}
+
+	sanitizeNode(&n)
+
+	out, err := json.Marshal(n)
+	if err != nil {
+		return "", fmt.Errorf("cannot marshal sanitized document: %w", err)
+	}
+
+	return string(out), nil
+}
+
+func sanitizeNode(n *Node) {
+	for i := range n.Marks {
+		sanitizeLinkMark(&n.Marks[i])
+	}
+	for i := range n.Content {
+		sanitizeNode(&n.Content[i])
+	}
+}
+
+func sanitizeLinkMark(m *Mark) {
+	if m.Type != MarkLink {
+		return
+	}
+
+	attrs, err := m.LinkAttrs()
+	if err != nil {
+		m.Attrs = []byte(`{"href":"#"}`)
+		return
+	}
+
+	attrs.Href = safeLinkHref(attrs.Href)
+	raw, err := json.Marshal(attrs)
+	if err != nil {
+		m.Attrs = []byte(`{"href":"#"}`)
+		return
+	}
+
+	m.Attrs = raw
+}