fix: normalize_path overflow shielding "//" path

include/boost/url/impl/url_base.hpp

@@ -2149,10 +2149,11 @@ normalize_path()
     // remove_dot_segments can produce output that
     // needs a 2-byte shield prefix, as explained
     // in step 2. The memmove below writes within
-    // the original path region (before shrink_impl)
-    // and always has room because ".." cancellation
-    // consumes >= 5 bytes but we only need 2 for the
-    // shield.
+    // the path region; when ".." cancellation has
+    // consumed >= 2 bytes there is already slack,
+    // but for short inputs that take the shield
+    // branch without any cancellation (e.g. "//"),
+    // the region has to grow to fit the prefix.
     //
     bool needs_shield = [&]()
     {
@@ -2204,7 +2205,14 @@ normalize_path()
     }();
     if (needs_shield)
     {
-        BOOST_ASSERT(n + 2 <= pn);
+        if (n + 2 > pn)
+        {
+            // No "..  cancellation slack — grow the path
+            // region to fit the 2-byte shield. p_dest may
+            // be invalidated by the underlying reallocation.
+            p_dest = resize_impl(id_path, n + 2, op);
+            pn = n + 2;
+        }
         std::memmove(p_dest + 2, p_dest, n);
         if (was_absolute)
         {

test/unit/url.cpp

@@ -1333,6 +1333,21 @@ struct url_test
             check("", "");
         }
 
+        // normalize path without authority: a "//" path must
+        // grow the buffer before prepending the "/." shield.
+        // https://github.com/boostorg/url/issues/992
+        {
+            auto check = [](core::string_view p,
+                            core::string_view e) {
+                url u = parse_origin_form(p).value();
+                u.normalize();
+                BOOST_TEST_EQ(u.encoded_path(), e);
+            };
+            check("//", "/.//");
+            check("///", "/.///");
+            check("////", "/.////");
+        }
+
         // inequality
         {
             auto check = [](core::string_view e1,