Avoid stack exhaustion in REXML::Formatters::Default#write_element - #358
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Fixes ruby#354 `REXML::Formatters::Default#write_element` recursed through `REXML::Formatters::Default#write` for every child element, so the nesting depth of the document became the depth of the machine stack. Writing a document nested about 2000 levels deep raised SystemStackError, after having already written the partial output to the caller's output object. Walk the descendants with an explicit stack instead. A String on the stack is an end tag waiting to be written, so the pending end tags cost one entry per open ancestor.
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Fixes #354
REXML::Formatters::Default#write_elementrecursed throughREXML::Formatters::Default#writefor every child element, so the nesting depth of the document became the depth of the machine stack. Writing a document nested about 2000 levels deep raised SystemStackError, after having already written the partial output to the caller's output object.Walk the descendants with an explicit stack instead. A String on the stack is an end tag waiting to be written, so the pending end tags cost one entry per open ancestor.