Fix duplicated <?xml-stylesheet?> processing instruction in XML output

httpie/output/formatters/xml.py

@@ -21,9 +21,15 @@ def parse_declaration(raw_body: str) -> Optional[str]:
     body = raw_body.strip()
     # XMLDecl ::= '<?xml' DECL_CONTENT '?>'
     if body.startswith(XML_DECLARATION_OPEN):
-        end = body.find(XML_DECLARATION_CLOSE)
-        if end != -1:
-            return body[:end + len(XML_DECLARATION_CLOSE)]
+        # The declaration target is exactly ``xml``; a processing instruction
+        # such as ``<?xml-stylesheet?>`` or ``<?xml-model?>`` merely starts with
+        # it and must not be treated as the declaration (which would duplicate
+        # the instruction in the prettified output).
+        rest = body[len(XML_DECLARATION_OPEN):]
+        if rest[:1] in ('', ' ', '\t', '\r', '\n', '?'):
+            end = body.find(XML_DECLARATION_CLOSE)
+            if end != -1:
+                return body[:end + len(XML_DECLARATION_CLOSE)]
 
 
 def pretty_xml(document: 'Document',

tests/test_xml.py

@@ -97,3 +97,16 @@ def test_invalid_xml(file):
     # No formatting done, data is simply printed as-is.
     r = http(DUMMY_URL)
     assert xml_data in r
+
+
+@pytest.mark.parametrize('target', ['xml-stylesheet', 'xml-model'])
+def test_xml_leading_processing_instruction_not_duplicated(target):
+    """A leading ``<?xml-...?>`` processing instruction (whose target merely
+    begins with ``xml``) must not be misdetected as the XML declaration and
+    duplicated in the prettified output.
+    """
+    from httpie.output.formatters.xml import XMLFormatter
+    formatter = XMLFormatter(format_options={'xml': {'format': True, 'indent': 2}})
+    body = f'<?{target} href="x"?><root><item>x</item></root>'
+    formatted = formatter.format_body(body, 'application/xml')
+    assert formatted.count(f'<?{target}') == 1