ns: Fix bug when not declared prefix resolved to a not bound namespace

Add `ResolveResult` with results of namespace resolution and replace
two-state `Option<Namespace>` with three-state `ResolveResult`.

Since now in that cases `ResolveResult::Unknown` is returned, which
can be converted to an `Error::UnknownPrefix` using `TryInto<Option<Namespace>>`

Author: Mingun

Changelog.md

@@ -15,7 +15,8 @@
 - [#387]: Allow overlapping between elements of sequence and other elements
   (using new feature `overlapped-lists`)
 - [#393]: New module `name` with `QName`, `LocalName`, `Namespace`, and `Prefix`
-  wrappers around byte arrays
+  wrappers around byte arrays and `ResolveResult` with the result of prefix
+  resolution to namespace
 
 ### Bug Fixes
 
@@ -25,6 +26,8 @@
 - [#387]: Internal deserializer state can be broken when deserializing a map with
   a sequence field (such as `Vec<T>`), where elements of this sequence contains
   another sequence. This error affects only users with the `serialize` feature enabled
+- [#393]: Now `event_namespace`, `attribute_namespace` and `read_event_namespaced`
+  returns `ResolveResult::Unknown` if prefix was not registered in namespace buffer
 
 ### Misc Changes
 
@@ -45,7 +48,8 @@
 - [#391]: Added code coverage
 
 - [#393]: `event_namespace` and `attribute_namespace` now accept `QName`
-  and returns `Namespace` and `LocalName`
+  and returns `ResolveResult` and `LocalName`, `read_event_namespaced` now
+  returns `ResolveResult` instead of `Option<[u8]>`
 
 ### New Tests
 

examples/issue68.rs

@@ -3,6 +3,7 @@
 use quick_xml::events::Event;
 use quick_xml::name::Namespace;
 use quick_xml::Reader;
+use std::convert::TryFrom;
 use std::io::Read;
 
 struct Resource {
@@ -83,7 +84,9 @@ fn parse_report(xml_data: &str) -> Vec<Resource> {
     loop {
         match reader.read_namespaced_event(&mut buf, &mut ns_buffer) {
             Ok((namespace_value, Event::Start(e))) => {
-                let namespace_value = namespace_value.unwrap_or(Namespace(b""));
+                let namespace_value = Option::<Namespace>::try_from(namespace_value)
+                    .unwrap_or_default() // Treat unknown prefixes as not bound to any namespace
+                    .unwrap_or(Namespace(b""));
                 match (depth, state, namespace_value.as_ref(), e.local_name()) {
                     (0, State::Root, b"DAV:", b"multistatus") => state = State::MultiStatus,
                     (1, State::MultiStatus, b"DAV:", b"response") => {
@@ -98,7 +101,9 @@ fn parse_report(xml_data: &str) -> Vec<Resource> {
                 depth += 1;
             }
             Ok((namespace_value, Event::End(e))) => {
-                let namespace_value = namespace_value.unwrap_or(Namespace(b""));
+                let namespace_value = Option::<Namespace>::try_from(namespace_value)
+                    .unwrap_or_default() // Treat unknown prefixes as not bound to any namespace
+                    .unwrap_or(Namespace(b""));
                 let local_name = e.local_name();
                 match (depth, state, namespace_value.as_ref(), local_name) {
                     (1, State::MultiStatus, b"DAV:", b"multistatus") => state = State::Root,

src/errors.rs

@@ -2,6 +2,7 @@
 
 use crate::escape::EscapeError;
 use crate::events::attributes::AttrError;
+use crate::utils::write_byte_string;
 use std::str::Utf8Error;
 
 /// The error type used by this crate.
@@ -32,6 +33,8 @@ pub enum Error {
     InvalidAttr(AttrError),
     /// Escape error
     EscapeError(EscapeError),
+    /// Specified namespace prefix is unknown, cannot resolve namespace for it
+    UnknownPrefix(Vec<u8>),
 }
 
 impl From<::std::io::Error> for Error {
@@ -93,6 +96,11 @@ impl std::fmt::Display for Error {
             ),
             Error::InvalidAttr(e) => write!(f, "error while parsing attribute: {}", e),
             Error::EscapeError(e) => write!(f, "{}", e),
+            Error::UnknownPrefix(prefix) => {
+                f.write_str("Unknown namespace prefix '")?;
+                write_byte_string(f, &prefix)?;
+                f.write_str("'")
+            }
         }
     }
 }