Fix list objects xml parsing bug and improve perf. (#59)

For list-object-versions there was a bug that would return delete
markers after all versions of an object. The server response contains
the order of versions and delete markers according to recency and the
list objects client call should preserve this. This is fixed in this
change.

XML parsing was using the `take_child` call, that mutates a vector
removing an element in it. For a response containing N items, using
take_child on item, causes the XML parsing cost to be O(N^2) (so 1
million operations for a 1000 item list) - this change makes the parsing
cost linear.

src/s3/error.rs

@@ -52,8 +52,8 @@ impl ErrorResponse {
     }
 }
 
-#[derive(Debug)]
 /// Error definitions
+#[derive(Debug)]
 pub enum Error {
     TimeParseError(chrono::ParseError),
     InvalidUrl(http::uri::InvalidUri),

src/s3/response/list_objects.rs

@@ -17,12 +17,14 @@ use std::collections::HashMap;
 use async_trait::async_trait;
 use bytes::Buf;
 use reqwest::header::HeaderMap;
-use xmltree::Element;
 
 use crate::s3::{
     error::Error,
     types::{FromS3Response, ListEntry, S3Request},
-    utils::{from_iso8601utc, get_default_text, get_option_text, get_text, urldecode},
+    utils::{
+        from_iso8601utc, urldecode,
+        xml::{Element, MergeXmlElements},
+    },
 };
 
 fn url_decode(
@@ -57,65 +59,73 @@ fn parse_common_list_objects_response(
     ),
     Error,
 > {
-    let encoding_type = get_option_text(root, "EncodingType");
-    let prefix = url_decode(&encoding_type, Some(get_default_text(root, "Prefix")))?;
+    let encoding_type = root.get_child_text("EncodingType");
+    let prefix = url_decode(
+        &encoding_type,
+        Some(root.get_child_text("Prefix").unwrap_or_default()),
+    )?;
     Ok((
-        get_text(root, "Name")?,
+        root.get_child_text_or_error("Name")?,
         encoding_type,
         prefix,
-        get_option_text(root, "Delimiter"),
-        match get_option_text(root, "IsTruncated") {
-            Some(v) => v.to_lowercase() == "true",
-            None => false,
-        },
-        match get_option_text(root, "MaxKeys") {
-            Some(v) => Some(v.parse::<u16>()?),
-            None => None,
-        },
+        root.get_child_text("Delimiter"),
+        root.get_child_text("IsTruncated")
+            .map(|x| x.to_lowercase() == "true")
+            .unwrap_or(false),
+        root.get_child_text("MaxKeys")
+            .map(|x| x.parse::<u16>())
+            .transpose()?,
     ))
 }
 
 fn parse_list_objects_contents(
     contents: &mut Vec<ListEntry>,
-    root: &mut xmltree::Element,
-    tag: &str,
+    root: &Element,
+    main_tag: &str,
     encoding_type: &Option<String>,
-    is_delete_marker: bool,
+    with_delete_marker: bool,
 ) -> Result<(), Error> {
-    while let Some(v) = root.take_child(tag) {
-        let content = v;
+    let children1 = root.get_matching_children(main_tag);
+    let children2 = if with_delete_marker {
+        root.get_matching_children("DeleteMarker")
+    } else {
+        vec![]
+    };
+    let mut merged = MergeXmlElements::new(&children1, &children2);
+    while let Some(content) = merged.next() {
         let etype = encoding_type.as_ref().cloned();
-        let key = url_decode(&etype, Some(get_text(&content, "Key")?))?.unwrap();
-        let last_modified = Some(from_iso8601utc(&get_text(&content, "LastModified")?)?);
-        let etag = get_option_text(&content, "ETag");
-        let v = get_default_text(&content, "Size");
-        let size = match v.is_empty() {
-            true => None,
-            false => Some(v.parse::<usize>()?),
-        };
-        let storage_class = get_option_text(&content, "StorageClass");
-        let is_latest = get_default_text(&content, "IsLatest").to_lowercase() == "true";
-        let version_id = get_option_text(&content, "VersionId");
-        let (owner_id, owner_name) = match content.get_child("Owner") {
-            Some(v) => (get_option_text(v, "ID"), get_option_text(v, "DisplayName")),
-            None => (None, None),
-        };
-        let user_metadata = match content.get_child("UserMetadata") {
-            Some(v) => {
-                let mut map: HashMap<String, String> = HashMap::new();
-                for xml_node in &v.children {
-                    let u = xml_node
-                        .as_element()
-                        .ok_or(Error::XmlError("unable to convert to element".to_string()))?;
-                    map.insert(
-                        u.name.to_string(),
-                        u.get_text().unwrap_or_default().to_string(),
-                    );
-                }
-                Some(map)
-            }
-            None => None,
-        };
+        let key = url_decode(&etype, Some(content.get_child_text_or_error("Key")?))?.unwrap();
+        let last_modified = Some(from_iso8601utc(
+            &content.get_child_text_or_error("LastModified")?,
+        )?);
+        let etag = content.get_child_text("ETag");
+        let size: Option<usize> = content
+            .get_child_text("Size")
+            .map(|x| x.parse::<usize>())
+            .transpose()?;
+        let storage_class = content.get_child_text("StorageClass");
+        let is_latest = content
+            .get_child_text("IsLatest")
+            .unwrap_or_default()
+            .to_lowercase()
+            == "true";
+        let version_id = content.get_child_text("VersionId");
+        let (owner_id, owner_name) = content
+            .get_child("Owner")
+            .map(|v| (v.get_child_text("ID"), v.get_child_text("DisplayName")))
+            .unwrap_or((None, None));
+        let user_metadata = content.get_child("UserMetadata").map(|v| {
+            v.get_xmltree_children()
+                .into_iter()
+                .map(|elem| {
+                    (
+                        elem.name.to_string(),
+                        elem.get_text().unwrap_or_default().to_string(),
+                    )
+                })
+                .collect::<HashMap<String, String>>()
+        });
+        let is_delete_marker = content.name() == "DeleteMarker";
 
         contents.push(ListEntry {
             name: key,
@@ -139,13 +149,16 @@ fn parse_list_objects_contents(
 
 fn parse_list_objects_common_prefixes(
     contents: &mut Vec<ListEntry>,
-    root: &mut Element,
+    root: &Element,
     encoding_type: &Option<String>,
 ) -> Result<(), Error> {
-    while let Some(v) = root.take_child("CommonPrefixes") {
-        let common_prefix = v;
+    for (_, common_prefix) in root.get_matching_children("CommonPrefixes") {
         contents.push(ListEntry {
-            name: url_decode(encoding_type, Some(get_text(&common_prefix, "Prefix")?))?.unwrap(),
+            name: url_decode(
+                encoding_type,
+                Some(common_prefix.get_child_text_or_error("Prefix")?),
+            )?
+            .unwrap(),
             last_modified: None,
             etag: None,
             owner_id: None,
@@ -187,18 +200,19 @@ impl FromS3Response for ListObjectsV1Response {
     ) -> Result<Self, Error> {
         let headers = resp.headers().clone();
         let body = resp.bytes().await?;
-        let mut root = Element::parse(body.reader())?;
+        let xmltree_root = xmltree::Element::parse(body.reader())?;
+        let root = Element::from(&xmltree_root);
 
         let (name, encoding_type, prefix, delimiter, is_truncated, max_keys) =
             parse_common_list_objects_response(&root)?;
-        let marker = url_decode(&encoding_type, get_option_text(&root, "Marker"))?;
-        let mut next_marker = url_decode(&encoding_type, get_option_text(&root, "NextMarker"))?;
+        let marker = url_decode(&encoding_type, root.get_child_text("Marker"))?;
+        let mut next_marker = url_decode(&encoding_type, root.get_child_text("NextMarker"))?;
         let mut contents: Vec<ListEntry> = Vec::new();
-        parse_list_objects_contents(&mut contents, &mut root, "Contents", &encoding_type, false)?;
+        parse_list_objects_contents(&mut contents, &root, "Contents", &encoding_type, false)?;
         if is_truncated && next_marker.is_none() {
             next_marker = contents.last().map(|v| v.name.clone())
         }
-        parse_list_objects_common_prefixes(&mut contents, &mut root, &encoding_type)?;
+        parse_list_objects_common_prefixes(&mut contents, &root, &encoding_type)?;
 
         Ok(ListObjectsV1Response {
             headers,
@@ -240,24 +254,21 @@ impl FromS3Response for ListObjectsV2Response {
     ) -> Result<Self, Error> {
         let headers = resp.headers().clone();
         let body = resp.bytes().await?;
-        let mut root = Element::parse(body.reader())?;
+        let xmltree_root = xmltree::Element::parse(body.reader())?;
+        let root = Element::from(&xmltree_root);
 
         let (name, encoding_type, prefix, delimiter, is_truncated, max_keys) =
             parse_common_list_objects_response(&root)?;
-        let text = get_option_text(&root, "KeyCount");
-        let key_count = match text {
-            Some(v) => match v.is_empty() {
-                true => None,
-                false => Some(v.parse::<u16>()?),
-            },
-            None => None,
-        };
-        let start_after = url_decode(&encoding_type, get_option_text(&root, "StartAfter"))?;
-        let continuation_token = get_option_text(&root, "ContinuationToken");
-        let next_continuation_token = get_option_text(&root, "NextContinuationToken");
+        let key_count = root
+            .get_child_text("KeyCount")
+            .map(|x| x.parse::<u16>())
+            .transpose()?;
+        let start_after = url_decode(&encoding_type, root.get_child_text("StartAfter"))?;
+        let continuation_token = root.get_child_text("ContinuationToken");
+        let next_continuation_token = root.get_child_text("NextContinuationToken");
         let mut contents: Vec<ListEntry> = Vec::new();
-        parse_list_objects_contents(&mut contents, &mut root, "Contents", &encoding_type, false)?;
-        parse_list_objects_common_prefixes(&mut contents, &mut root, &encoding_type)?;
+        parse_list_objects_contents(&mut contents, &root, "Contents", &encoding_type, false)?;
+        parse_list_objects_common_prefixes(&mut contents, &root, &encoding_type)?;
 
         Ok(ListObjectsV2Response {
             headers,
@@ -301,24 +312,18 @@ impl FromS3Response for ListObjectVersionsResponse {
     ) -> Result<Self, Error> {
         let headers = resp.headers().clone();
         let body = resp.bytes().await?;
-        let mut root = Element::parse(body.reader())?;
+        let xmltree_root = xmltree::Element::parse(body.reader())?;
+        let root = Element::from(&xmltree_root);
 
         let (name, encoding_type, prefix, delimiter, is_truncated, max_keys) =
             parse_common_list_objects_response(&root)?;
-        let key_marker = url_decode(&encoding_type, get_option_text(&root, "KeyMarker"))?;
-        let next_key_marker = url_decode(&encoding_type, get_option_text(&root, "NextKeyMarker"))?;
-        let version_id_marker = get_option_text(&root, "VersionIdMarker");
-        let next_version_id_marker = get_option_text(&root, "NextVersionIdMarker");
+        let key_marker = url_decode(&encoding_type, root.get_child_text("KeyMarker"))?;
+        let next_key_marker = url_decode(&encoding_type, root.get_child_text("NextKeyMarker"))?;
+        let version_id_marker = root.get_child_text("VersionIdMarker");
+        let next_version_id_marker = root.get_child_text("NextVersionIdMarker");
         let mut contents: Vec<ListEntry> = Vec::new();
-        parse_list_objects_contents(&mut contents, &mut root, "Version", &encoding_type, false)?;
-        parse_list_objects_common_prefixes(&mut contents, &mut root, &encoding_type)?;
-        parse_list_objects_contents(
-            &mut contents,
-            &mut root,
-            "DeleteMarker",
-            &encoding_type,
-            true,
-        )?;
+        parse_list_objects_contents(&mut contents, &root, "Version", &encoding_type, true)?;
+        parse_list_objects_common_prefixes(&mut contents, &root, &encoding_type)?;
 
         Ok(ListObjectVersionsResponse {
             headers,

src/s3/utils.rs

@@ -391,3 +391,157 @@ pub fn copy_slice(dst: &mut [u8], src: &[u8]) -> usize {
     }
     c
 }
+
+pub mod xml {
+    use std::collections::HashMap;
+
+    use crate::s3::error::Error;
+
+    struct XmlElementIndex {
+        children: HashMap<String, Vec<usize>>,
+    }
+
+    impl XmlElementIndex {
+        fn get_first(&self, tag: &str) -> Option<usize> {
+            let tag: String = tag.to_string();
+            let is = self.children.get(&tag)?;
+            is.first().map(|v| *v)
+        }
+
+        fn get(&self, tag: &str) -> Option<&Vec<usize>> {
+            let tag: String = tag.to_string();
+            self.children.get(&tag)
+        }
+    }
+
+    impl From<&xmltree::Element> for XmlElementIndex {
+        fn from(value: &xmltree::Element) -> Self {
+            let mut children = HashMap::new();
+            for (i, e) in value
+                .children
+                .iter()
+                .enumerate()
+                .filter_map(|(i, v)| v.as_element().map(|e| (i, e)))
+            {
+                children
+                    .entry(e.name.clone())
+                    .or_insert_with(Vec::new)
+                    .push(i);
+            }
+            Self { children }
+        }
+    }
+
+    pub struct Element<'a> {
+        inner: &'a xmltree::Element,
+        child_element_index: XmlElementIndex,
+    }
+
+    impl<'a> From<&'a xmltree::Element> for Element<'a> {
+        fn from(value: &'a xmltree::Element) -> Self {
+            let element_index = XmlElementIndex::from(value);
+            Self {
+                inner: value,
+                child_element_index: element_index,
+            }
+        }
+    }
+
+    impl Element<'_> {
+        pub fn name(&self) -> &str {
+            self.inner.name.as_str()
+        }
+
+        pub fn get_child_text(&self, tag: &str) -> Option<String> {
+            let index = self.child_element_index.get_first(tag)?;
+            self.inner.children[index]
+                .as_element()?
+                .get_text()
+                .map(|v| v.to_string())
+        }
+
+        pub fn get_child_text_or_error(&self, tag: &str) -> Result<String, Error> {
+            let i = self
+                .child_element_index
+                .get_first(tag)
+                .ok_or(Error::XmlError(format!("<{}> tag not found", tag)))?;
+            self.inner.children[i]
+                .as_element()
+                .unwrap()
+                .get_text()
+                .map(|x| x.to_string())
+                .ok_or(Error::XmlError(format!("text of <{}> tag not found", tag)))
+        }
+
+        // Returns all children with given tag along with their index.
+        pub fn get_matching_children(&self, tag: &str) -> Vec<(usize, Element)> {
+            self.child_element_index
+                .get(tag)
+                .unwrap_or(&vec![])
+                .into_iter()
+                .map(|i| (*i, self.inner.children[*i].as_element().unwrap().into()))
+                .collect()
+        }
+
+        pub fn get_child(&self, tag: &str) -> Option<Element> {
+            let index = self.child_element_index.get_first(tag)?;
+            Some(self.inner.children[index].as_element()?.into())
+        }
+
+        pub fn get_xmltree_children(&self) -> Vec<&xmltree::Element> {
+            self.inner
+                .children
+                .iter()
+                .filter_map(|v| v.as_element())
+                .collect()
+        }
+    }
+
+    // Helper type that implements merge sort in the iterator.
+    pub struct MergeXmlElements<'a> {
+        v1: &'a Vec<(usize, Element<'a>)>,
+        v2: &'a Vec<(usize, Element<'a>)>,
+        i1: usize,
+        i2: usize,
+    }
+
+    impl<'a> MergeXmlElements<'a> {
+        pub fn new(v1: &'a Vec<(usize, Element<'a>)>, v2: &'a Vec<(usize, Element<'a>)>) -> Self {
+            Self {
+                v1,
+                v2,
+                i1: 0,
+                i2: 0,
+            }
+        }
+    }
+
+    impl<'a> Iterator for MergeXmlElements<'a> {
+        type Item = &'a Element<'a>;
+
+        fn next(&mut self) -> Option<Self::Item> {
+            let c1 = self.v1.get(self.i1);
+            let c2 = self.v2.get(self.i2);
+            match (c1, c2) {
+                (Some(val1), Some(val2)) => {
+                    if val1.0 < val2.0 {
+                        self.i1 += 1;
+                        Some(&val1.1)
+                    } else {
+                        self.i2 += 1;
+                        Some(&val2.1)
+                    }
+                }
+                (Some(val1), None) => {
+                    self.i1 += 1;
+                    Some(&val1.1)
+                }
+                (None, Some(val2)) => {
+                    self.i2 += 1;
+                    Some(&val2.1)
+                }
+                (None, None) => None,
+            }
+        }
+    }
+}