memona-plugin-signature/src/dav.rs

//! WebDAV wire details: addressing, credentials, and the one XML reply shape
//! this provider reads.

use crate::sdk::ErrorCode;

const UNRESERVED: &[u8] = b"-._~";
const BASE64: &[u8; 64] =
  b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/// Percent-encodes one path segment. Slashes are excluded deliberately: the
/// caller joins segments, so an encoded name can never grow a path boundary.
fn segment(value: &str) -> String {
  let mut encoded = String::with_capacity(value.len());
  for byte in value.as_bytes() {
    if byte.is_ascii_alphanumeric() || UNRESERVED.contains(byte) {
      encoded.push(*byte as char);
    } else {
      encoded.push_str(&format!("%{byte:02X}"));
    }
  }
  encoded
}

/// Joins a connection-relative path onto the configured root.
pub fn url(root: &str, path: &str) -> String {
  let mut url = root.trim_end_matches('/').to_owned();
  for part in path.split('/').filter(|part| !part.is_empty()) {
    url.push('/');
    url.push_str(&segment(part));
  }
  if path.ends_with('/') && path.len() > 1 {
    url.push('/');
  }
  url
}

pub fn basic(username: &str, password: &str) -> String {
  let raw = format!("{username}:{password}");
  let bytes = raw.as_bytes();
  let mut encoded = String::from("Basic ");
  for chunk in bytes.chunks(3) {
    let mut block = [0u8; 3];
    block[..chunk.len()].copy_from_slice(chunk);
    let packed = u32::from(block[0]) << 16
      | u32::from(block[1]) << 8
      | u32::from(block[2]);
    for index in 0..4 {
      if index <= chunk.len() {
        let position = (packed >> (18 - index * 6)) & 0x3f;
        encoded.push(BASE64[position as usize] as char);
      } else {
        encoded.push('=');
      }
    }
  }
  encoded
}

/// What a WebDAV status means to a filesystem caller. A body is never shown to
/// the user, so only the code is translated.
pub fn outcome(status: u16) -> Result<(), ErrorCode> {
  match status {
    200..=299 => Ok(()),
    401 | 403 => Err(ErrorCode::Denied),
    404 | 410 => Err(ErrorCode::NotFound),
    405 | 409 | 412 => Err(ErrorCode::Conflict),
    // A lock is someone else holding the resource, which is a conflict to
    // resolve rather than a permission to grant.
    423 => Err(ErrorCode::Conflict),
    501 => Err(ErrorCode::Unsupported),
    _ => Err(ErrorCode::Unavailable),
  }
}

/// What a ranged GET's status means to a reader: whether a body follows. A
/// range that starts at the end of the file draws 416 from Apache and most
/// other servers, which for a reader is simply end of file, not a failure.
pub fn range_outcome(status: u16) -> Result<bool, ErrorCode> {
  match status {
    416 => Ok(false),
    _ => outcome(status).map(|()| true),
  }
}

/// A response body served in pieces no larger than the caller asked for. The
/// host may hand over more than one pull wanted, so the surplus waits here for
/// the next pull rather than being dropped or passed through oversize.
#[derive(Default)]
pub struct Body {
  spare: Vec<u8>,
  ended: bool,
}

impl Body {
  /// A body that was never sent: every pull reports end of file.
  pub fn empty() -> Self {
    Self {
      spare: Vec::new(),
      ended: true,
    }
  }

  /// Returns at most `limit` bytes, drawing on `next` until the limit is met
  /// or the body ends. `next` is asked for the shortfall and may answer with
  /// more or less than that.
  pub fn pull(
    &mut self,
    limit: usize,
    mut next: impl FnMut(u32) -> Result<Vec<u8>, ErrorCode>,
  ) -> Result<Vec<u8>, ErrorCode> {
    let mut data = std::mem::take(&mut self.spare);
    while data.len() < limit && !self.ended {
      let chunk = next((limit - data.len()) as u32)?;
      if chunk.is_empty() {
        self.ended = true;
      } else {
        data.extend_from_slice(&chunk);
      }
    }
    if data.len() > limit {
      self.spare = data.split_off(limit);
    }
    Ok(data)
  }
}

/// One entry from a multistatus reply.
pub struct Found {
  pub href: String,
  pub collection: bool,
  pub length: u64,
  pub etag: Option<String>,
}

/// Reads the `response` elements of a multistatus body, keeping only the four
/// properties this provider asked for. Unknown elements and namespaces other
/// than DAV: are skipped rather than rejected, because servers add their own.
pub fn multistatus(body: &[u8]) -> Result<Vec<Found>, ErrorCode> {
  let mut reader = quick_xml::Reader::from_reader(body);
  reader.config_mut().trim_text(true);
  let mut found = Vec::new();
  let mut current: Option<Found> = None;
  let mut field = None;
  let mut buffer = Vec::new();
  loop {
    let event = reader
      .read_event_into(&mut buffer)
      .map_err(|_| ErrorCode::Unavailable)?;
    match event {
      quick_xml::events::Event::Eof => break,
      quick_xml::events::Event::Start(tag) => match local(tag.name().0) {
        b"response" => {
          current = Some(Found {
            href: String::new(),
            collection: false,
            length: 0,
            etag: None,
          });
        }
        b"collection" => {
          if let Some(entry) = current.as_mut() {
            entry.collection = true;
          }
        }
        name @ (b"href" | b"getcontentlength" | b"getetag") => {
          field = Some(name.to_vec());
        }
        _ => field = None,
      },
      quick_xml::events::Event::Empty(tag) => {
        if local(tag.name().0) == b"collection"
          && let Some(entry) = current.as_mut()
        {
          entry.collection = true;
        }
      }
      quick_xml::events::Event::Text(text) => {
        let (Some(entry), Some(name)) = (current.as_mut(), field.as_deref())
        else {
          continue;
        };
        let value = text.unescape().map_err(|_| ErrorCode::Unavailable)?;
        match name {
          b"href" => entry.href = value.into_owned(),
          b"getcontentlength" => {
            entry.length = value.trim().parse().unwrap_or(0);
          }
          b"getetag" => entry.etag = Some(value.trim().to_owned()),
          _ => {}
        }
      }
      quick_xml::events::Event::End(tag) => {
        field = None;
        if local(tag.name().0) == b"response"
          && let Some(entry) = current.take()
        {
          found.push(entry);
        }
      }
      _ => {}
    }
    buffer.clear();
    if found.len() > 4096 {
      return Err(ErrorCode::Unavailable);
    }
  }
  Ok(found)
}

/// Strips any namespace prefix, so `d:href` and `D:href` read the same.
fn local(name: &[u8]) -> &[u8] {
  match name.iter().position(|byte| *byte == b':') {
    Some(index) => &name[index + 1..],
    None => name,
  }
}

/// The last non-empty segment of an href, percent-decoded.
pub fn name_of(href: &str) -> String {
  let trimmed = href.trim_end_matches('/');
  let tail = trimmed.rsplit('/').next().unwrap_or(trimmed);
  let bytes = tail.as_bytes();
  let mut decoded = Vec::with_capacity(bytes.len());
  let mut index = 0;
  while index < bytes.len() {
    if bytes[index] == b'%' && index + 2 < bytes.len() {
      let high = (bytes[index + 1] as char).to_digit(16);
      let low = (bytes[index + 2] as char).to_digit(16);
      if let (Some(high), Some(low)) = (high, low) {
        decoded.push((high * 16 + low) as u8);
        index += 3;
        continue;
      }
    }
    decoded.push(bytes[index]);
    index += 1;
  }
  String::from_utf8_lossy(&decoded).into_owned()
}

/// The direct children in a Depth 1 reply. The first response describes the
/// resource asked about, so it must be a collection before the rest can be
/// called its children.
pub fn children(found: &[Found]) -> Result<&[Found], ErrorCode> {
  match found.split_first() {
    None => Err(ErrorCode::NotFound),
    Some((first, rest)) if first.collection => Ok(rest),
    Some(_) => Err(ErrorCode::InvalidInput),
  }
}

/// Whether a Depth 1 reply describes something `remove` may delete: a file,
/// or a collection with no children. DELETE on a collection is recursive in
/// WebDAV, so this check is the only thing keeping a whole tree.
pub fn removable(found: &[Found]) -> Result<(), ErrorCode> {
  match children(found) {
    Ok([]) => Ok(()),
    Ok(_) => Err(ErrorCode::Conflict),
    Err(ErrorCode::InvalidInput) => Ok(()),
    Err(error) => Err(error),
  }
}

#[cfg(test)]
mod tests {
  use super::{
    Body, Found, basic, children, multistatus, name_of, outcome, range_outcome,
    removable, url,
  };
  use crate::sdk::ErrorCode;

  fn found(collection: bool) -> Found {
    Found {
      href: String::new(),
      collection,
      length: 0,
      etag: None,
    }
  }

  #[test]
  fn paths_join_onto_the_root_without_growing_a_boundary() {
    assert_eq!(
      url("https://dav.example/files/", "/"),
      "https://dav.example/files"
    );
    assert_eq!(
      url("https://dav.example/files", "/notes/today.md"),
      "https://dav.example/files/notes/today.md"
    );
    // A slash inside a name is encoded, so it cannot become a new segment.
    assert_eq!(
      url("https://dav.example", "/a b/c%d"),
      "https://dav.example/a%20b/c%25d"
    );
    assert_eq!(
      url("https://dav.example", "/papers/"),
      "https://dav.example/papers/"
    );
  }

  #[test]
  fn credentials_encode_the_way_basic_auth_expects() {
    assert_eq!(basic("user", "pass"), "Basic dXNlcjpwYXNz");
    assert_eq!(basic("a", ""), "Basic YTo=");
    assert_eq!(basic("ab", ""), "Basic YWI6");
  }

  #[test]
  fn statuses_keep_denied_missing_and_conflicting_apart() {
    assert!(outcome(207).is_ok());
    assert!(matches!(outcome(401), Err(ErrorCode::Denied)));
    assert!(matches!(outcome(404), Err(ErrorCode::NotFound)));
    assert!(matches!(outcome(412), Err(ErrorCode::Conflict)));
    // A locked resource is someone else holding it, not a permission to grant.
    assert!(matches!(outcome(423), Err(ErrorCode::Conflict)));
    assert!(matches!(outcome(500), Err(ErrorCode::Unavailable)));
  }

  #[test]
  fn a_range_past_the_end_is_end_of_file_not_a_failure() {
    assert_eq!(range_outcome(206), Ok(true));
    assert_eq!(range_outcome(200), Ok(true));
    assert_eq!(range_outcome(416), Ok(false));
    assert_eq!(range_outcome(404), Err(ErrorCode::NotFound));
  }

  #[test]
  fn a_body_never_hands_over_more_than_the_limit() -> Result<(), ErrorCode> {
    // The host answers every pull with a whole 10-byte piece, whatever the
    // shortfall asked for.
    let mut asked = Vec::new();
    let mut source = |wanted: u32| {
      asked.push(wanted);
      Ok(if asked.len() > 2 {
        Vec::new()
      } else {
        vec![asked.len() as u8; 10]
      })
    };
    let mut body = Body::default();
    assert_eq!(body.pull(4, &mut source)?, vec![1, 1, 1, 1]);
    // The surplus of the first piece is served before anything new is read.
    assert_eq!(body.pull(8, &mut source)?, vec![1, 1, 1, 1, 1, 1, 2, 2]);
    assert_eq!(body.pull(100, &mut source)?, vec![2; 8]);
    assert_eq!(body.pull(100, &mut source)?, Vec::<u8>::new());
    // Only the shortfall is asked for, and nothing after the body ended.
    assert_eq!(asked, vec![4, 2, 92]);
    Ok(())
  }

  #[test]
  fn a_body_keeps_reading_until_the_limit_is_met() -> Result<(), ErrorCode> {
    // A host that answers in pieces smaller than the limit is drained until
    // the limit is met, so a short piece is never mistaken for end of file.
    let mut body = Body::default();
    let mut left = 7;
    let mut source = |_wanted: u32| {
      let piece = left.min(3);
      left -= piece;
      Ok(vec![9; piece])
    };
    assert_eq!(body.pull(5, &mut source)?, vec![9; 5]);
    assert_eq!(body.pull(5, &mut source)?, vec![9; 2]);
    assert_eq!(Body::empty().pull(5, &mut source)?, Vec::<u8>::new());
    Ok(())
  }

  #[test]
  fn only_an_empty_directory_or_a_file_may_be_removed() {
    assert_eq!(removable(&[found(false)]), Ok(()));
    assert_eq!(removable(&[found(true)]), Ok(()));
    assert_eq!(
      removable(&[found(true), found(false)]),
      Err(ErrorCode::Conflict)
    );
    assert_eq!(removable(&[]), Err(ErrorCode::NotFound));
    assert_eq!(
      children(&[found(false)]).err(),
      Some(ErrorCode::InvalidInput)
    );
    assert_eq!(
      children(&[found(true), found(false), found(true)]).map(<[Found]>::len),
      Ok(2)
    );
  }

  #[test]
  fn a_multistatus_reply_yields_the_collection_then_its_children()
  -> Result<(), ErrorCode> {
    let body = br#"<?xml version="1.0"?>
      <D:multistatus xmlns:D="DAV:">
        <D:response>
          <D:href>/files/notes/</D:href>
          <D:propstat><D:prop><D:resourcetype><D:collection/></D:resourcetype>
          </D:prop><D:status>HTTP/1.1 200 OK</D:status></D:propstat>
        </D:response>
        <D:response>
          <D:href>/files/notes/a%20b.md</D:href>
          <D:propstat><D:prop>
            <D:resourcetype/>
            <D:getcontentlength>42</D:getcontentlength>
            <D:getetag>"v1"</D:getetag>
          </D:prop><D:status>HTTP/1.1 200 OK</D:status></D:propstat>
        </D:response>
      </D:multistatus>"#;
    let found = multistatus(body)?;
    assert_eq!(found.len(), 2);
    assert!(found[0].collection);
    assert!(!found[1].collection);
    assert_eq!(found[1].length, 42);
    assert_eq!(found[1].etag.as_deref(), Some("\"v1\""));
    assert_eq!(name_of(&found[0].href), "notes");
    assert_eq!(name_of(&found[1].href), "a b.md");
    Ok(())
  }
}