A file's revision reads the same after a save as it does when listed

src/dav.rs

@@ -185,7 +185,7 @@ pub fn multistatus(body: &[u8]) -> Result<Vec<Found>, ErrorCode> {
           b"getcontentlength" => {
             entry.length = value.trim().parse().unwrap_or(0);
           }
-          b"getetag" => entry.etag = Some(value.trim().to_owned()),
+          b"getetag" => entry.etag = Some(etag(&value)),
           _ => {}
         }
       }
@@ -207,6 +207,21 @@ pub fn multistatus(body: &[u8]) -> Result<Vec<Found>, ErrorCode> {
   Ok(found)
 }
 
+/// An entity tag in the quoted form an `ETag` header carries.
+///
+/// Some servers, WsgiDAV among them, list `getetag` bare while their `ETag`
+/// header quotes the same tag, so a revision read from a listing and one
+/// returned by a write would differ for the same bytes. Quoting a bare tag
+/// gives every revision one spelling, and `If-Match` accepts that form.
+pub fn etag(value: &str) -> String {
+  let value = value.trim();
+  if value.starts_with('"') || value.starts_with("W/") {
+    value.to_owned()
+  } else {
+    format!("\"{value}\"")
+  }
+}
+
 /// 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':') {
@@ -264,8 +279,8 @@ pub fn removable(found: &[Found]) -> Result<(), ErrorCode> {
 #[cfg(test)]
 mod tests {
   use super::{
-    Body, Found, basic, children, multistatus, name_of, outcome, range_outcome,
-    removable, url,
+    Body, Found, basic, children, etag, multistatus, name_of, outcome,
+    range_outcome, removable, url,
   };
   use crate::sdk::ErrorCode;
 
@@ -414,4 +429,11 @@ mod tests {
     assert_eq!(name_of(&found[1].href), "a b.md");
     Ok(())
   }
+
+  #[test]
+  fn bare_etags_are_quoted_like_headers() {
+    assert_eq!(etag("1996000-1790356720-264"), "\"1996000-1790356720-264\"");
+    assert_eq!(etag(" \"v1\" "), "\"v1\"");
+    assert_eq!(etag("W/\"v1\""), "W/\"v1\"");
+  }
 }

src/lib.rs

@@ -246,7 +246,7 @@ impl GuestWriter for DavWriter {
       .headers
       .iter()
       .find(|header| header.name.eq_ignore_ascii_case("etag"))
-      .map(|header| header.value.clone());
+      .map(|header| dav::etag(&header.value));
     Ok(Some(FileInfo {
       kind: EntryKind::File,
       size: self.written.get(),
@@ -369,7 +369,7 @@ impl Guest for Webdav {
         .headers
         .iter()
         .find(|header| header.name.eq_ignore_ascii_case("etag"))
-        .map(|header| header.value.clone())
+        .map(|header| dav::etag(&header.value))
     });
     Ok(Reader::new(DavReader {
       call,