Merge PR #1602 (rebased)

crates/engine/examples/sync_scan_bench.rs

@@ -0,0 +1,47 @@
+//! What a Synchronize Folder scan costs on a folder whose photos are all in the library and
+//! unchanged: the walk, and the check of every photo and its sidecar. Reads the folder; writes
+//! nothing there, and probes no file (the library is made from the folder's own listing).
+//!
+//! `cargo run --release -p lightcraft-engine --example sync_scan_bench -- <folder>`
+use std::time::Instant;
+
+use lightcraft_catalog::{Op, Photo, Source};
+use lightcraft_engine::Session;
+use lightcraft_engine::sync::{SyncInput, SyncProgress, scan_with};
+
+fn main() {
+    let Some(folder) = std::env::args().nth(1) else {
+        eprintln!("usage: sync_scan_bench <folder>");
+        return;
+    };
+    let mut s = Session::new();
+    let t = Instant::now();
+    let files = lightcraft_engine::import::expand(std::slice::from_ref(&folder), None);
+    println!("{} files listed in {:.1} s (the library is made of them)", files.len(), t.elapsed().as_secs_f64());
+    for f in &files {
+        let id = s.catalog.alloc_photo_id();
+        let name = std::path::Path::new(f).file_name().map(|n| n.to_string_lossy().to_string()).unwrap_or_default();
+        let p = Photo::new(id, Source::File { path: f.clone() }, &name, "JPEG", 6, 4, "2026-01-01T00:00:00");
+        if s.catalog.apply(Op::AddPhoto { photo: Box::new(p) }).is_err() {
+            eprintln!("could not add {name}");
+        }
+    }
+    for run in 0..2 {
+        let input = match SyncInput::new(&mut s, &folder, true) {
+            Ok(input) => input,
+            Err(e) => {
+                eprintln!("{e}");
+                return;
+            }
+        };
+        let t = Instant::now();
+        let changes = scan_with(input, &SyncProgress::default());
+        println!(
+            "run {run}: scan {:.1} s ({} new, {} missing, {} with sidecar news)",
+            t.elapsed().as_secs_f64(),
+            changes.new.len(),
+            changes.missing.len(),
+            changes.metadata.len()
+        );
+    }
+}

crates/engine/src/import.rs

@@ -382,6 +382,17 @@ pub fn expand(paths: &[String], skip: Option<&Path>) -> Vec<String> {
 
 /// [`expand`] with the walk bounded by `limits` → (files, whether a walk stopped at a bound).
 pub fn expand_within(paths: &[String], skip: Option<&Path>, limits: crate::walk::Limits) -> (Vec<String>, bool) {
+    expand_noting(paths, skip, limits, None)
+}
+
+/// [`expand_within`], handing the listing of every folder that is walked to `note`
+/// ([`crate::walk::files_in_noting`]).
+pub(crate) fn expand_noting(
+    paths: &[String],
+    skip: Option<&Path>,
+    limits: crate::walk::Limits,
+    mut note: Option<crate::walk::NoteFolder>,
+) -> (Vec<String>, bool) {
     let mut out = Vec::new();
     let mut truncated = false;
     for p in paths {
@@ -390,7 +401,10 @@ pub fn expand_within(paths: &[String], skip: Option<&Path>, limits: crate::walk:
             continue;
         }
         if p.is_dir() {
-            let w = crate::walk::files_in(p, skip, limits, is_supported);
+            let w = match note.as_mut() {
+                Some(note) => crate::walk::files_in_noting(p, skip, limits, is_supported, &mut **note),
+                None => crate::walk::files_in(p, skip, limits, is_supported),
+            };
             if w.truncated {
                 log::warn!(
                     "import: stopped looking in {} after {} entries (limits: {} entries, {} folder levels)",
@@ -638,6 +652,11 @@ impl ScanInput {
         (input, paths.to_vec())
     }
 
+    /// The folder a scan never enters (the library's own).
+    pub(crate) fn skip(&self) -> Option<&Path> {
+        self.skip.as_deref()
+    }
+
     /// The library as the scan sees it (shared, for [`crate::sync`], which looks at the same one).
     pub(crate) fn library(&self) -> std::sync::Arc<lightcraft_catalog::Catalog> {
         std::sync::Arc::clone(&self.library)
@@ -656,9 +675,14 @@ pub fn scan(s: &mut Session, paths: &[String]) -> Vec<ImportCandidate> {
 
 /// [`scan`] without the session, so it can run on a worker thread. Stops early (returning what it
 /// has) when `progress.cancel` is set.
-pub fn scan_with(mut input: ScanInput, paths: &[String], progress: &ScanProgress) -> ScanOutput {
+pub fn scan_with(input: ScanInput, paths: &[String], progress: &ScanProgress) -> ScanOutput {
-    use std::sync::atomic::Ordering::Relaxed;
     let files = expand(paths, input.skip.as_deref());
+    scan_files(input, files, progress)
+}
+
+/// [`scan_with`] for files that were found already (`paths` expanded, [`expand`]).
+pub(crate) fn scan_files(mut input: ScanInput, files: Vec<String>, progress: &ScanProgress) -> ScanOutput {
+    use std::sync::atomic::Ordering::Relaxed;
     let by_path = library_files(&input.library, &files);
     let todo: Vec<String> = files.iter().filter(|f| !by_path.contains_key(Path::new(f))).cloned().collect();
     progress.total.store(todo.len(), Relaxed);

crates/engine/src/sidecar.rs

@@ -66,6 +66,13 @@ pub fn sidecar_path(original: &str, naming: SidecarNaming) -> PathBuf {
 /// accepting `.XMP` and `.Xmp`, and a full name whose photo extension differs in case, such as
 /// `IMG_1.cr3.xmp` beside `IMG_1.CR3`, which a case-sensitive file system would otherwise miss).
 pub fn find_sidecar(original: &str, naming: SidecarNaming) -> Option<PathBuf> {
+    sidecar_candidates(original, naming).find(|p| p.is_file())
+}
+
+/// The names `original`'s sidecar can have, in the order they are looked for: under `naming`,
+/// then under the other naming (the full name also with the photo extension in the other case),
+/// each as `.xmp`, `.XMP` and `.Xmp`.
+pub(crate) fn sidecar_candidates(original: &str, naming: SidecarNaming) -> impl Iterator<Item = PathBuf> {
     let other = if naming == SidecarNaming::Stem { SidecarNaming::Full } else { SidecarNaming::Stem };
     let ext = Path::new(original).extension().map(|e| e.to_string_lossy().to_string()).unwrap_or_default();
     let base = Path::new(original).with_extension("");
@@ -73,7 +80,7 @@ pub fn find_sidecar(original: &str, naming: SidecarNaming) -> Option<PathBuf> {
     let full_variants: Vec<String> = [recased(ext.to_lowercase()), recased(ext.to_uppercase())].into_iter().flatten().collect();
     [naming, other]
         .into_iter()
-        .flat_map(|n| {
+        .flat_map(move |n| {
             let mut v = vec![sidecar_path(original, n)];
             if n == SidecarNaming::Full {
                 v.extend(full_variants.iter().map(|o| sidecar_path(o, n)));
@@ -81,7 +88,6 @@ pub fn find_sidecar(original: &str, naming: SidecarNaming) -> Option<PathBuf> {
             v
         })
         .flat_map(|p| [p.clone(), p.with_extension("XMP"), p.with_extension("Xmp")])
-        .find(|p| p.is_file())
 }
 
 /// What a sidecar (or embedded XMP packet) says about a photo. `None` = not stated.

crates/engine/src/sync.rs

@@ -38,7 +38,7 @@
 //! not cancelled midway.
 
 use std::collections::HashMap;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
 use lightcraft_catalog::{Catalog, Filter, Op, Photo, PhotoId, Sort, Source};
@@ -203,18 +203,37 @@ pub fn scan_with(input: SyncInput, progress: &SyncProgress) -> FolderChanges {
     let namings = crate::sidecar::namings_in(&labels, naming, &ids);
     let photos: Vec<(Arc<Photo>, SidecarNaming)> =
         ids.iter().zip(namings).filter_map(|(id, naming)| labels.photo(*id).map(|p| (Arc::clone(p), naming))).collect();
+    // The folder is walked once, and its listing answers for the library's photos in it: on a
+    // network share that is one request for a folder's names where it was several for each
+    // photo. Then both halves wait on the disk at once: the new files are probed (on their own
+    // threads) beside the checks of the photos, which ask the disk only what the listing can't say.
+    // (how many photos there are to check is known before the walk: the count shows while it runs)
+    progress.to_check.store(photos.len(), std::sync::atomic::Ordering::Relaxed);
+    let mut listing = Listing::default();
+    let (files, _) = crate::import::expand_noting(
+        std::slice::from_ref(&folder),
+        scan.skip(),
+        crate::walk::Limits::default(),
+        Some(&mut |dir, names, complete| listing.note(dir, names, complete)),
+    );
+    let listing = &listing;
     // (no threads on the web: one after the other there)
     #[cfg(target_arch = "wasm32")]
-    let (out, checks) = (crate::import::scan_with(scan, std::slice::from_ref(&folder), &progress.files), check_photos(&photos, &labels, progress));
+    let (out, checks) = (crate::import::scan_files(scan, files, &progress.files), check_photos(&photos, &labels, listing, progress));
     #[cfg(not(target_arch = "wasm32"))]
     let (out, checks) = std::thread::scope(|sc| {
         let photos = &photos;
         let labels = &labels;
-        let checks = sc.spawn(move || check_photos(photos, labels, progress));
+        let checks = sc.spawn(move || check_photos(photos, labels, listing, progress));
-        let out = crate::import::scan_with(scan, std::slice::from_ref(&folder), &progress.files);
+        let out = crate::import::scan_files(scan, files, &progress.files);
         // (a check that panicked counts as nothing found; the scan still reports the rest)
         (out, checks.join().unwrap_or_default())
     });
+    log::debug!(
+        "synchronize: {} folders listed; the disk was asked {} times about the library's photos",
+        listing.folders.len(),
+        listing.asked.load(std::sync::atomic::Ordering::Relaxed)
+    );
     let mut changes = FolderChanges { path: folder, seen, ..Default::default() };
     for ((p, _), check) in photos.iter().zip(checks) {
         let Some(file) = crate::cmd::missing::checked_path(p) else { continue };
@@ -335,11 +354,136 @@ enum Check {
     News,
 }
 
-fn check_photo(p: &Photo, naming: SidecarNaming, labels: &Catalog) -> Check {
+/// The listings of the folders a scan walked: what is in each, by name. It answers "is this file
+/// there" for the library's photos and their sidecars without asking the disk, which on a
+/// network share is a round trip for every name (and a photo without a sidecar took five).
+///
+/// It answers only what holds on any file system. A name is there when the folder lists exactly
+/// that name ([`asks_the_disk`] aside). It is not there when the folder was listed to its end,
+/// every name in the folder is ASCII, the name asked about is plain ASCII ([`plain_name`]), and
+/// no name in the folder is that name in another case. Anything else is not answered, and the
+/// disk is asked as before: a name that differs in case only, a link, a folder that was not
+/// walked or not to its end, and every name that is not listed in a folder that holds a name
+/// outside ASCII. (File systems hold names outside ASCII equal in more ways than can be listed
+/// here: by case, where the capital sharp s is `SS`; by how accents are written; and some pass
+/// over marks altogether.)
+///
+/// Where the listing and a look-up would differ, the listing is the folder as the scan saw it:
+/// a file it lists is there though it could not be opened by its path (a folder that may be
+/// listed but not searched, a path longer than the system takes), and a share that drops after
+/// the walk does not turn every photo the listing answers for into a missing one.
+#[derive(Default)]
+pub(crate) struct Listing {
+    folders: HashMap<PathBuf, Listed>,
+    /// How often the disk was asked after all (for the tests, and the log).
+    pub(crate) asked: std::sync::atomic::AtomicUsize,
+}
+
+/// One folder's listing.
+struct Listed {
+    /// name → what it is.
+    names: HashMap<String, crate::walk::EntryKind>,
+    /// The names, lower-cased (only kept for a folder whose names are all ASCII): what a plain
+    /// name asked about could be in another case.
+    lower: std::collections::HashSet<String>,
+    /// Every entry was read, and every name is ASCII: a plain name that is not listed, in any
+    /// case, is not there.
+    sure: bool,
+}
+
+/// Whether the disk is asked about `name` whatever the listing holds: a name Windows takes for
+/// a device (`NUL`, `aux.jpg`: there in every folder, or not openable where it is listed).
+fn asks_the_disk(name: &str) -> bool {
+    let stem = name.split('.').next().unwrap_or_default().trim_end_matches(' ').to_ascii_lowercase();
+    // (`COM` and `LPT` with any one digit, the superscript ones Windows also takes among them)
+    let numbered = ["com", "lpt"].iter().any(|port| {
+        let mut rest = stem.strip_prefix(port).unwrap_or_default().chars();
+        matches!((rest.next(), rest.next()), (Some(d), None) if d.is_ascii_digit() || ['\u{b9}', '\u{b2}', '\u{b3}'].contains(&d))
+    });
+    numbered || ["con", "prn", "aux", "nul", "conin$", "conout$", "clock$"].contains(&stem.as_str())
+}
+
+/// Whether `name` can only be found under itself in some case: ASCII, and nothing in it that a
+/// file system resolves another way (a short name with `~`, a stream after `:`, dots or spaces
+/// at its end, which Windows drops).
+fn plain_name(name: &str) -> bool {
+    name.is_ascii() && !name.contains(['~', ':']) && !name.ends_with(['.', ' ']) && !name.is_empty()
+}
+
+impl Listing {
+    /// Take the listing of `folder` from the walk ([`crate::walk::files_in_noting`]).
+    pub(crate) fn note(&mut self, folder: &Path, entries: Vec<(std::ffi::OsString, crate::walk::EntryKind)>, complete: bool) {
+        let mut sure = complete;
+        let mut names = HashMap::with_capacity(entries.len());
+        let mut lower = std::collections::HashSet::new();
+        for (name, kind) in entries {
+            match name.into_string() {
+                Ok(name) => {
+                    sure &= name.is_ascii();
+                    lower.insert(name.to_ascii_lowercase());
+                    names.insert(name, kind);
+                }
+                // (a name that is no text is not one a photo's path can be compared with)
+                Err(_) => sure = false,
+            }
+        }
+        if !sure {
+            lower.clear();
+        }
+        self.folders.insert(folder.to_path_buf(), Listed { names, lower, sure });
+    }
+
+    /// What the listing of `path`'s folder says of it: its kind when it is listed under exactly
+    /// this name, `Some(None)` when it is surely not there, `None` when the listing can't say.
+    fn kind(&self, path: &Path) -> Option<Option<crate::walk::EntryKind>> {
+        let name = path.file_name()?.to_str()?;
+        // (the path ends in that name: no `/.` or separator after it, which the disk would resolve)
+        if !path.to_str()?.ends_with(name) {
+            return None;
+        }
+        if asks_the_disk(name) {
+            return None;
+        }
+        let folder = self.folders.get(path.parent()?)?;
+        if let Some(kind) = folder.names.get(name) {
+            return Some(Some(*kind));
+        }
+        (folder.sure && plain_name(name) && !folder.lower.contains(&name.to_ascii_lowercase())).then_some(None)
+    }
+
+    /// `Path::exists`, where the listing can say.
+    pub(crate) fn exists(&self, path: &Path) -> Option<bool> {
+        match self.kind(path)? {
+            Some(crate::walk::EntryKind::Other) => None,
+            Some(_) => Some(true),
+            None => Some(false),
+        }
+    }
+
+    /// `Path::is_file`, where the listing can say.
+    pub(crate) fn is_file(&self, path: &Path) -> Option<bool> {
+        match self.kind(path)? {
+            Some(crate::walk::EntryKind::Other) => None,
+            Some(kind) => Some(kind == crate::walk::EntryKind::File),
+            None => Some(false),
+        }
+    }
+
+    /// The listing's answer, or the disk's where it has none.
+    fn or_ask(&self, answer: Option<bool>, ask: impl FnOnce() -> bool) -> bool {
+        answer.unwrap_or_else(|| {
+            self.asked.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+            ask()
+        })
+    }
+}
+
+fn check_photo(p: &Photo, naming: SidecarNaming, labels: &Catalog, listing: &Listing) -> Check {
     let Some(file) = crate::cmd::missing::checked_path(p) else { return Check::Nothing };
-    if !Path::new(file).exists() {
+    let file = Path::new(file);
+    if !listing.or_ask(listing.exists(file), || file.exists()) {
         Check::Missing
-    } else if p.copy_of.is_none() && sidecar_has_news(labels, p, naming) {
+    } else if p.copy_of.is_none() && sidecar_has_news(labels, p, naming, listing) {
         Check::News
     } else {
         Check::Nothing
@@ -348,12 +492,12 @@ fn check_photo(p: &Photo, naming: SidecarNaming, labels: &Catalog) -> Check {
 
 /// [`check_photo`] for each photo, in order, on several threads (each file is a wait on the
 /// disk, not work for the CPU). Photos not reached before a cancel count as nothing found.
-fn check_photos(photos: &[(Arc<Photo>, SidecarNaming)], labels: &Catalog, progress: &SyncProgress) -> Vec<Check> {
+fn check_photos(photos: &[(Arc<Photo>, SidecarNaming)], labels: &Catalog, listing: &Listing, progress: &SyncProgress) -> Vec<Check> {
     use std::sync::atomic::Ordering::Relaxed;
     progress.to_check.store(photos.len(), Relaxed);
     let cancel = &progress.files.cancel;
     let check = |p: &Photo, naming: SidecarNaming| {
-        let c = check_photo(p, naming, labels);
+        let c = check_photo(p, naming, labels, listing);
         progress.checked.fetch_add(1, Relaxed);
         c
     };
@@ -421,10 +565,17 @@ impl Session {
 
 /// Whether `p`'s XMP sidecar file was saved after the library last had its say about the photo
 /// (imported, or edited here) and says something the library doesn't.
-fn sidecar_has_news(labels: &Catalog, p: &Photo, naming: SidecarNaming) -> bool {
+fn sidecar_has_news(labels: &Catalog, p: &Photo, naming: SidecarNaming, listing: &Listing) -> bool {
     let Source::File { path } = &p.source else { return false };
-    let Some(file) = crate::sidecar::find_sidecar(path, naming) else { return false };
+    // ([`crate::sidecar::find_sidecar`], each name looked up in the listing before the disk)
-    let Ok(meta) = std::fs::metadata(&file) else { return false };
+    // (a name the listing holds is read for its date next, and one that can't be read is passed
+    // over for the next name, as a name that is no file was)
+    let found = crate::sidecar::sidecar_candidates(path, naming).find_map(|c| {
+        let there = listing.or_ask(listing.is_file(&c), || c.is_file());
+        let meta = there.then(|| std::fs::metadata(&c).ok()).flatten().filter(std::fs::Metadata::is_file)?;
+        Some((c, meta))
+    });
+    let Some((file, meta)) = found else { return false };
     if meta.len() > MAX_SIDECAR_BYTES {
         return false;
     }
@@ -708,3 +859,7 @@ impl SyncCommit {
         self.report
     }
 }
+
+#[cfg(test)]
+#[path = "sync/tests_listing.rs"]
+mod tests_listing;

crates/engine/src/sync/tests_listing.rs

@@ -0,0 +1,192 @@
+//! Synchronize Folder answers "is the photo's file there" and "has it a sidecar" from the
+//! listing of the folder its scan has just made, instead of asking the disk about every photo.
+//!
+//! On a network share each question is a round trip (about 5 ms a file on a hard disk over SMB,
+//! measured), and a photo without a sidecar took five of them; the listing has every name.
+//! The listing answers only what it can be sure of on any file system: where a name could match
+//! another way (case, letters outside ASCII, a link), the disk is asked as before.
+use super::*;
+use crate::walk::EntryKind::{File, Folder, Other};
+
+fn listing(folders: &[(&str, &[(&str, crate::walk::EntryKind)], bool)]) -> Listing {
+    let mut l = Listing::default();
+    for (dir, names, complete) in folders {
+        l.note(Path::new(dir), names.iter().map(|(n, k)| (std::ffi::OsString::from(n), *k)).collect(), *complete);
+    }
+    l
+}
+
+/// Given the listing of a folder, when it is asked about a name, then it says "there" for a
+/// name it lists, "not there" for one nothing in the folder could be taken for, and nothing
+/// (ask the disk) wherever a file system could see it otherwise.
+#[test]
+fn the_listing_answers_only_what_it_is_sure_of() {
+    let l = listing(&[
+        (
+            "/pics/trip",
+            &[("IMG_1.CR3", File), ("IMG_1.xmp", File), ("img_2.jpg", File), ("sub", Folder), ("link.jpg", Other), (".hidden.xmp", File)],
+            true,
+        ),
+        ("/pics/cut", &[("IMG_1.CR3", File)], false),
+        ("/pics/names", &[("IMG_1.CR3", File), ("Größe.JPG", File)], true),
+        // (the capital sharp s, which a file system that folds case takes for `SS`)
+        ("/pics/sharp", &[("IMG_1.CR3", File), ("STRA\u{1e9e}E.jpg", File)], true),
+        ("/pics/devices", &[("aux.jpg", File), ("COM\u{b9}.jpg", File), ("IMG_1.CR3", File)], true),
+    ]);
+    let (there, gone, ask) = (Some(true), Some(false), None);
+    for (path, exists, is_file) in [
+        ("/pics/trip/IMG_1.CR3", there, there),
+        ("/pics/trip/IMG_1.xmp", there, there),
+        ("/pics/trip/.hidden.xmp", there, there),
+        ("/pics/trip/sub", there, gone),
+        ("/pics/trip/IMG_9.CR3", gone, gone),
+        ("/pics/trip/IMG_1.XMP", ask, ask),
+        ("/pics/trip/IMG_2.JPG", ask, ask),
+        ("/pics/trip/link.jpg", ask, ask),
+        ("/pics/trip/SUB", ask, ask),
+        // a folder the walk did not list to its end, or not at all
+        ("/pics/cut/IMG_1.CR3", there, there),
+        ("/pics/cut/IMG_2.CR3", ask, ask),
+        ("/pics/other/IMG_1.CR3", ask, ask),
+        // a folder that holds a name outside ASCII: what it lists is there, and nothing is surely not there (file
+        // systems hold such names equal in more ways than one)
+        ("/pics/names/IMG_1.CR3", there, there),
+        ("/pics/names/Größe.JPG", there, there),
+        ("/pics/names/IMG_2.CR3", ask, ask),
+        ("/pics/names/größe.JPG", ask, ask),
+        ("/pics/trip/Größe.JPG", ask, ask),
+        ("/pics/sharp/strasse.jpg", ask, ask),
+        ("/pics/sharp/STRASSE.xmp", ask, ask),
+        ("/pics/sharp/IMG_2.CR3", ask, ask),
+        // names Windows resolves another way: a short name, a stream, dots or spaces at the end, a device
+        ("/pics/trip/IMG_10~1.CR3", ask, ask),
+        ("/pics/trip/IMG_1.CR3::$DATA", ask, ask),
+        ("/pics/trip/IMG_1.CR3.", ask, ask),
+        ("/pics/trip/IMG_1.CR3 ", ask, ask),
+        ("/pics/devices/aux.jpg", ask, ask),
+        ("/pics/devices/NUL", ask, ask),
+        ("/pics/devices/nul.xmp", ask, ask),
+        ("/pics/devices/COM1 .jpg", ask, ask),
+        ("/pics/devices/COM\u{b9}.jpg", ask, ask),
+        ("/pics/trip/con", ask, ask),
+        ("/pics/trip/PRN.txt", ask, ask),
+        ("/pics/trip/com2.jpg", ask, ask),
+        ("/pics/trip/LPT9.xmp", ask, ask),
+        ("/pics/trip/lpt0", ask, ask),
+        ("/pics/trip/nul.tar.gz", ask, ask),
+        ("/pics/trip/CONIN$", ask, ask),
+        ("/pics/trip/com10.jpg", gone, gone),
+        ("/pics/trip/computer.jpg", gone, gone),
+        ("/pics/devices/IMG_1.CR3", there, there),
+        ("/pics/devices/auxiliary.jpg", ask, ask),
+        ("/pics/trip/auxiliary.jpg", gone, gone),
+        // a path that does not end in its file's name is the disk's to resolve
+        ("/pics/trip/IMG_1.CR3/", ask, ask),
+        ("/pics/trip/IMG_1.CR3/.", ask, ask),
+        ("/pics/trip/", ask, ask),
+        ("", ask, ask),
+    ] {
+        assert_eq!((l.exists(Path::new(path)), l.is_file(Path::new(path))), (exists, is_file), "{path:?}");
+    }
+    // a name that is no text: nothing in its folder is surely not there
+    #[cfg(unix)]
+    {
+        use std::os::unix::ffi::OsStringExt;
+        let mut l = Listing::default();
+        l.note(
+            Path::new("/pics/raw"),
+            vec![(std::ffi::OsString::from("IMG_1.CR3"), File), (std::ffi::OsString::from_vec(vec![0x66, 0xff, 0x6f]), File)],
+            true,
+        );
+        assert_eq!((l.exists(Path::new("/pics/raw/IMG_1.CR3")), l.exists(Path::new("/pics/raw/IMG_2.CR3"))), (there, ask));
+    }
+}
+
+fn photo(c: &mut Catalog, path: &Path) -> Arc<Photo> {
+    let id = c.alloc_photo_id();
+    let name = path.file_name().unwrap().to_string_lossy().to_string();
+    c.apply(lightcraft_catalog::Op::AddPhoto {
+        photo: Box::new(Photo::new(id, Source::File { path: path.to_string_lossy().to_string() }, &name, "JPEG", 6, 4, "2020-01-01T00:00:00")),
+    })
+    .unwrap();
+    Arc::clone(c.photo(id).unwrap())
+}
+
+/// Given a folder on disk whose photos are there or gone, with sidecars under every name a
+/// sidecar can have, when each photo is checked with the folder's listing, then the answer is
+/// the one the disk gives; and for photos with plain names the disk was asked nothing but the
+/// sidecars' dates.
+#[test]
+fn a_check_from_the_listing_is_the_check_from_the_disk() {
+    let dir = std::env::temp_dir().join(format!("lc-listing-{}", std::process::id()));
+    let _ = std::fs::remove_dir_all(&dir);
+    std::fs::create_dir_all(dir.join("sub")).unwrap();
+    let news = r#"<x:xmpmeta xmlns:x="adobe:ns:meta/"><rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"><rdf:Description xmlns:xmp="http://ns.adobe.com/xap/1.0/" xmp:Rating="5"/></rdf:RDF></x:xmpmeta>"#;
+    for name in [
+        "plain.jpg",
+        "stem.jpg",
+        "stem.xmp",
+        "full.jpg",
+        "full.jpg.xmp",
+        "upper.jpg",
+        "upper.XMP",
+        "sub/deep.jpg",
+        "sub/deep.xmp",
+        "Größe.jpg",
+        "Größe.xmp",
+        "dir.jpg.xmp",
+    ] {
+        std::fs::write(dir.join(name), if name.ends_with("mp") || name.ends_with("MP") { news.as_bytes() } else { b"jpeg" }).unwrap();
+    }
+    let mut c = Catalog::new();
+    let photos: Vec<Arc<Photo>> =
+        ["plain.jpg", "stem.jpg", "full.jpg", "upper.jpg", "sub/deep.jpg", "Größe.jpg", "gone.jpg", "sub/gone.jpg", "dir.jpg"]
+            .iter()
+            .map(|n| photo(&mut c, &dir.join(n)))
+            .collect();
+    // (`dir.jpg` is gone, though a sidecar of its name is there)
+    let mut l = Listing::default();
+    let walked =
+        crate::walk::files_in_noting(&dir, None, crate::walk::Limits::default(), |_| true, &mut |d, names, complete| l.note(d, names, complete));
+    assert_eq!(walked.files.len(), 12);
+    let nothing = Listing::default();
+    for naming in [SidecarNaming::Stem, SidecarNaming::Full] {
+        for p in &photos {
+            let from_disk = check_photo(p, naming, &c, &nothing);
+            assert_eq!(check_photo(p, naming, &c, &l), from_disk, "{:?} {naming:?}", p.file_name);
+        }
+    }
+    let expected = [Check::Nothing, Check::News, Check::News, Check::News, Check::News, Check::News, Check::Missing, Check::Missing, Check::Missing];
+    assert_eq!(photos.iter().map(|p| check_photo(p, SidecarNaming::Stem, &c, &l)).collect::<Vec<_>>(), expected);
+
+    // what the disk was asked about the same photos, with the listing and without: fewer questions with it
+    let asked = |l: &Listing| {
+        for p in &photos {
+            check_photo(p, SidecarNaming::Stem, &c, l);
+        }
+        l.asked.load(std::sync::atomic::Ordering::Relaxed)
+    };
+    let mut with = Listing::default();
+    crate::walk::files_in_noting(&dir, None, crate::walk::Limits::default(), |_| true, &mut |d, names, complete| with.note(d, names, complete));
+    let (with, without) = (asked(&with), asked(&Listing::default()));
+    // (without: each photo's file, and the names of its sidecar up to the one that is there: 20 questions where
+    // names are found in any case, 21 where they are not)
+    assert!(without >= 20, "{without}");
+    // (with: this folder holds `Größe.*`, so what it does not list is still asked about: the nine names of the
+    // sidecar `plain.jpg` has not (`.xmp`, `.XMP` and `.Xmp` of `plain`, `plain.jpg` and `plain.JPG`), the three
+    // `full.jpg` has not, `upper.xmp`, and the two photos that are gone. What it lists is not asked about, nor
+    // anything in `sub`, whose names are all ASCII)
+    assert_eq!(with, 15, "asked {with} times with the listing, {without} without");
+
+    // a folder of plain names only: the disk is asked nothing at all
+    let sub = dir.join("sub");
+    let mut only_plain = Listing::default();
+    crate::walk::files_in_noting(&sub, None, crate::walk::Limits::default(), |_| true, &mut |d, names, complete| only_plain.note(d, names, complete));
+    let mut c2 = Catalog::new();
+    for (name, expected) in [("deep.jpg", Check::News), ("gone.jpg", Check::Missing), ("gone.jpg.xmp", Check::Missing)] {
+        let p = photo(&mut c2, &sub.join(name));
+        assert_eq!(check_photo(&p, SidecarNaming::Stem, &c2, &only_plain), expected, "{name}");
+    }
+    assert_eq!(only_plain.asked.load(std::sync::atomic::Ordering::Relaxed), 0, "every answer came from the listing");
+    let _ = std::fs::remove_dir_all(&dir);
+}

crates/engine/src/walk.rs

@@ -47,10 +47,35 @@ fn hidden(p: &Path) -> bool {
     p.file_name().is_some_and(|n| n.to_string_lossy().starts_with('.'))
 }
 
+/// What a folder's listing says an entry is.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum EntryKind {
+    File,
+    Folder,
+    /// A link, or an entry whose kind the listing did not say.
+    Other,
+}
+
+/// What [`files_in_noting`] hands over for each folder it lists: the folder, the name and kind
+/// of every entry in it (hidden ones too), and whether that is all of them (not when the walk
+/// stopped inside the folder, or an entry could not be read).
+pub type NoteFolder<'a> = &'a mut dyn FnMut(&Path, Vec<(std::ffi::OsString, EntryKind)>, bool);
+
 /// The files below folder `root` that `keep` accepts. Hidden entries (names starting with `.`)
 /// are left out, the folder `skip` (e.g. the library's own) is never entered, and the walk stops
 /// at `limits` (see [`Walked::truncated`]). Unreadable folders are skipped.
-pub fn files_in(root: &Path, skip: Option<&Path>, limits: Limits, mut keep: impl FnMut(&Path) -> bool) -> Walked {
+pub fn files_in(root: &Path, skip: Option<&Path>, limits: Limits, keep: impl FnMut(&Path) -> bool) -> Walked {
+    walk(root, skip, limits, keep, None)
+}
+
+/// [`files_in`], handing the listing of every folder it reads to `note`: who walks a folder to
+/// find its files has, with that, the answer to "is this name there" for everything in it, and
+/// need not ask the disk again (a round trip for each name on a network share).
+pub fn files_in_noting(root: &Path, skip: Option<&Path>, limits: Limits, keep: impl FnMut(&Path) -> bool, note: NoteFolder) -> Walked {
+    walk(root, skip, limits, keep, Some(note))
+}
+
+fn walk(root: &Path, skip: Option<&Path>, limits: Limits, mut keep: impl FnMut(&Path) -> bool, mut note: Option<NoteFolder>) -> Walked {
     let mut out = Walked::default();
     // canonical folders reached through a link, so a link loop is walked at most once
     let mut linked: HashSet<PathBuf> = HashSet::new();
@@ -77,17 +102,41 @@ pub fn files_in(root: &Path, skip: Option<&Path>, limits: Limits, mut keep: impl
         }
         let Ok(rd) = std::fs::read_dir(&path) else { continue };
         let mut children: Vec<(PathBuf, bool)> = Vec::new();
-        for e in rd.flatten() {
+        // (the folder's listing, for who asked for it; all of it unless something below says otherwise)
+        let mut listed = note.is_some().then(Vec::new);
+        let mut complete = true;
+        for e in rd {
+            let Ok(e) = e else {
+                complete = false;
+                continue;
+            };
             if out.entries >= limits.max_entries {
                 out.truncated = true;
+                complete = false;
                 break;
             }
             out.entries = out.entries.saturating_add(1);
             let p = e.path();
-            if hidden(&p) {
+            // (a hidden entry's kind is only asked for when the listing is wanted; every other
+            // entry's is asked once)
+            let is_hidden = hidden(&p);
+            if is_hidden && listed.is_none() {
                 continue;
             }
-            let Ok(ft) = e.file_type() else { continue };
+            let ft = e.file_type();
+            if let Some(listed) = listed.as_mut() {
+                let kind = match &ft {
+                    Ok(t) if t.is_symlink() => EntryKind::Other,
+                    Ok(t) if t.is_dir() => EntryKind::Folder,
+                    Ok(t) if t.is_file() => EntryKind::File,
+                    _ => EntryKind::Other,
+                };
+                listed.push((e.file_name(), kind));
+            }
+            if is_hidden {
+                continue;
+            }
+            let Ok(ft) = ft else { continue };
             let dir = if ft.is_symlink() {
                 // a link (or a Windows junction): a folder is entered once, a file is a file
                 match std::fs::metadata(&p) {
@@ -105,6 +154,9 @@ pub fn files_in(root: &Path, skip: Option<&Path>, limits: Limits, mut keep: impl
             };
             children.push((p, dir));
         }
+        if let (Some(note), Some(listed)) = (note.as_mut(), listed) {
+            note(&path, listed, complete);
+        }
         children.sort();
         let below = depth.saturating_add(1);
         stack.extend(children.into_iter().rev().map(|(p, d)| (p, below, d)));
@@ -123,6 +175,43 @@ mod tests {
         d
     }
 
+    /// Given a folder with files, a folder inside it, a hidden file and a link, when it is walked
+    /// with its listings noted, then the walk finds what it finds without them, and each folder's
+    /// listing holds every entry by name with its kind (hidden ones too, a link as neither file nor
+    /// folder) and says it is all of them; a walk that stops inside a folder says it is not.
+    #[test]
+    fn a_walk_notes_each_folders_listing() {
+        let root = tmp("noting");
+        std::fs::create_dir_all(root.join("sub")).unwrap();
+        for name in ["a.jpg", ".hidden", "sub/b.jpg"] {
+            std::fs::write(root.join(name), b"x").unwrap();
+        }
+        #[cfg(unix)]
+        std::os::unix::fs::symlink(root.join("a.jpg"), root.join("link.jpg")).unwrap();
+        let mut noted: Vec<(PathBuf, Vec<(String, EntryKind)>, bool)> = Vec::new();
+        let walked = files_in_noting(&root, None, Limits::default(), |_| true, &mut |dir, entries, complete| {
+            let mut entries: Vec<(String, EntryKind)> = entries.into_iter().map(|(n, k)| (n.to_string_lossy().to_string(), k)).collect();
+            entries.sort_by(|a, b| a.0.cmp(&b.0));
+            noted.push((dir.to_path_buf(), entries, complete));
+        });
+        assert_eq!(walked, files_in(&root, None, Limits::default(), |_| true), "the same walk, noted or not");
+        let named = |n: &str, k: EntryKind| (n.to_string(), k);
+        let mut top = vec![named(".hidden", EntryKind::File), named("a.jpg", EntryKind::File), named("sub", EntryKind::Folder)];
+        if cfg!(unix) {
+            top.insert(2, named("link.jpg", EntryKind::Other));
+        }
+        assert_eq!(noted, [(root.clone(), top, true), (root.join("sub"), vec![named("b.jpg", EntryKind::File)], true)]);
+
+        // stopped inside the first folder: its listing is not all of it
+        let mut cut = Vec::new();
+        let walked = files_in_noting(&root, None, Limits { max_depth: 8, max_entries: 2 }, |_| true, &mut |dir, entries, complete| {
+            cut.push((dir.to_path_buf(), entries.len(), complete))
+        });
+        assert!(walked.truncated);
+        assert_eq!(cut.first().map(|(dir, n, complete)| (dir.clone(), *n, *complete)), Some((root.clone(), 2, false)));
+        let _ = std::fs::remove_dir_all(&root);
+    }
+
     #[test]
     fn files_come_in_path_order_without_hidden_ones_or_the_skipped_folder() {
         let d = tmp("order");

docs/whats-new.md

@@ -265,6 +265,23 @@
   else (issue #1504): trimming or clearing the cache can't delete files outside it, and thumbnails are never read
   from or written through such a link.
 
+### Synchronize Folder asks a network share less
+- A Synchronize Folder scan no longer asks the disk about every photo of the library in the folder (is its file
+  there, has it a sidecar): it has just listed the folder, and reads the answers off the listing. On a network share
+  each of those questions is a round trip. Measured on a hard disk over SMB, for folders of 1,600 to 2,400 photos
+  that had not changed: 4.6 to 7.1 s of questions after the listing (about 3 ms a photo), now under 0.05 s. The
+  listing itself took 4 to 5 s and is as it was.
+- The disk is still asked wherever a listing can't be sure what a file system would say: about a name that is
+  there in another case, a link, a name Windows reads another way (`aux.jpg`, a `~`, a `:`, a dot or a space at the
+  end), a folder the scan did not walk to its end, and, in a folder that holds any name with a character outside
+  ASCII (an umlaut, a curly apostrophe, a dash), about every name the folder does not list. Names it does list are
+  answered there too. In such a folder the scan saves less, and its checks now start after the listing where they
+  ran beside it.
+- A photo the folder lists is there for the scan, also where its file could not be opened by its path (a folder
+  that may be listed but not searched, a path longer than the system takes): such photos used to be listed as
+  missing. And a share that drops after the scan has listed a folder no longer turns that folder's photos into
+  missing ones.
+
 ### Large libraries: Synchronize Folder, import and sidecars start sooner
 - Opening Synchronize Folder, an import review or an import no longer makes a table of every photo in the library
   before it looks at the first file. In a library of 3.5 million photos that held the window for about 6 s before