fix(content): keep text a merge writes off a new file's other links

core/crates/layerfs-content/src/merge/build.rs

@@ -4,9 +4,11 @@
 //! in the merge's records; the merged root is one backed update of the
 //! `into` side over them, so it is built by the same canonical producer, on
 //! the same route, as a Workspace Commit. A file value written at a name
-//! keeps its source serial when the serial is new to that side, already
-//! holds that value there, or is changed in place at its only binding;
-//! otherwise it gets a fresh serial, so one serial never carries two values.
+//! keeps its source serial when the serial is new to that side and `from`
+//! binds it at no other name, already holds that value there, or is changed
+//! in place at its only binding; otherwise it gets a fresh serial, so one
+//! serial never carries two values and a new file `from` also binds at other
+//! names keeps its own value there.
 
 use layerfs_telemetry::timer::{Active, TimingScope};
 
@@ -168,16 +170,17 @@ impl Plan<'_, '_> {
         }
     }
 
-    /// The serial that carries `value` for the source serial `source`.
+    /// The serial that carries `value` for the entry `like`.
     pub(super) fn assign(
         &mut self,
         resolver: &Resolver<'_>,
         into: usize,
         writer: &mut Writer<'_>,
-        source: u64,
+        like: EntryFacts,
         value: InodeValue,
         here: Option<EntryFacts>,
     ) -> ContentResult<u64> {
+        let source = like.serial;
         let stored = if self.assigned.may_hold(source) {
             self.spill.get(ASSIGNED, single(source))?
         } else {
@@ -194,6 +197,9 @@ impl Plan<'_, '_> {
         let first = given.is_empty();
         let serial = match resolver.value(into, source)? {
             _ if !first => None,
+            // `from` binds the file at other names too, which keep its own
+            // value: the value written here is another file.
+            None if like.value.namespace_ref_count > 1 => None,
             None => {
                 self.fresh(source)?;
                 Some(source)
@@ -233,11 +239,6 @@ impl Plan<'_, '_> {
         Ok(self.spill.get(VALUE, single(serial))?.is_some())
     }
 
-    /// True when a decision gave `source`'s entry a value.
-    pub(super) fn assigned(&self, source: u64) -> ContentResult<bool> {
-        Ok(self.assigned.may_hold(source) && self.spill.get(ASSIGNED, single(source))?.is_some())
-    }
-
     /// Adds one assignment to a source serial's record.
     fn record(
         &mut self,

core/crates/layerfs-content/src/merge/serial.rs

@@ -407,8 +407,9 @@ impl Ctx<'_, '_> {
                 let mut listing = self.listing_of(c, PathSlot::Name(self.from, s));
                 self.names(&mut listing, Mode::Emit)?;
             }
-            // A new file decided at a name already carries its result.
-            (None, Some(f), None) if !self.plan.assigned(s)? => {
+            // A new file keeps `from`'s value, unless a decision at a name
+            // gave this serial its result.
+            (None, Some(f), None) if !self.plan.valued(s)? => {
                 self.plan.fresh(s)?;
                 self.plan.value(s, f)?;
             }
@@ -547,7 +548,7 @@ impl Ctx<'_, '_> {
                 &self.resolver,
                 self.into,
                 &mut self.writer,
-                like.serial,
+                like,
                 value,
                 here,
             )?),

core/docs/architecture/81-compare-and-merge.md

@@ -150,6 +150,7 @@ text) go through `construct_bytes` into the same consumer first.
    | `from` changed a serial `into` deleted | decided as a deletion against a change; a kept file is placed at `from`'s path, its missing parents revived |
    | one side changed a directory's names, the other replaced it at the same path | decided at the path in either direction: a new directory there takes the changed directory's names; otherwise the entries decide (step 3), so a file or symlink there wins; a lost directory leaves its new serials unbound, a kept one stays whole |
    | a serial new on `from` that loses a collision | left unbound, with the new subtree below it; a file `from` hard-linked at other names only once every link is lost, else it stays at the names the merge keeps |
+   | a file new on `from`, hard-linked at other names, whose name is decided with written bytes (merged text or markers) | the bytes are a new file at that name; the other names keep the file and its own bytes, in a Branch merge and in the root a conflicted Commit installs |
 
 5. **Paths.** Only a serial that needs a path is named: a decision (for its
    conflict report and `/.lww` matching), a deletion against a change below
@@ -159,9 +160,13 @@ text) go through `construct_bytes` into the same consumer first.
    rewritten; link counts are derived from the changed bindings, and a serial
    left with none is released. A directory new to `into` is built from its
    bindings. A file value keeps its serial in three cases: the serial is new to
-   `into`, `into` already holds that value, or the value changes in place at
-   the serial's only binding. Otherwise the caller's `fresh` allocator supplies
-   a new serial, so one serial never carries two values.
+   `into` and `from` binds it at no other name, `into` already holds that
+   value, or the value changes in place at the serial's only binding.
+   Otherwise the caller's `fresh` allocator supplies a new serial, so one
+   serial never carries two values: a new file `from` also binds at other
+   names stays at those names with its own value, and the written value is a
+   fresh file at the decided name. Link counts follow the bindings the merge
+   keeps, for the file and for the fresh serial alike.
 
 **Working state (#355 (d), M1).** Memory does not grow with the merge. The
 serials deferred until moves are known are one run of 60 KiB chunks read once;

engine/crates/layerfs-api/tests/merge_by_serial.rs

@@ -67,18 +67,25 @@ fn merge(
     (left_root.to_owned(), merged)
 }
 
-fn view(fixture: &Fixture, root: &str) -> Vec<String> {
+fn viewer(fixture: &Fixture) -> view::Viewer {
     view::Viewer::open(
         &fixture.host.config.path,
         installed::BINDING,
         installed::CURSOR,
     )
-    .view(root, false)
 }
 
-fn merged_view(fixture: &Fixture, merged: Result<SdkValue, SdkRefusal>) -> Vec<String> {
+fn view(fixture: &Fixture, root: &str) -> Vec<String> {
+    viewer(fixture).view(root, false)
+}
+
+fn merged_root(merged: Result<SdkValue, SdkRefusal>) -> String {
     let reply = merged.unwrap_or_else(|refusal| panic!("merge refused: {refusal:?}"));
-    view(fixture, reply.get_text("root").unwrap())
+    reply.get_text("root").unwrap().to_owned()
+}
+
+fn merged_view(fixture: &Fixture, merged: Result<SdkValue, SdkRefusal>) -> Vec<String> {
+    view(fixture, &merged_root(merged))
 }
 
 /// The view's line for `path`, if the path exists.
@@ -89,8 +96,16 @@ fn line<'v>(view: &'v [String], path: &str) -> Option<&'v str> {
 }
 
 fn read(fixture: &Fixture, path: &str) -> Option<String> {
+    read_at(fixture, "m", path)
+}
+
+/// The text at `path` on `branch`, if the path exists.
+fn read_at(fixture: &Fixture, branch: &str, path: &str) -> Option<String> {
     fixture
-        .try_call("fs.read", vec![("branch", text("m")), ("path", text(path))])
+        .try_call(
+            "fs.read",
+            vec![("branch", text(branch)), ("path", text(path))],
+        )
         .ok()
         .map(|reply| reply.get_text("text").unwrap().to_owned())
 }
@@ -875,6 +890,201 @@ fn a_new_link_over_a_base_name_in_a_merged_directory_is_lost_once() {
     }
 }
 
+/// `l` replaces `/shared.txt` with a new file that it also links at
+/// `/uncontested.txt`; `r` edits `/shared.txt` and never touches the other
+/// name. Merging `branch` with `with` gives `/shared.txt` the merged text, a
+/// file of its own, and leaves `/uncontested.txt` the bytes `l` wrote, as one
+/// link.
+fn merge_beside_a_new_link(branch: &str, with: &str) {
+    let fixture = Fixture::new(&format!("serial-link-beside-{branch}"), |source| {
+        fs::write(source.join("shared.txt"), b"line1\nline2\nline3\n").unwrap();
+        1
+    });
+    fixture.fork("l");
+    fixture.fork("r");
+    fixture.apply("l", vec![rm("/shared.txt")]);
+    fixture.workspace_commit(
+        "l",
+        &[
+            Change::Create("/uncontested.txt", b"OUR_LINE1\nline2\nline3\n"),
+            Change::Link("/uncontested.txt", "/shared.txt"),
+        ],
+        1,
+        || {},
+    );
+    fixture.apply(
+        "r",
+        vec![write("/shared.txt", "line1\nline2\nTHEIR_LINE3\n")],
+    );
+    let merged = fixture.try_call(
+        "branch.merge",
+        vec![
+            ("branch", text(branch)),
+            ("with", text(with)),
+            ("name", text("m")),
+        ],
+    );
+    let root = merged_root(merged);
+    let merged_view = view(&fixture, &root);
+    assert_eq!(
+        read(&fixture, "/shared.txt").as_deref(),
+        Some("OUR_LINE1\nline2\nTHEIR_LINE3\n")
+    );
+    assert_eq!(
+        read(&fixture, "/uncontested.txt").as_deref(),
+        Some("OUR_LINE1\nline2\nline3\n")
+    );
+    for path in ["/shared.txt", "/uncontested.txt"] {
+        let found = line(&merged_view, path).expect("both names stay");
+        assert!(!found.contains(" link="), "{found}");
+        assert_eq!(viewer(&fixture).links(&root, path), 1, "{path}");
+    }
+    fixture.cleanup();
+}
+
+/// `l` is merged with `r`: the merge reads the changes of the branch that
+/// made the new file.
+#[test]
+fn text_merged_at_one_link_of_a_new_file_leaves_its_other_link_alone() {
+    merge_beside_a_new_link("l", "r");
+}
+
+/// `r` is merged with `l`: the merge reads the changes of the branch that
+/// only edited.
+#[test]
+fn text_merged_at_one_link_of_a_new_file_leaves_its_other_link_alone_in_the_other_order() {
+    merge_beside_a_new_link("r", "l");
+}
+
+/// The new file linked at `/a.txt` and `/b.txt`, both edited by `r`: each
+/// name holds its own merged text and the file keeps none of its own bytes.
+/// Two merged texts that differ are two files; two that are equal stay one
+/// file with two links.
+#[test]
+fn a_new_file_with_every_name_merged_keeps_none_of_its_own_bytes() {
+    for (label, edits, merged_texts, links) in [
+        (
+            "serial-links-merged-apart",
+            ["1\n2\nA3\n", "1\n2\nB3\n"],
+            ["O1\n2\nA3\n", "O1\n2\nB3\n"],
+            1,
+        ),
+        (
+            "serial-links-merged-alike",
+            ["1\n2\nT3\n", "1\n2\nT3\n"],
+            ["O1\n2\nT3\n", "O1\n2\nT3\n"],
+            2,
+        ),
+    ] {
+        let fixture = Fixture::new(label, |source| {
+            fs::write(source.join("a.txt"), b"1\n2\n3\n").unwrap();
+            fs::write(source.join("b.txt"), b"1\n2\n3\n").unwrap();
+            2
+        });
+        fixture.fork("l");
+        fixture.fork("r");
+        fixture.apply("l", vec![rm("/a.txt"), rm("/b.txt")]);
+        fixture.workspace_commit(
+            "l",
+            &[
+                Change::Create("/a.txt", b"O1\n2\n3\n"),
+                Change::Link("/a.txt", "/b.txt"),
+            ],
+            1,
+            || {},
+        );
+        fixture.apply(
+            "r",
+            vec![write("/a.txt", edits[0]), write("/b.txt", edits[1])],
+        );
+        let merged = fixture.try_call(
+            "branch.merge",
+            vec![
+                ("branch", text("l")),
+                ("with", text("r")),
+                ("name", text("m")),
+            ],
+        );
+        let root = merged_root(merged);
+        let merged_view = view(&fixture, &root);
+        for (path, merged_text) in ["/a.txt", "/b.txt"].into_iter().zip(merged_texts) {
+            let case = format!("{label} {path}");
+            assert_eq!(read(&fixture, path).as_deref(), Some(merged_text), "{case}");
+            let found = line(&merged_view, path).expect("both names stay");
+            assert_eq!(found.contains(" link="), links == 2, "{case}: {found}");
+            assert_eq!(viewer(&fixture).links(&root, path), links, "{case}");
+        }
+        fixture.cleanup();
+    }
+}
+
+/// A Workspace creates `/conflict.txt` and links it at `/clean_alias.txt`
+/// while another writer adds a different `/conflict.txt` to the Branch. The
+/// Commit conflicts at `/conflict.txt` alone and publishes nothing. The root
+/// it installs in the Workspace holds the markers at `/conflict.txt` and
+/// leaves `/clean_alias.txt` the bytes the Workspace wrote, as one link.
+#[test]
+fn a_conflict_at_one_link_of_a_new_file_leaves_its_other_link_alone() {
+    let fixture = Fixture::new("serial-link-conflict", |source| {
+        fs::write(source.join("readme.txt"), b"base\n").unwrap();
+        1
+    });
+    fixture.fork("work");
+    let committed = fixture.workspace_commit(
+        "work",
+        &[
+            Change::Create("/conflict.txt", b"our new file\n"),
+            Change::Link("/conflict.txt", "/clean_alias.txt"),
+        ],
+        1,
+        || {
+            fixture.apply("work", vec![write("/conflict.txt", "their new file\n")]);
+        },
+    );
+    let report = committed
+        .merge
+        .as_ref()
+        .expect("the head moved: a merge ran");
+    let conflicts: Vec<_> = report.conflicts.iter().map(|c| c.path.as_str()).collect();
+    assert_eq!(conflicts, ["/conflict.txt"]);
+    // Nothing is published: the Branch keeps only the other writer's file.
+    assert!(committed.history.is_none(), "{:?}", committed.history);
+    assert_eq!(
+        read_at(&fixture, "work", "/conflict.txt").as_deref(),
+        Some("their new file\n")
+    );
+    assert_eq!(read_at(&fixture, "work", "/clean_alias.txt"), None);
+    // The Workspace's own root binds one file at both names; the root the
+    // merge installed in its place binds a file of its own at each.
+    let written = report.written.clone();
+    let (shown, local) = committed
+        .replaced
+        .expect("a conflicted merge installs a root");
+    let (shown, local) = (view::hex(shown.as_bytes()), view::hex(local.as_bytes()));
+    let store = viewer(&fixture);
+    let shown_view = store.view(&shown, false);
+    for path in ["/conflict.txt", "/clean_alias.txt"] {
+        let found = line(&shown_view, path).expect("both names bound");
+        assert!(found.ends_with(" link=/clean_alias.txt"), "{found}");
+    }
+    let local_view = store.view(&local, false);
+    assert_eq!(store.text(&local, "/clean_alias.txt"), "our new file\n");
+    let marked = store.text(&local, "/conflict.txt");
+    assert!(
+        marked
+            .starts_with("<<<<<<< workspace\nour new file\n=======\ntheir new file\n>>>>>>> work@"),
+        "{marked}"
+    );
+    for path in ["/conflict.txt", "/clean_alias.txt"] {
+        let found = line(&local_view, path).expect("both names stay");
+        assert!(!found.contains(" link="), "{found}");
+        assert_eq!(store.links(&local, path), 1, "{path}");
+    }
+    // Only the conflicted name is reported as written into the Workspace.
+    assert_eq!(written, ["/conflict.txt"]);
+    fixture.cleanup();
+}
+
 #[test]
 fn a_one_sided_mode_change_is_kept() {
     let fixture = seeded("serial-chmod");