A component can name the files its build reads elsewhere

Freshness compared the commits under a component's own cwd, which is
right about where its files are and wrong about what its build reads.
tdep-survey's two clients each source one shared scripts/android-sdk-env.sh:
a fix committed there changed what every build does, moved no component's
subtree head, rebuilt nothing, and left every card correctly reporting
"current" while answering a narrower question than the reader was asking.

alsoWatch names the rest. The paths join the component's own in the same
subtree-head and dirty comparisons -- git takes several pathspecs, so it
stays one call each -- and it is part of the acceptance gate like any
other declared field. Declared rather than inferred: which files a build
reads is not knowable from here, and both wrong guesses are expensive.

Raised by the tdep-survey session, which traced why its dioxus client
could never become stale on the serving host.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/src/build_state.rs

@@ -556,10 +556,11 @@ impl BuildState {
         let Some(built) = built else {
             return Freshness::Unknown;
         };
-        if crate::git::subtree_dirty(&self.project_path, component.cwd()) != Some(false) {
+        if crate::git::subtree_dirty(&self.project_path, &component.watched_paths()) != Some(false)
+        {
             return Freshness::Unknown;
         }
-        match crate::git::subtree_head(&self.project_path, component.cwd()) {
+        match crate::git::subtree_head(&self.project_path, &component.watched_paths()) {
             Some(current) if current == built => Freshness::Current,
             Some(_) => Freshness::Behind,
             None => Freshness::Unknown,
@@ -611,7 +612,7 @@ impl BuildState {
         // tell; neither is evidence of staleness, and announcing one would
         // make every project built before this existed demand a rebuild.
         if let Some(built) = built_from.get(component.name())
-            && crate::git::subtree_head(&self.project_path, component.cwd())
+            && crate::git::subtree_head(&self.project_path, &component.watched_paths())
                 .is_some_and(|current| &current != built)
         {
             return true;
@@ -850,7 +851,8 @@ impl BuildState {
         //
         // A checkout that cannot be read records nothing, which reads
         // as unknown rather than as current -- see `component_is_stale`.
-        if let Some(sha) = crate::git::subtree_head(&self.project_path, component.cwd()) {
+        if let Some(sha) = crate::git::subtree_head(&self.project_path, &component.watched_paths())
+        {
             self.inner
                 .lock()
                 .unwrap()
@@ -1324,6 +1326,7 @@ mod tests {
                 build: crate::config::Command::from_line("true"),
                 cwd: Some(PathBuf::from("server")),
                 stale_when: None,
+                also_watch: Vec::new(),
                 service: None,
                 built_from: None,
             },
@@ -1332,6 +1335,7 @@ mod tests {
                 build: crate::config::Command::from_line("true"),
                 cwd: Some(PathBuf::from("app")),
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -1388,6 +1392,7 @@ mod tests {
             build: crate::config::Command::from_line("touch built-marker"),
             cwd: None,
             stale_when: None,
+            also_watch: Vec::new(),
             strip: false,
             package: None,
             built_from: None,
@@ -1441,6 +1446,7 @@ mod tests {
                 build: crate::config::Command::from_line("touch backend-built"),
                 cwd: Some(PathBuf::from("server")),
                 stale_when: None,
+                also_watch: Vec::new(),
                 service: None,
                 built_from: None,
             },
@@ -1449,6 +1455,7 @@ mod tests {
                 build: crate::config::Command::from_line("touch app-built"),
                 cwd: Some(PathBuf::from("app")),
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -1522,6 +1529,7 @@ mod tests {
                 }),
                 cwd: Some(PathBuf::from(name)),
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -1592,6 +1600,7 @@ mod tests {
                 }),
                 cwd: Some(PathBuf::from(name)),
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -1661,6 +1670,7 @@ mod tests {
                 build: crate::config::Command::from_line("touch a-built"),
                 cwd: Some(PathBuf::from("a")),
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -1670,6 +1680,7 @@ mod tests {
                 build: crate::config::Command::from_line("touch b-built"),
                 cwd: Some(PathBuf::from("b")),
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -1767,7 +1778,7 @@ mod tests {
         // Built now, from what is checked out now.
         for name in ["backend", "app"] {
             let cwd = PathBuf::from(if name == "backend" { "server" } else { "app" });
-            let sha = crate::git::subtree_head(root, Some(&cwd)).expect("a commit");
+            let sha = crate::git::subtree_head(root, std::slice::from_ref(&cwd)).expect("a commit");
             state
                 .inner
                 .lock()
@@ -1789,6 +1800,65 @@ mod tests {
         );
     }
 
+    /// The other half of that scoping: a component's build reads more
+    /// than its own directory, and what it names with `alsoWatch` counts
+    /// as its own.
+    ///
+    /// The case this is for had every piece defensible and the whole
+    /// silent: two clients sourcing one shared script that locates their
+    /// SDK and mints their signing key, a fix committed to that script,
+    /// and no component's subtree head moved -- so nothing rebuilt, and
+    /// every card correctly reported "current" while answering a
+    /// narrower question than the reader was asking. The component that
+    /// says nothing must still be left alone, which is the second
+    /// assertion here.
+    #[test]
+    fn a_commit_to_a_watched_path_makes_only_the_components_that_watch_it_stale() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let root = dir.path();
+        let mut components = two_component_checkout(root);
+        let Component::Apk { also_watch, .. } = &mut components[1] else {
+            panic!("the app is the second component");
+        };
+        *also_watch = vec![PathBuf::from("scripts")];
+        let state = state_for(root, components);
+
+        std::fs::create_dir_all(root.join("scripts")).expect("mkdir");
+        std::fs::write(root.join("scripts/env.sh"), "one").expect("write");
+        run(root, &["git", "add", "-A"]);
+        run(root, &["git", "commit", "-qm", "scripts"]);
+
+        for name in ["backend", "app"] {
+            let component = state
+                .components
+                .iter()
+                .find(|component| component.name() == name)
+                .expect("component");
+            let sha = crate::git::subtree_head(root, &component.watched_paths()).expect("a commit");
+            state
+                .inner
+                .lock()
+                .unwrap()
+                .built_from
+                .insert(name.to_string(), sha);
+        }
+        assert!(!stale(&state, "backend"), "just built");
+        assert!(!stale(&state, "app"), "just built");
+
+        // A change to neither component's own directory.
+        std::fs::write(root.join("scripts/env.sh"), "two").expect("write");
+        run(root, &["git", "commit", "-qam", "shared script"]);
+
+        assert!(
+            stale(&state, "app"),
+            "the app watches scripts/, so a commit there is a commit to it",
+        );
+        assert!(
+            !stale(&state, "backend"),
+            "the server said nothing about scripts/, so nothing about it changed",
+        );
+    }
+
     /// Nothing recorded is "we have never built this", not "this is out of
     /// date" -- otherwise every project that existed before this feature
     /// would demand a rebuild the moment it arrived.
@@ -1822,6 +1892,7 @@ mod tests {
                 build: crate::config::Command::from_line("true"),
                 cwd: None,
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: Some("0000000000000000000000000000000000000000".to_string()),

server/src/config.rs

@@ -237,6 +237,8 @@ pub enum Component {
         cwd: Option<PathBuf>,
         #[serde(default, skip_serializing_if = "Option::is_none")]
         stale_when: Option<StaleRule>,
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        also_watch: Vec<PathBuf>,
         /// Serve a debug-symbol-stripped copy (see `crate::strip`).
         ///
         /// Declared rather than detected: whether the symbols are worth the
@@ -279,6 +281,8 @@ pub enum Component {
         cwd: Option<PathBuf>,
         #[serde(default, skip_serializing_if = "Option::is_none")]
         stale_when: Option<StaleRule>,
+        #[serde(default, skip_serializing_if = "Vec::is_empty")]
+        also_watch: Vec<PathBuf>,
         /// How this server's service is driven: with the project's own
         /// script, or by dev-updater's built-in one. See [`Service`].
         #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -345,6 +349,38 @@ impl Component {
         }
     }
 
+    /// Every path in the checkout whose commits count as this
+    /// component's: its own directory, plus anything it declared with
+    /// `alsoWatch`. Relative to the project root, which is what git wants
+    /// as a pathspec.
+    ///
+    /// The list exists because scoping a component to its `cwd` alone is
+    /// right about where its *files* are and wrong about what its build
+    /// *reads*. A project whose clients each source one shared script --
+    /// which locates their SDK, their JDK, their signing key -- has a file
+    /// that changes what every build does and moves no component's
+    /// subtree head, so a commit to it left every card correctly
+    /// reporting "current" while answering a narrower question than the
+    /// reader was asking. Nothing rebuilt, and nothing said why.
+    ///
+    /// Declared rather than inferred: which files a build actually reads
+    /// is not something this server can know, and a guess here is the
+    /// expensive kind -- too wide rebuilds everything on every commit, too
+    /// narrow is the silence above.
+    pub fn watched_paths(&self) -> Vec<PathBuf> {
+        let also_watch = match self {
+            Self::Apk { also_watch, .. } | Self::Server { also_watch, .. } => also_watch,
+        };
+        // The component's own directory first, so a caller reading the
+        // list sees the same thing `dir` would answer. `.` for a
+        // component at the root, which is the pathspec for the whole
+        // checkout and what a component declaring no `cwd` has always
+        // meant.
+        std::iter::once(self.cwd().unwrap_or(Path::new(".")).to_path_buf())
+            .chain(also_watch.iter().cloned())
+            .collect()
+    }
+
     /// Whether these two say the same thing about what to *do*, ignoring
     /// what this server measured for itself.
     ///
@@ -365,6 +401,7 @@ impl Component {
                     build,
                     cwd,
                     stale_when,
+                    also_watch,
                     strip,
                     package: _,
                     built_from: _,
@@ -374,6 +411,7 @@ impl Component {
                     build: other_build,
                     cwd: other_cwd,
                     stale_when: other_stale_when,
+                    also_watch: other_also_watch,
                     strip: other_strip,
                     package: _,
                     built_from: _,
@@ -383,6 +421,7 @@ impl Component {
                     && build == other_build
                     && cwd == other_cwd
                     && stale_when == other_stale_when
+                    && also_watch == other_also_watch
                     && strip == other_strip
             }
             (
@@ -391,6 +430,7 @@ impl Component {
                     build,
                     cwd,
                     stale_when,
+                    also_watch,
                     service,
                     built_from: _,
                 },
@@ -399,6 +439,7 @@ impl Component {
                     build: other_build,
                     cwd: other_cwd,
                     stale_when: other_stale_when,
+                    also_watch: other_also_watch,
                     service: other_service,
                     built_from: _,
                 },
@@ -407,6 +448,7 @@ impl Component {
                     && build == other_build
                     && cwd == other_cwd
                     && stale_when == other_stale_when
+                    && also_watch == other_also_watch
                     && service == other_service
             }
             _ => false,

server/src/git.rs

@@ -79,15 +79,35 @@ pub fn status(project: &Path) -> Option<GitStatus> {
 /// submodule case correctly and without a special case: a submodule whose
 /// working tree has moved shows as a modified path, so whichever component
 /// contains it goes unknown and the others do not.
-pub fn subtree_dirty(project: &Path, within: Option<&Path>) -> Option<bool> {
-    let mut args = vec!["status", "--porcelain", "--"];
-    let within = within.map(|path| path.to_string_lossy().into_owned());
-    args.push(within.as_deref().unwrap_or("."));
+pub fn subtree_dirty(project: &Path, within: &[PathBuf]) -> Option<bool> {
+    let mut args = vec![
+        "status".to_string(),
+        "--porcelain".to_string(),
+        "--".to_string(),
+    ];
+    args.extend(pathspecs(within));
+    let args: Vec<&str> = args.iter().map(String::as_str).collect();
     git(project, &args).ok().map(|out| !out.trim().is_empty())
 }
 
-/// The newest commit touching `within` (a path relative to `project`), or
-/// the whole checkout when `within` is `None`.
+/// The paths to hand git, which is every one it was given or `.` for a
+/// caller that named none. One function because the empty case has to
+/// mean the same thing to both of the reads above and below -- an empty
+/// pathspec list is git's "everything", and getting that wrong in one of
+/// them would make a component's dirtiness and its commit disagree about
+/// which files they are talking about.
+fn pathspecs(within: &[PathBuf]) -> Vec<String> {
+    if within.is_empty() {
+        return vec![".".to_string()];
+    }
+    within
+        .iter()
+        .map(|path| path.to_string_lossy().into_owned())
+        .collect()
+}
+
+/// The newest commit touching any of `within` (paths relative to
+/// `project`), or the whole checkout when the list is empty.
 ///
 /// This is what a build is recorded against, and the scoping is the point.
 /// One checkout routinely produces several things -- a backend under
@@ -97,15 +117,23 @@ pub fn subtree_dirty(project: &Path, within: Option<&Path>) -> Option<bool> {
 /// touched the component's own directory makes "out of date" mean what a
 /// person reading the card expects it to.
 ///
+/// Several paths rather than one because a component's build reads more
+/// than its own directory: `alsoWatch` names the shared script both of a
+/// project's clients source, and a commit to it has to count as a commit
+/// to each of them. Git takes the pathspecs directly, so "the newest
+/// commit touching any of these" costs the same one call.
+///
 /// `None` when the checkout cannot be read, which is "we cannot tell"
 /// rather than an answer; see `BuildState::freshness`.
-pub fn subtree_head(project: &Path, within: Option<&Path>) -> Option<String> {
-    let mut args = vec!["log", "-1", "--format=%H"];
-    let within = within.map(|path| path.to_string_lossy().into_owned());
-    if let Some(path) = within.as_deref() {
-        args.push("--");
-        args.push(path);
-    }
+pub fn subtree_head(project: &Path, within: &[PathBuf]) -> Option<String> {
+    let mut args = vec![
+        "log".to_string(),
+        "-1".to_string(),
+        "--format=%H".to_string(),
+        "--".to_string(),
+    ];
+    args.extend(pathspecs(within));
+    let args: Vec<&str> = args.iter().map(String::as_str).collect();
     let sha = git(project, &args).ok()?;
     (!sha.is_empty()).then_some(sha)
 }

server/src/registry.rs

@@ -574,6 +574,7 @@ impl AppState {
                     build: crate::config::Command::default(),
                     cwd: None,
                     stale_when: None,
+                    also_watch: Vec::new(),
                     strip: false,
                     package,
                     built_from: None,
@@ -744,6 +745,7 @@ fn reconcile_self(config: &mut Config, self_project: &Path) -> bool {
                 build: crate::config::Command::default(),
                 cwd: None,
                 stale_when: None,
+                also_watch: Vec::new(),
                 strip: false,
                 package: None,
                 built_from: None,
@@ -914,6 +916,7 @@ mod tests {
             build: crate::config::Command::from_line(command),
             cwd: None,
             stale_when: None,
+            also_watch: Vec::new(),
             strip: false,
             package: None,
             built_from: None,
@@ -994,6 +997,7 @@ mod tests {
             build: crate::config::Command::default(),
             cwd: cwd.map(PathBuf::from),
             stale_when: None,
+            also_watch: Vec::new(),
             strip: false,
             package: None,
             built_from: None,

server/src/service.rs

@@ -398,6 +398,7 @@ mod tests {
             build: Command::default(),
             cwd: None,
             stale_when: None,
+            also_watch: Vec::new(),
             service,
             built_from: None,
         }