Add validation inputs

These were added to regular ninja in:
https://github.com/ninja-build/ninja/commit/04c410b15b70fb321928ffba19d697db15cb0121

Android uses them, so they're needed to port android to n2. (which is
something I'm exploring but not committing to)

6 files changed, 146 insertions, 7 deletions

diff --git a/src/graph.rs b/src/graph.rs
index f3577c1..245bd72 100644
--- a/src/graph.rs
+++ b/src/graph.rs
@@ -80,8 +80,9 @@ pub struct BuildIns {
     pub ids: Vec<FileId>,
     pub explicit: usize,
     pub implicit: usize,
-    // order_only count implied by other counts.
-    // pub order_only: usize,
+    pub order_only: usize,
+    // validation count implied by other counts.
+    // pub validation: usize,
 }
 
 /// Output files from a Build.
@@ -204,7 +205,15 @@ impl Build {
     /// Note that we don't order on discovered_ins, because they're not allowed to
     /// affect build order.
     pub fn ordering_ins(&self) -> &[FileId] {
-        &self.ins.ids
+        &self.ins.ids[0..(self.ins.order_only + self.ins.explicit + self.ins.implicit)]
+    }
+
+    /// Inputs that are needed before validating information.
+    /// Validation inputs will be built whenever this Build is built, but this Build will not
+    /// wait for them to complete before running. The validation inputs can fail to build, which
+    /// will cause the overall build to fail.
+    pub fn validation_ins(&self) -> &[FileId] {
+        &self.ins.ids[(self.ins.order_only + self.ins.explicit + self.ins.implicit)..]
     }
 
     /// Potentially update discovered_ins with a new set of deps, returning true if they changed.
diff --git a/src/load.rs b/src/load.rs
index f583309..7029d4c 100644
--- a/src/load.rs
+++ b/src/load.rs
@@ -82,7 +82,8 @@ impl Loader {
             ids: b.ins,
             explicit: b.explicit_ins,
             implicit: b.implicit_ins,
-            // order_only is unused
+            order_only: b.order_only_ins,
+            // validation is implied by the other counts
         };
         let outs = graph::BuildOuts {
             ids: b.outs,
diff --git a/src/parse.rs b/src/parse.rs
index 382f6f0..22ead3c 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -30,6 +30,7 @@ pub struct Build<'text, Path> {
     pub explicit_ins: usize,
     pub implicit_ins: usize,
     pub order_only_ins: usize,
+    pub validation_ins: usize,
     pub vars: VarList<'text>,
 }
 
@@ -215,7 +216,8 @@ impl<'text> Parser<'text> {
 
         if self.scanner.peek() == '|' {
             self.scanner.next();
-            if self.scanner.peek() == '|' {
+            let peek = self.scanner.peek();
+            if peek == '|' || peek == '@' {
                 self.scanner.back();
             } else {
                 self.read_paths_to(loader, &mut ins)?;
@@ -225,11 +227,22 @@ impl<'text> Parser<'text> {
 
         if self.scanner.peek() == '|' {
             self.scanner.next();
-            self.scanner.expect('|')?;
-            self.read_paths_to(loader, &mut ins)?;
+            if self.scanner.peek() == '@' {
+                self.scanner.back();
+            } else {
+                self.scanner.expect('|')?;
+                self.read_paths_to(loader, &mut ins)?;
+            }
         }
         let order_only_ins = ins.len() - implicit_ins - explicit_ins;
 
+        if self.scanner.peek() == '|' {
+            self.scanner.next();
+            self.scanner.expect('@')?;
+            self.read_paths_to(loader, &mut ins)?;
+        }
+        let validation_ins = ins.len() - order_only_ins - implicit_ins - explicit_ins;
+
         self.scanner.skip('\r');
         self.scanner.expect('\n')?;
         let vars = self.read_scoped_vars()?;
@@ -242,6 +255,7 @@ impl<'text> Parser<'text> {
             explicit_ins,
             implicit_ins,
             order_only_ins,
+            validation_ins,
             vars,
         })
     }
diff --git a/src/work.rs b/src/work.rs
index c8d6ca0..afdf2c2 100644
--- a/src/work.rs
+++ b/src/work.rs
@@ -216,6 +216,13 @@ impl BuildStates {
             self.want_file(graph, stack, id)?;
             ready = ready && graph.file(id).input.is_none();
         }
+        for &id in build.validation_ins() {
+            // This build doesn't technically depend on the validation inputs, so
+            // allocate a new stack. Validation inputs could in theory depend on this build's
+            // outputs.
+            let mut stack = Vec::new();
+            self.want_file(graph, &mut stack, id)?;
+        }
 
         if ready {
             self.set(id, build, BuildState::Ready);
diff --git a/tests/e2e/mod.rs b/tests/e2e/mod.rs
index b28e45d..a4cea68 100644
--- a/tests/e2e/mod.rs
+++ b/tests/e2e/mod.rs
@@ -4,6 +4,7 @@ mod basic;
 mod discovered;
 mod missing;
 mod regen;
+mod validations;
 
 pub fn n2_binary() -> std::path::PathBuf {
     std::env::current_exe()
@@ -72,6 +73,10 @@ impl TestSpace {
         std::fs::metadata(self.dir.path().join(path))
     }
 
+    pub fn path(&self) -> &std::path::Path {
+        self.dir.path()
+    }
+
     /// Invoke n2, returning process output.
     pub fn run(&self, cmd: &mut std::process::Command) -> std::io::Result<std::process::Output> {
         cmd.current_dir(self.dir.path()).output()
diff --git a/tests/e2e/validations.rs b/tests/e2e/validations.rs
new file mode 100644
index 0000000..1b1eb98
--- /dev/null
+++ b/tests/e2e/validations.rs
@@ -0,0 +1,103 @@
+use crate::e2e::*;
+
+#[test]
+fn basic_validation() -> anyhow::Result<()> {
+    let space = TestSpace::new()?;
+    space.write(
+        "build.ninja",
+        &[
+            TOUCH_RULE,
+            "build my_validation: touch",
+            "build out: touch |@ my_validation",
+            "",
+        ]
+        .join("\n"),
+    )?;
+    space.run_expect(&mut n2_command(vec!["out"]))?;
+    assert!(space.read("out").is_ok());
+    assert!(space.read("my_validation").is_ok());
+    Ok(())
+}
+
+#[cfg(unix)]
+#[test]
+fn build_starts_before_validation_finishes() -> anyhow::Result<()> {
+    let space = TestSpace::new()?;
+    space.write(
+        "build.ninja",
+        "
+rule build_slow
+  command = sleep 0.3 && touch $out
+
+rule build_fast
+  command = sleep 0.1 && touch $out
+
+build out: build_fast regular_input |@ validation_input
+build regular_input: build_fast
+build validation_input: build_slow
+",
+    )?;
+    let command = n2_command(vec!["out"])
+        .current_dir(space.path())
+        .stdout(std::process::Stdio::null())
+        .stderr(std::process::Stdio::null())
+        .stdin(std::process::Stdio::null())
+        .spawn()?;
+    std::thread::sleep(std::time::Duration::from_millis(50));
+    assert!(space.read("out").is_err());
+    assert!(space.read("regular_input").is_err());
+    assert!(space.read("validation_input").is_err());
+    std::thread::sleep(std::time::Duration::from_millis(100));
+    assert!(space.read("out").is_err());
+    assert!(space.read("regular_input").is_ok());
+    assert!(space.read("validation_input").is_err());
+    std::thread::sleep(std::time::Duration::from_millis(100));
+    assert!(space.read("out").is_ok());
+    assert!(space.read("regular_input").is_ok());
+    assert!(space.read("validation_input").is_err());
+    std::thread::sleep(std::time::Duration::from_millis(100));
+    assert!(space.read("out").is_ok());
+    assert!(space.read("regular_input").is_ok());
+    assert!(space.read("validation_input").is_ok());
+    assert!(command.wait_with_output()?.status.success());
+    Ok(())
+}
+
+#[cfg(unix)]
+#[test]
+fn build_fails_when_validation_fails() -> anyhow::Result<()> {
+    let space = TestSpace::new()?;
+    space.write(
+        "build.ninja",
+        "
+rule touch
+  command = touch $out
+
+rule fail
+  command = exit 1
+
+build out: touch |@ validation_input
+build validation_input: fail
+",
+    )?;
+    let output = space.run(&mut n2_command(vec!["out"]))?;
+    assert!(!output.status.success());
+    Ok(())
+}
+
+#[test]
+fn validation_inputs_break_cycles() -> anyhow::Result<()> {
+    let space = TestSpace::new()?;
+    space.write(
+        "build.ninja",
+        &[
+            TOUCH_RULE,
+            "build out: touch |@ validation_input",
+            "build validation_input: touch out",
+            "",
+        ]
+        .join("\n"),
+    )?;
+    space.run_expect(&mut n2_command(vec!["out"]))?;
+    Ok(())
+}