2 files changed, 213 insertions, 0 deletions
diff --git a/src/tools/expand-yaml-anchors/Cargo.toml b/src/tools/expand-yaml-anchors/Cargo.toml
new file mode 100644
index 000000000..9a25b6c1f
--- /dev/null
+++ b/src/tools/expand-yaml-anchors/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "expand-yaml-anchors"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+yaml-rust = "0.4.3"
+yaml-merge-keys = "0.4.0"
diff --git a/src/tools/expand-yaml-anchors/src/main.rs b/src/tools/expand-yaml-anchors/src/main.rs
new file mode 100644
index 000000000..8992d165d
--- /dev/null
+++ b/src/tools/expand-yaml-anchors/src/main.rs
@@ -0,0 +1,205 @@
+use std::error::Error;
+use std::path::{Path, PathBuf};
+use yaml_rust::{Yaml, YamlEmitter, YamlLoader};
+
+/// List of directories containing files to expand. The first tuple element is the source
+/// directory, while the second tuple element is the destination directory.
+#[rustfmt::skip]
+static TO_EXPAND: &[(&str, &str)] = &[
+    ("src/ci/github-actions", ".github/workflows"),
+];
+
+/// Name of a special key that will be removed from all the maps in expanded configuration files.
+/// This key can then be used to contain shared anchors.
+static REMOVE_MAP_KEY: &str = "x--expand-yaml-anchors--remove";
+
+/// Message that will be included at the top of all the expanded files. {source} will be replaced
+/// with the source filename relative to the base path.
+static HEADER_MESSAGE: &str = "\
+#############################################################
+#   WARNING: automatically generated file, DO NOT CHANGE!   #
+#############################################################
+
+# This file was automatically generated by the expand-yaml-anchors tool. The
+# source file that generated this one is:
+#
+#   {source}
+#
+# Once you make changes to that file you need to run:
+#
+#   ./x.py run src/tools/expand-yaml-anchors/
+#
+# The CI build will fail if the tool is not run after changes to this file.
+
+";
+
+enum Mode {
+    Check,
+    Generate,
+}
+
+struct App {
+    mode: Mode,
+    base: PathBuf,
+}
+
+impl App {
+    fn from_args() -> Result<Self, Box<dyn Error>> {
+        // Parse CLI arguments
+        let args = std::env::args().skip(1).collect::<Vec<_>>();
+        let (mode, base) = match args.iter().map(|s| s.as_str()).collect::<Vec<_>>().as_slice() {
+            ["generate", ref base] => (Mode::Generate, PathBuf::from(base)),
+            ["check", ref base] => (Mode::Check, PathBuf::from(base)),
+            _ => {
+                eprintln!("usage: expand-yaml-anchors <source-dir> <dest-dir>");
+                std::process::exit(1);
+            }
+        };
+
+        Ok(App { mode, base })
+    }
+
+    fn run(&self) -> Result<(), Box<dyn Error>> {
+        for (source, dest) in TO_EXPAND {
+            let source = self.base.join(source);
+            let dest = self.base.join(dest);
+            for entry in std::fs::read_dir(&source)? {
+                let path = entry?.path();
+                if !path.is_file() || path.extension().and_then(|e| e.to_str()) != Some("yml") {
+                    continue;
+                }
+
+                let dest_path = dest.join(path.file_name().unwrap());
+                self.expand(&path, &dest_path).with_context(|| match self.mode {
+                    Mode::Generate => format!(
+                        "failed to expand {} into {}",
+                        self.path(&path),
+                        self.path(&dest_path)
+                    ),
+                    Mode::Check => format!(
+                        "{} is not up to date; please run \
+                        `x.py run src/tools/expand-yaml-anchors`.",
+                        self.path(&dest_path)
+                    ),
+                })?;
+            }
+        }
+        Ok(())
+    }
+
+    fn expand(&self, source: &Path, dest: &Path) -> Result<(), Box<dyn Error>> {
+        let content = std::fs::read_to_string(source)
+            .with_context(|| format!("failed to read {}", self.path(source)))?;
+
+        let mut buf =
+            HEADER_MESSAGE.replace("{source}", &self.path(source).to_string().replace("\\", "/"));
+
+        let documents = YamlLoader::load_from_str(&content)
+            .with_context(|| format!("failed to parse {}", self.path(source)))?;
+        for mut document in documents.into_iter() {
+            document = yaml_merge_keys::merge_keys(document)
+                .with_context(|| format!("failed to expand {}", self.path(source)))?;
+            document = filter_document(document);
+
+            YamlEmitter::new(&mut buf).dump(&document).map_err(|err| WithContext {
+                context: "failed to serialize the expanded yaml".into(),
+                source: Box::new(err),
+            })?;
+            buf.push('\n');
+        }
+
+        match self.mode {
+            Mode::Check => {
+                let old = std::fs::read_to_string(dest)
+                    .with_context(|| format!("failed to read {}", self.path(dest)))?;
+                if old != buf {
+                    return Err(Box::new(StrError(format!(
+                        "{} and {} are different",
+                        self.path(source),
+                        self.path(dest),
+                    ))));
+                }
+            }
+            Mode::Generate => {
+                std::fs::write(dest, buf.as_bytes())
+                    .with_context(|| format!("failed to write to {}", self.path(dest)))?;
+            }
+        }
+        Ok(())
+    }
+
+    fn path<'a>(&self, path: &'a Path) -> impl std::fmt::Display + 'a {
+        path.strip_prefix(&self.base).unwrap_or(path).display()
+    }
+}
+
+fn filter_document(document: Yaml) -> Yaml {
+    match document {
+        Yaml::Hash(map) => Yaml::Hash(
+            map.into_iter()
+                .filter(|(key, _)| {
+                    if let Yaml::String(string) = &key { string != REMOVE_MAP_KEY } else { true }
+                })
+                .map(|(key, value)| (filter_document(key), filter_document(value)))
+                .collect(),
+        ),
+        Yaml::Array(vec) => Yaml::Array(vec.into_iter().map(filter_document).collect()),
+        other => other,
+    }
+}
+
+fn main() {
+    if let Err(err) = App::from_args().and_then(|app| app.run()) {
+        eprintln!("error: {}", err);
+
+        let mut source = err.as_ref() as &dyn Error;
+        while let Some(err) = source.source() {
+            eprintln!("caused by: {}", err);
+            source = err;
+        }
+
+        std::process::exit(1);
+    }
+}
+
+#[derive(Debug)]
+struct StrError(String);
+
+impl Error for StrError {}
+
+impl std::fmt::Display for StrError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(&self.0, f)
+    }
+}
+
+#[derive(Debug)]
+struct WithContext {
+    context: String,
+    source: Box<dyn Error>,
+}
+
+impl std::fmt::Display for WithContext {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.context)
+    }
+}
+
+impl Error for WithContext {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        Some(self.source.as_ref())
+    }
+}
+
+pub(crate) trait ResultExt<T> {
+    fn with_context<F: FnOnce() -> String>(self, f: F) -> Result<T, Box<dyn Error>>;
+}
+
+impl<T, E: Into<Box<dyn Error>>> ResultExt<T> for Result<T, E> {
+    fn with_context<F: FnOnce() -> String>(self, f: F) -> Result<T, Box<dyn Error>> {
+        match self {
+            Ok(ok) => Ok(ok),
+            Err(err) => Err(WithContext { source: err.into(), context: f() }.into()),
+        }
+    }
+}