Repository information in version

build.rs

@@ -0,0 +1,108 @@
+use std::io::Write;
+use std::path::{absolute, Path, PathBuf};
+
+/// Fetch the contents of the given file, and also tell cargo that we looked
+/// in there.
+fn file_contents<P: AsRef<Path>>(path: P) -> String {
+    let path =
+        absolute(path.as_ref()).expect("Unable to make the path absolute");
+
+    let mut stdout = std::io::stdout();
+    stdout
+        .write_all(b"cargo::rerun-if-changed=")
+        .expect("Unable to write stdout");
+    stdout
+        .write_all(path.as_os_str().as_encoded_bytes())
+        .expect("Unable to write path to stdout");
+    stdout
+        .write_all(b"\n")
+        .expect("Unable to write newline to stdout");
+
+    std::fs::read_to_string(path).expect("Unable to read file")
+}
+
+/// Emit the current git commit.
+fn emit_git_commit() {
+    // Fetch the current commit from the head. We do it this way instead of
+    // asking `git rev-parse` to do it for us because we want to reliably
+    // tell cargo which files it should monitor for changes.
+    let head = file_contents("./.git/HEAD");
+    let rev = if let Some(r) = head.strip_prefix("ref: ") {
+        let mut ref_path = PathBuf::from("./.git/");
+        ref_path.push(r.trim());
+        file_contents(ref_path)
+    } else {
+        head
+    };
+
+    // But *now* we ask git rev-parse to make this into a short hash (a) to
+    // make sure we got it right and (b) because git knows how to quickly
+    // determine how much of a commit is required to be unique. We don't need
+    // to tell cargo anything here, no file that git consults will be
+    // mutable.
+    let output = std::process::Command::new("git")
+        .arg("rev-parse")
+        .arg("--short")
+        .arg(rev.trim())
+        .output()
+        .expect("could not spawn `git` to get the hash");
+    if !output.status.success() {
+        let stderr = std::str::from_utf8(&output.stderr)
+            .expect("git failed and stderr was not utf8");
+        eprintln!("`git rev-parse --short HEAD` failed, stderr: {stderr}");
+        panic!("`git rev-parse --short HEAD` failed");
+    }
+    let rev =
+        std::str::from_utf8(&output.stdout).expect("git did not output utf8");
+
+    println!("cargo::rustc-env=REPO_REV={rev}");
+}
+
+fn emit_git_dirty() {
+    // Here is the way to see if anything is up with the repository: run `git
+    // status --porcelain=v1`. The status output in the v1 porcelain format
+    // has one line for every file that's modified in some way: staged,
+    // changed but unstaged, untracked, you name it. Files in the working
+    // tree that are up to date with the repository are not emitted. This is
+    // exactly what we want.
+    //
+    // (Yes, I want to track untracked files, because they can mess with the
+    // build too. The only good build is a clean build!)
+    let output = std::process::Command::new("git")
+        .arg("status")
+        .arg("-z")
+        .arg("--porcelain=v1")
+        .output()
+        .expect("could not spawn `git` to get repository status");
+    if !output.status.success() {
+        let stderr = std::str::from_utf8(&output.stderr)
+            .expect("git failed and stderr was not utf8");
+        eprintln!("`git status` failed, stderr: {stderr}");
+        panic!("`git status` failed");
+    }
+    let output =
+        std::str::from_utf8(&output.stdout).expect("git did not output utf8");
+
+    // If there *was* any output, parse it and tell cargo to re-run if any of
+    // these files changed. (Maybe they get reverted! Then the repo status
+    // will change.)
+    for line in output.lines() {
+        let fields: Vec<_> = line.split('\x00').collect();
+        let parts: Vec<_> = fields[0].split(' ').collect();
+        let path = parts[1];
+        println!("cargo::rerun-if-changed={path}");
+    }
+
+    // Emit the repository status.
+    let dirty = if output.trim().is_empty() {
+        ""
+    } else {
+        " *dirty*"
+    };
+    println!("cargo::rustc-env=REPO_DIRTY={dirty}");
+}
+
+fn main() {
+    emit_git_commit();
+    emit_git_dirty();
+}