Fix parallel package installation (#5979)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

Cargo.lock

@@ -2896,6 +2896,7 @@ dependencies = [
  "dirs",
  "ecow",
  "env_proxy",
+ "fastrand",
  "flate2",
  "fontdb",
  "native-tls",

Cargo.toml

@@ -55,6 +55,7 @@ ctrlc = "3.4.1"
 dirs = "6"
 ecow = { version = "0.2", features = ["serde"] }
 env_proxy = "0.4"
+fastrand = "2.3"
 flate2 = "1"
 fontdb = { version = "0.23", default-features = false }
 fs_extra = "1.3"

crates/typst-kit/Cargo.toml

@@ -19,6 +19,7 @@ typst-utils = { workspace = true }
 dirs = { workspace = true, optional = true }
 ecow = { workspace = true }
 env_proxy = { workspace = true, optional = true }
+fastrand = { workspace = true, optional = true }
 flate2 = { workspace = true, optional = true }
 fontdb = { workspace = true, optional = true }
 native-tls = { workspace = true, optional = true }
@@ -43,7 +44,7 @@ fonts = ["dep:fontdb", "fontdb/memmap", "fontdb/fontconfig"]
 downloads = ["dep:env_proxy", "dep:native-tls", "dep:ureq", "dep:openssl"]
 
 # Add package downloading utilities, implies `downloads`
-packages = ["downloads", "dep:dirs", "dep:flate2", "dep:tar"]
+packages = ["downloads", "dep:dirs", "dep:flate2", "dep:tar", "dep:fastrand"]
 
 # Embeds some fonts into the binary:
 # - For text: Libertinus Serif, New Computer Modern

crates/typst-kit/src/package.rs

@@ -1,6 +1,7 @@
 //! Download and unpack packages and package indices.
 
 use std::fs;
+use std::io;
 use std::path::{Path, PathBuf};
 
 use ecow::eco_format;
@@ -100,7 +101,7 @@ impl PackageStorage {
 
             // Download from network if it doesn't exist yet.
             if spec.namespace == DEFAULT_NAMESPACE {
-                self.download_package(spec, &dir, progress)?;
+                self.download_package(spec, cache_dir, progress)?;
                 if dir.exists() {
                     return Ok(dir);
                 }
@@ -167,7 +168,7 @@ impl PackageStorage {
     pub fn download_package(
         &self,
         spec: &PackageSpec,
-        package_dir: &Path,
+        cache_dir: &Path,
         progress: &mut dyn Progress,
     ) -> PackageResult<()> {
         assert_eq!(spec.namespace, DEFAULT_NAMESPACE);
@@ -191,11 +192,52 @@ impl PackageStorage {
             }
         };
 
+        // The directory in which the package's version lives.
+        let base_dir = cache_dir.join(format!("{}/{}", spec.namespace, spec.name));
+
+        // The place at which the specific package version will live in the end.
+        let package_dir = base_dir.join(format!("{}", spec.version));
+
+        // To prevent multiple Typst instances from interferring, we download
+        // into a temporary directory first and then move this directory to
+        // its final destination.
+        //
+        // In the `rename` function's documentation it is stated:
+        // > This will not work if the new name is on a different mount point.
+        //
+        // By locating the temporary directory directly next to where the
+        // package directory will live, we are (trying our best) making sure
+        // that `tempdir` and `package_dir` are on the same mount point.
+        let tempdir = Tempdir::create(base_dir.join(format!(
+            ".tmp-{}-{}",
+            spec.version,
+            fastrand::u32(..),
+        )))
+        .map_err(|err| error("failed to create temporary package directory", err))?;
+
+        // Decompress the archive into the temporary directory.
         let decompressed = flate2::read::GzDecoder::new(data.as_slice());
-        tar::Archive::new(decompressed).unpack(package_dir).map_err(|err| {
-            fs::remove_dir_all(package_dir).ok();
-            PackageError::MalformedArchive(Some(eco_format!("{err}")))
-        })
+        tar::Archive::new(decompressed)
+            .unpack(&tempdir)
+            .map_err(|err| PackageError::MalformedArchive(Some(eco_format!("{err}"))))?;
+
+        // When trying to move (i.e., `rename`) the directory from one place to
+        // another and the target/destination directory is empty, then the
+        // operation will succeed (if it's atomic, or hardware doesn't fail, or
+        // power doesn't go off, etc.). If however the target directory is not
+        // empty, i.e., another instance already successfully moved the package,
+        // then we can safely ignore the `DirectoryNotEmpty` error.
+        //
+        // This means that we do not check the integrity of an existing moved
+        // package, just like we don't check the integrity if the package
+        // directory already existed in the first place. If situations with
+        // broken packages still occur even with the rename safeguard, we might
+        // consider more complex solutions like file locking or checksums.
+        match fs::rename(&tempdir, &package_dir) {
+            Ok(()) => Ok(()),
+            Err(err) if err.kind() == io::ErrorKind::DirectoryNotEmpty => Ok(()),
+            Err(err) => Err(error("failed to move downloaded package directory", err)),
+        }
     }
 }
 
@@ -207,6 +249,36 @@ struct MinimalPackageInfo {
     version: PackageVersion,
 }
 
+/// A temporary directory that is a automatically cleaned up.
+struct Tempdir(PathBuf);
+
+impl Tempdir {
+    /// Creates a directory at the path and auto-cleans it.
+    fn create(path: PathBuf) -> io::Result<Self> {
+        std::fs::create_dir_all(&path)?;
+        Ok(Self(path))
+    }
+}
+
+impl Drop for Tempdir {
+    fn drop(&mut self) {
+        _ = fs::remove_dir_all(&self.0);
+    }
+}
+
+impl AsRef<Path> for Tempdir {
+    fn as_ref(&self) -> &Path {
+        &self.0
+    }
+}
+
+/// Enriches an I/O error with a message and turns it into a
+/// `PackageError::Other`.
+#[cold]
+fn error(message: &str, err: io::Error) -> PackageError {
+    PackageError::Other(Some(eco_format!("{message}: {err}")))
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;