@@ -1,5 +1,6 @@
 # Changelog
 
+- **Fixed** Tasks that run at the same time and hit the same cache entry no longer intermittently fail with `File exists (os error 17)` while restoring cached outputs ([#846](https://github.com/voidzero-dev/vite-task/issues/846), [#???](https://github.com/voidzero-dev/vite-task/pull/???)).
 - **Fixed** A mistyped or misplaced setting in a task now gets an error that names it. For example, `cache: { outputs: [...] }` now fails with an `unknown field` error that names `outputs` and lists the accepted settings, instead of `data did not match any variant of untagged enum UserTaskDefinition` ([vite-plus#2635](https://github.com/voidzero-dev/vite-plus/issues/2635), [#815](https://github.com/voidzero-dev/vite-task/pull/815)).
 - **Fixed** In cached tasks on Linux and macOS, programs that pass `-1` as a directory descriptor to calls such as `openat` no longer abort. For example, `ps -p <pid>` from procps-ng 4.0.6 or later aborted when the process had already exited ([#862](https://github.com/voidzero-dev/vite-task/pull/862)).
 - **Fixed** `vp run` no longer panics before running a script that contains non-ASCII characters, such as `echo こんにちは`, and no longer cuts off such commands where it displays them ([#861](https://github.com/voidzero-dev/vite-task/pull/861)).
 
@@ -33,6 +33,7 @@ serde_json = { workspace = true }
 supports-color = { workspace = true }
 thiserror = { workspace = true }
 tar = { workspace = true }
+tempfile = { workspace = true }
 tokio = { workspace = true, features = [
   "rt-multi-thread",
   "io-std",
@@ -66,9 +67,6 @@ vt_workspace = { workspace = true }
 wax = { workspace = true }
 zstd = { workspace = true }
 
-[dev-dependencies]
-tempfile = { workspace = true }
-
 [target.'cfg(any(target_os = "windows", target_os = "macos", target_os = "linux"))'.dependencies]
 fspy = { workspace = true }
 
 
@@ -3,6 +3,7 @@
 use std::{
     fs::File,
     io::{self, Read},
+    path::Component,
 };
 
 use vt_path::{AbsolutePath, RelativePathBuf};
@@ -48,7 +49,9 @@ pub fn create_output_archive(
 
 /// Extract a tar.zst archive, restoring files relative to workspace root.
 ///
-/// Parent directories are created automatically. Existing files are overwritten.
+/// Parent directories are created automatically. Existing files are replaced
+/// atomically, so concurrent extractions of the same archive into one
+/// workspace don't fail and never expose a partly written file.
 ///
 /// # Errors
 ///
@@ -61,7 +64,67 @@ pub fn extract_output_archive(
     let decoder = zstd::Decoder::new(file)?;
     let mut archive = tar::Archive::new(decoder);
 
-    archive.unpack(workspace_root.as_path())?;
+    for entry in archive.entries()? {
+        let mut entry = entry?;
+        if entry.header().entry_type().is_file() {
+            unpack_file_atomically(&mut entry, workspace_root)?;
+        } else {
+            entry.unpack_in(workspace_root.as_path())?;
+        }
+    }
+    Ok(())
+}
+
+/// Unpack a regular file entry by writing a temporary file next to the target
+/// and renaming it over the target.
+///
+/// `tar` removes an existing target and then creates it with `create_new`, so
+/// two extractions of the same file can make one of them fail with
+/// `File exists`. The rename can't fail that way, and whichever extraction
+/// renames last wins with identical bytes.
+fn unpack_file_atomically(
+    entry: &mut tar::Entry<'_, impl Read>,
+    workspace_root: &AbsolutePath,
+) -> anyhow::Result<()> {
+    let rel_path = entry.path()?.into_owned();
+    if !rel_path.components().all(|component| matches!(component, Component::Normal(_))) {
+        return Err(anyhow::Error::msg(vt_str::format!(
+            "archive entry `{}` is not a path inside the workspace",
+            rel_path.display(),
+        )));
+    }
+    let target = workspace_root.as_path().join(&rel_path);
+    let parent = target.parent().expect("a joined entry path has a parent");
+    // Like `Entry::unpack_in`, refuse entries that resolve outside the workspace
+    // through a symlinked directory, and check before creating missing directories.
+    let canonical_root = workspace_root.as_path().canonicalize()?;
+    #[expect(clippy::disallowed_types, reason = "checks std paths at the filesystem boundary")]
+    let ensure_inside_workspace = |dir: &std::path::Path| -> anyhow::Result<()> {
+        if dir.canonicalize()?.starts_with(&canonical_root) {
+            return Ok(());
+        }
+        Err(anyhow::Error::msg(vt_str::format!(
+            "archive entry `{}` resolves outside the workspace",
+            rel_path.display(),
+        )))
+    };
+    let mut deepest_existing = parent;
+    while deepest_existing.symlink_metadata().is_err()
+        && let Some(ancestor) = deepest_existing.parent()
+    {
+        deepest_existing = ancestor;
+    }
+    ensure_inside_workspace(deepest_existing)?;
+    std::fs::create_dir_all(parent)?;
+    ensure_inside_workspace(parent)?;
+    // The temporary path is removed on drop, so a failed unpack or rename leaves nothing behind.
+    // Plain `fs::rename` rather than `TempPath::persist`. On Windows both call `MoveFileExW`,
+    // which fails with access denied when the target is open or being replaced at the same
+    // time, but only std then retries with a POSIX-semantics rename. That replaces a target
+    // a reader has open with `FILE_SHARE_DELETE`, which std passes for every file it opens.
+    let temp_path = tempfile::NamedTempFile::new_in(parent)?.into_temp_path();
+    entry.unpack(&temp_path)?;
+    std::fs::rename(&temp_path, &target)?;
     Ok(())
 }
 
@@ -84,10 +147,286 @@ pub fn check_output_archive(reader: impl Read) -> io::Result<()> {
 
 #[cfg(test)]
 mod tests {
+    use std::{
+        sync::{
+            Barrier,
+            atomic::{AtomicBool, Ordering},
+        },
+        thread,
+    };
+
     use vt_path::AbsolutePathBuf;
 
     use super::*;
 
+    #[test]
+    fn concurrent_extractions_into_one_workspace_succeed() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let outputs: Vec<RelativePathBuf> = (0..20)
+            .map(|i| RelativePathBuf::new(vt_str::format!("dist/sub/out{i}.txt")).unwrap())
+            .collect();
+        for (i, output) in outputs.iter().enumerate() {
+            let path = dir.join(output);
+            std::fs::create_dir_all(path.as_path().parent().unwrap()).unwrap();
+            std::fs::write(path.as_path(), vt_str::format!("content {i}\n").repeat(2000)).unwrap();
+        }
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &outputs, &archive_path).unwrap();
+
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        std::fs::create_dir(workspace.as_path()).unwrap();
+        for _ in 0..50 {
+            let barrier = Barrier::new(2);
+            thread::scope(|scope| {
+                for _ in 0..2 {
+                    scope.spawn(|| {
+                        barrier.wait();
+                        extract_output_archive(&workspace, &archive_path).unwrap();
+                    });
+                }
+            });
+            for (i, output) in outputs.iter().enumerate() {
+                assert_eq!(
+                    std::fs::read_to_string(workspace.join(output).as_path()).unwrap(),
+                    vt_str::format!("content {i}\n").repeat(2000),
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn extraction_replaces_existing_files_and_leaves_nothing_behind_on_failure() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let output = RelativePathBuf::new("dist/out.txt").unwrap();
+        let path = dir.join(&output);
+        std::fs::create_dir_all(path.as_path().parent().unwrap()).unwrap();
+        // Incompressible content, so that truncating the archive cuts into the entry.
+        let content: Vec<u8> =
+            (0..100_000_u32).map(|i| (i.wrapping_mul(2_654_435_761) >> 24) as u8).collect();
+        std::fs::write(path.as_path(), &content).unwrap();
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &[output], &archive_path).unwrap();
+
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        let restored = workspace.join("dist/out.txt");
+        std::fs::create_dir_all(restored.as_path().parent().unwrap()).unwrap();
+        std::fs::write(restored.as_path(), "stale").unwrap();
+        extract_output_archive(&workspace, &archive_path).unwrap();
+        assert_eq!(std::fs::read(restored.as_path()).unwrap(), content);
+
+        let archive = std::fs::read(archive_path.as_path()).unwrap();
+        let truncated = dir.join("truncated.tar.zst");
+        std::fs::write(truncated.as_path(), &archive[..archive.len() / 2]).unwrap();
+        let fresh = AbsolutePathBuf::new(tmp.path().join("fresh")).unwrap();
+        std::fs::create_dir(fresh.as_path()).unwrap();
+        assert!(extract_output_archive(&fresh, &truncated).is_err());
+        // Neither the target nor a temporary file may be left behind.
+        let left_behind =
+            std::fs::read_dir(fresh.join("dist").as_path()).map_or(0, Iterator::count);
+        assert_eq!(left_behind, 0);
+    }
+
+    #[test]
+    fn failed_rename_leaves_no_temporary_file() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let output = RelativePathBuf::new("dist/out.txt").unwrap();
+        let path = dir.join(&output);
+        std::fs::create_dir_all(path.as_path().parent().unwrap()).unwrap();
+        std::fs::write(path.as_path(), "built\n").unwrap();
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &[output], &archive_path).unwrap();
+
+        // A non-empty directory at the target makes the final rename fail.
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        let blocker = workspace.join("dist/out.txt/child");
+        std::fs::create_dir_all(blocker.as_path()).unwrap();
+        assert!(extract_output_archive(&workspace, &archive_path).is_err());
+        let names: Vec<_> = std::fs::read_dir(workspace.join("dist").as_path())
+            .unwrap()
+            .map(|entry| entry.unwrap().file_name())
+            .collect();
+        assert_eq!(names, ["out.txt"]);
+    }
+
+    #[test]
+    fn readers_never_see_a_missing_or_partial_file_while_restores_run() {
+        const FILES: usize = 8;
+        let expected = |i: usize| vt_str::format!("content {i}\n").repeat(20_000);
+
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let outputs: Vec<RelativePathBuf> = (0..FILES)
+            .map(|i| RelativePathBuf::new(vt_str::format!("dist/out{i}.txt")).unwrap())
+            .collect();
+        for (i, output) in outputs.iter().enumerate() {
+            let path = dir.join(output);
+            std::fs::create_dir_all(path.as_path().parent().unwrap()).unwrap();
+            std::fs::write(path.as_path(), expected(i)).unwrap();
+        }
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &outputs, &archive_path).unwrap();
+
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        std::fs::create_dir(workspace.as_path()).unwrap();
+        // The files exist before the readers start, as after an earlier cache hit.
+        extract_output_archive(&workspace, &archive_path).unwrap();
+
+        let done = AtomicBool::new(false);
+        thread::scope(|scope| {
+            let readers: Vec<_> = (0..2)
+                .map(|_| {
+                    scope.spawn(|| {
+                        let mut reads = 0_u32;
+                        // Read at least once more after the restores finished.
+                        loop {
+                            let finished = done.load(Ordering::SeqCst);
+                            for (i, output) in outputs.iter().enumerate() {
+                                // `fs::read` fails if the file is missing.
+                                let bytes = std::fs::read(workspace.join(output).as_path())
+                                    .unwrap_or_else(|err| panic!("{output}: {err}"));
+                                assert!(bytes == expected(i).as_bytes(), "{output}: partial file");
+                                reads += 1;
+                            }
+                            if finished {
+                                return reads;
+                            }
+                        }
+                    })
+                })
+                .collect();
+            let restorers: Vec<_> = (0..2)
+                .map(|_| {
+                    scope.spawn(|| {
+                        for _ in 0..30 {
+                            extract_output_archive(&workspace, &archive_path).unwrap();
+                        }
+                    })
+                })
+                .collect();
+            for restorer in restorers {
+                restorer.join().unwrap();
+            }
+            done.store(true, Ordering::SeqCst);
+            for reader in readers {
+                assert!(reader.join().unwrap() > 0);
+            }
+        });
+    }
+
+    /// A tar.zst archive with one regular file entry whose path is written to
+    /// the header verbatim (`tar::Builder` itself refuses `..`).
+    fn archive_with_raw_path(path: &str) -> Vec<u8> {
+        let mut header = tar::Header::new_gnu();
+        header.as_old_mut().name[..path.len()].copy_from_slice(path.as_bytes());
+        header.set_size(5);
+        header.set_mode(0o644);
+        header.set_entry_type(tar::EntryType::Regular);
+        header.set_cksum();
+        let mut builder = tar::Builder::new(Vec::new());
+        builder.append(&header, &b"evil\n"[..]).unwrap();
+        zstd::encode_all(builder.into_inner().unwrap().as_slice(), 0).unwrap()
+    }
+
+    #[test]
+    fn entries_outside_the_workspace_are_rejected() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        std::fs::create_dir(workspace.as_path()).unwrap();
+        let outside = tmp.path().join("evil.txt");
+        let absolute = outside.to_str().unwrap().to_owned();
+
+        for (i, path) in
+            ["../evil.txt", "dist/../../evil.txt", absolute.as_str()].into_iter().enumerate()
+        {
+            let archive_path =
+                AbsolutePathBuf::new(tmp.path().join(vt_str::format!("evil{i}.tar.zst").as_str()))
+                    .unwrap();
+            std::fs::write(archive_path.as_path(), archive_with_raw_path(path)).unwrap();
+            assert!(
+                extract_output_archive(&workspace, &archive_path).is_err(),
+                "`{path}` must be rejected",
+            );
+            assert!(!outside.exists(), "`{path}` escaped the workspace");
+        }
+    }
+
+    #[test]
+    fn entries_through_a_symlinked_directory_out_of_the_workspace_are_rejected() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        let outside = tmp.path().join("outside");
+        std::fs::create_dir(&outside).unwrap();
+        std::fs::create_dir(workspace.as_path()).unwrap();
+        #[cfg(unix)]
+        std::os::unix::fs::symlink(&outside, workspace.join("dist").as_path()).unwrap();
+        #[cfg(windows)]
+        std::os::windows::fs::symlink_dir(&outside, workspace.join("dist").as_path()).unwrap();
+
+        let archive_path = AbsolutePathBuf::new(tmp.path().join("output.tar.zst")).unwrap();
+        // Neither the file nor the missing `sub` directory may be created outside.
+        for path in ["dist/evil.txt", "dist/sub/evil.txt"] {
+            std::fs::write(archive_path.as_path(), archive_with_raw_path(path)).unwrap();
+            assert!(extract_output_archive(&workspace, &archive_path).is_err(), "{path}");
+            assert_eq!(std::fs::read_dir(&outside).unwrap().count(), 0, "{path}");
+        }
+    }
+
+    #[test]
+    fn restored_files_keep_their_modification_time() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let output = RelativePathBuf::new("dist/out.txt").unwrap();
+        let path = dir.join(&output);
+        std::fs::create_dir_all(path.as_path().parent().unwrap()).unwrap();
+        std::fs::write(path.as_path(), "built\n").unwrap();
+        // A whole second in the past, as tar stores mtime in seconds.
+        let mtime = std::time::UNIX_EPOCH + std::time::Duration::from_secs(1_600_000_000);
+        File::options().write(true).open(path.as_path()).unwrap().set_modified(mtime).unwrap();
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &[output], &archive_path).unwrap();
+
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        std::fs::create_dir(workspace.as_path()).unwrap();
+        extract_output_archive(&workspace, &archive_path).unwrap();
+        let restored = std::fs::metadata(workspace.join("dist/out.txt").as_path()).unwrap();
+        assert_eq!(restored.modified().unwrap(), mtime);
+    }
+
+    #[cfg(unix)]
+    #[test]
+    fn restored_files_keep_their_permissions() {
+        use std::os::unix::fs::PermissionsExt;
+
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let modes = [("dist/run.sh", 0o755), ("dist/data.txt", 0o644), ("dist/private.txt", 0o600)];
+        let outputs: Vec<RelativePathBuf> =
+            modes.iter().map(|(name, _)| RelativePathBuf::new(*name).unwrap()).collect();
+        std::fs::create_dir(dir.join("dist").as_path()).unwrap();
+        for ((name, mode), output) in modes.iter().zip(&outputs) {
+            let path = dir.join(output);
+            std::fs::write(path.as_path(), name).unwrap();
+            std::fs::set_permissions(path.as_path(), std::fs::Permissions::from_mode(*mode))
+                .unwrap();
+        }
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &outputs, &archive_path).unwrap();
+
+        let workspace = AbsolutePathBuf::new(tmp.path().join("workspace")).unwrap();
+        std::fs::create_dir(workspace.as_path()).unwrap();
+        // Extract twice: the second run replaces files that already exist.
+        for _ in 0..2 {
+            extract_output_archive(&workspace, &archive_path).unwrap();
+            for (name, mode) in modes {
+                let restored = std::fs::metadata(workspace.join(name).as_path()).unwrap();
+                assert_eq!(restored.permissions().mode() & 0o777, mode, "{name}");
+            }
+        }
+    }
+
     #[test]
     fn check_accepts_only_complete_archives() {
         let tmp = tempfile::TempDir::new().unwrap();