build_helper: try to rename dir before delete

Author: ChrisDenton

Cargo.lock

@@ -332,6 +332,7 @@ dependencies = [
 name = "build_helper"
 version = "0.1.0"
 dependencies = [
+ "fastrand",
  "serde",
  "serde_derive",
 ]

src/bootstrap/Cargo.lock

@@ -82,6 +82,7 @@ dependencies = [
 name = "build_helper"
 version = "0.1.0"
 dependencies = [
+ "fastrand",
  "serde",
  "serde_derive",
 ]
@@ -233,6 +234,12 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "fastrand"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "37909eebbb50d72f9059c3b6d82c0463f2ff062c9e95845c43a6c9c0355411be"
+
 [[package]]
 name = "fd-lock"
 version = "4.0.2"

src/build_helper/Cargo.toml

@@ -6,5 +6,6 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+fastrand = "2.3.0"
 serde = "1"
 serde_derive = "1"

src/build_helper/src/fs/mod.rs

@@ -1,4 +1,5 @@
 //! Misc filesystem related helpers for use by bootstrap and tools.
+use std::ffi::{OsStr, OsString};
 use std::fs::Metadata;
 use std::path::Path;
 use std::{fs, io};
@@ -100,6 +101,30 @@ where
 
 pub fn remove_and_create_dir_all<P: AsRef<Path>>(path: P) -> io::Result<()> {
     let path = path.as_ref();
-    recursive_remove(path)?;
+
+    // Attempt to rename the directory in case removing fails.
+    // We allow either the rename to fail or the remove to fail but not both.
+    let rm_path = rand_name(path.as_os_str());
+    if fs::rename(path, &rm_path).is_err() {
+        // Rename failed, try to remove the original path
+        recursive_remove(&path)?;
+    } else {
+        // Rename succeeded, try to remove the renamed path
+        let _ = recursive_remove(&rm_path);
+    }
     fs::create_dir_all(path)
 }
+
+fn rand_name(prefix: &OsStr) -> OsString {
+    let mut name: OsString = prefix.into();
+    name.push("-");
+    let mut rand_suffix = [0; 8];
+    for n in rand_suffix.iter_mut() {
+        *n = fastrand::alphanumeric() as u8;
+    }
+    // SAFETY: `fastrand::alphanumeric` only returns valid ascii.
+    // Since an `OsStr` is a superset of UTF-8, ascii must be a valid `OsStr`.
+    let ascii = unsafe { OsStr::from_encoded_bytes_unchecked(&rand_suffix) };
+    name.push(ascii);
+    name
+}