fix(util): preserve some file permissions during write_atomic

Preseves u/g/o r/w/x permissions on unix platforms, and the "readonly" property
on non-unix platforms.

Fixes the two unit tests added in the previous commit.

crates/cargo-util/src/paths.rs

@@ -4,7 +4,7 @@ use anyhow::{Context, Result};
 use filetime::FileTime;
 use std::env;
 use std::ffi::{OsStr, OsString};
-use std::fs::{self, File, Metadata, OpenOptions};
+use std::fs::{self, File, Metadata, OpenOptions, Permissions};
 use std::io;
 use std::io::prelude::*;
 use std::iter;
@@ -185,10 +185,51 @@ pub fn write<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Result<()>
 /// write_atomic uses tempfile::persist to accomplish atomic writes.
 pub fn write_atomic<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Result<()> {
     let path = path.as_ref();
+
+    // On unix platforms, get the permissions of the original file. Copy only the user/group/other
+    // read/write/execute permission bits. The tempfile lib defaults to an initial mode of 0o600,
+    // and we'll set the proper permissions after creating the file.
+    #[cfg(unix)]
+    let perms = path.metadata().ok().map(|meta| {
+        use std::os::unix::fs::PermissionsExt;
+
+        // these constants are u16 on macOS
+        let mask = u32::from(libc::S_IRWXU | libc::S_IRWXG | libc::S_IRWXO);
+        let mode = meta.permissions().mode() & mask;
+
+        Permissions::from_mode(mode)
+    });
+
+    // On non-unix platforms, get the "readonly" permission of the original file.
+    #[cfg(not(unix))]
+    let readonly = path
+        .metadata()
+        .ok()
+        .map(|meta| meta.permissions().readonly());
+
     let mut tmp = TempFileBuilder::new()
         .prefix(path.file_name().unwrap())
         .tempfile_in(path.parent().unwrap())?;
     tmp.write_all(contents.as_ref())?;
+
+    // On unix platforms, set the permissions on the newly created file. We can use fchmod (called
+    // by the std lib; subject to change) which ignores the umask so that the new file has the same
+    // permissions as the old file.
+    #[cfg(unix)]
+    if let Some(perms) = perms {
+        tmp.as_file().set_permissions(perms)?;
+    }
+
+    // On non-unix platforms, update the readonly permissions on the newly created file.
+    #[cfg(not(unix))]
+    if let Some(readonly) = readonly {
+        if let Ok(metadata) = tmp.as_file().metadata() {
+            let mut perms = metadata.permissions();
+            perms.set_readonly(readonly);
+            tmp.as_file().set_permissions(perms)?;
+        }
+    }
+
     tmp.persist(path)?;
     Ok(())
 }