cli: Refactor workspace root directory creation

* Add file_util::create_or_reuse_dir() which is needed by all init
  functionality regardless of the backend.

cli/src/commands/init.rs

@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::io;
 use std::io::Write;
-use std::{fs, io};
 
 use clap::ArgGroup;
 use itertools::Itertools as _;
@@ -51,16 +51,11 @@ pub(crate) fn cmd_init(
     command: &CommandHelper,
     args: &InitArgs,
 ) -> Result<(), CommandError> {
-    let wc_path = command.cwd().join(&args.destination);
-    match fs::create_dir(&wc_path) {
-        Ok(()) => {}
-        Err(_) if wc_path.is_dir() => {}
-        Err(e) => return Err(user_error_with_message("Failed to create workspace", e)),
-    }
-    let wc_path = wc_path
-        .canonicalize()
-        .map_err(|e| user_error_with_message("Failed to create workspace", e))?; // raced?
     let cwd = command.cwd().canonicalize().unwrap();
+    let wc_path = cwd.join(&args.destination);
+    let wc_path = file_util::create_or_reuse_dir(&wc_path)
+        .and_then(|p| p.canonicalize())
+        .map_err(|e| user_error_with_message("Failed to create workspace", e))?;
     let relative_wc_path = file_util::relative_path(&cwd, &wc_path);
 
     if let Some(git_store_str) = &args.git_repo {

lib/src/file_util.rs

@@ -14,7 +14,7 @@
 
 #![allow(missing_docs)]
 
-use std::fs::File;
+use std::fs::{self, File};
 use std::path::{Component, Path, PathBuf};
 use std::{io, iter};
 
@@ -42,6 +42,22 @@ impl<T> IoResultExt<T> for io::Result<T> {
     }
 }
 
+/// Creates a directory or does nothing if the directory already exists.
+///
+/// Returns the &Path if successful, so it can be chained with .and_then() and
+/// friends which take a closure with a single param.
+///
+/// Returns the underlying error if the directory can't be created.
+/// The function will also fail if intermediate directories on the path do not
+/// already exist.
+pub fn create_or_reuse_dir(dirname: &Path) -> io::Result<&Path> {
+    match fs::create_dir(dirname) {
+        Ok(()) => Ok(dirname),
+        Err(_) if dirname.is_dir() => Ok(dirname),
+        Err(e) => Err(e),
+    }
+}
+
 /// Turns the given `to` path into relative path starting from the `from` path.
 ///
 /// Both `from` and `to` paths are supposed to be absolute and normalized in the