cli: make concurrent description-editing not use the same file

If you ran two concurrent `jj describe` (for example) before this
patch, they'd both try to open an editor on the same file. This patch
fixes that by randomizing the filename. It also deletes the file at
the end so the `.jj/` directory is not cluttered by these files.

Cargo.lock

@@ -559,6 +559,7 @@ dependencies = [
  "pest_derive",
  "protobuf",
  "protobuf-codegen-pure",
+ "rand 0.8.0",
  "regex",
  "serde_json",
  "tempfile",

Cargo.toml

@@ -35,6 +35,7 @@ pest = "2.1.3"
 pest_derive = "2.1.0"
 protobuf = { version = "2.22.1", features = ["with-bytes"] }
 protobuf-codegen-pure = "2.22.1"
+rand = "0.8.0"
 serde_json = "1.0.60"
 tempfile = "3.1.0"
 thiserror = "1.0.22"

src/commands.rs

@@ -1310,13 +1310,12 @@ fn cmd_obslog(
 }
 
 fn edit_description(repo: &ReadonlyRepo, description: &str) -> String {
-    // TODO: Where should this file live? The current location prevents two
-    // concurrent `jj describe` calls.
-    let description_file_path = repo.repo_path().join("description");
+    let random: u32 = rand::random();
+    let description_file_path = repo.repo_path().join(format!("description-{}", random));
     {
         let mut description_file = OpenOptions::new()
             .write(true)
-            .create(true)
+            .create_new(true)
             .truncate(true)
             .open(&description_file_path)
             .unwrap_or_else(|_| panic!("failed to open {:?} for write", &description_file_path));
@@ -1342,7 +1341,11 @@ fn edit_description(repo: &ReadonlyRepo, description: &str) -> String {
         .unwrap_or_else(|_| panic!("failed to open {:?} for read", &description_file_path));
     let mut buf = vec![];
     description_file.read_to_end(&mut buf).unwrap();
-    String::from_utf8(buf).unwrap()
+    let description = String::from_utf8(buf).unwrap();
+    // Delete the file only if everything went well.
+    // TODO: Tell the user the name of the file we left behind.
+    std::fs::remove_file(description_file_path).ok();
+    description
 }
 
 fn cmd_describe(