package: use a consistent timestamp

For each entry in the tar archive, we generate a new timestamp.
Normally cargo will be fast enough that we get a consistent timestamp,
but that need not be the case.  There's very little reason to produce
different timestamps for different files and it's slightly more
efficient not to need to make multiple queries, so let's instead
generate a single timestamp for all entries that we generate.

Author: bk2204

src/cargo/ops/cargo_package.rs

@@ -472,6 +472,13 @@ fn check_repo_state(
     }
 }
 
+fn timestamp() -> u64 {
+    SystemTime::now()
+        .duration_since(SystemTime::UNIX_EPOCH)
+        .unwrap()
+        .as_secs()
+}
+
 fn tar(
     ws: &Workspace<'_>,
     ar_files: Vec<ArchiveFile>,
@@ -491,6 +498,7 @@ fn tar(
 
     let base_name = format!("{}-{}", pkg.name(), pkg.version());
     let base_path = Path::new(&base_name);
+    let time = timestamp();
     for ar_file in ar_files {
         let ArchiveFile {
             rel_path,
@@ -525,12 +533,7 @@ fn tar(
                 };
                 header.set_entry_type(EntryType::file());
                 header.set_mode(0o644);
-                header.set_mtime(
-                    SystemTime::now()
-                        .duration_since(SystemTime::UNIX_EPOCH)
-                        .unwrap()
-                        .as_secs(),
-                );
+                header.set_mtime(time);
                 header.set_size(contents.len() as u64);
                 header.set_cksum();
                 ar.append_data(&mut header, &ar_path, contents.as_bytes())