pull,context: preserve file mtimes when copying sources

std::fs::copy resets the destination mtime to "now", which breaks
timestamp-based build systems (autotools/gnulib). Packages like
'hello' from Debian sid ship pre-generated files alongside their
prerequisites; when the copy flattens all mtimes, make considers the
generated targets out-of-date and tries to regenerate them with tools
(e.g. gperf) that are not declared build-dependencies, failing the
build.

Restore the source modification and access times after every file copy
in pull::copy_dir_all, pull::fetch_archive_sources merge step, and
context::unshare copy_dir_recursive/ensure_available.

src/pull.rs

@@ -113,13 +113,46 @@ fn copy_dir_all(src: &Path, dst: &Path) -> Result<(), Box<dyn Error>> {
         } else if src_path.is_dir() {
             copy_dir_all(&src_path, &dst_path)?;
         } else {
+            // Copy the file, preserving the source modification and access
+            // times. This is important for autotools/gnulib-based packages
+            // (e.g. 'hello' from Debian sid) that ship pre-generated files
+            // alongside their prerequisites: if the copy resets the mtime to
+            // "now", the prerequisites appear as new as the generated targets
+            // and `make` tries to regenerate them using tools (like gperf)
+            // that are not declared build-dependencies.
             std::fs::copy(&src_path, &dst_path)?;
+            copy_file_times(&src_path, &dst_path)?;
         }
     }
 
     Ok(())
 }
 
+/// Restore the modification and access times of `dest` to match `src`.
+///
+/// `std::fs::copy` resets the destination mtime to "now", which breaks
+/// timestamp-based build systems (autotools/gnulib) that ship pre-generated
+/// files alongside their prerequisites. Restoring the original timestamps
+/// prevents `make` from needlessly regenerating those files with tools that
+/// may not be installed (e.g. `gperf`).
+fn copy_file_times(src: &Path, dest: &Path) -> Result<(), Box<dyn Error>> {
+    let metadata = std::fs::metadata(src)?;
+    let mut times = std::fs::FileTimes::new();
+    let mut have_times = false;
+    if let Ok(mtime) = metadata.modified() {
+        times = times.set_modified(mtime);
+        have_times = true;
+    }
+    if let Ok(atime) = metadata.accessed() {
+        times = times.set_accessed(atime);
+        have_times = true;
+    }
+    if have_times && let Ok(dest_file) = std::fs::File::open(dest) {
+        let _ = dest_file.set_times(times);
+    }
+    Ok(())
+}
+
 /// Helper function to extract tar archive with progress tracking
 fn extract_tar_archive<D, F>(
     file_path: &Path,
@@ -467,6 +500,7 @@ async fn fetch_archive_sources(
                             copy_dir_all(&sub_path, &target_path)?;
                         } else {
                             std::fs::copy(&sub_path, &target_path)?;
+                            copy_file_times(&sub_path, &target_path)?;
                         }
                     }
                     std::fs::remove_dir_all(&src_dir)?;