Fix race in blocktree.insert_shreds (#6550)

* Add guard for blocktree insert_shreds

* Add test

ledger/tests/blocktree.rs

@@ -0,0 +1,51 @@
+#[macro_use]
+extern crate solana_ledger;
+
+use solana_ledger::blocktree::{self, get_tmp_ledger_path, Blocktree};
+use solana_ledger::entry;
+use solana_sdk::hash::Hash;
+use std::sync::Arc;
+use std::thread::Builder;
+
+#[test]
+fn test_multiple_threads_insert_shred() {
+    let blocktree_path = get_tmp_ledger_path!();
+    let blocktree = Arc::new(Blocktree::open(&blocktree_path).unwrap());
+
+    for _ in 0..100 {
+        let num_threads = 10;
+
+        // Create `num_threads` different ticks in slots 1..num_therads + 1, all
+        // with parent = slot 0
+        let threads: Vec<_> = (0..num_threads)
+            .map(|i| {
+                let entries = entry::create_ticks(1, Hash::default());
+                let shreds = blocktree::entries_to_test_shreds(entries, i + 1, 0, false);
+                let blocktree_ = blocktree.clone();
+                Builder::new()
+                    .name("blocktree-writer".to_string())
+                    .spawn(move || {
+                        blocktree_.insert_shreds(shreds, None).unwrap();
+                    })
+                    .unwrap()
+            })
+            .collect();
+
+        for t in threads {
+            t.join().unwrap()
+        }
+
+        // Check slot 0 has the correct children
+        let mut meta0 = blocktree.meta(0).unwrap().unwrap();
+        meta0.next_slots.sort();
+        let expected_next_slots: Vec<_> = (1..num_threads + 1).collect();
+        assert_eq!(meta0.next_slots, expected_next_slots);
+
+        // Delete slots for next iteration
+        blocktree.purge_slots(0, None);
+    }
+
+    // Cleanup
+    drop(blocktree);
+    Blocktree::destroy(&blocktree_path).expect("Expected successful database destruction");
+}