From abd2ae4b5d8128ed7aae5b697ea14242512d20f4 Mon Sep 17 00:00:00 2001
From: Braydon Fuller <courier@braydon.com>
Date: Thu, 28 Feb 2019 11:04:46 -0800
Subject: [PATCH] blockstore: prevent blocks writes at the same position

---
 lib/blockstore/file.js  |  9 +++++++++
 test/blockstore-test.js | 25 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/lib/blockstore/file.js b/lib/blockstore/file.js
index 231dc573..e735b442 100644
--- a/lib/blockstore/file.js
+++ b/lib/blockstore/file.js
@@ -45,6 +45,8 @@ class FileBlockStore extends AbstractBlockStore {
 
     if (options.network != null)
       this.network = Network.get(options.network);
+
+    this.writing = false;
   }
 
   /**
@@ -231,6 +233,11 @@ class FileBlockStore extends AbstractBlockStore {
    */
 
   async write(hash, data) {
+    if (this.writing)
+      throw new Error('Already writing.');
+
+    this.writing = true;
+
     const mlength = 8;
     const blength = data.length;
     const length = data.length + mlength;
@@ -280,6 +287,8 @@ class FileBlockStore extends AbstractBlockStore {
     b.put(layout.R.encode(), bw.writeU32(fileno).render());
 
     await b.write();
+
+    this.writing = false;
   }
 
   /**
diff --git a/test/blockstore-test.js b/test/blockstore-test.js
index 53043516..a7bea129 100644
--- a/test/blockstore-test.js
+++ b/test/blockstore-test.js
@@ -477,6 +477,31 @@ describe('BlockStore', function() {
       }
     });
 
+    it('will not write blocks at the same position', (done) => {
+      let err = null;
+      let finished = 0;
+
+      for (let i = 0; i < 16; i++) {
+        const block = random.randomBytes(128);
+        const hash = random.randomBytes(32);
+
+        // Accidently don't use `await` and attempt to
+        // write multiple blocks in parallel and at the
+        // same file position.
+        const promise = store.write(hash, block);
+        promise.catch((e) => {
+          err = e;
+        }).finally(() => {
+          finished += 1;
+          if (finished >= 16) {
+            assert(err);
+            assert(err.message, 'Already writing.');
+            done();
+          }
+        });
+      }
+    });
+
     it('will return null if block not found', async () => {
       const hash = random.randomBytes(32);
       const block = await store.read(hash);