add max suggested block size (#481)

* add max suggested block size

* fix tests

Author: mafintosh

index.js

@@ -29,6 +29,10 @@ const {
 const promises = Symbol.for('hypercore.promises')
 const inspect = Symbol.for('nodejs.util.inspect.custom')
 
+// Hypercore actually does not have any notion of max/min block sizes
+// but we enforce 15mb to ensure smooth replication (each block is transmitted atomically)
+const MAX_SUGGESTED_BLOCK_SIZE = 15 * 1024 * 1024
+
 module.exports = class Hypercore extends EventEmitter {
   constructor (storage, key, opts) {
     super()
@@ -134,6 +138,8 @@ module.exports = class Hypercore extends EventEmitter {
       indent + ')'
   }
 
+  static MAX_SUGGESTED_BLOCK_SIZE = MAX_SUGGESTED_BLOCK_SIZE
+
   static key (manifest, { compat } = {}) {
     return compat ? manifest.signers[0].publicKey : manifestHash(createManifest(manifest))
   }
@@ -1024,6 +1030,11 @@ module.exports = class Hypercore extends EventEmitter {
         buffers[i] = this._encode(this.valueEncoding, blocks[i])
       }
     }
+    for (const b of buffers) {
+      if (b.byteLength > MAX_SUGGESTED_BLOCK_SIZE) {
+        throw BAD_ARGUMENT('Appended block exceeds the maximum suggested block size')
+      }
+    }
 
     return this.core.append(buffers, { keyPair, signature, preappend })
   }

test/basic.js

@@ -410,3 +410,21 @@ test('writable session on a readable only core', async function (t) {
     t.pass(err.code, 'SESSION_NOT_WRITABLE')
   }
 })
+
+test('append above the max suggested block size', async function (t) {
+  t.plan(1)
+
+  const core = new Hypercore(RAM)
+
+  try {
+    await core.append(Buffer.alloc(Hypercore.MAX_SUGGESTED_BLOCK_SIZE))
+  } catch (e) {
+    t.fail('should not throw')
+  }
+
+  try {
+    await core.append(Buffer.alloc(Hypercore.MAX_SUGGESTED_BLOCK_SIZE + 1))
+  } catch {
+    t.pass('should throw')
+  }
+})