fix: publish empty batches to avoid staled rounds

publish/index.js

@@ -44,27 +44,35 @@ export const publish = async ({
     maxMeasurements
   ])
   if (measurements.length === 0) {
-    logger.log('No measurements to publish.')
-    return
+    // IMPORTANT: We still need to publish an empty batch,
+    // otherwise the current round will never advance.
+    logger.log('WARNING: No measurements to publish. Publishing an empty batch.')
   }
 
   // Fetch the count of all unpublished measurements - we need this for monitoring
   // Note: this number will be higher than `measurements.length` because spark-api adds more
   // measurements in between the previous and the next query.
   const totalCount = (await pgPool.query(
     'SELECT COUNT(*) FROM measurements'
-  )).rows[0].count
+  )).rows[0]?.count || 0
 
   logger.log(`Publishing ${measurements.length} measurements. Total unpublished: ${totalCount}. Batch size: ${maxMeasurements}.`)
 
   // Share measurements
   const start = new Date()
-  const file = new File(
-    [measurements.map(m => JSON.stringify(m)).join('\n')],
-    'measurements.ndjson',
-    { type: 'application/json' }
-  )
-  const cid = await web3Storage.uploadFile(file)
+
+  let cid
+  if (measurements.length) {
+    const file = new File(
+      [measurements.map(m => JSON.stringify(m)).join('\n')],
+      'measurements.ndjson',
+      { type: 'application/json' }
+    )
+    cid = await web3Storage.uploadFile(file)
+  } else {
+    // bafkqaaa is a CID for an empty file
+    cid = 'bafkqaaa'
+  }
   const uploadMeasurementsDuration = new Date().getTime() - start.getTime()
   logger.log(`Measurements packaged in ${cid}`)
 

publish/test/test.js

@@ -109,7 +109,7 @@ describe('unit', () => {
     assert.strictEqual(removeConfirmedCalls.length, 1)
   })
 
-  it('noops when measurements empty', async () => {
+  it('publishes an empty batch with CID bafkqaaa when measurements empty', async () => {
     const clientStatements = []
     const client = {
       connect () {
@@ -122,9 +122,40 @@ describe('unit', () => {
       }
     }
 
-    const web3Storage = {}
-    const ieContract = {}
-    const stuckTransactionsCanceller = {}
+    const web3Storage = {
+      async uploadFile (file) {
+        throw new Error('web3Storage.uploadFile should not be called')
+      }
+    }
+
+    const ieContractMeasurementCIDs = []
+    const ieContract = {
+      async addMeasurements (cid) {
+        ieContractMeasurementCIDs.push(cid)
+        return {
+          async wait () {
+            return {
+              logs: [{}]
+            }
+          }
+        }
+      },
+      interface: {
+        parseLog () {
+          return {
+            args: [
+              null,
+              1
+            ]
+          }
+        }
+      }
+    }
+
+    const stuckTransactionsCanceller = {
+      async addPending (tx) { },
+      async removeConfirmed (tx) { }
+    }
 
     const { recordTelemetry } = createTelemetryRecorderStub()
 
@@ -138,7 +169,7 @@ describe('unit', () => {
       stuckTransactionsCanceller
     })
 
-    assert.strictEqual(clientStatements.length, 1)
+    assert.deepStrictEqual(ieContractMeasurementCIDs, ['bafkqaaa'])
   })
 })