Set up cloud function for writing txns

common/txn.ts

@@ -0,0 +1,31 @@
+// A txn (pronounced "texan") respresents a payment between two ids on Manifold
+// Shortened from "transaction" to distinguish from Firebase transactions (and save chars)
+export type Txn = {
+  id: string
+  createdTime: number
+
+  fromId: string
+  fromName: string
+  fromUsername: string
+  fromAvatarUrl?: string
+
+  toId: string
+  toName: string
+  toUsername: string
+  toAvatarUrl?: string
+
+  amount: number
+
+  category: TxnCategory
+  // Human-readable description
+  description?: string
+  // Structured metadata for different kinds of txns
+  data?: TxnData
+}
+
+export type TxnCategory = 'TO_CHARITY' // | 'TIP' | 'BET' | ...
+export type TxnData = CharityData // | TipData | BetData | ...
+
+export type CharityData = {
+  // TODO: Could fill this in
+}

firestore.rules

@@ -64,5 +64,9 @@ service cloud.firestore {
       allow read;
       allow write: if request.auth.uid == userId;
     }
+
+    match /txns/{txnId} {
+      allow read;
+    }
   }
 }

functions/src/transact.ts

@@ -0,0 +1,82 @@
+import * as functions from 'firebase-functions'
+import * as admin from 'firebase-admin'
+
+import { User } from '../../common/user'
+import { Txn, TxnCategory, TxnData } from '../../common/txn'
+
+export const transact = functions.runWith({ minInstances: 1 }).https.onCall(
+  async (
+    data: {
+      amount: number
+      toId: string
+      category: TxnCategory
+      description?: string
+      txnData?: TxnData
+    },
+    context
+  ) => {
+    const fromId = context?.auth?.uid
+    if (!fromId) return { status: 'error', message: 'Not authorized' }
+
+    const { amount, toId, category, description, txnData } = data
+
+    if (amount <= 0 || isNaN(amount) || !isFinite(amount))
+      return { status: 'error', message: 'Invalid amount' }
+
+    // run as transaction to prevent race conditions
+    return await firestore.runTransaction(async (transaction) => {
+      const fromDoc = firestore.doc(`users/${fromId}`)
+      const fromSnap = await transaction.get(fromDoc)
+      if (!fromSnap.exists) {
+        return { status: 'error', message: 'User not found' }
+      }
+      const fromUser = fromSnap.data() as User
+
+      const toDoc = firestore.doc(`users/${toId}`)
+      const toSnap = await transaction.get(toDoc)
+      if (!toSnap.exists) {
+        return { status: 'error', message: 'User not found' }
+      }
+      const toUser = toSnap.data() as User
+
+      if (fromUser.balance < amount) {
+        return {
+          status: 'error',
+          message: `Insufficient balance: ${fromUser.username} needed ${amount} but only had ${fromUser.balance} `,
+        }
+      }
+
+      const newTxnDoc = firestore.collection(`txns/`).doc()
+
+      const txn: Txn = {
+        id: newTxnDoc.id,
+        // @ts-ignore - this is a firestore doc
+        createdTime: admin.firestore.FieldValue.serverTimestamp(),
+
+        fromId,
+        fromName: fromUser.name,
+        fromUsername: fromUser.username,
+        fromAvatarUrl: fromUser.avatarUrl,
+
+        toId,
+        toName: toUser.name,
+        toUsername: toUser.username,
+        toAvatarUrl: toUser.avatarUrl,
+
+        amount,
+
+        category,
+        description,
+        data: txnData,
+      }
+
+      transaction.create(newTxnDoc, txn)
+      transaction.update(fromDoc, { balance: fromUser.balance - amount })
+      transaction.update(toDoc, { balance: toUser.balance + amount })
+
+      return { status: 'success', txnId: newTxnDoc.id }
+    })
+  }
+)
+
+const firestore = admin.firestore()