Implement "sell all shares" functionality in sellshares and expose API (#696)

* Change `sellshares` to be able to sell all shares

* Sell all shares properly on bet panel UI

* Add API route for selling shares, document

docs/docs/api.md

@@ -579,6 +579,26 @@ $ curl https://manifold.markets/api/v0/market/{marketId}/resolve -X POST \
                  ]}'
 ```
 
+### `POST /v0/market/[marketId]/sell`
+
+Sells some quantity of shares in a market on behalf of the authorized user.
+
+Parameters:
+
+- `outcome`: Required. One of `YES`, `NO`, or a `number` indicating the numeric
+  bucket ID, depending on the market type.
+- `shares`: Optional. The amount of shares to sell of the outcome given
+  above. If not provided, all the shares you own will be sold.
+
+Example request:
+
+```
+$ curl https://manifold.markets/api/v0/market/{marketId}/sell -X POST \
+    -H 'Content-Type: application/json' \
+    -H 'Authorization: Key {...}' \
+    --data-raw '{"outcome": "YES", "shares": 10}'
+```
+
 ### `GET /v0/bets`
 
 Gets a list of bets, ordered by creation date descending.

functions/src/sell-shares.ts

@@ -16,7 +16,7 @@ import { redeemShares } from './redeem-shares'
 
 const bodySchema = z.object({
   contractId: z.string(),
-  shares: z.number(),
+  shares: z.number().optional(), // leave it out to sell all shares
   outcome: z.enum(['YES', 'NO']),
 })
 
@@ -49,11 +49,12 @@ export const sellshares = newEndpoint({}, async (req, auth) => {
 
     const outcomeBets = userBets.filter((bet) => bet.outcome == outcome)
     const maxShares = sumBy(outcomeBets, (bet) => bet.shares)
+    const sharesToSell = shares ?? maxShares
 
-    if (!floatingLesserEqual(shares, maxShares))
+    if (!floatingLesserEqual(sharesToSell, maxShares))
       throw new APIError(400, `You can only sell up to ${maxShares} shares.`)
 
-    const soldShares = Math.min(shares, maxShares)
+    const soldShares = Math.min(sharesToSell, maxShares)
 
     const unfilledBetsSnap = await transaction.get(
       getUnfilledBetsQuery(contractDoc)

web/components/bet-panel.tsx

@@ -771,7 +771,9 @@ export function SellPanel(props: {
   const betDisabled = isSubmitting || !amount || error
 
   // Sell all shares if remaining shares would be < 1
-  const sellQuantity = amount === Math.floor(shares) ? shares : amount
+  const isSellingAllShares = amount === Math.floor(shares)
+
+  const sellQuantity = isSellingAllShares ? shares : amount
 
   async function submitSell() {
     if (!user || !amount) return
@@ -780,7 +782,7 @@ export function SellPanel(props: {
     setIsSubmitting(true)
 
     await sellShares({
-      shares: sellQuantity,
+      shares: isSellingAllShares ? undefined : amount,
       outcome: sharesOutcome,
       contractId: contract.id,
     })

web/pages/api/v0/market/[id]/sell.ts

@@ -0,0 +1,28 @@
+import { NextApiRequest, NextApiResponse } from 'next'
+import {
+  CORS_ORIGIN_MANIFOLD,
+  CORS_ORIGIN_LOCALHOST,
+} from 'common/envs/constants'
+import { applyCorsHeaders } from 'web/lib/api/cors'
+import { fetchBackend, forwardResponse } from 'web/lib/api/proxy'
+
+export const config = { api: { bodyParser: true } }
+
+export default async function route(req: NextApiRequest, res: NextApiResponse) {
+  await applyCorsHeaders(req, res, {
+    origin: [CORS_ORIGIN_MANIFOLD, CORS_ORIGIN_LOCALHOST],
+    methods: 'POST',
+  })
+
+  const { id } = req.query
+  const contractId = id as string
+
+  if (req.body) req.body.contractId = contractId
+  try {
+    const backendRes = await fetchBackend(req, 'sellshares')
+    await forwardResponse(res, backendRes)
+  } catch (err) {
+    console.error('Error talking to cloud function: ', err)
+    res.status(500).json({ message: 'Error communicating with backend.' })
+  }
+}