txn.Begin does not appear to work

package main

// Summary: Using txn := db.Begin() followed by txn.Exec(foo),
// txn.{Commit,Rollback} as shown below does not appear to work (see log output at
// bottom).  However passing everything through db.Exec(foo) does,
// including sending db.Exec(`BEGIN`), db.Exec('ROLLBACK'), etc. (see
// code in PR at https://github.com/cockroachdb/docs/pull/5049 for the
// db.Exec version).

import (
    "fmt"
    "log"
    "math"
    "math/rand"
    "time"

    // Import GORM-related packages.
    "github.com/jinzhu/gorm"
    _ "github.com/jinzhu/gorm/dialects/postgres"

    // Necessary in order to check for transaction retry error codes.
    // See implementation below in `transferFunds`.
    "github.com/lib/pq"
)

// Account is our model, which corresponds to the "accounts" database
// table.
type Account struct {
    ID      int `gorm:"primary_key"`
    Balance int
}

func main() {
    // Connect to the "bank" database as the "maxroach" user.
    const addr = "postgresql://root@localhost:26257/bank?sslmode=disable"
    db, err := gorm.Open("postgres", addr)
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()

    // Set to `true` and GORM will print out all DB queries.
    db.LogMode(true)

    // Automatically create the "accounts" table based on the Account
    // model.
    db.AutoMigrate(&Account{})

    // Insert two rows into the "accounts" table.
    db.Create(&Account{ID: 1, Balance: 1000})
    db.Create(&Account{ID: 2, Balance: 250})

    // The sequence of steps in this section is:
    // 1. Print account balances.
    // 2. Set up some Accounts and transfer funds between them.
    // 3. Print account balances again to verify the transfer occurred.

    // Print balances before transfer.
    printBalances(db)

    var amount = 100
    var fromAccount Account
    var toAccount Account

    db.First(&fromAccount, 1)
    db.First(&toAccount, 2)

    // Transfer funds between accounts.  To handle any possible
    // transaction retry errors, we add a retry loop with exponential
    // backoff to the transfer logic (see implementation below).
    if err := transferFunds(db, fromAccount, toAccount, amount); err != nil {
        // If the error is returned, it's either:
        //   1. Not a transaction retry error, i.e., some other kind of
        //   database error that you should handle here.
        //   2. A transaction retry error that has occurred more than N
        //   times (defined by the `maxRetries` variable inside
        //   `transferFunds`), in which case you will need to figure out
        //   why your database access is resulting in so much contention
        //   (see 'Understanding and avoiding transaction contention':
        //   https://www.cockroachlabs.com/docs/stable/performance-best-practices-overview.html#understanding-and-avoiding-transaction-contention)
        fmt.Println(err)
    }

    // Print balances after transfer to ensure that it worked.
    printBalances(db)

    // Delete accounts so we can start fresh when we want to run this
    // program again.
    deleteAccounts(db)
}

func transferFunds(db *gorm.DB, fromAccount Account, toAccount Account, amount int) error {
    if fromAccount.Balance < amount {
        return fmt.Errorf("account %d balance %d is lower than transfer amount %d", fromAccount.ID, fromAccount.Balance, amount)
    }

    var maxRetries = 3
    for retries := 0; retries <= maxRetries; retries++ {
        if retries == maxRetries {
            return fmt.Errorf("hit max of %d retries, aborting", retries)
        }

        // db.Exec("BEGIN")
        txn := db.Begin()
        txn.Exec(`SELECT now()`) // disable server-side auto-retries
        if err := txn.Exec(
            `SELECT crdb_internal.force_retry('1s'::INTERVAL)`,

            // `UPSERT INTO accounts (id, balance) VALUES
            //    (?, ((SELECT balance FROM accounts WHERE id = ?) - ?)),
            //    (?, ((SELECT balance FROM accounts WHERE id = ?) + ?))`,
            // fromAccount.ID, fromAccount.ID, amount,
            // toAccount.ID, toAccount.ID, amount
        ).Error; err != nil {

            // We need to cast GORM's db.Error to *pq.Error so we can
            // detect the Postgres transaction retry error code and
            // handle retries appropriately.
            pqErr := err.(*pq.Error)

            if pqErr.Code == "40001" {
                // Since this is a transaction retry error, we
                // ROLLBACK the transaction and sleep a little before
                // trying again.  Each time through the loop we sleep
                // for a little longer than the last time
                // (A.K.A. exponential backoff).
                txn.Rollback()
                // db.Exec("ROLLBACK")
                var sleepMs = math.Pow(2, float64(retries)) * 100 * (rand.Float64() + 0.5)
                time.Sleep(time.Millisecond * time.Duration(sleepMs))
            } else {
                return err
            }
        } else {
            // Happy case.  All went well, so we commit and break out
            // of the retry loop.
            txn.Commit()
            // db.Exec("COMMIT")
            break
        }
    }
    return nil
}

func printBalances(db *gorm.DB) {
    var accounts []Account
    db.Find(&accounts)
    fmt.Printf("Balance at '%s':\n", time.Now())
    for _, account := range accounts {
        fmt.Printf("%d %d\n", account.ID, account.Balance)
    }
}

func deleteAccounts(db *gorm.DB) error {
    // Used to tear down the accounts table so we can re-run this
    // program.
    err := db.Exec("DELETE from accounts where ID > 0").Error
    if err != nil {
        return err
    }
    return nil
}

// -*- mode: compilation; default-directory: "~/work/code/deemphasize-savepoints/golang/" -*-
// Compilation started at Wed Jul 24 13:30:55

// make -k
// go run gorm-sample.go

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:43)
// [2019-07-24 13:30:55]  [0.77ms]  INSERT INTO "accounts" ("id","balance") VALUES ('1','1000') RETURNING "accounts"."id"
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:44)
// [2019-07-24 13:30:55]  [0.69ms]  INSERT INTO "accounts" ("id","balance") VALUES ('2','250') RETURNING "accounts"."id"
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:140)
// [2019-07-24 13:30:55]  [0.76ms]  SELECT * FROM "accounts"
// [2 rows affected or returned ]
// Balance at '2019-07-24 13:30:55.617543 -0400 EDT m=+0.019774217':
// 1 1000
// 2 250

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:58)
// [2019-07-24 13:30:55]  [0.48ms]  SELECT * FROM "accounts"  WHERE ("accounts"."id" = 1) ORDER BY "accounts"."id" ASC LIMIT 1
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:59)
// [2019-07-24 13:30:55]  [0.50ms]  SELECT * FROM "accounts"  WHERE ("accounts"."id" = 2) ORDER BY "accounts"."id" ASC LIMIT 1
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:98)
// [2019-07-24 13:30:55]  [0.61ms]  SELECT now()
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:99)
// [2019-07-24 13:30:55]  pq: restart transaction: crdb_internal.force_retry(): TransactionRetryWithProtoRefreshError: forced by crdb_internal.force_retry() 

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:99)
// [2019-07-24 13:30:55]  [0.90ms]  SELECT crdb_internal.force_retry('1s'::INTERVAL)
// [0 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:98)
// [2019-07-24 13:30:55]  [0.79ms]  SELECT now()
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:99)
// [2019-07-24 13:30:55]  pq: restart transaction: crdb_internal.force_retry(): TransactionRetryWithProtoRefreshError: forced by crdb_internal.force_retry() 

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:99)
// [2019-07-24 13:30:55]  [1.07ms]  SELECT crdb_internal.force_retry('1s'::INTERVAL)
// [0 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:98)
// [2019-07-24 13:30:56]  [0.74ms]  SELECT now()
// [1 rows affected or returned ]

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:99)
// [2019-07-24 13:30:56]  pq: restart transaction: crdb_internal.force_retry(): TransactionRetryWithProtoRefreshError: forced by crdb_internal.force_retry() 

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:99)
// [2019-07-24 13:30:56]  [1.56ms]  SELECT crdb_internal.force_retry('1s'::INTERVAL)
// [0 rows affected or returned ]
// hit max of 3 retries, aborting

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:140)
// [2019-07-24 13:30:56]  [1.44ms]  SELECT * FROM "accounts"
// [2 rows affected or returned ]
// Balance at '2019-07-24 13:30:56.533603 -0400 EDT m=+0.935833949':
// 1 1000
// 2 250

// (/Users/rloveland/work/code/deemphasize-savepoints/golang/gorm-sample.go:150)
// [2019-07-24 13:30:56]  [3.81ms]  DELETE from accounts where ID > 0
// [2 rows affected or returned ]

// Compilation finished at Wed Jul 24 13:30:56