fix: keep space for potential client info update segments.

[BobdenOs]

As part of @cap-js/hana for @sap/cds while implementing prepare statement re-use. Seemingly random stmt.exec calls started failing with the error Packet size limit exceeded.

After further investigations the following reproduction script showed the root cause:

const prepare = await tx.dbc._prepare(`WITH A AS (SELECT ? AS JSON FROM DUMMY UNION ALL SELECT TO_BLOB(NULL) AS JSON FROM DUMMY) SELECT * FROM DUMMY`)

const gen = async function* () {
  for (let i = 0; i < 64; i++) yield Buffer.allocUnsafe(1024)
}
tx.dbc.set({'size': '64'}) // connection.getClientInfo().setProperty('size','64')
const res = await new Promise((resolve, reject) => {
  prepare.exec([Readable.from(gen(), { objectMode: false })], (err, res) => { if (err) { reject(err) } else { resolve(res) } })
})

When only using hdb it should look something like:

const gen = async function* () {
  for (let i = 0; i < 64; i++) yield Buffer.allocUnsafe(1024)
}

connection.prepare(`WITH A AS (SELECT ? AS JSON FROM DUMMY UNION ALL SELECT TO_BLOB(NULL) AS JSON FROM DUMMY) SELECT * FROM DUMMY`, (err, stmt) => {
  connection.getClientInfo().setProperty('size', '64') // <== additional information not counted for total packet size
  stmt.exec([Readable.from(gen(), { objectMode: false })],(err, res) => {
    if(err) process.exit(1)
    process.exit(0)
  })
})

Currently uploading LOB parameters works with hdb only when another operation has happened between getClientInfo().setProperty(...) and stmt.exec. Which usually is fulfilled by calling prepare before stmt.exec.

[davidbrandow]

To confirm, was this with 2.26.4 or later, or with an earlier version?

[BobdenOs]

@davidbrandow this issue was reproduced with hdb@0.19.12 and hdb@2.27.1. The function getAvailableSize seems to have never considered the client info being pre pended to the message.