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.

[linjun-he-sap]

Hi @BobdenOs, thanks for your PR!
You change has been consumed in our internal repo, and merged with modifications and tests.
This change (23aa0e6) has been merged into this external repo by merge commit f26df55, and has been included in the patch release v2.29.6 (https://www.npmjs.com/package/hdb/v/2.29.6)