Bun.sh generating multipart cloud-init user data for devops

user-data.ts

import { Glob } from 'bun'
import MagicString from 'magic-string'

type AttachmentType = 'cloud-config'

interface Attachment {
    content: string
    filename: string
    type: AttachmentType
}

function resolveAttachmentType(
    filename: string,
    content: string,
): AttachmentType {
    if (
        filename.endsWith('.yml') ||
        (filename.endsWith('.yaml') &&
            content.trim().startsWith('#cloud-config'))
    ) {
        return 'cloud-config'
    } else {
        throw new Error()
    }
}

async function collectAttachments(dir: string) {
    const result: Array<Attachment> = []
    for (const filename of new Glob('*').scanSync(dir)) {
        const content = await Bun.file(`${dir}/${filename}`).text()
        const type = resolveAttachmentType(filename, content)
        result.push({ type, content, filename })
    }
    return result
}

type TemplateExpression = {
    index: number
    innie: string
    outie: string
}

function templateContent(content: string) {
    const regex = new RegExp(/{{\s*(.*)\s*}}/g)
    let match: RegExpExecArray | null
    const expressions: Array<TemplateExpression> = []
    while ((match = regex.exec(content)) != null) {
        expressions.push({
            index: match.index,
            innie: match[1],
            outie: match[0],
        })
    }
    if (!expressions.length) {
        return content
    }
    const ms = new MagicString(content)
    for (const expression of expressions) {
        ms.update(
            expression.index,
            expression.index + expression.outie.length,
            evaluateExpression(expression.innie),
        )
    }
    return ms.toString()
}

function evaluateExpression(expression: string): string {
    let match: RegExpMatchArray | null
    if ((match = expression.match(/env\(\s*'(.*)'\s*\)/)) != null) {
        const envVarKey = match[1]
        if (!/[A-Z_]+/.test(envVarKey)) {
            throw new Error(
                `env var expression \`${envVarKey}\` is not valid syntax`,
            )
        }
        const envVarValue = Bun.env[envVarKey]
        if (!envVarValue) {
            throw new Error(`env var \`${envVarKey}\` does not exist`)
        }
        return envVarValue
    }

    throw new Error('unsupported expression: ' + expression)
}

const attachments = await collectAttachments('init')
const boundary = Bun.randomUUIDv7()

let result = `
Content-Type: multipart/form-data; boundary=${boundary}
MIME-Version: 1.0
Number-Attachments: ${attachments.length}
`

function appendAttachment(attachment: Attachment) {
    let content: string
    try {
        content = templateContent(attachment.content)
    } catch (e) {
        console.error(`error templating ${attachment.filename}: ${e.message}`)
    }
}

for (const attachment of attachments) {
    let content: string
    try {
        content = templateContent(attachment.content)
    } catch (e) {
        console.error(`error templating ${attachment.filename}: ${e.message}`)
        process.exit(1)
    }
    result += `
Content-Type: text/${attachment.type}; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="${attachment.filename}"

${content}
--${boundary}
`
}

console.log(result)