fix(core): terminate exhausted Markdown code budgets

packages/core/src/tool/html-markdown.ts

@@ -246,6 +246,8 @@ export function convertHTMLToMarkdown(html: string) {
         block()
         return
       }
+      // No amount of trimming can make the empty fenced block fit.
+      if (!payload) return
       const excess = valueBytes - Math.max(0, CONTENT_BYTES - outputBytes)
       payload = sliceBytes(payload, Math.max(0, encoder.encode(payload).byteLength - Math.ceil(excess)))
     }

packages/core/test/fixture/html-markdown.ts

@@ -0,0 +1,7 @@
+import { writeSync } from "node:fs"
+import { convertHTMLToMarkdown } from "../../src/tool/html-markdown"
+
+const html = await Bun.stdin.text()
+// Flush readiness before entering a conversion that may block the child event loop.
+writeSync(1, "ready\n")
+await Bun.write(Bun.stdout, convertHTMLToMarkdown(html))

packages/core/test/tool-html-markdown-budget.test.ts

@@ -0,0 +1,69 @@
+import { expect, test } from "bun:test"
+import { fileURLToPath } from "node:url"
+import { MAX_MARKDOWN_BYTES } from "../src/tool/html-markdown"
+
+const budget = MAX_MARKDOWN_BYTES - 64 * 1024
+
+test.each([
+  ["exhausted budget", budget, "x", "", false],
+  ["one byte short of an empty fence", budget - 13, "x", "", false],
+  ["small fitting block", 64, "x", "\n\n```\nx\n```", false],
+  ["last payload byte fits", budget - 15, "x", "\n\n```\nx\n```", false],
+  ["only an empty fence fits", budget - 14, "x", "\n\n```\n\n```", false],
+  ["Unicode payload truncates at a code point", budget - 18, "😀é", "\n\n```\n😀\n```", false],
+  ["quoted payload fits", budget - 23, "x", "\n\n> ```\n> x\n> ```", true],
+  ["only an empty quoted fence fits", budget - 22, "x", "\n\n> ```\n> \n> ```", true],
+  ["one byte short of an empty quoted fence", budget - 21, "x", "", true],
+] as const)(
+  "finishes bounded code conversion: %s",
+  async (_name, count, payload, suffix, quoted) => {
+    const code = `<pre>${payload}</pre>`
+    const html = `<p>${"x".repeat(count)}</p>${quoted ? `<blockquote>${code}</blockquote>` : code}`
+    expect(Buffer.byteLength(html)).toBeLessThanOrEqual(MAX_MARKDOWN_BYTES)
+    const child = Bun.spawn({
+      cmd: [process.execPath, fileURLToPath(new URL("./fixture/html-markdown.ts", import.meta.url))],
+      stdin: new Blob([html]),
+      stdout: "pipe",
+      stderr: "pipe",
+    })
+    let ready = false
+    let timeout: "startup" | "conversion" | undefined
+    let timer = setTimeout(() => {
+      timeout = "startup"
+      child.kill("SIGKILL")
+    }, 10_000)
+    const stdout = (async () => {
+      let output = ""
+      for await (const chunk of child.stdout.pipeThrough(new TextDecoderStream())) {
+        output += chunk
+        if (ready || !output.startsWith("ready\n")) continue
+        ready = true
+        clearTimeout(timer)
+        // This watchdog runs outside the possibly stuck synchronous converter.
+        timer = setTimeout(() => {
+          timeout = "conversion"
+          child.kill("SIGKILL")
+        }, 3_000)
+      }
+      return output.slice("ready\n".length)
+    })()
+    const stderr = new Response(child.stderr).text()
+    try {
+      const exitCode = await child.exited
+      const output = await stdout
+      expect({ ready, timeout, exitCode, stderr: await stderr }).toEqual({
+        ready: true,
+        timeout: undefined,
+        exitCode: 0,
+        stderr: "",
+      })
+      expect(Buffer.byteLength(output)).toBeLessThanOrEqual(MAX_MARKDOWN_BYTES)
+      expect(output).toBe("x".repeat(Math.min(count, budget - 2)) + suffix)
+    } finally {
+      clearTimeout(timer)
+      if (child.exitCode === null && child.signalCode === null) child.kill("SIGKILL")
+      await child.exited
+    }
+  },
+  15_000,
+)