DecompressionStream and trailing bytes in PDF streams
A PDF stream's bytes sit between stream\n and endstream, but writers put a line break (\n or \r\n) before endstream – the PDF specification asks for one. DecompressionStream('deflate') treats those bytes as junk after the end of the data and the whole read fails – in Chrome new Response(stream).arrayBuffer() rejects with TypeError: Failed to fetch, in Node with TypeError: Trailing junk found after the end of the compressed stream. You lose even the data that was already inflated. Trim trailing CR/LF before inflating, read with getReader() chunk by chunk, and if the error arrives after some data, keep what you have.
Obsah proudu PDF leží mezi stream\n a endstream, ale před endstream zapisovače dávají konec řádku – specifikace PDF ho žádá. DecompressionStream('deflate') ho bere jako bajty za koncem dat a celé čtení spadne – v Chrome TypeError: Failed to fetch z arrayBuffer(), v Node TypeError: Trailing junk found after the end of the compressed stream. Přijdete i o to, co se už rozbalilo. Před rozbalením ořízněte koncové CR/LF, čtěte po kouscích přes getReader() a když chyba přijde až po datech, platí to, co už přišlo.
Symptom
A check "is this PDF a scan without text?" looked for fonts or text operators (/Font, Tj, TJ) inside compressed content streams. Every stream came back empty, so a text PDF whose fonts live only in compressed object streams was reported as "unknown", and a real scan was never recognised as a scan.
Cause
4 0 obj << /Length 25 /Filter /FlateDecode >>
stream
<25 bytes of zlib data>
endstreamSlicing from after stream\n to the index of endstream includes the line break before endstream. The zlib data ends before it; the decompressor sees extra bytes and errors. Response.arrayBuffer() reports only the error, never the partial result.
Fix
async function inflate(bytes: Uint8Array): Promise<Uint8Array | undefined> {
let end = bytes.length;
while (end > 0 && (bytes[end - 1] === 0x0a || bytes[end - 1] === 0x0d)) end--;
const parts: Uint8Array[] = [];
let total = 0;
try {
const reader = new Blob([bytes.subarray(0, end)]).stream()
.pipeThrough(new DecompressionStream('deflate')).getReader();
for (;;) {
const { done, value } = await reader.read();
if (done) break;
if (value) { parts.push(value); total += value.length; }
}
} catch { if (!total) return undefined; } // error after data: keep the data
const out = new Uint8Array(total);
let o = 0;
for (const p of parts) { out.set(p, o); o += p.length; }
return out;
}Trimming alone covers the usual case; reading in chunks also covers streams whose /Length is shorter than the slice for other reasons.
How to verify
Compress a short content stream with CompressionStream('deflate'), append \r\n, and decompress it with new Response(...).arrayBuffer(). It rejects. Without the \r\n it returns the original bytes.