Differential downloads don't work when the blockmap HTTP response is compressed

[NoahAndrews]

Skipping the version info and such because I can point to the problem in the source code (and I'll be submitting a pull request).

The content-length header specifies the size of the raw data in the HTTP body. If the data is uncompressed, that is the same thing as the size of the file that was downloaded. However, if compression was enabled for the response, content-length specifies the compressed size of the data.

electron-updater treats content-length as the size of the uncompressed data (node/Electron handles the decompression transparently), which is incorrect.

---

This code creates a result buffer based on the value of content-length

electron-builder/packages/builder-util-runtime/src/httpExecutor.ts

Lines 242 to 252 in 7c05c36

	const contentLength = safeGetHeader(response, "content-length")
	let position = -1
	if (contentLength != null) {
	const size = parseInt(contentLength, 10)
	if (size > 0) {
	if (size > 524288000) {
	callback(new Error("Maximum allowed size is 500 MB"))
	return
	}
	result = Buffer.alloc(size)

And this code raises an error if the size of the received data does not match the size of that result buffer

electron-builder/packages/builder-util-runtime/src/httpExecutor.ts

Lines 270 to 272 in 7c05c36

	response.on("end", () => {
	if (result != null && position !== -1 && position !== result.length) {
	callback(new Error(`Received data length ${position} is not equal to expected ${result.length}`))

[NoahAndrews]

I accidentally submitted this sooner than I meant to. The issue description has been updated.