explain the options curl passes to bootstrap

also fixes a discrepency where the rust side doesn't use -L

must not be merged before rust-lang#129134

docs are only on the rust side, since duplicated prose
has a tendancy to get out-of-sync, and also because there
are talks of removing the python script all together eventually.

src/bootstrap/bootstrap.py

@@ -115,6 +115,10 @@ def _download(path, url, probably_big, verbose, exception):
  extra_flags = []
  if curl_version() > (7, 70):
  extra_flags = [ "--retry-all-errors" ]
+ # options should be kept in sync with
+ # src/bootstrap/src/core/download.rs
+ # for consistency.
+ # they are also more compreprensivly explained in that file.
  run(["curl", option] + extra_flags + [
  "-L", # Follow redirect.
  "-y", "30", "-Y", "10", # timeout if speed is < 10 bytes/sec for > 30 seconds

src/bootstrap/src/core/download.rs

@@ -227,20 +227,35 @@ impl Config {
  fn download_http_with_retries(&self, tempfile: &Path, url: &str, help_on_error: &str) {
  println!("downloading {url}");
  // Try curl. If that fails and we are on windows, fallback to PowerShell.
+ // options should be kept in sync with
+ // src/bootstrap/src/core/download.rs
+ // for consistency
  let mut curl = command("curl");
  curl.args([
+ // follow redirect
+ "-L",
+ // timeout if speed is < 10 bytes/sec for > 30 seconds
  "-y",
  "30",
  "-Y",
- "10", // timeout if speed is < 10 bytes/sec for > 30 seconds
+ "10",
+ // timeout if cannot connect within 30 seconds
  "--connect-timeout",
- "30", // timeout if cannot connect within 30 seconds
+ "30",
+ // output file
  "-o",
  tempfile.to_str().unwrap(),
+ // if there is an error, don't restart the download,
+ // instead continue where it left off.
  "--continue-at",
  "-",
+ // retry up to 3 times. note that this means a maximum of 4
+ // attempts will be made, since the first attempt isn't a *re*try.
  "--retry",
  "3",
+ // -S: show errors, even if -s is specified
+ // -R: set timestamp of downloaded file to that of the server
+ // -f: fail on non-ok http status
  "-SRf",
  ]);
  // Don't print progress in CI; the \r wrapping looks bad and downloads don't take long enough for progress to be useful.