Rebase the speed limit on top of the re-sync'd installer script

Author: grahamc

nix-installer.sh

@@ -453,20 +453,34 @@ check_help_for() {
 }
 
 # Check if curl supports the --retry flag, then pass it to the curl invocation.
+# Note that --speed-limit and --speed-time were in the very first commit of curl.
+# So this should be pretty much ubiquitously safe.
 check_curl_for_retry_support() {
-    local _retry_supported=""
-    # "unspecified" is for arch, allows for possibility old OS using macports, homebrew, etc.
-    if check_help_for "notspecified" "curl" "--retry"; then
-        _retry_supported="--retry 3"
-        if check_help_for "notspecified" "curl" "--continue-at"; then
-            # "-C -" tells curl to automatically find where to resume the download when retrying.
-            _retry_supported="--retry 3 -C -"
-        fi
-    fi
-
-    RETVAL="$_retry_supported"
+  local _retry_part=""
+  local _continue_part=""
+  local _speed_limit_part=""
+
+  # "unspecified" is for arch, allows for possibility old OS using macports, homebrew, etc.
+  if check_help_for "notspecified" "curl" "--retry"; then
+      _retry_supported="--retry 3"
+
+      if check_help_for "notspecified" "curl" "--continue-at"; then
+          # "-C -" tells curl to automatically find where to resume the download when retrying.
+          _continue_part="--continue-at -"
+      fi
+
+      if check_help_for "notspecified" "curl" "--speed-limit" \
+          && check_help_for "notspecified" "curl" "--speed-time"; then
+        # 250000 is approximately 20% of the bandwidth of typical DSL
+        # these limits mean users below these limits will see failures.
+        _speed_limit_part="--speed-limit 250000 --speed-time 15"
+      fi
+  fi
+
+  RETVAL="$_retry_part $_continue_part $_speed_limit_part"
 }
 
+
 # Return cipher suite string specified by user, otherwise return strong TLS 1.2-1.3 cipher suites
 # if support by local tools is detected. Detection currently supports these curl backends:
 # GnuTLS and OpenSSL (possibly also LibreSSL and BoringSSL). Return value can be empty.