Replace the API download retry loop with a single retry

Signed-off-by: Patrick Linnane <patrick@linnane.io>

Library/Homebrew/cmd/update.sh

@@ -398,6 +398,21 @@ EOWARN
   trap - SIGINT
 }
 
+api_curl_download() {
+  local json_url="$1"
+  local cache_path="$2"
+  shift 2
+
+  curl \
+    "${CURL_DISABLE_CURLRC_ARGS[@]}" \
+    --fail --compressed --silent \
+    --speed-limit "${HOMEBREW_CURL_SPEED_LIMIT}" --speed-time "${HOMEBREW_CURL_SPEED_TIME}" \
+    --location --remote-time --output "${cache_path}" \
+    "$@" \
+    --user-agent "${HOMEBREW_USER_AGENT_CURL}" \
+    "${json_url}"
+}
+
 fetch_api_file() {
   local filename="$1"
   local update_failed_file="$2"
@@ -436,23 +451,15 @@ fetch_api_file() {
     do
       time_cond+=("${arg}")
     done < <(api_time_cond_args "${cache_path}")
-    while true
-    do
-      curl \
-        "${CURL_DISABLE_CURLRC_ARGS[@]}" \
-        --fail --compressed --silent \
-        --speed-limit "${HOMEBREW_CURL_SPEED_LIMIT}" --speed-time "${HOMEBREW_CURL_SPEED_TIME}" \
-        --location --remote-time --output "${cache_path}" \
-        "${time_cond[@]}" \
-        --user-agent "${HOMEBREW_USER_AGENT_CURL}" \
-        "${json_url}"
+    api_curl_download "${json_url}" "${cache_path}" "${time_cond[@]}"
+    curl_exit_code=$?
+    # A conditional request can fail with a receive error (curl exit code 56) when
+    # an unconditional request for the same URL succeeds, so retry exactly once.
+    if [[ ${curl_exit_code} -eq 56 ]] && [[ ${#time_cond[@]} -gt 0 ]]
+    then
+      api_curl_download "${json_url}" "${cache_path}"
       curl_exit_code=$?
-      if [[ ${curl_exit_code} -ne 56 ]] || [[ ${#time_cond[@]} -eq 0 ]]
-      then
-        break
-      fi
-      time_cond=()
-    done
+    fi
     last_json_url="${json_url}"
     [[ ${curl_exit_code} -eq 0 ]] && break
   done < <(api_urls "${filename}")