software_spec: Add HOMEBREW_BOTTLE_DEFAULT_DOMAIN as a fallback option

Library/Homebrew/env_config.rb

@@ -46,8 +46,12 @@ module EnvConfig
       },
       HOMEBREW_BOTTLE_DOMAIN:                 {
         description:  "Use this URL as the download mirror for bottles. " \
+                      "If bottles at that URL are temporarily unavailable, " \
+                      "the default bottle domain will be used as a fallback mirror. " \
                       "For example, `HOMEBREW_BOTTLE_DOMAIN=http://localhost:8080` will cause all bottles to " \
-                      "download from the prefix `http://localhost:8080/`.",
+                      "download from the prefix `http://localhost:8080/`. " \
+                      "If bottles are not available at `HOMEBREW_BOTTLE_DOMAIN` " \
+                      "they will be downloaded from the default bottle domain.",
         default_text: "macOS: `https://homebrew.bintray.com/`, " \
                       "Linux: `https://linuxbrew.bintray.com/`.",
         default:      HOMEBREW_BOTTLE_DEFAULT_DOMAIN,

Library/Homebrew/software_spec.rb

@@ -2,6 +2,7 @@
 # frozen_string_literal: true
 
 require "resource"
+require "download_strategy"
 require "checksum"
 require "version"
 require "options"
@@ -292,7 +293,7 @@ def extname
 
   attr_reader :name, :resource, :prefix, :cellar, :rebuild
 
-  def_delegators :resource, :url, :fetch, :verify_download_integrity
+  def_delegators :resource, :url, :verify_download_integrity
   def_delegators :resource, :cached_download, :clear_cache
 
   def initialize(formula, spec)
@@ -314,6 +315,18 @@ def initialize(formula, spec)
     @rebuild = spec.rebuild
   end
 
+  def fetch(verify_download_integrity: true)
+    # add the default bottle domain as a fallback mirror
+    if @resource.download_strategy == CurlDownloadStrategy &&
+       @resource.url.start_with?(Homebrew::EnvConfig.bottle_domain)
+      fallback_url = @resource.url
+                              .sub(/^#{Regexp.escape(Homebrew::EnvConfig.bottle_domain)}/,
+                                   HOMEBREW_BOTTLE_DEFAULT_DOMAIN)
+      @resource.mirror(fallback_url) if [@resource.url, *@resource.mirrors].exclude?(fallback_url)
+    end
+    @resource.fetch(verify_download_integrity: verify_download_integrity)
+  end
+
   def compatible_locations?
     @spec.compatible_locations?
   end

docs/Manpage.md

@@ -1729,7 +1729,7 @@ example, run `export HOMEBREW_NO_INSECURE_REDIRECT=1` rather than just
   <br>If set, use Bootsnap to speed up repeated `brew` calls. A no-op when using Homebrew's vendored, relocatable Ruby on macOS (as it doesn't work).
 
 - `HOMEBREW_BOTTLE_DOMAIN`
-  <br>Use this URL as the download mirror for bottles. For example, `HOMEBREW_BOTTLE_DOMAIN=http://localhost:8080` will cause all bottles to download from the prefix `http://localhost:8080/`.
+  <br>Use this URL as the download mirror for bottles. If bottles at that URL are temporarily unavailable, the default bottle domain will be used as a fallback mirror. For example, `HOMEBREW_BOTTLE_DOMAIN=http://localhost:8080` will cause all bottles to download from the prefix `http://localhost:8080/`. If bottles are not available at `HOMEBREW_BOTTLE_DOMAIN` they will be downloaded from the default bottle domain.
 
   *Default:* macOS: `https://homebrew.bintray.com/`, Linux: `https://linuxbrew.bintray.com/`.
 

manpages/brew.1

@@ -2415,7 +2415,7 @@ If set, use Bootsnap to speed up repeated \fBbrew\fR calls\. A no\-op when using
 \fBHOMEBREW_BOTTLE_DOMAIN\fR
 .
 .br
-Use this URL as the download mirror for bottles\. For example, \fBHOMEBREW_BOTTLE_DOMAIN=http://localhost:8080\fR will cause all bottles to download from the prefix \fBhttp://localhost:8080/\fR\.
+Use this URL as the download mirror for bottles\. If bottles at that URL are temporarily unavailable, the default bottle domain will be used as a fallback mirror\. For example, \fBHOMEBREW_BOTTLE_DOMAIN=http://localhost:8080\fR will cause all bottles to download from the prefix \fBhttp://localhost:8080/\fR\. If bottles are not available at \fBHOMEBREW_BOTTLE_DOMAIN\fR they will be downloaded from the default bottle domain\.
 .
 .IP
 \fIDefault:\fR macOS: \fBhttps://homebrew\.bintray\.com/\fR, Linux: \fBhttps://linuxbrew\.bintray\.com/\fR\.