Better handle missing environment variables when updating

Try to re-exec the Homebrew binary to get the environment variables we
need and give up if that doesn't work.

Library/Homebrew/brew.rb

@@ -10,7 +10,21 @@ if RUBY_X < 2 || (RUBY_X == 2 && RUBY_Y < 3)
   raise "Homebrew must be run under Ruby 2.3! You're running #{RUBY_VERSION}."
 end
 
+begin
   require_relative "global"
+rescue MissingEnvironmentVariables => e
+  raise e if ENV["HOMEBREW_MISSING_ENV_RETRY"]
+
+  if ENV["HOMEBREW_DEVELOPER"]
+    $stderr.puts <<~EOS
+      Warning: #{e.message}
+      Retrying with `exec #{ENV["HOMEBREW_BREW_FILE"]}`!
+    EOS
+  end
+
+  ENV["HOMEBREW_MISSING_ENV_RETRY"] = "1"
+  exec ENV["HOMEBREW_BREW_FILE"], *ARGV
+end
 
 begin
   trap("INT", std_trap) # restore default CTRL-C handler

Library/Homebrew/config.rb

@@ -1,18 +1,18 @@
-def get_env_or_raise(env, message = nil)
-  message ||= <<~EOS
-    don't worry, you likely hit a bug auto-updating from an old version.
-    Rerun your command, everything is up-to-date and fine now
-  EOS
-  unless ENV[env]
-    abort <<~EOS
-      Error: #{env} was not exported!\nPlease #{message.chomp}.
-    EOS
-  end
-  ENV[env]
+unless ENV["HOMEBREW_BREW_FILE"]
+  raise "HOMEBREW_BREW_FILE was not exported! Please call bin/brew directly!"
 end
 
 # Path to `bin/brew` main executable in `HOMEBREW_PREFIX`
-HOMEBREW_BREW_FILE = Pathname.new(get_env_or_raise("HOMEBREW_BREW_FILE", "call bin/brew directly"))
+HOMEBREW_BREW_FILE = Pathname.new(ENV["HOMEBREW_BREW_FILE"])
+
+class MissingEnvironmentVariables < RuntimeError; end
+
+def get_env_or_raise(env)
+  unless ENV[env]
+    raise MissingEnvironmentVariables, "#{env} was not exported!"
+  end
+  ENV[env]
+end
 
 # Where we link under
 HOMEBREW_PREFIX = Pathname.new(get_env_or_raise("HOMEBREW_PREFIX"))