From 1b6a200809ce5963795a0d60bacd72dc40d7692e Mon Sep 17 00:00:00 2001
From: Xu Cheng <xucheng@me.com>
Date: Tue, 10 May 2016 20:16:25 +0800
Subject: [PATCH] update: use lock

Closes #181.

Signed-off-by: Xu Cheng <xucheng@me.com>
---
 Library/Homebrew/cmd/update.sh | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index 9679c0f8e2..0ad66b8b44 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -5,6 +5,8 @@
 #:    If `--merge` is specified then `git merge` is used to include updates
 #:      (rather than `git rebase`).
 
+source "$HOMEBREW_LIBRARY/Homebrew/utils/lock.sh"
+
 brew() {
   "$HOMEBREW_BREW_FILE" "$@"
 }
@@ -307,6 +309,9 @@ EOS
   # ensure GIT_CONFIG is unset as we need to operate on .git/config
   unset GIT_CONFIG
 
+  # only allow one instance of brew update
+  lock update
+
   chdir "$HOMEBREW_REPOSITORY"
   git_init_if_necessary
   # rename Taps directories