brew: check xcode license status

The xcrun code is stolen right out of the INSTALL script.

Check for user agreement of the Xcode license before permitting any other brew usage to continue.
This prevents the situation where people are instructed to "please re-run as root via sudo" on brew commands.
The check can only fail when Xcode is installed & the active developer dir.

Closes Homebrew/homebrew#44974.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Library/brew.rb

@@ -25,18 +25,30 @@ elsif ARGV.first == "-v"
   exit 0 if ARGV.length == 1
 end
 
-# Check for bad xcode-select before anything else, because `doctor` and
-# many other things will hang
-# Note that this bug was fixed in 10.9
-if OS.mac? && MacOS.version < :mavericks && MacOS.active_developer_dir == "/"
-  odie <<-EOS.undent
-  Your xcode-select path is currently set to '/'.
-  This causes the `xcrun` tool to hang, and can render Homebrew unusable.
-  If you are using Xcode, you should:
-    sudo xcode-select -switch /Applications/Xcode.app
-  Otherwise, you should:
-    sudo rm -rf /usr/share/xcode-select
-  EOS
+if OS.mac?
+  # Check for bad xcode-select before other checks, because `doctor` and
+  # many other things will hang. Note that this bug was fixed in 10.9
+  if MacOS.version < :mavericks && MacOS.active_developer_dir == "/"
+    odie <<-EOS.undent
+      Your xcode-select path is currently set to '/'.
+      This causes the `xcrun` tool to hang, and can render Homebrew unusable.
+      If you are using Xcode, you should:
+        sudo xcode-select -switch /Applications/Xcode.app
+      Otherwise, you should:
+        sudo rm -rf /usr/share/xcode-select
+    EOS
+  end
+
+  # Check for user agreement of the Xcode license before permitting
+  # any other brew usage to continue. This prevents the situation where
+  # people are instructed to "please re-run as root via sudo" on brew commands.
+  # The check can only fail when Xcode is installed & the active developer dir.
+  if MacOS::Xcode.installed? && `/usr/bin/xcrun clang 2>&1` =~ /license/ && !$?.success?
+    odie <<-EOS.undent
+      You have not agreed to the Xcode license. Please resolve this by running:
+        sudo xcodebuild -license
+    EOS
+  end
 end
 
 case HOMEBREW_PREFIX.to_s
@@ -45,7 +57,7 @@ when "/", "/usr"
   abort "Cowardly refusing to continue at this prefix: #{HOMEBREW_PREFIX}"
 end
 
-if OS.mac? and MacOS.version < "10.6"
+if OS.mac? && MacOS.version < "10.6"
   abort <<-EOABORT.undent
     Homebrew requires Snow Leopard or higher. For Tiger and Leopard support, see:
     https://github.com/mistydemeo/tigerbrew
@@ -88,7 +100,7 @@ begin
   sudo_check = %w[ install link pin unpin upgrade ]
 
   if sudo_check.include? cmd
-    if Process.uid.zero? and not File.stat(HOMEBREW_BREW_FILE).uid.zero?
+    if Process.uid.zero? && !File.stat(HOMEBREW_BREW_FILE).uid.zero?
       raise <<-EOS.undent
         Cowardly refusing to `sudo brew #{cmd}`
         You can use brew with sudo, but only if the brew executable is owned by root.