Check the lock directory is writable.

And, if it isn't, print more helpful debugging messages.

Fixes #1456.

Library/Homebrew/diagnostic.rb

@@ -348,6 +348,20 @@ module Homebrew
         EOS
       end
 
+      def check_access_lock_dir
+        return unless HOMEBREW_LOCK_DIR.exist?
+        return if HOMEBREW_LOCK_DIR.writable_real?
+
+        <<-EOS.undent
+          #{HOMEBREW_LOCK_DIR} isn't writable.
+          Homebrew writes lock files to this location.
+
+          You should change the ownership and permissions of #{HOMEBREW_LOCK_DIR}
+          back to your user account.
+            sudo chown -R $(whoami) #{HOMEBREW_LOCK_DIR}
+        EOS
+      end
+
       def check_access_logs
         return unless HOMEBREW_LOGS.exist?
         return if HOMEBREW_LOGS.writable_real?

Library/Homebrew/utils/lock.sh

@@ -6,6 +6,14 @@ lock() {
   local lock_dir="$HOMEBREW_PREFIX/var/homebrew/locks"
   local lock_file="$lock_dir/$name"
   [[ -d "$lock_dir" ]] || mkdir -p "$lock_dir"
+  if ! [[ -w "$lock_dir" ]]
+  then
+    odie <<EOS
+Can't create $name lock in $lock_dir!
+Fix permissions by running:
+  sudo chown -R \$(whoami) $HOMEBREW_PREFIX/var/homebrew
+EOS
+  fi
   # 200 is the file descriptor used in the lock.
   # This FD should be used exclusively for lock purpose.
   # Any value except 0(stdin), 1(stdout) and 2(stderr) can do the job.
@@ -17,10 +25,10 @@ lock() {
   exec 200>&-
   # open the lock file to FD, so the shell process can hold the lock.
   exec 200>"$lock_file"
-  if ! _create_lock 200
+  if ! _create_lock 200 "$name"
   then
     odie <<EOS
-Another active Homebrew process is already in progress.
+Another active Homebrew $name process is already in progress.
 Please wait for it to finish or terminate it to continue.
 EOS
   fi
@@ -28,6 +36,7 @@ EOS
 
 _create_lock() {
   local lock_fd="$1"
+  local name="$2"
   local ruby="/usr/bin/ruby"
   [[ -x "$ruby" ]] || ruby="$(which ruby 2>/dev/null)"
 
@@ -38,6 +47,6 @@ _create_lock() {
   then
     flock -n "$lock_fd"
   else
-    onoe "Cannot create lock, please avoid running brew in parallel."
+    onoe "Cannot create $name lock, please avoid running Homebrew in parallel."
   fi
 }