Prevent race condition when deleting lock files.

Library/Homebrew/lock_file.rb

@@ -5,6 +5,9 @@ require "fcntl"
 
 # A lock file to prevent multiple Homebrew processes from modifying the same path.
 class LockFile
+  class OpenFileChangedOnDisk < RuntimeError; end
+  private_constant :OpenFileChangedOnDisk
+
   attr_reader :path
 
   sig { params(type: Symbol, locked_path: Pathname).void }
@@ -18,10 +21,44 @@ class LockFile
   sig { void }
   def lock
     ignore_interrupts do
-      create_lockfile
+      next if @lockfile.present?
 
-      next if @lockfile.flock(File::LOCK_EX | File::LOCK_NB)
+      path.dirname.mkpath
 
+      begin
+        lockfile = begin
+          path.open(File::RDWR | File::CREAT)
+        rescue Errno::EMFILE
+          odie "The maximum number of open files on this system has been reached. " \
+               "Use `ulimit -n` to increase this limit."
+        end
+        lockfile.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
+
+        if lockfile.flock(File::LOCK_EX | File::LOCK_NB)
+          # This prevents a race condition in case the file we locked doesn't exist on disk anymore, e.g.:
+          #
+          # 1. Process A creates and opens the file.
+          # 2. Process A locks the file.
+          # 3. Process B opens the file.
+          # 4. Process A unlinks the file.
+          # 5. Process A unlocks the file.
+          # 6. Process B locks the file.
+          # 7. Process C creates and opens the file.
+          # 8. Process C locks the file.
+          # 9. Process B and C hold locks to files with different inode numbers. 💥
+          if !path.exist? || lockfile.stat.ino != path.stat.ino
+            lockfile.close
+            raise OpenFileChangedOnDisk
+          end
+
+          @lockfile = lockfile
+          next
+        end
+      rescue OpenFileChangedOnDisk
+        retry
+      end
+
+      lockfile.close
       raise OperationInProgressError, @locked_path
     end
   end
@@ -29,15 +66,12 @@ class LockFile
   sig { params(unlink: T::Boolean).void }
   def unlock(unlink: false)
     ignore_interrupts do
-      next if @lockfile.nil? || @lockfile.closed?
+      next if @lockfile.nil?
 
+      @path.unlink if unlink
       @lockfile.flock(File::LOCK_UN)
       @lockfile.close
-
-      if unlink
-        @path.unlink if @path.exist?
-        @lockfile = nil
-      end
+      @lockfile = nil
     end
   end
 
@@ -47,21 +81,6 @@ class LockFile
   ensure
     unlock
   end
-
-  private
-
-  def create_lockfile
-    return if @lockfile.present? && !@lockfile.closed?
-
-    @path.dirname.mkpath
-
-    begin
-      @lockfile = @path.open(File::RDWR | File::CREAT)
-    rescue Errno::EMFILE
-      odie "The maximum number of open files on this system has been reached. Use `ulimit -n` to increase this limit."
-    end
-    @lockfile.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
-  end
 end
 
 # A lock file for a formula.