From 800d139a3db273bf9b2a4d6562c9c6dfc012975b Mon Sep 17 00:00:00 2001
From: Luka Vandervelden <lukc@upyum.com>
Date: Wed, 24 Jun 2020 21:45:45 +0200
Subject: [PATCH] WIP for lock files and atomic operations.

---
 spec/test.cr      | 22 ++++++++++++++++++++++
 src/dodb.cr       | 25 +++++++++++++++++++++++++
 src/dodb/index.cr | 15 +++++++++++++++
 src/dodb/lib_c.cr |  9 +++++++++
 4 files changed, 71 insertions(+)
 create mode 100644 src/dodb/lib_c.cr

diff --git a/spec/test.cr b/spec/test.cr
index ac4204f..43863d1 100644
--- a/spec/test.cr
+++ b/spec/test.cr
@@ -396,6 +396,28 @@ describe "DODB::DataBase" do
 		end
 	end
 
+	describe "atomic operations" do
+		it "safe_get and safe_get?" do
+			db = DODB::SpecDataBase.new
+
+			db_ships_by_name = db.new_index "name", &.name
+
+			Ship.all_ships.each do |ship|
+				db << ship
+			end
+
+			Ship.all_ships.each do |ship|
+				db_ships_by_name.safe_get ship.name do |results|
+					results.should eq(ship)
+				end
+
+				db_ships_by_name.safe_get? ship.name do |results|
+					results.should eq(ship)
+				end
+			end
+		end
+	end
+
 	describe "tools" do
 		it "rebuilds indexes" do
 			db = DODB::SpecDataBase.new
diff --git a/src/dodb.cr b/src/dodb.cr
index 28fad7d..2a34db6 100644
--- a/src/dodb.cr
+++ b/src/dodb.cr
@@ -8,6 +8,7 @@ class DODB::DataBase(V)
 
 	def initialize(@directory_name : String)
 		Dir.mkdir_p data_path
+		Dir.mkdir_p locks_directory
 
 		begin
 			self.last_index
@@ -43,6 +44,22 @@ class DODB::DataBase(V)
 		end
 	end
 
+	def request_lock(name)
+		r = -1
+		file_path = get_lock_file_path name
+		file_perms = 0o644
+
+		flags = LibC::O_EXCL | LibC::O_CREAT
+		while (r = LibC.open file_path, flags, file_perms) == -1
+			sleep 1.milliseconds
+		end
+
+		LibC.close r
+	end
+	def release_lock(name)
+		File.delete get_lock_file_path name
+	end
+
 	##
 	# name is the name that will be used on the file system.
 	def new_partition(name : String, &block : Proc(V, String))
@@ -279,6 +296,14 @@ class DODB::DataBase(V)
 		"#{data_path}/%010i.json" % key
 	end
 
+	private def locks_directory : String
+		"#{@directory_name}/locks"
+	end
+
+	private def get_lock_file_path(name : String)
+		"#{locks_directory}/#{name}.lock"
+	end
+
 	private def read(file_path : String)
 		V.from_json ::File.read file_path
 	end
diff --git a/src/dodb/index.cr b/src/dodb/index.cr
index df79068..5c518f3 100644
--- a/src/dodb/index.cr
+++ b/src/dodb/index.cr
@@ -68,6 +68,21 @@ class DODB::Index(V) < DODB::Indexer(V)
 		nil
 	end
 
+	def safe_get(index : String) : Nil
+		internal_key = get_key(index).to_s
+		@storage.request_lock internal_key
+
+		yield get index
+
+		@storage.release_lock internal_key
+	end
+
+	def safe_get?(index : String, &block : Proc(V | Nil, Nil)) : Nil
+		safe_get index, &block
+	rescue MissingEntry
+		yield nil
+	end
+
 	def get_key(index : String) : Int32
 		file_path = file_path_index index
 
diff --git a/src/dodb/lib_c.cr b/src/dodb/lib_c.cr
new file mode 100644
index 0000000..5a32b78
--- /dev/null
+++ b/src/dodb/lib_c.cr
@@ -0,0 +1,9 @@
+
+lib LibC
+	{% if flag?(:linux) %}
+		O_EXCL = 0o200
+	{% elsif flag?(:openbsd) %}
+		O_EXCL = 0x0800
+	{% end %}
+end
+