feat: add error command

Closes #17

README.md

@@ -126,6 +126,22 @@ migrator.to_latest
 migrator.current_version # => 10
 ```
 
+### Errors
+
+A special command `+migrate error` is available. It raises `Migrate::Migration::Error` when a specific migration file is run. A error can be either top-level or direction-specific. This is useful to point out irreversible migrations:
+
+```sql
+-- +migrate up
+CREATE TABLE foo;
+
+-- +migrate down
+-- +migrate error Could not migrate down from this point
+```
+
+```sql
+-- +migrate error Could not run this migration file at all
+```
+
 ### [Cakefile](https://github.com/axvm/cake)
 
 Note that `Cakefile` doesn't support task arguments (that means that `Migrator#to` will not be available). Also see [cake-bake](https://github.com/vladfaust/cake-bake.cr) for baking Cakefiles (this could be helpful in Docker deployments).

spec/migrate/migrator_with_errors_spec.cr

@@ -0,0 +1,43 @@
+require "../spec_helper"
+
+db = DB.open(ENV["DATABASE_URL"])
+
+{% for table in %w(foo bar baz) %}
+  def {{table.id}}_exists?(db)
+    db.scalar("SELECT COUNT(*) FROM {{table.id}}").as(Int64)
+  rescue
+    false
+  end
+{% end %}
+
+describe "Migrate::Migrator with errors" do
+  drop_db
+
+  migrator = Migrate::Migrator.new(
+    db,
+    nil,
+    File.join("spec", "migrations_with_errors")
+  )
+
+  describe "direction-specific errors" do
+    it do
+      migrator.up
+
+      expect_raises Migrate::Migration::Error do
+        migrator.down
+      end
+
+      migrator.current_version.should eq 1
+    end
+  end
+
+  describe "top-level errors" do
+    it do
+      expect_raises Migrate::Migration::Error do
+        migrator.up
+      end
+
+      migrator.current_version.should eq 1
+    end
+  end
+end

spec/migrations_with_errors/1.sql

@@ -0,0 +1,11 @@
+-- +migrate up
+CREATE TABLE foo (
+  id      SERIAL PRIMARY KEY,
+  content TEXT NOT NULL
+);
+
+-- Indexes
+CREATE UNIQUE INDEX foo_content_index ON foo (content);
+
+-- +migrate down
+-- +migrate error Could not migrate down from version 1

spec/migrations_with_errors/2.sql

@@ -0,0 +1 @@
+-- +migrate error Nope

src/migrate/migration.cr

@@ -1,9 +1,12 @@
+require "./migration/*"
+
 module Migrate
   # :nodoc:
   struct Migration
     CMD_PREFIX = "-- +migrate"
 
     getter queries_up = Array(String).new, queries_down = Array(String).new
+    getter error : Exception | Nil, error_up : Exception | Nil, error_down : Exception | Nil
 
     def initialize(lines : Iterator)
       direction : Direction? = nil
@@ -12,12 +15,21 @@ module Migrate
 
       lines.each do |line|
         if line.starts_with?(CMD_PREFIX)
-          cmd = line[CMD_PREFIX.size..-1].strip.downcase
+          cmd = /^#{Regex.escape(CMD_PREFIX)} (?<cmd>\w+)(?:\s.+)?/.match(line).not_nil!["cmd"].strip.downcase
+
           case cmd
           when "up"
             direction = Direction::Up
           when "down"
             direction = Direction::Down
+          when "error"
+            message = /^#{Regex.escape(CMD_PREFIX)} error (?<message>.+)$/.match(line).try &.["message"]
+
+            case direction
+            when Direction::Up   then @error_up ||= Error.new(message)
+            when Direction::Down then @error_down ||= Error.new(message)
+            else                      @error ||= Error.new(message)
+            end
           else
             raise "Unknown command #{cmd}!"
           end

src/migrate/migration/error.cr

@@ -0,0 +1,7 @@
+module Migrate
+  struct Migration
+    # Could be raised after `error` command. See README for examples.
+    class Error < Exception
+    end
+  end
+end

src/migrate/migrator.cr

@@ -129,20 +129,30 @@ module Migrate
 
       applied_files.reverse! if direction == Direction::Down
 
-      migrations = applied_files.map do |file_path|
-        migration = Migration.new(File.join(@dir, file_path))
+      migrations = applied_files.map { |path| Migration.new(File.join(@dir, path)) }
+
+      migrations.each do |migration|
+        if error = migration.error
+          raise error
+        end
 
         case direction
         when Direction::Up
-          migration.queries_up
+          if error = migration.error_up
+            raise error
+          end
+
+          queries = migration.queries_up
         when Direction::Down
-          migration.queries_down
-        end.not_nil!
-      end
+          if error = migration.error_down
+            raise error
+          end
+
+          queries = migration.queries_down
+        end
 
-      migrations.each do |migration|
         @db.transaction do |tx|
-          migration.each do |query|
+          queries.not_nil!.each do |query|
             @logger.try &.debug(query)
             tx.connection.exec(query)
           end