Add File.absolute_path? (#2198)

In order to check whether a path is absolute or not in a portable way.

[Feature #15868]

file.c

@@ -4106,6 +4106,25 @@ s_absolute_path(int c, const VALUE * v, VALUE _)
     return rb_file_s_absolute_path(c, v);
 }
 
+/*
+ *  call-seq:
+ *     File.absolute_path?(file_name)  ->  true or false
+ *
+ *  Returns <code>true</code> if +file_name+ is an absolute path, and
+ *  <code>false</code> otherwise.
+ *
+ *     File.absolute_path?("c:/foo")     #=> false (on Linux), true (on Windows)
+ */
+
+static VALUE
+s_absolute_path_p(VALUE klass, VALUE fname)
+{
+    VALUE path = rb_get_path(fname);
+
+    if (!rb_is_absolute_path(RSTRING_PTR(path))) return Qfalse;
+    return Qtrue;
+}
+
 enum rb_realpath_mode {
     RB_REALPATH_CHECK,
     RB_REALPATH_DIR,
@@ -6504,6 +6523,7 @@ Init_File(void)
     rb_define_singleton_method(rb_cFile, "mkfifo", rb_file_s_mkfifo, -1);
     rb_define_singleton_method(rb_cFile, "expand_path", s_expand_path, -1);
     rb_define_singleton_method(rb_cFile, "absolute_path", s_absolute_path, -1);
+    rb_define_singleton_method(rb_cFile, "absolute_path?", s_absolute_path_p, 1);
     rb_define_singleton_method(rb_cFile, "realpath", rb_file_s_realpath, -1);
     rb_define_singleton_method(rb_cFile, "realdirpath", rb_file_s_realdirpath, -1);
     rb_define_singleton_method(rb_cFile, "basename", rb_file_s_basename, -1);

spec/ruby/core/file/absolute_path_spec.rb

@@ -1,5 +1,58 @@
 require_relative '../../spec_helper'
 
+ruby_version_is "2.7" do
+  describe "File.absolute_path?" do
+    before :each do
+      @abs = File.expand_path(__FILE__)
+    end
+
+    it "returns true if it's an absolute pathname" do
+      File.absolute_path?(@abs).should be_true
+    end
+
+    it "returns false if it's a relative path" do
+      File.absolute_path?(File.basename(__FILE__)).should be_false
+    end
+
+    it "returns false if it's a tricky relative path" do
+      File.absolute_path?("C:foo\\bar").should be_false
+    end
+
+    it "does not expand '~' to a home directory." do
+      File.absolute_path?('~').should be_false
+    end
+
+    it "does not expand '~user' to a home directory." do
+      path = File.dirname(@abs)
+      Dir.chdir(path) do
+        File.absolute_path?('~user').should be_false
+      end
+    end
+
+    it "calls #to_path on its argument" do
+      mock = mock_to_path(File.expand_path(__FILE__))
+
+      File.absolute_path?(mock).should be_true
+    end
+
+    platform_is_not :windows do
+      it "takes into consideration the platform's root" do
+        File.absolute_path?("C:\\foo\\bar").should be_false
+        File.absolute_path?("C:/foo/bar").should be_false
+        File.absolute_path?("/foo/bar\\baz").should be_true
+      end
+    end
+
+    platform_is :windows do
+      it "takes into consideration the platform path separator(s)" do
+        File.absolute_path?("C:\\foo\\bar").should be_true
+        File.absolute_path?("C:/foo/bar").should be_true
+        File.absolute_path?("/foo/bar\\baz").should be_false
+      end
+    end
+  end
+end
+
 describe "File.absolute_path" do
   before :each do
     @abs = File.expand_path(__FILE__)