os: add os.open_stdin/0 and os.File.get_line/0

vlib/os/file.v

@@ -1,5 +1,7 @@
 module os
 
+import strings
+
 pub struct File {
 	cfile  voidptr // Using void* instead of FILE*
 pub:
@@ -88,3 +90,63 @@ pub fn (mut f File) flush() {
 	}
 	C.fflush(f.cfile)
 }
+
+// open_stdin - return an os.File for stdin, so that you can use .get_line on it too.
+pub fn open_stdin() File {
+	return File{
+		fd: 0
+		cfile: C.stdin
+		is_opened: true
+	}
+}
+
+// File.get_line - get a single line from the file. NB: the ending newline is *included*.
+pub fn (mut f File) get_line() ?string {
+	if !f.is_opened {
+		return error('file is closed')
+	}
+	$if !windows {
+		mut zbuf := byteptr(0)
+		mut zblen := size_t(0)
+		mut zx := 0
+		unsafe {
+			zx = C.getline(&zbuf, &zblen, f.cfile)
+			if zx == -1 {
+				C.free(zbuf)
+				if C.errno == 0 {
+					return error('end of file')
+				}
+				return error(posix_get_error_msg(C.errno))
+			}
+			return zbuf.vstring_with_len(zx)
+		}
+	}
+	//
+	// using C.fgets is less efficient than f.get_line_getline,
+	// but is available everywhere, while C.getline does not work
+	// on windows
+	//
+	buf := [4096]byte{}
+	mut res := strings.new_builder(1024)
+	mut x := 0
+	for {
+		unsafe {
+			x = C.fgets(charptr(buf), 4096, f.cfile)
+		}
+		if x == 0 {
+			if res.len > 0 {
+				break
+			}
+			return error('end of file')
+		}
+		bufbp := byteptr(buf)
+		mut blen := vstrlen(bufbp)
+		res.write_bytes(bufbp, blen)
+		unsafe {
+			if blen == 0 || bufbp[blen-1] == `\n` || bufbp[blen-1] == `\r` {
+				break
+			}
+		}
+	}
+	return res.str()
+}