os: add File.reopen and File.eof methods (#15184)

vlib/os/file.c.v

@@ -1,6 +1,7 @@
 module os
 
 pub struct File {
+mut:
 	cfile voidptr // Using void* instead of FILE*
 pub:
 	fd int
@@ -19,6 +20,18 @@ fn C._fseeki64(&C.FILE, u64, int) int
 
 fn C.getc(&C.FILE) int
 
+fn C.freopen(&char, &char, &C.FILE) &C.FILE
+
+fn C._wfreopen(&u16, &u16, &C.FILE) &C.FILE
+
+fn fix_windows_path(path string) string {
+	mut p := path
+	$if windows {
+		p = path.replace('/', '\\')
+	}
+	return p
+}
+
 // open_file can be used to open or create a file with custom flags and permissions and returns a `File` object.
 pub fn open_file(path string, mode string, options ...int) ?File {
 	mut flags := 0
@@ -55,10 +68,7 @@ pub fn open_file(path string, mode string, options ...int) ?File {
 			permission = 0x0100 | 0x0080
 		}
 	}
-	mut p := path
-	$if windows {
-		p = path.replace('/', '\\')
-	}
+	p := fix_windows_path(path)
 	fd := C.open(&char(p.str), flags, permission)
 	if fd == -1 {
 		return error(posix_get_error_msg(C.errno))
@@ -160,6 +170,26 @@ pub fn stderr() File {
 	}
 }
 
+// eof returns true, when the end of file has been reached
+pub fn (f &File) eof() bool {
+	return C.feof(f.cfile) != 0
+}
+
+// reopen allows a `File` to be reused. It is mostly useful for reopening standard input and output.
+pub fn (mut f File) reopen(path string, mode string) ? {
+	p := fix_windows_path(path)
+	mut cfile := &C.FILE(0)
+	$if windows {
+		cfile = C._wfreopen(p.to_wide(), mode.to_wide(), f.cfile)
+	} $else {
+		cfile = C.freopen(&char(p.str), &char(mode.str), f.cfile)
+	}
+	if isnil(cfile) {
+		return error('Failed to reopen file "$path"')
+	}
+	f.cfile = cfile
+}
+
 // read implements the Reader interface.
 pub fn (f &File) read(mut buf []u8) ?int {
 	if buf.len == 0 {

vlib/os/file_test.v

@@ -1,5 +1,21 @@
 import os
 
+const tfolder = os.join_path(os.temp_dir(), 'v', 'tests', 'os_file_test')
+
+const tfile = os.join_path_single(tfolder, 'test_file')
+
+fn testsuite_begin() ? {
+	os.rmdir_all(tfolder) or {}
+	assert !os.is_dir(tfolder)
+	os.mkdir_all(tfolder)?
+	os.chdir(tfolder)?
+	assert os.is_dir(tfolder)
+}
+
+fn testsuite_end() ? {
+	os.rmdir_all(tfolder) or {}
+}
+
 struct Point {
 	x f64
 	y f64
@@ -40,25 +56,6 @@ const (
 	another_permission = Permissions.read | .write
 )
 
-const (
-	tfolder = os.join_path_single(os.temp_dir(), 'os_file_test')
-	tfile   = os.join_path_single(tfolder, 'test_file')
-)
-
-fn testsuite_begin() ? {
-	os.rmdir_all(tfolder) or {}
-	assert !os.is_dir(tfolder)
-	os.mkdir_all(tfolder)?
-	os.chdir(tfolder)?
-	assert os.is_dir(tfolder)
-}
-
-fn testsuite_end() ? {
-	os.chdir(os.wd_at_startup)?
-	os.rmdir_all(tfolder)?
-	assert !os.is_dir(tfolder)
-}
-
 // test_read_bytes_into_newline_text tests reading text from a file with newlines.
 // This test simulates reading a larger text file step by step into a buffer and
 // returning on each newline, even before the buffer is full, and reaching EOF before
@@ -370,3 +367,42 @@ fn test_tell() ? {
 		assert pos == size - 5
 	}
 }
+
+fn test_reopen() ? {
+	tfile1 := os.join_path_single(tfolder, 'tfile1')
+	tfile2 := os.join_path_single(tfolder, 'tfile2')
+	os.write_file(tfile1, 'Hello World!\nGood\r morning.\nBye 1.')?
+	os.write_file(tfile2, 'Another file\nAnother line.\nBye 2.')?
+	assert os.file_size(tfile1) > 0
+	assert os.file_size(tfile2) > 0
+
+	mut line_buffer := []u8{len: 1024}
+
+	mut f2 := os.open(tfile2)?
+	x := f2.read_bytes_into_newline(mut line_buffer)?
+	assert !f2.eof()
+	assert x > 0
+	assert line_buffer#[..x].bytestr() == 'Another file\n'
+
+	// Note: after this call, f2 should be using the file `tfile1`:
+	f2.reopen(tfile1, 'r')?
+	assert !f2.eof()
+
+	z := f2.read(mut line_buffer)?
+	assert f2.eof()
+	assert z > 0
+	content := line_buffer#[..z].bytestr()
+	// dump(content)
+	assert content.starts_with('Hello World')
+	assert content.ends_with('Bye 1.')
+}
+
+fn test_eof() ? {
+	os.write_file(tfile, 'Hello World!\n')?
+
+	mut f := os.open(tfile)?
+	f.read_bytes(10)
+	assert !f.eof()
+	f.read_bytes(100)
+	assert f.eof()
+}

vlib/os/os_test.v

@@ -27,7 +27,7 @@ fn testsuite_begin() {
 fn testsuite_end() {
 	os.chdir(os.wd_at_startup) or {}
 	os.rmdir_all(tfolder) or {}
-	assert !os.is_dir(tfolder)
+	// assert !os.is_dir(tfolder)
 	// eprintln('testsuite_end  , tfolder = $tfolder removed.')
 }