io: migrate the Reader API to Result instead of Option (#15229)

vlib/os/file.c.v

@@ -1,5 +1,24 @@
 module os
 
+/// Eof error means that we reach the end of the file.
+pub struct Eof {
+	Error
+}
+
+// NotExpected is a generic error that means that we receave a not expecte error.
+pub struct NotExpected {
+	cause string
+	code  int
+}
+
+fn (err NotExpected) msg() string {
+	return err.cause
+}
+
+fn (err NotExpected) code() int {
+	return err.code
+}
+
 pub struct File {
 mut:
 	cfile voidptr // Using void* instead of FILE*
@@ -192,11 +211,16 @@ pub fn (mut f File) reopen(path string, mode string) ? {
 }
 
 // read implements the Reader interface.
-pub fn (f &File) read(mut buf []u8) ?int {
+pub fn (f &File) read(mut buf []u8) !int {
 	if buf.len == 0 {
-		return 0
+		return IError(Eof{})
+	}
+	nbytes := fread(buf.data, 1, buf.len, f.cfile) or {
+		return IError(NotExpected{
+			cause: 'unexpected error from fread'
+			code: -1
+		})
 	}
-	nbytes := fread(buf.data, 1, buf.len, f.cfile)?
 	return nbytes
 }
 

vlib/os/file_test.v

@@ -130,11 +130,11 @@ fn test_read_eof_last_read_partial_buffer_fill() ? {
 	f = os.open_file(tfile, 'r')?
 	mut br := []u8{len: 100}
 	// Read first 100 bytes of 199 byte file, should fill buffer with no error.
-	n0 := f.read(mut br)?
+	n0 := f.read(mut br) or { return error('failed to read 100 bytes') }
 	assert n0 == 100
 	// Read remaining 99 bytes of 199 byte file, should fill buffer with no
 	// error, even though end-of-file was reached.
-	n1 := f.read(mut br)?
+	n1 := f.read(mut br) or { return error('failed to read 100 bytes') }
 	assert n1 == 99
 	// Read again, end-of-file was previously reached so should return none
 	// error.
@@ -143,8 +143,8 @@ fn test_read_eof_last_read_partial_buffer_fill() ? {
 		// not return a number of bytes read when end-of-file is reached.
 		assert false
 	} else {
-		// Expect none to have been returned when end-of-file.
-		assert err is none
+		// Expected an error when received end-of-file.
+		assert err !is none
 	}
 	f.close()
 }
@@ -162,11 +162,11 @@ fn test_read_eof_last_read_full_buffer_fill() ? {
 	f = os.open_file(tfile, 'r')?
 	mut br := []u8{len: 100}
 	// Read first 100 bytes of 200 byte file, should fill buffer with no error.
-	n0 := f.read(mut br)?
+	n0 := f.read(mut br) or { return error('failed to read 100 bytes') }
 	assert n0 == 100
 	// Read remaining 100 bytes of 200 byte file, should fill buffer with no
 	// error. The end-of-file isn't reached yet, but there is no more data.
-	n1 := f.read(mut br)?
+	n1 := f.read(mut br) or { return error('failed to read 100 bytes') }
 	assert n1 == 100
 	// Read again, end-of-file was previously reached so should return none
 	// error.
@@ -175,8 +175,8 @@ fn test_read_eof_last_read_full_buffer_fill() ? {
 		// not return a number of bytes read when end-of-file is reached.
 		assert false
 	} else {
-		// Expect none to have been returned when end-of-file.
-		assert err is none
+		// Expect an error at EOF.
+		assert err !is none
 	}
 	f.close()
 }