os: add fn `read_bytes_into` with buffer argument (#6365)

vlib/os/file.v

@@ -78,21 +78,42 @@ pub fn (mut f File) write_bytes_at(data voidptr, size, pos int) int {
 }
 
 // **************************** Read ops  ***************************
-// read_bytes reads an amount of bytes from the beginning of the file
+// read_bytes reads bytes from the beginning of the file
 pub fn (f &File) read_bytes(size int) []byte {
 	return f.read_bytes_at(size, 0)
 }
 
-// read_bytes_at reads an amount of bytes at the given position in the file
+// read_bytes_at reads bytes at the given position in the file
 pub fn (f &File) read_bytes_at(size, pos int) []byte {
-	// mut arr := [`0`].repeat(size)
 	mut arr := []byte{len: size}
-	C.fseek(f.cfile, pos, C.SEEK_SET)
-	nreadbytes := C.fread(arr.data, 1, size, f.cfile)
-	C.fseek(f.cfile, 0, C.SEEK_SET)
+	nreadbytes := f.read_bytes_into(pos, arr) or {
+		// return err
+		return []
+	}
 	return arr[0..nreadbytes]
 }
 
+// read_bytes_from fills `buf` with bytes at the given position in the file.
+// `buf` must have length greater than zero.
+// Returns number of bytes read or an error.
+pub fn (f &File) read_bytes_into(pos int, mut buf []byte) ?int {
+	if buf.len == 0 {
+		panic(@FN + ': `buf.len` == 0')
+	}
+	// Note: fseek errors if pos == os.file_size, which we accept
+	C.fseek(f.cfile, pos, C.SEEK_SET)
+	// errno is only set if fread fails, so clear it first to tell
+	C.errno = 0
+	nbytes := C.fread(buf.data, 1, buf.len, f.cfile)
+	if C.errno != 0 {
+		return error(posix_get_error_msg(C.errno))
+	}
+	$if debug {
+		C.fseek(f.cfile, 0, C.SEEK_SET)
+	}
+	return nbytes
+}
+
 // **************************** Utility  ops ***********************
 // write any unwritten data in stream's buffer
 pub fn (mut f File) flush() {

vlib/os/os_test.v

@@ -142,11 +142,19 @@ fn test_write_and_read_bytes() {
 	// read_bytes_at with second parameter zeroed (size, 0).
 	rbytes := file_read.read_bytes(5)
 
-	file_read.close()
 	// eprintln('rbytes: $rbytes')
 	// eprintln('payload: $payload')
 	assert rbytes == payload
 
+	// check that trying to read data from EOF doesn't error and returns 0
+	mut a := []byte{len: 5}
+	nread := file_read.read_bytes_into(5, a) or {
+		eprintln(err)
+		int(-1)
+	}
+	assert nread == 0
+
+	file_read.close()
 	// We finally delete the test file.
 	os.rm(file_name)
 }