Fix filecount plugin size tests (#6038)

plugins/inputs/filecount/filesystem_helpers.go

@@ -0,0 +1,73 @@
+package filecount
+
+import (
+	"errors"
+	"io"
+	"os"
+	"time"
+)
+
+/*
+	The code below is lifted from numerous articles and originates from Andrew Gerrand's 10 things you (probably) don't know about Go.
+	it allows for mocking a filesystem; this allows for consistent testing of this code across platforms (directory sizes reported
+	differently by different platforms, for example), while preserving the rest of the functionality as-is, without modification.
+*/
+
+type fileSystem interface {
+	Open(name string) (file, error)
+	Stat(name string) (os.FileInfo, error)
+}
+
+type file interface {
+	io.Closer
+	io.Reader
+	io.ReaderAt
+	io.Seeker
+	Stat() (os.FileInfo, error)
+}
+
+// osFS implements fileSystem using the local disk
+type osFS struct{}
+
+func (osFS) Open(name string) (file, error)        { return os.Open(name) }
+func (osFS) Stat(name string) (os.FileInfo, error) { return os.Stat(name) }
+
+/*
+	The following are for mocking the filesystem - this allows us to mock Stat() files. This means that we can set file attributes, and know that they
+	will be the same regardless of the platform sitting underneath our tests (directory sizes vary see https://github.com/influxdata/telegraf/issues/6011)
+
+	NOTE: still need the on-disk file structure to mirror this because the 3rd party library ("github.com/karrick/godirwalk") uses its own
+	walk functions, that we cannot mock from here.
+*/
+
+type fakeFileSystem struct {
+	files map[string]fakeFileInfo
+}
+
+type fakeFileInfo struct {
+	name     string
+	size     int64
+	filemode uint32
+	modtime  time.Time
+	isdir    bool
+	sys      interface{}
+}
+
+func (f fakeFileInfo) Name() string       { return f.name }
+func (f fakeFileInfo) Size() int64        { return f.size }
+func (f fakeFileInfo) Mode() os.FileMode  { return os.FileMode(f.filemode) }
+func (f fakeFileInfo) ModTime() time.Time { return f.modtime }
+func (f fakeFileInfo) IsDir() bool        { return f.isdir }
+func (f fakeFileInfo) Sys() interface{}   { return f.sys }
+
+func (f fakeFileSystem) Open(name string) (file, error) {
+	return nil, &os.PathError{Op: "Open", Path: name, Err: errors.New("Not implemented by fake filesystem")}
+}
+
+func (f fakeFileSystem) Stat(name string) (os.FileInfo, error) {
+	if fakeInfo, found := f.files[name]; found {
+		return fakeInfo, nil
+	}
+	return nil, &os.PathError{Op: "Stat", Path: name, Err: errors.New("No such file or directory")}
+
+}