util: add ReadFile function

util/util.go

@@ -222,3 +222,53 @@ func getUnderlyingAndPath(fs billy.Basic, path string) (billy.Basic, string) {
 
 	return u.Underlying(), path
 }
+
+// ReadFile reads the named file and returns the contents from the given filesystem.
+// A successful call returns err == nil, not err == EOF.
+// Because ReadFile reads the whole file, it does not treat an EOF from Read
+// as an error to be reported.
+func ReadFile(fs billy.Basic, name string) ([]byte, error) {
+	f, err := fs.Open(name)
+	if err != nil {
+		return nil, err
+	}
+
+	defer f.Close()
+
+	var size int
+	if info, err := fs.Stat(name); err == nil {
+		size64 := info.Size()
+		if int64(int(size64)) == size64 {
+			size = int(size64)
+		}
+	}
+
+	size++ // one byte for final read at EOF
+	// If a file claims a small size, read at least 512 bytes.
+	// In particular, files in Linux's /proc claim size 0 but
+	// then do not work right if read in small pieces,
+	// so an initial read of 1 byte would not work correctly.
+
+	if size < 512 {
+		size = 512
+	}
+
+	data := make([]byte, 0, size)
+	for {
+		if len(data) >= cap(data) {
+			d := append(data[:cap(data)], 0)
+			data = d[:len(data)]
+		}
+
+		n, err := f.Read(data[len(data):cap(data)])
+		data = data[:len(data)+n]
+
+		if err != nil {
+			if err == io.EOF {
+				err = nil
+			}
+
+			return data, err
+		}
+	}
+}

util/util_test.go

@@ -41,3 +41,24 @@ func TestTempDir(t *testing.T) {
 		}
 	}
 }
+
+func TestReadFile(t *testing.T) {
+	fs := memfs.New()
+	f, err := util.TempFile(fs, "", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	f.Write([]byte("foo"))
+	f.Close()
+
+	data, err := util.ReadFile(fs, f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(data) != "foo" || err != nil {
+		t.Errorf("ReadFile(%q, %q) = %v, %v", fs, f.Name(), data, err)
+	}
+
+}