Add ReadFile helper

Author: adammck

ioutil.go

@@ -1,6 +1,7 @@
 package vfs
 
 import (
+	"bytes"
 	"io"
 	"os"
 )
@@ -24,3 +25,59 @@ func WriteFile(fs Filesystem, filename string, data []byte, perm os.FileMode) er
 	}
 	return err
 }
+
+// ReadFile reads the file named by filename and returns the contents. 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.
+//
+// This is a port of the stdlib ioutil.ReadFile function.
+func ReadFile(fs Filesystem, filename string) ([]byte, error) {
+	f, err := fs.OpenFile(filename, os.O_RDONLY, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	// It's a good but not certain bet that FileInfo will tell us exactly how
+	// much to read, so let's try it but be prepared for the answer to be wrong.
+	var n int64
+	if fi, err := fs.Stat(filename); err == nil {
+		if size := fi.Size(); size < 1e9 {
+			n = size
+		}
+	}
+
+    // As initial capacity for readAll, use n + a little extra in case Size is
+    // zero, and to avoid another allocation after Read has filled the buffer.
+    // The readAll call will read into its allocated internal buffer cheaply. If
+    // the size was wrong, we'll either waste some space off the end or
+    // reallocate as needed, but in the overwhelmingly common case we'll get it
+    // just right.
+	return readAll(f, n+bytes.MinRead)
+}
+
+// readAll reads from r until an error or EOF and returns the data it read from
+// the internal buffer allocated with a specified capacity.
+//
+// This is a paste of the stdlib ioutil.readAll function.
+func readAll(r io.Reader, capacity int64) (b []byte, err error) {
+	buf := bytes.NewBuffer(make([]byte, 0, capacity))
+
+	// If the buffer overflows, we will get bytes.ErrTooLarge.
+	// Return that as an error. Any other panic remains.
+	defer func() {
+		e := recover()
+		if e == nil {
+			return
+		}
+		if panicErr, ok := e.(error); ok && panicErr == bytes.ErrTooLarge {
+			err = panicErr
+		} else {
+			panic(e)
+		}
+	}()
+
+	_, err = buf.ReadFrom(r)
+	return buf.Bytes(), err
+}

ioutil_test.go

@@ -31,3 +31,24 @@ func TestWriteFile(t *testing.T) {
 		t.Fatalf("Bad file mode: %o (expected %o)", info.Mode(), testmode)
 	}
 }
+
+func TestReadFile(t *testing.T) {
+	fs := memfs.Create()
+
+	f, _ := fs.OpenFile(testpath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, testmode)
+	f.Write(testdata)
+	f.Close()
+
+	data, err := vfs.ReadFile(fs, testpath)
+	if err != nil {
+		t.Fatalf("ReadFile failed: %s", err)
+	}
+	if len(data) != len(testdata) {
+		t.Fatalf("Bad data length: %d bytes (expected %d)", len(data), len(testdata))
+	}
+
+	_, err = vfs.ReadFile(fs, "/doesnt-exist.txt")
+	if err == nil {
+		t.Fatalf("ReadFile failed: expected error")
+	}
+}