embed: implement openFile.ReadAt

Implementation copied from testing/fstest/mapfs.go

Fixes #57803

Change-Id: I531682b50ab6663511bac41fce7614ab9197bf38
GitHub-Last-Rev: d9bcc1c0bf4d339edcdabaf4d4d67b848ad8818d
GitHub-Pull-Request: golang/go#59489
Reviewed-on: https://go-review.googlesource.com/c/go/+/483235
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 57 insertions, 0 deletions

diff --git a/src/embed/internal/embedtest/embed_test.go b/src/embed/internal/embedtest/embed_test.go
index cbd58ee846..c925942191 100644
--- a/src/embed/internal/embedtest/embed_test.go
+++ b/src/embed/internal/embedtest/embed_test.go
@@ -6,6 +6,7 @@ package embedtest
 
 import (
 	"embed"
+	"io"
 	"reflect"
 	"testing"
 	"testing/fstest"
@@ -176,3 +177,59 @@ func TestAliases(t *testing.T) {
 	check(helloBytes)
 	check(helloString)
 }
+
+func TestOffset(t *testing.T) {
+	file, err := testDirAll.Open("testdata/hello.txt")
+	if err != nil {
+		t.Fatal("Open:", err)
+	}
+
+	const want = "hello, world\n"
+
+	// Read the entire file.
+	got := make([]byte, len(want))
+	n, err := file.Read(got)
+	if err != nil {
+		t.Fatal("Read:", err)
+	}
+	if n != len(want) {
+		t.Fatal("Read:", n)
+	}
+	if string(got) != want {
+		t.Fatalf("Read: %q", got)
+	}
+
+	// Try to read one byte; confirm we're at the EOF.
+	var buf [1]byte
+	n, err = file.Read(buf[:])
+	if err != io.EOF {
+		t.Fatal("Read:", err)
+	}
+	if n != 0 {
+		t.Fatal("Read:", n)
+	}
+
+	// Use seek to get the offset at the EOF.
+	seeker := file.(io.Seeker)
+	off, err := seeker.Seek(0, io.SeekCurrent)
+	if err != nil {
+		t.Fatal("Seek:", err)
+	}
+	if off != int64(len(want)) {
+		t.Fatal("Seek:", off)
+	}
+
+	// Use ReadAt to read the entire file, ignoring the offset.
+	at := file.(io.ReaderAt)
+	got = make([]byte, len(want))
+	n, err = at.ReadAt(got, 0)
+	if err != nil {
+		t.Fatal("ReadAt:", err)
+	}
+	if n != len(want) {
+		t.Fatal("ReadAt:", n)
+	}
+	if string(got) != want {
+		t.Fatalf("ReadAt: %q", got)
+	}
+}