Fix Azure blob object `Seek` (#32974)

This bug was found in a
[test](https://github.com/go-gitea/gitea/actions/runs/12464294025/job/34788152194?pr=32883#step:9:105)
of #32883

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/storage/azureblob.go

@@ -70,7 +70,7 @@ func (a *azureBlobObject) Seek(offset int64, whence int) (int64, error) {
 	case io.SeekCurrent:
 		offset += a.offset
 	case io.SeekEnd:
-		offset = a.Size - offset
+		offset = a.Size + offset
 	default:
 		return 0, errors.New("Seek: invalid whence")
 	}

modules/storage/azureblob_test.go

@@ -4,6 +4,8 @@
 package storage
 
 import (
+	"bytes"
+	"io"
 	"os"
 	"testing"
 
@@ -54,3 +56,46 @@ func TestAzureBlobStoragePath(t *testing.T) {
 	assert.Equal(t, "base/a", m.buildAzureBlobPath("/a"))
 	assert.Equal(t, "base/a/b", m.buildAzureBlobPath("/a/b/"))
 }
+
+func Test_azureBlobObject(t *testing.T) {
+	if os.Getenv("CI") == "" {
+		t.Skip("azureBlobStorage not present outside of CI")
+		return
+	}
+
+	s, err := NewStorage(setting.AzureBlobStorageType, &setting.Storage{
+		AzureBlobConfig: setting.AzureBlobStorageConfig{
+			// https://learn.microsoft.com/azure/storage/common/storage-use-azurite?tabs=visual-studio-code#ip-style-url
+			Endpoint: "http://devstoreaccount1.azurite.local:10000",
+			// https://learn.microsoft.com/azure/storage/common/storage-use-azurite?tabs=visual-studio-code#well-known-storage-account-and-key
+			AccountName: "devstoreaccount1",
+			AccountKey:  "Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==",
+			Container:   "test",
+		},
+	})
+	assert.NoError(t, err)
+
+	data := "Q2xTckt6Y1hDOWh0"
+	_, err = s.Save("test.txt", bytes.NewBufferString(data), int64(len(data)))
+	assert.NoError(t, err)
+	obj, err := s.Open("test.txt")
+	assert.NoError(t, err)
+	offset, err := obj.Seek(2, io.SeekStart)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 2, offset)
+	buf1 := make([]byte, 3)
+	read, err := obj.Read(buf1)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 3, read)
+	assert.Equal(t, data[2:5], string(buf1))
+	offset, err = obj.Seek(-5, io.SeekEnd)
+	assert.NoError(t, err)
+	assert.EqualValues(t, len(data)-5, offset)
+	buf2 := make([]byte, 4)
+	read, err = obj.Read(buf2)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 4, read)
+	assert.Equal(t, data[11:15], string(buf2))
+	assert.NoError(t, obj.Close())
+	assert.NoError(t, s.Delete("test.txt"))
+}