Follow file symlinks in the UI to their target (#28835)

Symlinks are followed when you click on a link next to an entry, either
until a file has been found or until we know that the link is dead.
When the link cannot be accessed, we fall back to the current behavior
of showing the document containing the target.

---------

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

modules/git/tree_entry_common_test.go

@@ -0,0 +1,76 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/util"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFollowLink(t *testing.T) {
+	r, err := openRepositoryWithDefaultContext("tests/repos/repo1_bare")
+	require.NoError(t, err)
+	defer r.Close()
+
+	commit, err := r.GetCommit("37991dec2c8e592043f47155ce4808d4580f9123")
+	require.NoError(t, err)
+
+	// get the symlink
+	{
+		lnkFullPath := "foo/bar/link_to_hello"
+		lnk, err := commit.Tree.GetTreeEntryByPath("foo/bar/link_to_hello")
+		require.NoError(t, err)
+		assert.True(t, lnk.IsLink())
+
+		// should be able to dereference to target
+		res, err := EntryFollowLink(commit, lnkFullPath, lnk)
+		require.NoError(t, err)
+		assert.Equal(t, "hello", res.TargetEntry.Name())
+		assert.Equal(t, "foo/nar/hello", res.TargetFullPath)
+		assert.False(t, res.TargetEntry.IsLink())
+		assert.Equal(t, "b14df6442ea5a1b382985a6549b85d435376c351", res.TargetEntry.ID.String())
+	}
+
+	{
+		// should error when called on a normal file
+		entry, err := commit.Tree.GetTreeEntryByPath("file1.txt")
+		require.NoError(t, err)
+		res, err := EntryFollowLink(commit, "file1.txt", entry)
+		assert.ErrorIs(t, err, util.ErrUnprocessableContent)
+		assert.Nil(t, res)
+	}
+
+	{
+		// should error for broken links
+		entry, err := commit.Tree.GetTreeEntryByPath("foo/broken_link")
+		require.NoError(t, err)
+		assert.True(t, entry.IsLink())
+		res, err := EntryFollowLink(commit, "foo/broken_link", entry)
+		assert.ErrorIs(t, err, util.ErrNotExist)
+		assert.Equal(t, "nar/broken_link", res.SymlinkContent)
+	}
+
+	{
+		// should error for external links
+		entry, err := commit.Tree.GetTreeEntryByPath("foo/outside_repo")
+		require.NoError(t, err)
+		assert.True(t, entry.IsLink())
+		res, err := EntryFollowLink(commit, "foo/outside_repo", entry)
+		assert.ErrorIs(t, err, util.ErrNotExist)
+		assert.Equal(t, "../../outside_repo", res.SymlinkContent)
+	}
+
+	{
+		// testing fix for short link bug
+		entry, err := commit.Tree.GetTreeEntryByPath("foo/link_short")
+		require.NoError(t, err)
+		res, err := EntryFollowLink(commit, "foo/link_short", entry)
+		assert.ErrorIs(t, err, util.ErrNotExist)
+		assert.Equal(t, "a", res.SymlinkContent)
+	}
+}