[PATCH v3] MdePkg/UefiFileHandleLib: Fix the Root directory determination.

["Marvin Häuser" <Marvin.Haeuser@outlook.com>]

The current implementation of the FileHandleGetFileName() function
assumes that the Root directory always has the FileName '\0'.
However, the only requirement the UEFI specification defines is that
a prepended '\\' must be supported to access files and folders
relative to the Root directory.
This patch removes this assumption and supports constructing valid
paths for any value of FileName for the Root Directory.

In practice, this fixes compatibility issues with File System drivers
that report '\\' as the FileName of the Root directory, which
currently is both generating an invalid path ("\\\\") and resulting
in an EFI_NOT_FOUND result from the CurrentHandle->Open() call.

V2:
  - Do not change the copyright date as requested.

V3:
  - Simplify the root check to not check the length of FileName.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marvin Haeuser <Marvin.Haeuser@outlook.com>
---
 MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c | 26 ++++++++++++++------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c b/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c
index a768c3e23ae5..0a316231f984 100644
--- a/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c
+++ b/MdePkg/Library/UefiFileHandleLib/UefiFileHandleLib.c
@@ -820,10 +820,24 @@ FileHandleGetFileName (
         Status = EFI_OUT_OF_RESOURCES;
         break;
       } else {
+        //
+        // Prepare to move to the parent directory.
+        // Also determine whether CurrentHandle refers to the Root directory.
+        //
+        Status = CurrentHandle->Open (CurrentHandle, &NextHigherHandle, L"..", EFI_FILE_MODE_READ, 0);
         //
         // We got info... do we have a name? if yes precede the current path with it...
         //
-        if (StrLen (FileInfo->FileName) == 0) {
+        if (EFI_ERROR (Status)) {
+          //
+          // EFI_ERROR() suggests that CurrentHandle refers to the Root
+          // directory.  As this loop ensures FullFileName is starting with
+          // '\\' at all times, signal success and exit the loop.
+          // While FileInfo->FileName could theoretically be a value other than
+          // '\0' or '\\', '\\' is guaranteed to be supported by the
+          // specification and hence its value can safely be ignored.
+          //
+          Status = EFI_SUCCESS;
           if (*FullFileName == NULL) {
             ASSERT((*FullFileName == NULL && Size == 0) || (*FullFileName != NULL));
             *FullFileName = StrnCatGrowLeft(FullFileName, &Size, L"\\", 0);
@@ -841,15 +855,11 @@ FileHandleGetFileName (
           FreePool(FileInfo);
         }
       }
-      //
-      // Move to the parent directory
-      //
-      Status = CurrentHandle->Open (CurrentHandle, &NextHigherHandle, L"..", EFI_FILE_MODE_READ, 0);
-      if (EFI_ERROR (Status)) {
-        break;
-      }
 
       FileHandleClose(CurrentHandle);
+      //
+      // Move to the parent directory
+      //
       CurrentHandle = NextHigherHandle;
     }
   } else if (Status == EFI_NOT_FOUND) {
-- 
2.18.0.windows.1