[llvm] [llvm] Use the underlying VFS when constructing `RedirectingFileSystem` (PR #160942)

[Jan Svoboda via llvm-commits]

https://github.com/jansvoboda11 updated https://github.com/llvm/llvm-project/pull/160942

>From 66d43d6d90ca1cdbbe4d54492efaafa33572bf45 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Fri, 26 Sep 2025 13:22:13 -0700
Subject: [PATCH 1/2] [llvm][support] Use the underlying VFS when constructing
 RedirectingFileSystem

---
 llvm/lib/Support/VirtualFileSystem.cpp        |  2 +-
 .../Support/VirtualFileSystemTest.cpp         | 31 ++++++++++++++++++-
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
index cf784595c2f1c..7ff62d43ba205 100644
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -1973,7 +1973,7 @@ class llvm::vfs::RedirectingFileSystemParser {
           EC = FS->makeAbsolute(FullPath, Name);
           Name = canonicalize(Name);
         } else {
-          EC = sys::fs::make_absolute(Name);
+          EC = FS->makeAbsolute(Name);
         }
         if (EC) {
           assert(NameValueNode && "Name presence should be checked earlier");
diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
index 6228de8aa897a..e6c75a60a6a80 100644
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -1941,7 +1941,7 @@ TEST_F(VFSFromYAMLTest, ReturnsExternalPathVFSHit) {
   EXPECT_EQ(0, NumDiagnostics);
 }
 
-TEST_F(VFSFromYAMLTest, RootRelativeTest) {
+TEST_F(VFSFromYAMLTest, RootRelativeToOverlayDirTest) {
   auto Lower = makeIntrusiveRefCnt<DummyFileSystem>();
   Lower->addDirectory("//root/foo/bar");
   Lower->addRegularFile("//root/foo/bar/a");
@@ -2004,6 +2004,35 @@ TEST_F(VFSFromYAMLTest, RootRelativeTest) {
 #endif
 }
 
+TEST_F(VFSFromYAMLTest, RootRelativeToCWDTest) {
+  auto Lower = makeIntrusiveRefCnt<DummyFileSystem>();
+  Lower->addDirectory("//root/foo/bar");
+  Lower->addRegularFile("//root/foo/bar/a");
+  Lower->addDirectory("//root/foo/bar/cwd");
+  Lower->addRegularFile("//root/foo/bar/cwd/a");
+  Lower->setCurrentWorkingDirectory("//root/foo/bar/cwd");
+  IntrusiveRefCntPtr<vfs::FileSystem> FS =
+      getFromYAMLString("{\n"
+                        "  'case-sensitive': false,\n"
+                        "  'root-relative': 'cwd',\n"
+                        "  'roots': [\n"
+                        "    { 'name': 'b', 'type': 'file',\n"
+                        "      'external-contents': '//root/foo/bar/a'\n"
+                        "    }\n"
+                        "  ]\n"
+                        "}",
+                        Lower, "//root/foo/bar/overlay");
+
+  ASSERT_NE(FS.get(), nullptr);
+
+  ErrorOr<vfs::Status> S1 = FS->status("//root/foo/bar/b");
+  ASSERT_TRUE(S1.getError());
+
+  ErrorOr<vfs::Status> S2 = FS->status("//root/foo/bar/cwd/b");
+  ASSERT_FALSE(S2.getError());
+  EXPECT_EQ("//root/foo/bar/a", S2->getName());
+}
+
 TEST_F(VFSFromYAMLTest, ReturnsInternalPathVFSHit) {
   auto BaseFS = makeIntrusiveRefCnt<vfs::InMemoryFileSystem>();
   BaseFS->addFile("//root/foo/realname", 0,

>From 622afa6eb176aec22257c224f55b7cc8184f3a78 Mon Sep 17 00:00:00 2001
From: Jan Svoboda <jan_svoboda at apple.com>
Date: Mon, 29 Sep 2025 09:28:33 -0700
Subject: [PATCH 2/2] Set CWD on the underlying VFS

---
 llvm/unittests/Support/VirtualFileSystemTest.cpp | 1 +
 1 file changed, 1 insertion(+)

diff --git a/llvm/unittests/Support/VirtualFileSystemTest.cpp b/llvm/unittests/Support/VirtualFileSystemTest.cpp
index e6c75a60a6a80..d47a4ee986778 100644
--- a/llvm/unittests/Support/VirtualFileSystemTest.cpp
+++ b/llvm/unittests/Support/VirtualFileSystemTest.cpp
@@ -2518,6 +2518,7 @@ TEST_F(VFSFromYAMLTest, RelativePaths) {
   SmallString<128> CWD;
   EC = llvm::sys::fs::current_path(CWD);
   ASSERT_FALSE(EC);
+  Lower->setCurrentWorkingDirectory(CWD);
 
   // Filename at root level without a parent directory.
   IntrusiveRefCntPtr<vfs::FileSystem> FS = getFromYAMLString(