[llvm] [Path] Fix off-by-one in finding filename for win style paths (PR #78055)

[Matheus Izvekov via llvm-commits]

https://github.com/mizvekov updated https://github.com/llvm/llvm-project/pull/78055

>From eda99a5a78f0eaae60fc5fa9e03d001f465512b8 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Sat, 13 Jan 2024 14:02:50 -0300
Subject: [PATCH] [Path] Dix off-by-one in finding filename for win style paths

This fixes a crash where `path::parent_path` causes an invalid
access on a string upon receiving a path that consists of a single colon.

On Windows machine, with runtime checks enabled build, upon `clang -I: test.cc` produces:
```
Assertion failed: Index < Length && "Invalid index!", file llvm\include\llvm/ADT/StringRef.h, line 232
...
 #6 0x00007ff7816201eb `anonymous namespace'::parent_path_end llvm\lib\Support\Path.cpp:144:0
 #7 0x00007ff781620135 llvm::sys::path::parent_path(class llvm::StringRef, enum llvm::sys::path::Style) llvm\lib\Support\Path.cpp:470:0
```

Ideally, we can look for the last colon starting from the last
character, but we can instead start from second to last, and handle
empty paths by abusing `0 - 1 == npos`.
---
 llvm/lib/Support/Path.cpp       | 2 +-
 llvm/unittests/Support/Path.cpp | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
index c2456dcac0974a..1c441100679ed7 100644
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -104,7 +104,7 @@ namespace {
 
     if (is_style_windows(style)) {
       if (pos == StringRef::npos)
-        pos = str.find_last_of(':', str.size() - 2);
+        pos = str.find_last_of(':', str.size() - 1);
     }
 
     if (pos == StringRef::npos || (pos == 1 && is_separator(str[0], style)))
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
index a7b7e6a0f5044d..837ca03216f87a 100644
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -190,6 +190,7 @@ TEST(Support, Path) {
   paths.push_back("c:\\foo\\");
   paths.push_back("c:\\foo/");
   paths.push_back("c:/foo\\bar");
+  paths.push_back(":");
 
   for (SmallVector<StringRef, 40>::const_iterator i = paths.begin(),
                                                   e = paths.end();