[llvm-rc] Handle driveless absolute windows paths when loading external files

[mstorsjo]

Summary

When llvm-rc loads an external file, it looks for it relative to a number of include directories and the current working directory. If the path is considered absolute, llvm-rc tries to open the filename as such, and doesn't try to open it relative to other paths.

On Windows, a path name like "\dir\file" isn't considered absolute as it lacks the drive name, but by appending it on top of the search dirs, it's not found.

LLVM's sys::path::append just appends such a path (same with a properly absolute posix path) after the paths it's supposed to be relative to.

This fix doesn't handle the case if the resource script and the external file are on a different drive than the current working directory; to fix that, we'd have to make LLVM's sys::path::append handle appending fully absolute and partially absolute paths (ones lacking a drive prefix but containing a root directory), or switch to C++17's std::filesystem.

There's an existing testcase for fully absolute paths, using lit's %t to expand to an absolute path. Making a testcase for this would probably require a bit more scripting in the test file, to strip out the drive name prefix from the absolute path, and embed that into a resource script.

[thakis]

If you're fine with this randomly breaking one day when someone changes path handling, I think this is fine as is. It is difficult to test.

[thakis]

(It'd maybe be nice to have a PR that has manual repro steps and link to that in the commit message at least.)

[mstorsjo]

In D92558#2431144, @thakis wrote:

If you're fine with this randomly breaking one day when someone changes path handling, I think this is fine as is. It is difficult to test.

Actually, testing should be fairly straightforward with a diff like this:

diff --git a/llvm/test/tools/llvm-rc/absolute.test b/llvm/test/tools/llvm-rc/absolute.test
index 95aff3e42440..fd8b2d68d41e 100644
--- a/llvm/test/tools/llvm-rc/absolute.test 
+++ b/llvm/test/tools/llvm-rc/absolute.test
@@ -1,3 +1,7 @@ 
 ; RUN: touch %t.manifest
 ; RUN: echo "1 24 \"%t.manifest\"" > %t.rc
 ; RUN: llvm-rc -- %t.rc
+;; On Windows, try stripping out the drive name from the absolute path,
+;; and make sure the path still is found.
+; RUN: cat %t.rc | sed 's/"[a-zA-Z]:/"/' > %t2.rc
+; RUN: llvm-rc -- %t2.rc

But can we rely on tools like sed being available on windows test setups? (I faintly remember someone summarizing what tools we can rely on being available somewhere - @rnk?) I guess it also should be possible to do that replacement with some sort of shell expansion, like ABS="%t"; ROOTLESS="${ABS#*:\\}"; echo "1 24 \"$ROOTLESS\"" > %t2.rc

[mstorsjo]

Concluded that sed is one of the tools that is supposed to be available when running tests on windows, so it should be safe to add a test that uses it.

[rnk]

lgtm

llvm/test/tools/llvm-rc/absolute.test

6

Yep, it's available.

llvm/test/tools/llvm-rc/absolute.test

 ; RUN: touch %t.manifest
 ; RUN: echo "1 24 \"%t.manifest\"" > %t.rc
 ; RUN: llvm-rc -- %t.rc
+;; On Windows, try stripping out the drive name from the absolute path,
+;; and make sure the path still is found.
+; RUN: cat %t.rc | sed 's/"[a-zA-Z]:/"/' > %t2.rc
+; RUN: llvm-rc -- %t2.rc

llvm/tools/llvm-rc/ResourceFileWriter.cpp

 }
 
 Expected<std::unique_ptr<MemoryBuffer>>
 ResourceFileWriter::loadFile(StringRef File) const {
   SmallString<128> Path;
   SmallString<128> Cwd;
   std::unique_ptr<MemoryBuffer> Result;
 
-  // 0. The file path is absolute and the file exists.
-  if (sys::path::is_absolute(File))
+  // 0. The file path is absolute or has a root directory, so we shouldn't
+  // try to append it on top of other base directories. (An absolute path
+  // must have a root directory, but e.g. the path "\dir\file" on windows
+  // isn't considered absolute, but it does have a root directory. As long as
+  // sys::path::append doesn't handle appending an absolute path or a path
+  // starting with a root directory on top of a base, we must handle this
+  // case separately at the top. C++17's path::append handles that case
+  // properly though, so if using that to append paths below, this early
+  // exception case could be removed.)
+  if (sys::path::has_root_directory(File))
     return errorOrToExpected(MemoryBuffer::getFile(File, -1, false));
 
   // 1. The current working directory.
   sys::fs::current_path(Cwd);
   Path.assign(Cwd.begin(), Cwd.end());
   sys::path::append(Path, File);
   if (sys::fs::exists(Path))
     return errorOrToExpected(MemoryBuffer::getFile(Path, -1, false));