[llvm-ar] Fix when llvm-ar fails to replace existing members when updating a thin archive

[gbreynoo]

Summary

As seen in https://github.com/llvm/llvm-project/issues/55023 when a thin archive is updated when not in the CWD replacement does not work as expected. This change fixes the relative file path comparison so the correct files are updated.

[MaskRay]

llvm/test/tools/llvm-ar/thin-archive-relative-path.test

10

If I do llvm-ar cr --thin foo/lib.a foo/elf.o foo/elf.o, what's the expected behavior?

[MaskRay]

This as an incremental improvement looks good.

[gbreynoo]

llvm/test/tools/llvm-ar/thin-archive-relative-path.test

10

We follow GNU ar, two copies of foo/elf.o are added, replacing an existing member if it is already in the archive.

llvm/test/tools/llvm-ar/thin-archive-relative-path.test

+RUN: rm -rf %t && mkdir -p %t/foo %t/bar 
+
+RUN: yaml2obj %S/Inputs/elf.yaml -o %t/foo/elf.o
+RUN: cp %t/foo/elf.o %t/bar/elf.o
+RUN: cd %t
+
+Test a case in which CWD does not contain the archive, ensure replacement behaves as expected
+
+RUN: llvm-ar cr --thin foo/lib.a foo/elf.o
+RUN: llvm-ar t foo/lib.a  | FileCheck %s --check-prefix=FOO --implicit-check-not {{.}}
+
+RUN: llvm-ar cr --thin foo/lib.a bar/elf.o
+RUN: llvm-ar t foo/lib.a  | FileCheck %s --check-prefixes=FOO,BAR --implicit-check-not {{.}}
+
+RUN: llvm-ar cr --thin foo/lib.a foo/elf.o
+RUN: llvm-ar t foo/lib.a  | FileCheck %s --check-prefixes=FOO,BAR --implicit-check-not {{.}}
+
+FOO: foo/elf.o
+BAR: foo/../bar/elf.o

llvm/tools/llvm-ar/llvm-ar.cpp

static InsertAction computeInsertAction(ArchiveOperation Operation,
                                         StringRef Name,
                                         std::vector<StringRef>::iterator &Pos,
                                         StringMap<int> &MemberCount) {
   if (!isValidInBitMode(Member))
     return IA_AddOldMember;
 
   if (Operation == QuickAppend || Members.empty())
     return IA_AddOldMember;
-  auto MI = find_if(
-      Members, [Name](StringRef Path) { return comparePaths(Name, Path); });
+
+  auto MI = find_if(Members, [Name](StringRef Path) {
+    if (Thin && !sys::path::is_absolute(Path)) {
+      Expected<std::string> PathOrErr =
+          computeArchiveRelativePath(ArchiveName, Path);
+      return comparePaths(Name, PathOrErr ? *PathOrErr : Path);
+    } else {
+      return comparePaths(Name, Path);
+    }
+  });
 
   if (MI == Members.end())
     return IA_AddOldMember;
 
   Pos = MI;
 
   if (Operation == Delete) {
     if (CountParam && ++MemberCount[Name] != CountParam)