[llvm-ar] -s: don't convert a thin archive to a regular one

[MaskRay]

Summary

In binutils, ar -s and randlib don't convert a thin archive to a regular one.
This behavior makes sense and this patch ports the behavior.

[MaskRay]

If D116979 is accepted, I'd like to use --thin for new tests.

---

Unrelated to this patch:

% rm -f x.a && fllvm-ar rcT x.a a.o b.o
% ar q x.a c.o   # q or r
ar: Cannot convert existing thin library x.a to normal format

Do we want such a diagnostic? llvm-ar current behavior is to convert the thin archive to a regular one (untested, though).

The opposite has a diagnostic, like GNU ar. The diagnostic makes sense to me because it is difficult to ensure the filesystem has the listed files.

% 'rm' -f x.a && fllvm-ar rc x.a a.o b.o
% fllvm-ar rT x.a c.o
/tmp/RelA/bin/llvm-ar: error: cannot convert a regular archive to a thin one

[MaskRay]

Ping

[MaskRay]

So looks like this may miss the 14.0.0 branch point (https://llvm.discourse.group/t/llvm-14-0-0-release-schedule/5846). I'll hope that 14.0.0 includes this.

[gbreynoo]

Hi @MaskRay, apologies for the wait in this review. The change LGTM. Regarding the implicit conversion from thin to full archives I think there is a problem there, I've created https://reviews.llvm.org/D118693 to discuss it further.

llvm/test/tools/llvm-ar/symtab.test

 # RUN:   | FileCheck %s --check-prefix=NO-SYMTAB
 
 ## Thin Archive:
 # RUN: rm -f %t-thin.a
 # RUN: llvm-ar rcT %t-thin.a %t.o
 # RUN: llvm-nm --print-armap %t-thin.a \
 # RUN:   | FileCheck %s --check-prefix=SYMTAB
 
+## llvm-ranlib does not change a thin archive to a regular one.
+# RUN: rm -f %t-thin.a && llvm-ar rcTS %t-thin.a %t.o
+# RUN: llvm-nm --print-armap %t-thin.a | FileCheck %s --check-prefix=NO-SYMTAB
+# RUN: llvm-ranlib %t-thin.a
+# RUN: llvm-nm --print-armap %t-thin.a | FileCheck %s --check-prefix=SYMTAB
+# RUN: FileCheck --input-file=%t-thin.a %s --check-prefixes=THIN
+
+## llvm-ar -s is identical to ranlib and a duplicated operation does not change the output.
+# RUN: rm -f %t-thin2.a && llvm-ar rcTS %t-thin2.a %t.o
+# RUN: llvm-ar s %t-thin2.a && cmp %t-thin.a %t-thin2.a
+# RUN: llvm-ar s %t-thin2.a && cmp %t-thin.a %t-thin2.a
+
 # SYMTAB:        symbol in
 # NO-SYMTAB-NOT: symbol in
 
+# THIN: !<thin>
+
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Sections:
   - Name: .text
     Type: SHT_PROGBITS
 Symbols:
   - Name:    symbol
     Binding: STB_GLOBAL
     Section: .text

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

static void createSymbolTable(object::Archive *OldArchive) {
   // resulting archive will have a current symbol table. If the S option
   // is given, it will have no symbol table.
   // In summary, we only need to update the symbol table if we have none.
   // This is actually very common because of broken build systems that think
   // they have to run ranlib.
   if (OldArchive->hasSymbolTable())
     return;
 
+  if (OldArchive->isThin())
+    Thin = true;
   performWriteOperation(CreateSymTab, OldArchive, nullptr, nullptr);
 }
 
 static void performOperation(ArchiveOperation Operation,
                              object::Archive *OldArchive,
                              std::unique_ptr<MemoryBuffer> OldArchiveBuf,
                              std::vector<NewArchiveMember> *NewMembers) {
   switch (Operation) {