[WebAssembly] Error out on archives without and index

[sbc100]

Summary

This is fairly common with wasm since GNU ar (most likely the system
ar) doesn't support the wasm object format so user who don't override
AR will end up with archives without an index. We don't want to
silently ignore this issue.

An alternative would be to behave like the ELF version lld and read the
symbols from each object file in the archive if they are all of the
same type. However, error'ing out seem like a conservative appoach for
now.

Fixes: PR42376

[ruiu]

LGTM

This patch looks good by itself, but I believe that lld/wasm should handle each archive member as a lazy object file if there's no symbol table in an archive, just like lld/ELF does.

lld/trunk/test/wasm/archive-no-index.ll

+; Tests error on archive file without a symbol table
+; RUN: llvm-as -o %t.o %s
+; RUN: llvm-as -o %t.archive.o %S/Inputs/archive1.ll
+; RUN: rm -f %t.a
+; RUN: llvm-ar crS %t.a %t.archive.o
+
+; RUN: not wasm-ld -o out.wasm %t.o %t.a 2>&1 | FileCheck %s
+
+define i32 @_start() {
+  ret i32 0
+}
+
+; CHECK: archive has no index; run ranlib to add one

lld/trunk/wasm/Driver.cpp

 void LinkerDriver::addFile(StringRef Path) {
   Optional<MemoryBufferRef> Buffer = readFile(Path);
   if (!Buffer.hasValue())
     return;
   MemoryBufferRef MBRef = *Buffer;
 
   switch (identify_magic(MBRef.getBuffer())) {
   case file_magic::archive: {
+    SmallString<128> ImportFile = Path;
+    path::replace_extension(ImportFile, ".imports");
+    if (fs::exists(ImportFile))
+      readImportFile(ImportFile.str());
+
     // Handle -whole-archive.
     if (InWholeArchive) {
       for (MemoryBufferRef &M : getArchiveMembers(MBRef))
         Files.push_back(createObjectFile(M, Path));
       return;
     }
 
-    SmallString<128> ImportFile = Path;
-    path::replace_extension(ImportFile, ".imports");
-    if (fs::exists(ImportFile))
-      readImportFile(ImportFile.str());
+    std::unique_ptr<Archive> File =
+        CHECK(Archive::create(MBRef), Path + ": failed to parse archive");
+
+    if (!File->isEmpty() && !File->hasSymbolTable()) {
+      error(MBRef.getBufferIdentifier() +
+            ": archive has no index; run ranlib to add one");
+    }
 
     Files.push_back(make<ArchiveFile>(MBRef));
     return;
   }
   case file_magic::bitcode:
   case file_magic::wasm_object:
     Files.push_back(createObjectFile(MBRef));
     break;