[lld-macho] Save all thin archive members in repro tarball (#97169)

Previously, we only saved those members of thin archives into a repro
file that were actually used during linking. However, -ObjC handling
requires us to inspect all members, even those that don't end up being
loaded.

We weren't handling missing members correctly and crashed with an
"unhandled `Error`" failure in LLVM_ENABLE_ABI_BREAKING_CHECKS builds.

To fix this, we now eagerly load all object files and warn when
encountering missing members (in the instances where it wasn't a hard
error before). To avoid having to patch out the checks when dealing
with older repro files, the `--no-warn-thin-archive-missing-members`
flag is added as an escape hatch.

lld/MachO/Config.h

@@ -212,6 +212,7 @@ struct Configuration {
   bool csProfileGenerate = false;
   llvm::StringRef csProfilePath;
   bool pgoWarnMismatch;
+  bool warnThinArchiveMissingMembers;
 
   bool callGraphProfileSort = false;
   llvm::StringRef printSymbolOrder;

lld/MachO/Driver.cpp

@@ -270,6 +270,20 @@ struct ArchiveFileInfo {
 
 static DenseMap<StringRef, ArchiveFileInfo> loadedArchives;
 
+static void saveThinArchiveToRepro(ArchiveFile const *file) {
+  assert(tar && file->getArchive().isThin());
+
+  Error e = Error::success();
+  for (const object::Archive::Child &c : file->getArchive().children(e)) {
+    MemoryBufferRef mb = CHECK(c.getMemoryBufferRef(),
+                               toString(file) + ": failed to get buffer");
+    tar->append(relativeToRoot(CHECK(c.getFullName(), file)), mb.getBuffer());
+  }
+  if (e)
+    error(toString(file) +
+          ": Archive::children failed: " + toString(std::move(e)));
+}
+
 static InputFile *addFile(StringRef path, LoadType loadType,
                           bool isLazy = false, bool isExplicit = true,
                           bool isBundleLoader = false,
@@ -301,6 +315,9 @@ static InputFile *addFile(StringRef path, LoadType loadType,
       if (!archive->isEmpty() && !archive->hasSymbolTable())
         error(path + ": archive has no index; run ranlib to add one");
       file = make<ArchiveFile>(std::move(archive), isForceHidden);
+
+      if (tar && file->getArchive().isThin())
+        saveThinArchiveToRepro(file);
     } else {
       file = entry->second.file;
       // Command-line loads take precedence. If file is previously loaded via
@@ -330,9 +347,13 @@ static InputFile *addFile(StringRef path, LoadType loadType,
               reason = "-all_load";
               break;
           }
-          if (Error e = file->fetch(c, reason))
-            error(toString(file) + ": " + reason +
-                  " failed to load archive member: " + toString(std::move(e)));
+          if (Error e = file->fetch(c, reason)) {
+            if (config->warnThinArchiveMissingMembers)
+              warn(toString(file) + ": " + reason +
+                   " failed to load archive member: " + toString(std::move(e)));
+            else
+              llvm::consumeError(std::move(e));
+          }
         }
         if (e)
           error(toString(file) +
@@ -349,7 +370,18 @@ static InputFile *addFile(StringRef path, LoadType loadType,
         Error e = Error::success();
         for (const object::Archive::Child &c : file->getArchive().children(e)) {
           Expected<MemoryBufferRef> mb = c.getMemoryBufferRef();
-          if (!mb || !hasObjCSection(*mb))
+          if (!mb) {
+            // We used to create broken repro tarballs that only included those
+            // object files from thin archives that ended up being used.
+            if (config->warnThinArchiveMissingMembers)
+              warn(toString(file) + ": -ObjC failed to open archive member: " +
+                   toString(mb.takeError()));
+            else
+              llvm::consumeError(mb.takeError());
+            continue;
+          }
+
+          if (!hasObjCSection(*mb))
             continue;
           if (Error e = file->fetch(c, "-ObjC"))
             error(toString(file) + ": -ObjC failed to load archive member: " +
@@ -1699,6 +1731,9 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
   config->csProfilePath = args.getLastArgValue(OPT_cs_profile_path);
   config->pgoWarnMismatch =
       args.hasFlag(OPT_pgo_warn_mismatch, OPT_no_pgo_warn_mismatch, true);
+  config->warnThinArchiveMissingMembers =
+      args.hasFlag(OPT_warn_thin_archive_missing_members,
+                   OPT_no_warn_thin_archive_missing_members, true);
   config->generateUuid = !args.hasArg(OPT_no_uuid);
 
   for (const Arg *arg : args.filtered(OPT_alias)) {

lld/MachO/InputFiles.cpp

@@ -2200,10 +2200,6 @@ Error ArchiveFile::fetch(const object::Archive::Child &c, StringRef reason) {
   if (!mb)
     return mb.takeError();
 
-  // Thin archives refer to .o files, so --reproduce needs the .o files too.
-  if (tar && c.getParent()->isThin())
-    tar->append(relativeToRoot(CHECK(c.getFullName(), this)), mb->getBuffer());
-
   Expected<TimePoint<std::chrono::seconds>> modTime = c.getLastModified();
   if (!modTime)
     return modTime.takeError();

lld/MachO/Options.td

@@ -153,6 +153,9 @@ def cs_profile_path: Joined<["--"], "cs-profile-path=">,
 defm pgo_warn_mismatch: BB<"pgo-warn-mismatch",
   "turn on warnings about profile cfg mismatch (default)",
   "turn off warnings about profile cfg mismatch">, Group<grp_lld>;
+defm warn_thin_archive_missing_members : BB<"warn-thin-archive-missing-members",
+  "Warn on missing object files referenced by thin archives (default)",
+  "Do not warn on missing object files referenced by thin archives">, Group<grp_lld>;
 
 // This is a complete Options.td compiled from Apple's ld(1) manpage
 // dated 2018-03-07 and cross checked with ld64 source code in repo

lld/test/MachO/reproduce-thin-archive-objc.s

@@ -0,0 +1,25 @@
+# REQUIRES: x86
+
+## For a long time, LLD only included those members from thin archives that were actually used
+## during linking. However, we need to iterate over all members for -ObjC, check that we don't
+## crash when we encounter a missing member.
+
+# RUN: rm -rf %t; mkdir %t
+# RUN: sed s/SYM/_main/   %s | llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/main.o
+# RUN: sed s/SYM/_unused/ %s | llvm-mc -filetype=obj -triple=x86_64-apple-macos -o %t/unused.o
+
+# RUN: cd %t; llvm-ar rcsT unused.a unused.o; rm unused.o
+## FIXME: Absolute paths don't end up relativized in the repro file.
+
+# RUN: %no-fatal-warnings-lld %t/main.o %t/unused.a -ObjC -o /dev/null 2>&1 \
+# RUN:                      | FileCheck %s --check-prefix=WARN
+
+# RUN: %lld %t/main.o %t/unused.a -ObjC --no-warn-thin-archive-missing-members -o /dev/null \
+# RUN:    | FileCheck %s --implicit-check-not 'warning' --allow-empty
+
+# WARN: ld64.lld: warning: {{.*}}unused.a: -ObjC failed to open archive member: 'unused.o'
+
+.text
+.globl SYM
+SYM:
+    ret

lld/test/MachO/reproduce-thin-archives.s

@@ -1,10 +1,11 @@
 # REQUIRES: x86
 
-# RUN: rm -rf %t.dir
-# RUN: mkdir -p %t.dir
-# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %s -o %t.dir/foo.o
+# RUN: rm -rf %t.dir; split-file %s %t.dir
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t.dir/foo.s -o %t.dir/foo.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t.dir/unused.s -o %t.dir/unused.o
 # RUN: cd %t.dir
-# RUN: llvm-ar rcsT foo.a foo.o
+# RUN: llvm-ar rcsT foo.a foo.o unused.o
 
 # RUN: %lld foo.a -o /dev/null --reproduce repro.tar
 # RUN: tar tf repro.tar | FileCheck -DPATH='repro/%:t.dir' %s
@@ -12,9 +13,19 @@
 # RUN: %lld -all_load foo.a -o /dev/null --reproduce repro2.tar
 # RUN: tar tf repro2.tar | FileCheck -DPATH='repro2/%:t.dir' %s
 
+# RUN: %lld -ObjC foo.a -o /dev/null --reproduce repro3.tar
+# RUN: tar tf repro3.tar | FileCheck -DPATH='repro3/%:t.dir' %s
+
 # CHECK-DAG: [[PATH]]/foo.a
 # CHECK-DAG: [[PATH]]/foo.o
+# CHECK-DAG: [[PATH]]/unused.o
 
+#--- foo.s
 .globl _main
 _main:
   nop
+
+#--- unused.s
+.globl _unused
+_unused:
+  nop