[lld-link] emit an error when writing a PDB > 4 GiB

[thakis]

Summary

Maybe there's a way to make them work, but until I've investigated
if tools can consume large PDBs, erroring out is better than slowly
and silently consuming all available ram due to internal invariants
being violated.

(Patch to make writing larger files work at
https://bugs.chromium.org/p/chromium/issues/detail?id=1179085#c25
but I haven't had time to check if windbg & co can consume these
large PDBs. llvm-pdbutil can't, but we can fix that one at least :) )

[rnk]

lgtm

I considered and rejected the idea of adding a test for this.

[thakis]

In D98788#2632073, @rnk wrote:

lgtm

Thanks!

I considered and rejected the idea of adding a test for this.

Me too :)

[akrieger]

For the record, since this commit added the relevant comment, it does not appear that either VS or windbg can successfully load the resulting pdbs even if we patched lld to support emitting them.

DBGHELP: ........ DIA E_PDB_FILE_SYSTEM error from 875
DBGHELP: Failed copying the file 'O:\<redacted>\Codename.pdb' to the cache
DBGHELP: ........ DIA E_PDB_FILE_SYSTEM error from 953
DBGHELP: O:\<redacted>\Codename.pdb - drive not ready
...
************* Symbol Loading Error Summary **************
Module name            Error
Codename               - Unknown error: 0x400.

Bruce Dawson opened https://developercommunity.visualstudio.com/t/pdb-limit-of-4-gib-is-likely-to-be-a-problem-in-a/904784 last year and it seems to be an acknowledged issue that is not yet resolved.

llvm/lib/DebugInfo/MSF/MSFBuilder.cpp

 Expected<FileBufferByteStream> MSFBuilder::commit(StringRef Path,
                                                   MSFLayout &Layout) {
   Expected<MSFLayout> L = generateLayout();
   if (!L)
     return L.takeError();
 
   Layout = std::move(*L);
 
-  uint64_t FileSize = Layout.SB->BlockSize * Layout.SB->NumBlocks;
+  uint64_t FileSize = uint64_t(Layout.SB->BlockSize) * Layout.SB->NumBlocks;
+  if (FileSize > UINT32_MAX) {
+    // FIXME: Changing the BinaryStream classes to use 64-bit numbers lets
+    // us create PDBs larger than 4 GiB successfully. The file format is
+    // block-based and as long as each stream is small enough, PDBs larger than
+    // 4 GiB might work. Check if tools can handle these large PDBs, and if so
+    // add support for writing them.
+    return make_error<MSFError>(msf_error_code::invalid_format,
+                                "Output larger than 4 GiB");
+  }
+
   auto OutFileOrError = FileOutputBuffer::create(Path, FileSize);
   if (auto EC = OutFileOrError.takeError())
     return std::move(EC);
 
   FileBufferByteStream Buffer(std::move(*OutFileOrError),
                               llvm::support::little);
   BinaryStreamWriter Writer(Buffer);