TarWriter: Only use 137 of the 155 prefix bytes.

[thakis]

Summary

This makes lld's --reproduce output more compatible with tar 1.13 and
before. This is a very old version of tar, but it's the version in
both gnuwin and unxutils, and the cost for supporting them are very
low, so we might as well just do that.

https://bugs.chromium.org/p/chromium/issues/detail?id=1073524#c21
and onward has more details.

[hans]

Looks great to me. Thanks for figuring this out!

llvm/lib/Support/TarWriter.cpp

 // Otherwise, returns false.
 static bool splitUstar(StringRef Path, StringRef &Prefix, StringRef &Name) {
   if (Path.size() < sizeof(UstarHeader::Name)) {
     Prefix = "";
     Name = Path;
     return true;
   }
 
-  size_t Sep = Path.rfind('/', sizeof(UstarHeader::Prefix) + 1);
+  // tar 1.13 and earlier unconditionally look at the tar header interpreted
+  // as an 'oldgnu_header', which has an 'isextended' byte at offset 482 in the
+  // header, corresponding to offset 137 in the prefix. That's the version of
+  // tar in gnuwin, so only use 137 of the 155 bytes in the prefix. This means
+  // we'll need a pax header after 237 bytes of path instead of after 255,
+  // but in return paths up to 237 bytes work with gnuwin, instead of just
+  // 137 bytes of directory + 100 bytes of basename previously.
+  // (tar-1.13 also doesn't support pax headers, but in practice all paths in
+  // llvm's test suite are short enough for that to not matter.)
+  const int MaxPrefix = 137;
+  size_t Sep = Path.rfind('/', MaxPrefix + 1);
   if (Sep == StringRef::npos)
     return false;
   if (Path.size() - Sep - 1 >= sizeof(UstarHeader::Name))
     return false;
 
   Prefix = Path.substr(0, Sep);
   Name = Path.substr(Sep + 1);
   return true;