[Support] Move llvm::MemoryBuffer to sys::fs::file_t

Summary:
On Windows, Posix integer file descriptors are a compatibility layer
over native file handles provided by the C runtime. There is a hard
limit on the maximum number of file descriptors that a process can open,
and the limit is 8192. LLD typically doesn't run into this limit because
it opens input files, maps them into memory, and then immediately closes
the file descriptor. This prevents it from running out of FDs.

For various reasons, I'd like to open handles to every input file and
keep them open during linking. That requires migrating MemoryBuffer over
to taking open native file handles instead of integer FDs.

Reviewers: aganea, Bigcheese

Reviewed By: aganea

Subscribers: smeenai, silvas, mehdi_amini, hiraditya, steven_wu, dexonsmith, dang, llvm-commits, zturner

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63453

llvm-svn: 365588

1 files changed, 47 insertions, 2 deletions

diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index c64c0df19ad..58c15023bab 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -990,9 +990,54 @@ Expected<file_t> openNativeFileForRead(const Twine &Name, OpenFlags Flags,
   return ResultFD;
 }
 
-void closeFile(file_t &F) {
-  ::close(F);
+file_t getStdinHandle() { return 0; }
+file_t getStdoutHandle() { return 1; }
+file_t getStderrHandle() { return 2; }
+
+std::error_code readNativeFile(file_t FD, MutableArrayRef<char> Buf,
+                               size_t *BytesRead) {
+  *BytesRead = sys::RetryAfterSignal(-1, ::read, FD, Buf.data(), Buf.size());
+  if (ssize_t(*BytesRead) == -1)
+    return std::error_code(errno, std::generic_category());
+  return std::error_code();
+}
+
+std::error_code readNativeFileSlice(file_t FD, MutableArrayRef<char> Buf,
+                                    size_t Offset) {
+  char *BufPtr = Buf.data();
+  size_t BytesLeft = Buf.size();
+
+#ifndef HAVE_PREAD
+  // If we don't have pread, seek to Offset.
+  if (lseek(FD, Offset, SEEK_SET) == -1)
+    return std::error_code(errno, std::generic_category());
+#endif
+
+  while (BytesLeft) {
+#ifdef HAVE_PREAD
+    ssize_t NumRead = sys::RetryAfterSignal(-1, ::pread, FD, BufPtr, BytesLeft,
+                                            Buf.size() - BytesLeft + Offset);
+#else
+    ssize_t NumRead = sys::RetryAfterSignal(-1, ::read, FD, BufPtr, BytesLeft);
+#endif
+    if (NumRead == -1) {
+      // Error while reading.
+      return std::error_code(errno, std::generic_category());
+    }
+    if (NumRead == 0) {
+      memset(BufPtr, 0, BytesLeft); // zero-initialize rest of the buffer.
+      break;
+    }
+    BytesLeft -= NumRead;
+    BufPtr += NumRead;
+  }
+  return std::error_code();
+}
+
+std::error_code closeFile(file_t &F) {
+  file_t TmpF = F;
   F = kInvalidFile;
+  return Process::SafelyCloseFileDescriptor(TmpF);
 }
 
 template <typename T>