[llvm] [Support] Advance the buffer after partial file writes (PR #209480)

[via llvm-commits]

https://github.com/NotPppp1116 created https://github.com/llvm/llvm-project/pull/209480

## What

Advance both the buffer pointer and byte count while retrying partial writes in `copy_file_internal`, and treat a zero-byte write as an I/O error.

## Why

The previous loop decreased `BytesRead` after a short write but always passed the original buffer pointer to the next write. A short write therefore duplicated the buffer prefix and omitted later bytes. A zero-byte write could also loop forever.

## Impact

Descriptor-based file copies now preserve the complete buffer across partial writes and terminate cleanly on a non-progressing write.

## Checks

- built `SupportTests`
- `FileSystemTest.CopyFile` passed
- focused ReturnGuard scan reports no partial-transfer diagnostic for the corrected loop


>From ca723399911493809c85a73090c6badc7693a228 Mon Sep 17 00:00:00 2001
From: Pppp1116 <ACCOUNT1_NOREPLY>
Date: Tue, 14 Jul 2026 14:25:09 +0100
Subject: [PATCH] [Support] Advance the buffer after partial file writes

---
 llvm/lib/Support/Path.cpp | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
index 65e294020291f..2ca76b8ee95cc 100644
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -1016,26 +1016,24 @@ std::error_code create_directories(const Twine &Path, bool IgnoreExisting,
 
 static std::error_code copy_file_internal(int ReadFD, int WriteFD) {
   const size_t BufSize = 4096;
-  char *Buf = new char[BufSize];
-  int BytesRead = 0, BytesWritten = 0;
+  auto Buf = std::make_unique<char[]>(BufSize);
   for (;;) {
-    BytesRead = read(ReadFD, Buf, BufSize);
-    if (BytesRead <= 0)
-      break;
-    while (BytesRead) {
-      BytesWritten = write(WriteFD, Buf, BytesRead);
+    int BytesRead = read(ReadFD, Buf.get(), BufSize);
+    if (BytesRead < 0)
+      return errnoAsErrorCode();
+    if (BytesRead == 0)
+      return std::error_code();
+
+    int Offset = 0;
+    while (Offset < BytesRead) {
+      int BytesWritten = write(WriteFD, Buf.get() + Offset, BytesRead - Offset);
       if (BytesWritten < 0)
-        break;
-      BytesRead -= BytesWritten;
+        return errnoAsErrorCode();
+      if (BytesWritten == 0)
+        return make_error_code(errc::io_error);
+      Offset += BytesWritten;
     }
-    if (BytesWritten < 0)
-      break;
   }
-  delete[] Buf;
-
-  if (BytesRead < 0 || BytesWritten < 0)
-    return errnoAsErrorCode();
-  return std::error_code();
 }
 
 #ifndef __APPLE__