[LLVMbugs] [Bug 17960] New: Windows Rewriter/MemoryBuffer errors with 16KB large files

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=17960

            Bug ID: 17960
           Summary: Windows Rewriter/MemoryBuffer errors with 16KB large
                    files
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: alp at nuanti.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Various parts of tooling, the modernizers and migrators are mis-using the
Rewriter interface and causing Windows errors or crashes when rewriting large
files.

A typical error:

"Error while writing file: Error opening output file \'1.cpp\': The requested
operation cannot be performed on a file with a user-mapped section open"

The problematic pattern appearing frequently in clang's codebase is:

      llvm::raw_fd_ostream FileStream(FileName.c_str(), ErrorInfo,
                                      llvm::sys::fs::F_Binary);
      Rewrite.getEditBuffer(ID).write(FileStream);

The fix is often simple as just replacing with:

      Rewrite.overwriteChangedFiles()

overwriteChangedFiles() knows how to write to a temporary file and move into
place without writing over the mmaped source MemoryBuffer.

My analysis of the underlying problem:

    The previous implementation was attempting to stream back directly to the
    original file and failing if it was already memory mapped by MemoryBuffer,
    an operation unsupported by Windows.

    MemoryBuffer generally mmaps files larger than the physical page.

I've so far fixed two instances of the problem:

 * r193594: RefactoringTool
 * r194250: clang-format (PR17216)

There are a others yet to be fixed, some more involved, which I'm filing this
bug to track.

The clang lit tests have mostly missed this so far because there's a tendency
to keep test inputs checked into version control small, but in practice this
gets hit often when running the rewrite-based tools on Windows with real code.

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