[libc++] Fix filebuf resetting its underlying buffer upon close() (#168947)

When closing a filebuf, we would previously call setbuf(0, 0). Not only
does this get rid of any underlying buffer set by the user, but it also
sets the unbuffered mode.

This means that if the filebuf is then reopened to keep writing to a
file, it would have lost track of the original user-provided buffer (or
the default one which has a size of 4096), and instead it would use the
unbuffered mode, which is terribly slow.

This led to a bug report where users were complaining that closing and
reopening the filebuf led to a significantly worse performance than
using it without having closed and reopened. While this is a slightly
unusual usage pattern, it should definitely work.

rdar://161833214

libcxx/docs/ImplementationDefinedBehavior.rst

@@ -62,6 +62,13 @@ E.g.,
 - ``std::hermite(unsigned n, T x)`` for ``n >= 128``
 
 
+`[filebuf.virtuals] <https://eel.is/c++draft/filebuf.virtual>`_ Effect of calling ``basic_filebuf::setbuf`` with nonzero arguments
+----------------------------------------------------------------------------------------------------------------------------------
+
+Libc++ uses the provided buffer as the underlying buffer for input and output, and
+does not discard that buffer even when the stream is closed.
+
+
 `[stringbuf.cons] <http://eel.is/c++draft/stringbuf.cons>`_ Whether sequence pointers are initialized to null pointers
 ----------------------------------------------------------------------------------------------------------------------
 

libcxx/include/fstream

@@ -759,7 +759,12 @@ basic_filebuf<_CharT, _Traits>* basic_filebuf<_CharT, _Traits>::close() {
     if (fclose(__h.release()))
       __rt = nullptr;
     __file_ = nullptr;
-    setbuf(0, 0);
+    // Reset the get and the put areas without getting rid of the underlying buffers,
+    // which might have been configured by the user. Make sure to keep the buffers
+    // since the user may re-open the stream.
+    this->setg(nullptr, nullptr, nullptr);
+    this->setp(nullptr, nullptr);
+    __cm_ = __no_io_operations;
   }
   return __rt;
 }

libcxx/test/extensions/libcxx/input.output/file.streams/fstreams/filebuf.members/close.dont-get-rid-of-buffer.pass.cpp

@@ -0,0 +1,131 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// This test requires the fix to std::filebuf::close() (which is defined in the
+// built library) from https://github.com/llvm/llvm-project/pull/168947.
+// UNSUPPORTED: using-built-library-before-llvm-22
+
+// UNSUPPORTED: no-localization, no-filesystem
+
+// <fstream>
+
+// basic_filebuf<charT,traits>* close();
+
+//
+// Ensure that basic_filebuf::close() does not get rid of the underlying buffer set
+// via pubsetbuf(). Otherwise, reopening the stream will result in not reusing the
+// same buffer, which is conforming but also very surprising.
+//
+
+#include <cassert>
+#include <fstream>
+#include <string>
+
+#include "platform_support.h"
+#include "test_macros.h"
+
+struct overflow_detecting_filebuf : std::filebuf {
+  explicit overflow_detecting_filebuf(bool* overflow_monitor) : did_overflow_(overflow_monitor) {
+    assert(overflow_monitor != nullptr && "must provide an overflow monitor");
+  }
+
+  using Traits = std::filebuf::traits_type;
+  virtual std::filebuf::int_type overflow(std::filebuf::int_type ch = Traits::eof()) {
+    *did_overflow_ = true;
+    return std::filebuf::overflow(ch);
+  }
+
+private:
+  bool* did_overflow_;
+};
+
+int main(int, char**) {
+  std::string temp = get_temp_file_name();
+
+  bool did_overflow;
+  overflow_detecting_filebuf buf(&did_overflow);
+
+  // Set a custom buffer (of size 32, reused below)
+  char underlying_buffer[32];
+  buf.pubsetbuf(underlying_buffer, sizeof(underlying_buffer));
+
+  // (1) Open a file and insert a first character. That should overflow() and set the underlying
+  //     put area to our internal buffer set above.
+  {
+    buf.open(temp, std::ios::out | std::ios::trunc);
+    did_overflow = false;
+    buf.sputc('c');
+    assert(did_overflow == true);
+  }
+
+  // (2) Now, confirm that we can still insert 30 more characters without calling
+  //     overflow, since we should be writing to the internal buffer.
+  {
+    did_overflow = false;
+    for (int i = 0; i != 30; ++i) {
+      buf.sputc('c');
+      assert(did_overflow == false);
+    }
+  }
+
+  // (3) Writing the last character may or may not call overflow(), depending on whether
+  //     the library implementation wants to flush as soon as the underlying buffer is
+  //     full, or on the next attempt to insert. For libc++, it doesn't overflow yet.
+  {
+    did_overflow = false;
+    buf.sputc('c');
+    LIBCPP_ASSERT(!did_overflow);
+  }
+
+  // (4) Writing one-too-many characters will overflow (with libc++).
+  {
+    did_overflow = false;
+    buf.sputc('c');
+    LIBCPP_ASSERT(did_overflow);
+  }
+
+  // Close the stream. This should NOT unset the underlying buffer we set at the beginning
+  // Unfortunately, the only way to check that is to repeat the above tests which tries to
+  // tie the presence of our custom set buffer to whether overflow() gets called. This is
+  // not entirely portable since implementations are free to call overflow() whenever they
+  // want, but in practice this works pretty portably.
+  buf.close();
+
+  // Repeat (1)
+  {
+    buf.open(temp, std::ios::out | std::ios::trunc);
+    did_overflow = false;
+    buf.sputc('c');
+    assert(did_overflow == true);
+  }
+
+  // Repeat (2)
+  {
+    did_overflow = false;
+    for (int i = 0; i != 30; ++i) {
+      buf.sputc('c');
+      assert(did_overflow == false);
+    }
+  }
+
+  // Repeat (3)
+  {
+    did_overflow = false;
+    buf.sputc('c');
+    LIBCPP_ASSERT(!did_overflow);
+  }
+
+  // Repeat (4)
+  {
+    did_overflow = false;
+    buf.sputc('c');
+    LIBCPP_ASSERT(did_overflow);
+  }
+
+  return 0;
+}