[libcxx-commits] [libcxx] [libc++][format] Don't leave the output buffer full after a bulk write (PR #226791)

[A. Jiang via libcxx-commits]

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@@ -250,6 +250,10 @@ class __output_buffer {
       __first += __chunk;
       __n -= __chunk;
     } while (__n);
+
+    // push_back needs a free code unit, which the last chunk may have used.
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frederick-vs-ja wrote:

To me, it was `push_back` that did a suspicious thing (https://github.com/llvm/llvm-project/issues/154670#issuecomment-3342606881). IIUC we need to add these `__prepare_write` calls if we want to keep the original optimization.

https://github.com/llvm/llvm-project/pull/226791