Add optional buffer size to pythonbuf::d_buffer constructor (#1687)

In some cases the user of pythonbuf needs to allocate the internal
buffer to a specific size e.g. for performance or to enable synchronous
writes to the buffer.
By changing `pythonbuf::d_buffer` to be dynamically allocated we can now
enable these use-cases while still providing the default behavior of
allocating a 1024 byte internal buffer (through a default parameter).

include/pybind11/iostream.h

@@ -25,7 +25,8 @@ class pythonbuf : public std::streambuf {
 private:
     using traits_type = std::streambuf::traits_type;
 
-    char d_buffer[1024];
+    const size_t buf_size;
+    std::unique_ptr<char[]> d_buffer;
     object pywrite;
     object pyflush;
 
@@ -51,10 +52,13 @@ private:
     }
 
 public:
-    pythonbuf(object pyostream)
-        : pywrite(pyostream.attr("write")),
+
+    pythonbuf(object pyostream, size_t buffer_size = 1024)
+        : buf_size(buffer_size),
+          d_buffer(new char[buf_size]),
+          pywrite(pyostream.attr("write")),
           pyflush(pyostream.attr("flush")) {
-        setp(d_buffer, d_buffer + sizeof(d_buffer) - 1);
+        setp(d_buffer.get(), d_buffer.get() + buf_size - 1);
     }
 
     /// Sync before destroy