Issue #17045 has been reported by byroot (Jean Boussier).

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Bug #17045: ObjectSpace.dump_all should allocate as little as possible in the GC heap
https://bugs.ruby-lang.org/issues/17045

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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For context I'm working on a [heap profiler](https://github.com/Shopify/heap-profiler). In short the use case is like this (pseudo code):

```ruby
GC.disable
ObjectSpace.dump_all(output: file1)
# run the user code
ObjectSpace.dump_all(output: file2)
compute_the_diff_and_report_statistics(file1, file2)
```

Ideally I would need `ObjectSpace.dump_all` to not modify the GC heap at all so that an empty user code would report an empty diff.

However as showcased [in this test case](https://github.com/Shopify/ruby/commit/c9c0f46318e8018e269840f3d173d62eeebb1176), `dump_all(output: <#File:/path.json>)` currently allocates 4 objects:

  - The `File` instance passed as `output:` is re-opened by `rb_io_stdio_file` in `dump_output` and I don't quite understand why.
  - The `scan_args` in `dump_all` allocates a `Hash` instance. Would using the new `Primitive` interface avoid that?
  - Another hash is allocated but I'm unsure where it comes from.
  - An `IMEMO "imemo_type"=>"callcache"` is allocated. Surprisingly it can be avoided by calling `ObjectSpace.dump_all(**opts)`

Could any of these be eliminated?



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