Add Read::is_append_only method to optimize read_to_string #89711
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I've added a new method called `is_append_only` to the `Read` trait. It returns true if `read_to_end` and `read_to_string` can be trusted not to read or overwrite any existing data in their input buffer. These readers use it to signal that they have optimized `read_to_end` methods: * `[u8]` extends the input buffer to the full slice size all at once instead of doubling its size repeatedly until it's big enough. * `File` pre-allocates enough space for the full file size, allowing for fewer `read` syscalls and reducing buffer reallocations. These readers take advantage of it: * If `BufReader` wraps a `[u8]` or `File` its `read_to_string` will call their optimized `read_to_end`s. Overall, this fixes a slow path where `BufReader::read_to_string` would be forced to read into a side buffer when passed a non-empty input buffer.
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This is what I came up with to fix the slow path identified in #89582 (comment). Introducing a new trait method feels like swatting a fly with a bazooka. I don't have any better ideas, though. (Except, of course, doing nothing.) cc @a1phyr |
jkugelman
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Oct 9, 2021
| /// | ||
| /// [`read_to_end`]: Read::read_to_end | ||
| /// [`read_to_string`]: Read::read_to_string | ||
| #[unstable(feature = "append_only", issue = "none")] |
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Placeholder for a new feature / tracking issue.
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I'm going to submit a different idea. |
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I've added a new method called
is_append_onlyto theReadtrait. It returns true ifread_to_endandread_to_stringcan be trusted not to read or overwrite any existing data in their input buffer.These readers use it to signal that they have optimized
read_to_endmethods:[u8]extends the input buffer to the full slice size all at once instead of doubling its size repeatedly until it's big enough.Filepre-allocates enough space for the full file size, allowing for fewerreadsyscalls and reducing buffer reallocations.These readers take advantage of it:
BufReaderwraps a[u8]orFileitsread_to_stringwill call their optimizedread_to_ends.Overall, this fixes a slow path where
BufReader::read_to_stringwould be forced to read into a side buffer when passed a non-empty input buffer.