From: nobu@...
Date: 2014-07-03T01:26:59+00:00
Subject: [ruby-core:63526] [ruby-trunk - Bug #9847] Cannot create new String when using File.read(size, buffer)

Issue #9847 has been updated by Nobuyoshi Nakada.

Duplicated by Bug #10007: IO#read does not respect String Copy-On-Write in some cases added

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Bug #9847: Cannot create new String when using File.read(size, buffer) 
https://bugs.ruby-lang.org/issues/9847#change-47575

* Author: Kenneth Guerin
* Status: Closed
* Priority: Normal
* Assignee: 
* Category: core
* Target version: current: 2.2.0
* ruby -v: 2.1.2p95
* Backport: 2.0.0: DONE, 2.1: DONE
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This bug was first noticed in version 1.9.2 and is still present in 2.1.2p95

The attached script does the following to highlight this bug:

- create a file of 13 fixed-length records of a specific size: all records contain a repeated letter, A for the first, B for the second, through M
- Test `#1`: read all records from the file and store them into an array of Strings, using `File.read(size)` and storing via '`cache << buffer`'
- Test `#2`: read all records from the file and store them into an Array of Strings, using `File.read(size, buffer)` and storing via '`cache << String.new(buffer)`'; buffer will be reused during each read, cache will hold copies
- test cycle is run using a record length of 23 and a record length of 24, highlighting Ruby's optimization of short strings

Results of running this script:

- with a record size of 23, Tests `#1` & `#2` show the cache containing all records:  `[ A, B, ... M ]`
- with a record size of 24, Test `#1` shows the cache containing all records: `[ A, B, ... M ]`
- with a record size of 24, Test `#2` shows the cache containing: `[ M, M, ... M ]`

Diagnosis & Notes:

- with a record size > 23 and reading a file using `File.read(size, buffer)`, buffer is in such a state as to prevent a new unique String from being derived
- variations of this script showed that `String.new` was a creating new String object on each record read, but the contents of all of the Strings stored in the cache Array were being overwritten on each call to `File.read`
- this shows that `String.new(buffer)` is creating new String objects, but that the underlying values of all strings based on buffer were sharing the same internal memory
- this behavior exists in the "long string" variation of String; short optimized Strings do not share this property


---Files--------------------------------
strbug.rb (2.07 KB)


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