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Bug #9847

Updated by nobu (Nobuyoshi Nakada) almost 10 years ago

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`: #1: read all records from the file and store them into an array of Strings, using `File.read(size)` File.read(size) and storing via '`cache 'cache << buffer`' buffer' 
 - Test `#2`: #2: read all records from the file and store them into an Array of Strings, using `File.read(size, buffer)` File.read(size, buffer) and storing via '`cache 'cache << String.new(buffer)`'; 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` #1 & `#2` #2 show the cache containing all records:    `[ [ A, B, ... M ]` ] 
 - with a record size of 24, Test `#1` #1 shows the cache containing all records: `[ [ A, B, ... M ]` ] 
 - with a record size of 24, Test `#2` #2 shows the cache containing: `[ [ M, M, ... M ]` ] 

 Diagnosis & Notes: 

 
 - with a record size > 23 and reading a file using `File.read(size, buffer)`, 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` 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` File.read 
 - this shows that `String.new(buffer)` 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 

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