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stdlib/String: fix a race in _StringBuffer.grow()

Browse Source 
rdar://17855614


Swift SVN r20960
This commit is contained in:
Dmitri Hrybenko
2014-08-02 22:50:45 +00:00
parent 63304b57b4
commit 6d4f8adbf5
9 changed files with 582 additions and 7 deletions
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23
stdlib/core/StringBuffer.swift
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@@ -28,7 +28,10 @@ struct _StringBufferIVars {
self.capacityAndElementShift = byteCapacity + (elementWidth - 1)
}
// This stored property should be stored at offset zero. We perform atomic
// operations on it using HeapBuffer's pointer.
var usedEnd: UnsafeMutablePointer<RawByte>
var capacityAndElementShift: Int
var byteCapacity: Int {
return capacityAndElementShift & ~0x1
@@ -200,13 +203,23 @@ public struct _StringBuffer {
return true
}
// FIXME: this function is currently NOT THREADSAFE. The test +
// assignment below should be replaced by a CAS
// <rdar://problem/17855614> _StringBuffer.grow() is racy
if usedEnd == subRange.endIndex {
usedEnd = newUsedEnd
// Optimization: even if the buffer is shared, but the substring we are
// trying to grow is located at the end of the buffer, it can be grown in
// place. The operation should be implemented in a thread-safe way,
// though.
//
// if usedEnd == subRange.endIndex {
// usedEnd = newUsedEnd
// return true
// }
let usedEndPhysicalPtr =
UnsafeMutablePointer<UnsafeMutablePointer<RawByte>>(_storage._value)
var expected = UnsafeMutablePointer<RawByte>(subRange.endIndex)
if _stdlib_atomicCompareExchangeStrongPtr(
object: usedEndPhysicalPtr, expected: &expected, desired: newUsedEnd) {
return true
}
return false
}