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fmt: fix usage of sync.Pool

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The current usage of sync.Pool is leaky because it stores an arbitrary
sized buffer into the pool. However, sync.Pool assumes that all items in the
pool are interchangeable from a memory cost perspective. Due to the unbounded
size of a buffer that may be added, it is possible for the pool to eventually
pin arbitrarily large amounts of memory in a live-lock situation.

As a simple fix, we just set a maximum size that we permit back into the pool.

We do not need to fix the use of a sync.Pool in scan.go since the free method
has always enforced a maximum capacity since the first commit of the scan logic.

Fixes #27740
Updates #23199

Change-Id: I875278f7dba42625405df36df3e9b028252ce5e3
Reviewed-on: https://go-review.googlesource.com/136116
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Joe Tsai 2018-09-18 12:52:39 -07:00 committed by Joe Tsai
parent 620bd5a3bc
commit e82d152e66
1 changed files with 10 additions and 0 deletions
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10
src/fmt/print.go
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@ -139,6 +139,16 @@ func newPrinter() *pp {
// free saves used pp structs in ppFree; avoids an allocation per invocation.
func (p *pp) free() {
// Proper usage of a sync.Pool requires each entry to have approximately
// the same memory cost. To obtain this property when the stored type
// contains a variably-sized buffer, we add a hard limit on the maximum buffer
// to place back in the pool.
//
// See https://golang.org/issue/23199
if cap(p.buf) > 64<<10 {
return
}
p.buf = p.buf[:0]
p.arg = nil
p.value = reflect.Value{}