SSA compiler on AMD64 may spill Duff-adjusted address as scalar. If
the object is on stack and the stack moves, the spilled address become
invalid.
Making the spill pointer-typed does not work. The Duff-adjusted address
points to the memory before the area to be zeroed and may be invalid.
This may cause stack scanning code panic.
Fix it by doing Duff-adjustment in genValue, so the intermediate value
is not seen by the reg allocator, and will not be spilled.
Add a test to cover both cases. As it depends on allocation, it may
be not always triggered.
Fixes#16515.
Change-Id: Ia81d60204782de7405b7046165ad063384ede0db
Reviewed-on: https://go-review.googlesource.com/25309
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Updates #16396
Change-Id: I7b4f85610e66f2c77c17cf8898cc41d81b2efc8c
Reviewed-on: https://go-review.googlesource.com/25283
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Mutator goroutines that allocate memory during the concurrent mark
phase are required to spend some time assisting the garbage
collector. The magnitude of this mandatory assistance is proportional
to the goroutine's allocation debt and subject to the assistance
ratio as calculated by the pacer.
When assisting the garbage collector, a mutator goroutine will go
beyond paying off its allocation debt. It will build up extra credit
to amortize the overhead of the assist.
In fast-allocating applications with high assist ratios, building up
this credit can take the affected goroutine's entire time slice.
Reduce the penalty on each goroutine being selected to assist the GC
in two ways, to spread the responsibility more evenly.
First, do a consistent amount of extra scan work without regard for
the pacer's assistance ratio. Second, reduce the magnitude of the
extra scan work so it can be completed within a few hundred
microseconds.
Commentary on gcOverAssistWork is by Austin Clements, originally in
https://golang.org/cl/24704
Updates #14812Fixes#16432
Change-Id: I436f899e778c20daa314f3e9f0e2a1bbd53b43e1
Reviewed-on: https://go-review.googlesource.com/25155
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
https://golang.org/cl/25233 was detecting the OS X release at compile
time, not run time. Detect it at run time instead.
Fixes#16473 (again)
Change-Id: I6bec4996e57aa50c52599c165aa6f1fae7423fa7
Reviewed-on: https://go-review.googlesource.com/25281
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Updates x/net/http2 to git rev 6a513af for:
http2: return flow control for closed streams
https://golang.org/cl/25231
http2: make Transport prefer HTTP response header recv before body write error
https://golang.org/cl/24984
http2: make Transport treat "Connection: close" the same as Request.Close
https://golang.org/cl/24982Fixesgolang/go#16481
Change-Id: Iaddb166387ca2df1cfbbf09a166f8605578bec49
Reviewed-on: https://go-review.googlesource.com/25282
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Currently the pprof package gives almost no guidance for how to use it
and, despite the standard boilerplate used to create CPU and memory
profiles, this boilerplate appears nowhere in the pprof documentation.
Update the pprof package documentation to give the standard
boilerplate in a form people can copy, paste, and tweak. This
boilerplate is based on rsc's 2011 blog post on profiling Go programs
at https://blog.golang.org/profiling-go-programs, which is where I
always go when I need to copy-paste the boilerplate.
Change-Id: I74021e494ea4dcc6b56d6fb5e59829ad4bb7b0be
Reviewed-on: https://go-review.googlesource.com/25182
Reviewed-by: Rick Hudson <rlh@golang.org>
Conservative fix for the OS X 10.8 crash. We can unify them back together
during the Go 1.8 dev cycle.
Fixes#16473
Change-Id: If07228deb2be36093dd324b3b3bcb31c23a95035
Reviewed-on: https://go-review.googlesource.com/25233
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Adds a test case for calling context.WithDeadline() where the deadline
exists in the past. This change increases the code coverage of the
context package.
Change-Id: Ib486bf6157e779fafd9dab2b7364cdb5a06be36e
Reviewed-on: https://go-review.googlesource.com/25007
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
From at least Go 1.4 to Go 1.6, Transport.RoundTrip would return the
error value from net.Conn.Read directly when the initial Read (1 byte
Peek) failed while reading the HTTP response, if a request was
outstanding. While never a documented or tested promise, Go 1.7 changed the
behavior (starting at https://golang.org/cl/23160).
This restores the old behavior and adds a test (but no documentation
promises yet) while keeping the fix for spammy logging reported in #15446.
This looks larger than it is: it just changes errServerClosedConn from
a variable to a type, where the type preserves the underlying
net.Conn.Read error, for unwrapping later in Transport.RoundTrip.
Fixes#16465
Change-Id: I6fa018991221e93c0cfe3e4129cb168fbd98bd27
Reviewed-on: https://go-review.googlesource.com/25153
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Noticed when investigating a separate issue.
No external bug report or repro yet.
Change-Id: I8a1641a43163f22b09accd3beb25dd9e2a68a238
Reviewed-on: https://go-review.googlesource.com/25152
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
It was removed in upstream Chrome https://codereview.chromium.org/2016863004
Rather than update to the latest version, make the minimal change for Go 1.7 and
change the "showToUser" boolean from true to false.
Tested by hand that it goes away after this change.
Updates #16247
Change-Id: I051f49da878e554b1a34a88e9abc70ab50e18780
Reviewed-on: https://go-review.googlesource.com/25117
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This is:
(1) a simple trick that cuts the number of phi-nodes
(temporarily) inserted into the ssa representation by a factor
of 10, and can cut the user time to compile tricky inputs like
gogo/protobuf tests from 13 user minutes to 9.5, and memory
allocation from 3.4GB to 2.4GB.
(2) a fix to sparse lookup, that does not rely on
an assumption proven false by at least one pathological
input "etldlen".
These two changes fix unrelated compiler performance bugs,
both necessary to obtain good performance compiling etldlen.
Without them it takes 20 minutes or longer, with them it
completes in 2 minutes, without a gigantic memory footprint.
Updates #16407
Change-Id: Iaa8aaa8c706858b3d49de1c4865a7fd79e6f4ff7
Reviewed-on: https://go-review.googlesource.com/23136
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
entry:
x = MOVQconst [7]
...
b1:
goto b2
b2:
v = Phi(x, y, z)
Transform that program to:
entry:
...
b1:
x = MOVQconst [7]
goto b2
b2:
v = Phi(x, y, z)
This CL moves constant-generating instructions used by a phi to the
appropriate immediate predecessor of the phi's block.
We used to put all constants in the entry block. Unfortunately, in
large functions we have lots of constants at the start of the
function, all of which are used by lots of phis throughout the
function. This leads to the constants being live through most of the
function (especially if there is an outer loop). That's an O(n^2)
problem.
Note that most of the non-phi uses of constants have already been
folded into instructions (ADDQconst, MOVQstoreconst, etc.).
This CL may be generally useful for other instances of compiler
slowness, I'll have to check. It may cause some programs to run
slower, but probably not by much, as rematerializeable values like
these constants are allocated late (not at their originally scheduled
location) anyway.
This CL is definitely a minimal change that can be considered for 1.7.
We probably want to do a better job in the tighten pass generally, not
just for phi args. Leaving that for 1.8.
Update #16407
Change-Id: If112a8883b4ef172b2f37dea13e44bda9346c342
Reviewed-on: https://go-review.googlesource.com/25046
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
The omission of this instruction could confuse the traceback code if a
SIGPROF occurred during a signal handler. The traceback code would
trace up to sigtramp, but would then get confused because it would see a
PC address that did not appear to be in the function.
Fixes#16453.
Change-Id: I2b3d53e0b272fb01d9c2cb8add22bad879d3eebc
Reviewed-on: https://go-review.googlesource.com/25104
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Most operations need an upper bound on the physical page size, which
is what sys.PhysPageSize is for (this is checked at runtime init on
Linux). However, a few operations need a *lower* bound on the physical
page size. Introduce a "minPhysPageSize" constant to act as this lower
bound and use it where it makes sense:
1) In addrspace_free, we have to query each page in the given range.
Currently we increment by the upper bound on the physical page
size, which means we may skip over pages if the true size is
smaller. Worse, we currently pass a result buffer that only has
enough room for one page. If there are actually multiple pages in
the range passed to mincore, the kernel will overflow this buffer.
Fix these problems by incrementing by the lower-bound on the
physical page size and by passing "1" for the length, which the
kernel will round up to the true physical page size.
2) In the write barrier, the bad pointer check tests for pointers to
the first physical page, which are presumably small integers
masquerading as pointers. However, if physical pages are smaller
than we think, we may have legitimate pointers below
sys.PhysPageSize. Hence, use minPhysPageSize for this test since
pointers should never fall below that.
In particular, this applies to ARM64 and MIPS. The runtime is
configured to use 64kB pages on ARM64, but by default Linux uses 4kB
pages. Similarly, the runtime assumes 16kB pages on MIPS, but both 4kB
and 16kB kernel configurations are common. This also applies to ARM on
systems where the runtime is recompiled to deal with a larger page
size. It is also a step toward making the runtime use only a
dynamically-queried page size.
Change-Id: I1fdfd18f6e7cbca170cc100354b9faa22fde8a69
Reviewed-on: https://go-review.googlesource.com/25020
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Austin Clements <austin@google.com>
When a non-Go thread calls into Go, the runtime needs an M to run the Go
code. The runtime keeps a list of extra M's available. When the last
extra M is allocated, the needextram field is set to tell it to allocate
a new extra M as soon as it is running in Go. This ensures that an extra
M will always be available for the next thread.
However, if many threads need an extra M at the same time, this
serializes them all. One thread will get an extra M with the needextram
field set. All the other threads will see that there is no M available
and will go to sleep. The one thread that succeeded will create a new
extra M. One lucky thread will get it. All the other threads will see
that there is no M available and will go to sleep. The effect is
thundering herd, as all the threads looking for an extra M go through
the process one by one. This seems to have a particularly bad effect on
the FreeBSD scheduler for some reason.
With this change, we track the number of threads waiting for an M, and
create all of them as soon as one thread gets through. This still means
that all the threads will fight for the lock to pick up the next M. But
at least each thread that gets the lock will succeed, instead of going
to sleep only to fight again.
This smooths out the performance greatly on FreeBSD, reducing the
average wall time of `testprogcgo CgoCallbackGC` by 74%. On GNU/Linux
the average wall time goes down by 9%.
Fixes#13926Fixes#16396
Change-Id: I6dc42a4156085a7ed4e5334c60b39db8f8ef8fea
Reviewed-on: https://go-review.googlesource.com/25047
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
This copies the frozen wording from the log/syslog package.
Fixes#16436
Change-Id: If5d478023328925299399f228d8aaf7fb117c1b4
Reviewed-on: https://go-review.googlesource.com/25080
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Rather than saying "stop-the-world", say "garbage collection pauses".
Change-Id: Ifb2931781ab3094e04bea93f01f18f1acb889bdc
Reviewed-on: https://go-review.googlesource.com/25018
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Because,
* The CGI spec defines that incoming request header "Foo: Bar" maps to
environment variable HTTP_FOO == "Bar". (see RFC 3875 4.1.18)
* The HTTP_PROXY environment variable is conventionally used to configure
the HTTP proxy for HTTP clients (and is respected by default for
Go's net/http.Client and Transport)
That means Go programs running in a CGI environment (as a child
process under a CGI host) are vulnerable to an incoming request
containing "Proxy: attacker.com:1234", setting HTTP_PROXY, and
changing where Go by default proxies all outbound HTTP requests.
This is CVE-2016-5386, aka https://httpoxy.org/Fixes#16405
Change-Id: I6f68ade85421b4807785799f6d98a8b077e871f0
Reviewed-on: https://go-review.googlesource.com/25010
Run-TryBot: Chris Broadfoot <cbro@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Most of the runtime improvements are hard to quantify or summarize,
but it's worth mentioning some of the substantial improvements in STW
time, and that the scavenger now actually works on ARM64, PPC64, and
MIPS.
Change-Id: I0e951038516378cc3f95b364716ef1c183f3445a
Reviewed-on: https://go-review.googlesource.com/24966
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Only run TestDialerDualStack on the builders, as to not annoy or
otherwise distract users when it's not their fault.
Even though the intention is to only run this on the builders, very
few of the builders have IPv6 support. Oh well. We'll get some
coverage.
Updates #13324
Change-Id: I13e7e3bca77ac990d290cabec88984cc3d24fb67
Reviewed-on: https://go-review.googlesource.com/24985
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Change https://golang.org/cl/19895 caused a regression
where the last character in a string would be dropped if it was
accompanied by an io.EOF.
This change fixes the logic so that the last byte is still returned
without a problem.
Fixes#16393
Change-Id: I7a4d0abf761c2c15454136a79e065fe002d736ea
Reviewed-on: https://go-review.googlesource.com/24981
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
We decided that ppc64 should maintain power5 compatibility.
ppc64le requires power8.
Fixes#16372.
Change-Id: If5b309a0563f55a3c1fe9c853d29a463f5b71101
Reviewed-on: https://go-review.googlesource.com/24915
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This fixes erroneous handling of the more result parameter of
runtime.Frames.Next.
Fixes#16349.
Change-Id: I4f1c0263dafbb883294b31dbb8922b9d3e650200
Reviewed-on: https://go-review.googlesource.com/24911
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Regression from Go 1.6 to Go 1.7rc1: we had broken the ability for
users to vendor "golang.org/x/net/http2" or "golang.org/x/net/route"
because we were vendoring them ourselves and cmd/go and cmd/compile do
not understand multiple vendor directories across multiple GOPATH
workspaces (e.g. user's $GOPATH and default $GOROOT).
As a short-term fix, since fixing cmd/go and cmd/compile is too
invasive at this point in the cycle, just rename "golang.org" to
"golang_org" for the standard library's vendored copy.
Fixes#16333
Change-Id: I9bfaed91e9f7d4ca6bab07befe80d71d437a21af
Reviewed-on: https://go-review.googlesource.com/24902
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Document that the http.Server is now stricter about rejecting
requests with invalid HTTP versions, and also that it rejects plaintext
HTTP/2 requests, except for `PRI * HTTP/2.0` upgrade requests.
The relevant CL is https://golang.org/cl/24505.
Updates #15810.
Change-Id: Ibbace23e001b5e2eee053bd341de50f9b6d3fde8
Reviewed-on: https://go-review.googlesource.com/24731
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
New Gophers sometimes misconstrue the advice in the "Generality" section
as "export interfaces instead of implementations" and add needless
interfaces to their code as a result. Down the road, they end up
needing to add methods and either break existing callers or have to
resort to unpleasant hacks (e.g. using "magic method" type-switches).
Weaken the first paragraph of this section to only advise leaving types
unexported when they will never need additional methods.
Change-Id: I32a1ae44012b5896faf167c02e192398a4dfc0b8
Reviewed-on: https://go-review.googlesource.com/24892
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Fixes#12272
Change-Id: I0306ce0ef4a87df2158df3b7d4d8d93a1cb6dabc
Reviewed-on: https://go-review.googlesource.com/24864
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
The cgocallback function picked up a ctxt parameter in CL 22508.
That CL updated the assembler implementation, but there are a few
mentions in Go code that were not updated. This CL fixes that.
Fixes#16326
Change-Id: I5f68e23565c6a0b11057aff476d13990bff54a66
Reviewed-on: https://go-review.googlesource.com/24848
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
The reflect package was returning a non-empty PkgPath for an unnamed
type with methods, such as a type whose methods have a pointer
receiver.
Fixes#16328.
Change-Id: I733e93981ebb5c5c108ef9b03bf5494930b93cf3
Reviewed-on: https://go-review.googlesource.com/24862
Reviewed-by: David Crawshaw <crawshaw@golang.org>
This is a copy of the "FANOUT" benchmark recently added to RE2 with the
following comment:
// This has quite a high degree of fanout.
// NFA execution will be particularly slow.
Most of the benchmarks on the regexp package have very little fanout and
are designed for comparing the regexp package's NFA with backtracking
engines found in other regular expression libraries. This benchmark
exercises the performance of the NFA on expressions with high fanout.Reviewed-on: https://go-review.googlesource.com/24846
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
"
This reverts commit fc803874d3.
Reason for revert: Breaks the -race build because the benchmark takes too long to run.
Change-Id: I6ed4b466f74a4108d8bcd5b019b9abe971eb483e
Reviewed-on: https://go-review.googlesource.com/24861
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
This is a copy of the "FANOUT" benchmark recently added to RE2 with the
following comment:
// This has quite a high degree of fanout.
// NFA execution will be particularly slow.
Most of the benchmarks on the regexp package have very little fanout and
are designed for comparing the regexp package's NFA with backtracking
engines found in other regular expression libraries. This benchmark
exercises the performance of the NFA on expressions with high fanout.
Change-Id: Ie9c8e3bbeffeb1fe9fb90474ddd19e53f2f57a52
Reviewed-on: https://go-review.googlesource.com/24846
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
A follow-on to https://golang.org/cl/24852 that mentions the
documentation clarifications.
Updates #16308.
Change-Id: Ic2a6e1d4938d74352f93a6649021fb610efbfcd0
Reviewed-on: https://go-review.googlesource.com/24857
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
There are no synchronization points protecting the readVal and readPos
variables. This leads to a race when Read is called concurrently.
Fix this by adding methods to lockedSource, which is the case where
a race matters.
Fixes#16308.
Change-Id: Ic028909955700906b2d71e5c37c02da21b0f4ad9
Reviewed-on: https://go-review.googlesource.com/24852
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
For better or for worse, it's IsExist, not IsExists.
Change-Id: I4503f961486edd459c0c81cf3f32047dff7703a4
Reviewed-on: https://go-review.googlesource.com/24819
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
In the beta version of the macOS Sierra (10.12) release, the
gettimeofday system call changed on x86. Previously it always returned
the time in the AX/DX registers. Now, if AX is returned as 0, it means
that the system call has stored the values into the memory pointed to by
the first argument, just as the libc gettimeofday function does. The
libc function handles both cases, and we need to do so as well.
Fixes#16272.
Change-Id: Ibe5ad50a2c5b125e92b5a4e787db4b5179f6b723
Reviewed-on: https://go-review.googlesource.com/24812
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
The shrinkstack code locks all the channels a goroutine is waiting for,
but didn't handle the case of the same channel appearing in the list
multiple times. This led to a deadlock. The channels are sorted so it's
easy to avoid locking the same channel twice.
Fixes#16286.
Change-Id: Ie514805d0532f61c942e85af5b7b8ac405e2ff65
Reviewed-on: https://go-review.googlesource.com/24815
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
