Takes a bit too long to run it all the time.

Fixes #17217
Update #17104

Change-Id: I4802190ea16ee0f436a7f95b093ea0f995f5b11d
Reviewed-on: https://go-review.googlesource.com/29751
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Unify the OS-specific versions of msigsave, msigrestore, sigblock,
updatesigmask, and unblocksig into single versions in signal_unix.go.
To do this, make sigprocmask work the same way on all systems, which
required adding a definition of sigprocmask for linux and openbsd.
Also add a single OS-specific function sigmaskToSigset.

Change-Id: I7cbf75131dddb57eeefe648ef845b0791404f785
Reviewed-on: https://go-review.googlesource.com/29689
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

Change-Id: I6071881a5f7b9638bca0bfef76d6f4f45c9202a6
Reviewed-on: https://go-review.googlesource.com/29396
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Change-Id: I8e594e059448879a9f451801064729186ac7c11b
Reviewed-on: https://go-review.googlesource.com/29395
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

This CL turns some special section marker symbols into real symbols
laid out in the sections they mark. This is to deal with the fact
that dyld on OS X resolves the section marker symbols in any dlopen-ed
Go program to the original section marker symbols in the host program.

More details in a comment in cmd/link/internal/ld/data.go.

Change-Id: Ie9451cfbf06d0bdcccb9959219c791b829f3f771
Reviewed-on: https://go-review.googlesource.com/29394Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

The 2-byte rule was firing before the 4-byte rule, preventing
the 4-byte rule from firing.  Update the 4-byte rule to use
the results of the 2-byte rule instead.

Add some tests to make sure we don't regress again.

Fixes #17147

Change-Id: Icfeccd9f2b96450981086a52edd76afb3191410a
Reviewed-on: https://go-review.googlesource.com/29382
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Before this change a nil check on s390x could be scheduled after the
target pointer has been dereferenced.

Change-Id: I7ea40a4b52f975739f6db183a2794be4981c4e3d
Reviewed-on: https://go-review.googlesource.com/29730
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

This adds the instructions frim, frip, and friz to the ppc64x
assembler for use in implementing the math.Floor, math.Ceil, and
math.Trunc functions to improve performance.

Fixes #17185

BenchmarkCeil-128                    21.4          6.99          -67.34%
BenchmarkFloor-128                   13.9          6.37          -54.17%
BenchmarkTrunc-128                   12.7          6.33          -50.16%

Change-Id: I96131bd4e8c9c8dbafb25bfeb544cf9d2dbb4282
Reviewed-on: https://go-review.googlesource.com/29654
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Michael Munday <munday@ca.ibm.com>

Attempt to fix the linux-amd64-clang builder, which broke
with CL 29472.

Turns out pthread_yield is a non-portable Linux function, and
should have #define _GNU_SOURCE before #include <pthread.h>.
GCC doesn't complain about this, but Clang does:

	./raceprof.go:44:3: warning: implicit declaration of function 'pthread_yield' is invalid in C99 [-Wimplicit-function-declaration]

(Though the error, while explicable, certainly could be clearer.)

There is a portable POSIX equivalent, sched_yield, so this
CL uses it instead.

Change-Id: I58ca7a3f73a2b3697712fdb02e72a8027c391169
Reviewed-on: https://go-review.googlesource.com/29675
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Change-Id: I76981d1d83da401178226634d076371a04f5ccb7
Reviewed-on: https://go-review.googlesource.com/29392
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Inspired by difficulties with plugin support on darwin.

Change-Id: I2cef8410837946454e75d00e94e46791f03f2267
Reviewed-on: https://go-review.googlesource.com/29391Reviewed-by: Ian Lance Taylor <iant@golang.org>

Instrumenting copy and append for the race detector changes them to call
different functions. In the runtime package the alternate functions are
not marked as nosplit. This caused a crash in the SIGPROF handler when
invoked on a non-Go thread in a program built with the race detector. In
some cases the handler can call copy, the race detector changed that to
a call to a non-nosplit function, the function tried to check the stack
guard, and crashed because it was running on a non-Go thread. The
SIGPROF handler is written carefully to avoid such problems, but hidden
function calls are difficult to avoid.

Fix this by changing the compiler to not instrument copy and append when
compiling the runtime package. Change the runtime package to add
explicit race checks for the only code I could find where copy is used
to write to user data (append is never used).

Change-Id: I11078a66c0aaa459a7d2b827b49f4147922050af
Reviewed-on: https://go-review.googlesource.com/29472
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

  var x *X = ...
  defer x.foo()

As part of the defer, we need to calculate &(*X).foo·f.  This expression
is the address of the static closure that will call (*X).foo when a
pointer to that closure is used in a call/defer/go.  This pointer is not
currently properly typed in SSA.  It is a pointer type, but the base
type is nil, not a proper type.

This turns out not to be a problem currently because we never use the
type of these SSA values.  But I'm trying to change that (to be able to
spill them) in CL 28391.  To fix, use uint8 as the fake type of the
closure.

Change-Id: Ieee388089c9af398ed772ee8c815122c347cb633
Reviewed-on: https://go-review.googlesource.com/29444
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Add an "errorcheckwithauto" action which performs error check
including lines with auto-generated functions (excluded by
default). Comment "// ERRORAUTO" matches these lines.

Add testcase for CL 29570 (as an example).

Updates #16016, #17186.

Change-Id: Iaba3727336cd602f3dda6b9e5f97dafe0848e632
Reviewed-on: https://go-review.googlesource.com/29652
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

On link-register machines we uses RET (sym), instead of JMP (sym),
for tail call (so the assembler knows and may rewrite it to
restore link register if necessary). Add RET to the analysis.

Fixes #17186.
Fixes #16016 on link-register machines.

Change-Id: I8690ac57dd9d49beeea76a5f291988e9a1d3afe5
Reviewed-on: https://go-review.googlesource.com/29570
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

After renegotiation support was added (af125a51) it's possible for a
Write to block on a Read when racing to complete the handshake:
   1. The Write determines that a handshake is needed and tries to
      take the neccesary locks in the correct order.
   2. The Read also determines that a handshake is needed and wins
      the race to take the locks.
   3. The Read goroutine completes the handshake and wins a race
      to unlock and relock c.in, which it'll hold when waiting for
      more network data.

If the application-level protocol requires the Write to complete before
data can be read then the system as a whole will deadlock.

Unfortunately it doesn't appear possible to reverse the locking order of
c.in and handshakeMutex because we might read a renegotiation request at
any point and need to be able to do a handshake without unlocking.

So this change adds a sync.Cond that indicates that a goroutine has
committed to doing a handshake. Other interested goroutines can wait on
that Cond when needed.

The test for this isn't great. I was able to reproduce the deadlock with
it only when building with -race. (Because -race happened to alter the
timing just enough.)

Fixes #17101.

Change-Id: I4e8757f7b82a84e46c9963a977d089f0fb675495
Reviewed-on: https://go-review.googlesource.com/29164Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

This change modifies URL.String to prepend "./" to a relative URL which
contains a colon in the first path segment.

Per RFC 3986 §4.2:

> A path segment that contains a colon character (e.g., "this:that")
> cannot be used as the first segment of a relative-path reference, as
> it would be mistaken for a scheme name.  Such a segment must be
> preceded by a dot-segment (e.g., "./this:that") to make a relative-
> path reference.

https://go-review.googlesource.com/27440 corrects the behavior for http.FileServer,
but URL.String will still return an invalid URL. This CL reverts the changes to
http.FileServer as they are unnecessary with this fix.

Fixes #17184

Change-Id: I9211ae20f82c91b785d1b079b2cd766487d94225
Reviewed-on: https://go-review.googlesource.com/29610Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Abandoned earlier efforts to expose zero register,
but left it in numbering to decrease squirrelyness of
register allocator.

ISELrelOp used in code generation of bool := x relOp y.
Some patterns added to better elide zero case and
some sign extension.

Updates: #17109

Change-Id: Ida7839f0023ca8f0ffddc0545f0ac269e65b05d9
Reviewed-on: https://go-review.googlesource.com/29380
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

Updates #16360.

Adds examples uing:
+ Writer, Reader
+ Reader.Multistream to concatenate and then
individually retrieve multiple gzipped files
+ Reset

Change-Id: I9ad9b92729a5cd58f7368eaf2db05f1cdf21063d
Reviewed-on: https://go-review.googlesource.com/29218Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

See https://groups.google.com/d/topic/golang-nuts/stbum5gZbAc/discussion

Change-Id: I2e78e8d0dadd78c8b0389514cad3c45d061b663b
Reviewed-on: https://go-review.googlesource.com/29496Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Requires adding a sigfwd function for Solaris, as previously
signal2_unix.go was not built for Solaris.

Change-Id: Iea3ff0ddfa15af573813eb075bead532b324a3fc
Reviewed-on: https://go-review.googlesource.com/29550
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

This change reverts CL 18835 which is a workaroud for older DragonFly
BSD kernels, and fixes #14051, #14052 and #14067 in a more general way
the same as other platforms except NetBSD.

This change also bumps the minimum required version of DragonFly BSD
kernel to 4.4.4.

Fixes #16329.

Change-Id: I0b44b6afa675f5ed9523914226bd9ec4809ba5ae
Reviewed-on: https://go-review.googlesource.com/29491Reviewed-by: Ian Lance Taylor <iant@golang.org>

Updates #16357.

Change-Id: Ia837dd44bad76931baa9469e64371bc253d6694b
Reviewed-on: https://go-review.googlesource.com/29219
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

Change-Id: I128b142aee5e1b917e7ba63b48512972f053ea0b
Reviewed-on: https://go-review.googlesource.com/29531
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Some applications built with Go on ppc64x with external linking
can fail to link with relocation truncation errors if the elf
text section that is generated is larger than 2^26 bytes and that
section contains a call instruction (bl) which calls a function
beyond the limit addressable by the 24 bit field in the
instruction.

This solution consists of generating multiple text sections where
each is small enough to allow the GNU linker to resolve the calls
by generating long branch code where needed.  Other changes were added
to handle differences in processing when multiple text sections exist.

Some adjustments were required to the computation of a method's address
when using the method offset table when there are multiple text sections.

The number of possible section headers was increased to allow for up
to 128 text sections.  A test case was also added.

Fixes #15823.

Change-Id: If8117b0e0afb058cbc072258425a35aef2363c92
Reviewed-on: https://go-review.googlesource.com/27790
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

partially addresses #16360

Change-Id: I8274825b9ca6aec46294c8513b4795b0eb3062a2
Reviewed-on: https://go-review.googlesource.com/28992Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

The new Resolver type (a struct) has 9 Lookup methods, all taking a
context.Context.

There's now a new DefaultResolver global, like http's
DefaultTransport and DefaultClient.

net.Dialer now has an optional Resolver field to set the Resolver.

This also does finishes some resolver cleanup internally, deleting
lookupIPMerge and renaming lookupIPContext into Resolver.LookupIPAddr.

The Resolver currently doesn't let you tweak much, but it's a struct
specifically so we can add knobs in the future. Currently I just added
a bool to force the pure Go resolver. In the future we could let
people provide an interface to implement the methods, or add a Timeout
time.Duration, which would wrap all provided contexts in a
context.WithTimeout.

Fixes #16672

Change-Id: I7ba1f886704f06def7b6b5c4da9809db51bc1495
Reviewed-on: https://go-review.googlesource.com/29440
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

This change fills the ContentLength field in the http.Response returned by
ResponseRecorder.Result.

Fixes #16952.

Change-Id: I9c49b1bf83e3719b5275b03a43aff5033156637d
Reviewed-on: https://go-review.googlesource.com/28302Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Change-Id: I6ea8650927e7946c6fd4659f400fd91ddaae68af
Reviewed-on: https://go-review.googlesource.com/29510Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

These used to be necessary when racing with updates to the mark bit,
but since the mark bit is no longer in the bitmap and the checkmark is
only updated with the world stopped, we can now always use regular
writes to update the type information in the heap bitmap.

Somewhat surprisingly, this has basically no overall performance
effect beyond the usual noise, but it does clean up the code.

Change-Id: I3933d0b4c0bc1c9bcf6313613515c0b496212105
Reviewed-on: https://go-review.googlesource.com/29277
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

Change-Id: I808fab97076493a95b0b5eb0ad15645099f54aee
Reviewed-on: https://go-review.googlesource.com/29492Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

This change fixes the broken build caused by CL 23780.

Change-Id: I142cf8a1af033d036d57ac56e9e21ea925d922d4
Reviewed-on: https://go-review.googlesource.com/29490
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Change-Id: I6634f70d6bd1a4eced47eda69a2d9b207d222a1b
Reviewed-on: https://go-review.googlesource.com/29470
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

Change-Id: Idcb5a8d6676aa38b4ebd0975edd2068386f5ca83
Reviewed-on: https://go-review.googlesource.com/29449
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

It is possible (and common) for Windows systems to use a different codepage
for console applications from that used on normal windowed application
(called ANSI codepage); for instance, most of the western Europe uses
CP850 for console (for backward compatibility with MS-DOS), while
windowed applications use a different codepage depending on the country
(eg: CP1252 aka Latin-1). The usage being changed with this commit is
specifically related to decoding input coming from the console, so the
previous usage of the ANSI codepage was wrong.

Also fixes an issue that previous did convert bytes as NFD. Go is
designed to handle single Unicode code point. This fix change behaivor
to NFC.

Fixes #16857.

Change-Id: I4f41ae83ece47321b6e9a79a2087ecbb8ac066dd
Reviewed-on: https://go-review.googlesource.com/27575Reviewed-by: Hiroshi Ioka <hirochachacha@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

Updates #16360

Change-Id: I80b981aa291a8e16d2986d4a2dfd84d3819bf488
Reviewed-on: https://go-review.googlesource.com/29443
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

Fixes #17170

Change-Id: I939f087df133710495fdf6f09040051cb9b176d7
Reviewed-on: https://go-review.googlesource.com/29442Reviewed-by: Ian Lance Taylor <iant@golang.org>

Move a value to the block which is the lowest common ancestor in the
dominator tree of all of its uses.  Make sure not to move a value into a
loop.

Makes the tighten pass on average (across go1 benchmarks) 40% slower.
Still not a big contributor to overall compile time.

Binary size is just a tad smaller.

name                      old time/op    new time/op    delta
BinaryTree17-12              2.77s ± 9%     2.76s ± 9%     ~     (p=0.878 n=8+8)
Fannkuch11-12                2.75s ± 1%     2.74s ± 1%     ~     (p=0.232 n=8+7)
FmtFprintfEmpty-12          48.9ns ± 9%    47.7ns ± 0%     ~     (p=0.431 n=8+8)
FmtFprintfString-12          143ns ± 8%     142ns ± 1%     ~     (p=0.257 n=8+7)
FmtFprintfInt-12             123ns ± 1%     122ns ± 1%   -1.04%  (p=0.026 n=7+8)
FmtFprintfIntInt-12          195ns ± 7%     185ns ± 0%   -5.32%  (p=0.000 n=8+8)
FmtFprintfPrefixedInt-12     194ns ± 4%     195ns ± 0%   +0.81%  (p=0.015 n=7+7)
FmtFprintfFloat-12           267ns ± 0%     268ns ± 0%   +0.37%  (p=0.001 n=7+6)
FmtManyArgs-12               800ns ± 0%     762ns ± 1%   -4.78%  (p=0.000 n=8+8)
GobDecode-12                7.67ms ± 2%    7.60ms ± 2%     ~     (p=0.234 n=8+8)
GobEncode-12                6.55ms ± 0%    6.57ms ± 1%     ~     (p=0.336 n=7+8)
Gzip-12                      237ms ± 0%     238ms ± 0%   +0.40%  (p=0.017 n=7+7)
Gunzip-12                   40.8ms ± 0%    40.2ms ± 0%   -1.52%  (p=0.000 n=7+8)
HTTPClientServer-12          208µs ± 3%     209µs ± 3%     ~     (p=0.955 n=8+7)
JSONEncode-12               16.2ms ± 1%    17.2ms ±11%   +5.80%  (p=0.001 n=7+8)
JSONDecode-12               57.3ms ±12%    55.5ms ± 3%     ~     (p=0.867 n=8+7)
Mandelbrot200-12            4.68ms ± 6%    4.46ms ± 1%     ~     (p=0.442 n=8+8)
GoParse-12                  4.27ms ±44%    3.42ms ± 1%  -19.95%  (p=0.005 n=8+8)
RegexpMatchEasy0_32-12      75.1ns ± 0%    75.8ns ± 1%   +0.99%  (p=0.002 n=7+7)
RegexpMatchEasy0_1K-12       963ns ± 0%    1021ns ± 6%   +5.98%  (p=0.001 n=7+7)
RegexpMatchEasy1_32-12      72.4ns ±11%    70.8ns ± 1%     ~     (p=0.368 n=8+8)
RegexpMatchEasy1_1K-12       394ns ± 1%     399ns ± 0%   +1.23%  (p=0.000 n=8+7)
RegexpMatchMedium_32-12      114ns ± 0%     115ns ± 1%   +0.63%  (p=0.021 n=7+7)
RegexpMatchMedium_1K-12     35.9µs ± 0%    37.6µs ± 1%   +4.72%  (p=0.000 n=7+8)
RegexpMatchHard_32-12       1.93µs ± 2%    1.91µs ± 0%   -0.91%  (p=0.001 n=7+7)
RegexpMatchHard_1K-12       60.2µs ± 3%    61.2µs ±10%     ~     (p=0.442 n=8+8)
Revcomp-12                   404ms ± 1%     406ms ± 1%     ~     (p=0.054 n=8+7)
Template-12                 64.6ms ± 1%    63.5ms ± 1%   -1.66%  (p=0.000 n=8+8)
TimeParse-12                 347ns ± 8%     309ns ± 0%  -11.13%  (p=0.000 n=8+7)
TimeFormat-12                343ns ± 4%     331ns ± 0%   -3.34%  (p=0.000 n=8+7)

Change-Id: Id6da1239ddd4d0cb074ff29cffb06302d1c6d08f
Reviewed-on: https://go-review.googlesource.com/28712
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

Change-Id: If2cf3c5a29afc6cf74c3b08b9745e950231ead37
Reviewed-on: https://go-review.googlesource.com/29441Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Don't panic, crash, or return references to uninitialized memory when
ParseDirent is passed invalid input.

Move common dirent parsing to syscall.go with minimal platform-specific
functions in syscall_$GOOS.go.

Fixes #15653

Change-Id: I5602475e02321fe381064488401c14b33bec6886
Reviewed-on: https://go-review.googlesource.com/23780
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>