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<!--{

"Title": "Go 1.9 Release Notes",

"Path": "/doc/go1.9",

"Template": true

}-->

<!--

NOTE: In this document and others in this directory, the convention is to

set fixed-width phrases with non-fixed-width spaces, as in

<code>hello</code> <code>world</code>.

Do not send CLs removing the interior tags from such phrases.

-->

<style>

ul li { margin: 0.5em 0; }

</style>

<h2 id="introduction">Introduction to Go 1.9</h2>

<p>

The latest Go release, version 1.9, arrives six months

after <a href="go1.8">Go 1.8</a> and is the tenth release in

the <a href="https://golang.org/doc/devel/release.html">Go 1.x

series</a>.

There are two <a href="#language">changes to the language</a>:

adding support for type aliases and defining when implementations

may fuse floating point operations.

Most of the changes are in the implementation of the toolchain,

runtime, and libraries.

As always, the release maintains the Go 1

<a href="/doc/go1compat.html">promise of compatibility</a>.

We expect almost all Go programs to continue to compile and run as

before.

</p>

<p>

The release

adds <a href="#monotonic-time">transparent monotonic time support</a>,

<a href="#parallel-compile">parallelizes compilation of functions</a> within a package,

better supports <a href="#test-helper">test helper functions</a>,

includes a new <a href="#math-bits">bit manipulation package</a>,

and has a new <a href="#sync-map">concurrent map type</a>.

</p>

<h2 id="language">Changes to the language</h2>

<p>

There are two changes to the language.

</p>

<p>

Go now supports type aliases to support gradual code repair while

moving a type between packages.

The <a href="https://golang.org/design/18130-type-alias">type alias

design document</a>

and <a href="https://talks.golang.org/2016/refactor.article">an

article on refactoring</a> cover the problem in detail.

In short, a type alias declaration has the form:

</p>

<pre>

type T1 = T2

</pre>

<p>

This declaration introduces an alias name <code>T1</code>—an

alternate spelling—for the type denoted by <code>T2</code>; that is,

both <code>T1</code> and <code>T2</code> denote the same type.

</p>

A smaller language change is that the

<a href="/ref/spec#Floating_point_operators">language specification

now states</a> when implementations are allowed to fuse floating

point operations together, such as by using an architecture's "fused

multiply and add" (FMA) instruction to compute <code>x*y</code> <code>+</code> <code>z</code>

without rounding the intermediate result <code>x*y</code>.

To force the intermediate rounding, write <code>float64(x*y)</code> <code>+</code> <code>z</code>.

</p>

<h2 id="ports">Ports</h2>

<p>

There are no new supported operating systems or processor

architectures in this release.

</p>

<h3 id="power8">ppc64x requires POWER8</h3>

Both <code>GOARCH=ppc64</code> and <code>GOARCH=ppc64le</code> now

require at least POWER8 support. In previous releases,

only <code>GOARCH=ppc64le</code> required POWER8 and the big

endian <code>ppc64</code> architecture supported older

hardware.

<p>

<h3 id="freebsd">FreeBSD</h3>

<p>

Go 1.9 is the last release that will run on FreeBSD 9.3,

which is already

<a href="https://www.freebsd.org/security/unsupported.html">unsupported by FreeBSD</a>.

Go 1.10 will require FreeBSD 10.3+.

</p>

<h3 id="openbsd">OpenBSD 6.0</h3>

Go 1.9 now enables PT_TLS generation for cgo binaries and thus

requires OpenBSD 6.0 or newer. Go 1.9 no longer supports

OpenBSD 5.9.

<p>

<h3 id="known_issues">Known Issues</h3>

<p>

There are some instabilities on FreeBSD that are known but not understood.

These can lead to program crashes in rare cases.

See <a href="https://golang.org/issue/15658">issue 15658</a>.

Any help in solving this FreeBSD-specific issue would be appreciated.

</p>

<p>

Go stopped running NetBSD builders during the Go 1.9 development

cycle due to NetBSD kernel crashes, up to and including NetBSD 7.1.

As Go 1.9 is being released, NetBSD 7.1.1 is being released with a fix.

However, at this time we have no NetBSD builders passing our test suite.

Any help investigating the

<a href="https://github.com/golang/go/labels/OS-NetBSD">various NetBSD issues</a>

would be appreciated.

</p>

<h2 id="tools">Tools</h2>

<h3 id="parallel-compile">Parallel Compilation</h3>

<p>

The Go compiler now supports compiling a package's functions in parallel, taking

advantage of multiple cores. This is in addition to the <code>go</code> command's

existing support for parallel compilation of separate packages.

Parallel compilation is on by default, but it can be disabled by setting the

environment variable <code>GO19CONCURRENTCOMPILATION</code> to <code>0</code>.

</p>

<h3 id="vendor-dotdotdot">Vendor matching with ./...</h3>

By popular request, <code>./...</code> no longer matches packages

in <code>vendor</code> directories in tools accepting package names,

such as <code>go</code> <code>test</code>. To match vendor

directories, write <code>./vendor/...</code>.

</p>

<h3 id="goroot">Moved GOROOT</h3>

The <a href="/cmd/go/">go tool</a> will now use the path from which it

was invoked to attempt to locate the root of the Go install tree.

This means that if the entire Go installation is moved to a new

location, the go tool should continue to work as usual.

This may be overridden by setting <code>GOROOT</code> in the environment,

which should only be done in unusual circumstances.

Note that this does not affect the result of

the <a href="/pkg/runtime/#GOROOT">runtime.GOROOT</a> function, which

will continue to report the original installation location;

this may be fixed in later releases.

</p>

<h3 id="compiler">Compiler Toolchain</h3>

Complex division is now C99-compatible. This has always been the

case in gccgo and is now fixed in the gc toolchain.

</p>

The linker will now generate DWARF information for cgo executables on Windows.

</p>

The compiler now includes lexical scopes in the generated DWARF if the

<code>-N -l</code> flags are provided, allowing

debuggers to hide variables that are not in scope. The <code>.debug_info</code>

section is now DWARF version 4.

</p>

The values of <code>GOARM</code> and <code>GO386</code> now affect a

compiled package's build ID, as used by the <code>go</code> tool's

dependency caching.

</p>

<h3 id="asm">Assembler</h3>

The four-operand ARM <code>MULA</code> instruction is now assembled correctly,

with the addend register as the third argument and the result

In previous releases, the two meanings were reversed.

The three-operand form, in which the fourth argument is implicitly

the same as the third, is unaffected.

Code using four-operand <code>MULA</code> instructions

will need to be updated, but we believe this form is very rarely used.

<code>MULAWT</code> and <code>MULAWB</code> were already

using the correct order in all forms and are unchanged.

</p>

The assembler now supports <code>ADDSUBPS/PD</code>, completing the

two missing x86 SSE3 instructions.

</p>

Long lists of arguments are now truncated. This improves the readability

of <code>go</code> <code>doc</code> on some generated code.

</p>

Viewing documentation on struct fields is now supported.

For example, <code>go</code> <code>doc</code> <code>http.Client.Jar</code>.

</p>

The new <code>go</code> <code>env</code> <code>-json</code> flag

enables JSON output, instead of the default OS-specific output

format.

</p>

The <a href="/cmd/go/#hdr-Description_of_testing_flags"><code>go</code> <code>test</code></a>

command accepts a new <code>-list</code> flag, which takes a regular

expression as an argument and prints to stdout the name of any

tests, benchmarks, or examples that match it, without running them.

</p>

<h3 id="go-tool-pprof">Pprof</h3>

Profiles produced by the <code>runtime/pprof</code> package now

include symbol information, so they can be viewed

in <code>go</code> <code>tool</code> <code>pprof</code>

without the binary that produced the profile.

</p>

The <code>go</code> <code>tool</code> <code>pprof</code> command now

uses the HTTP proxy information defined in the environment, using

<a href="/pkg/net/http/#ProxyFromEnvironment"><code>http.ProxyFromEnvironment</code></a>.

</p>

<p>

The <a href="/cmd/vet/"><code>vet</code> command</a>

has been better integrated into the

<a href="/cmd/go/"><code>go</code> tool</a>,

so <code>go</code> <code>vet</code> now supports all standard build

flags while <code>vet</code>'s own flags are now available

from <code>go</code> <code>vet</code> as well as

from <code>go</code> <code>tool</code> <code>vet</code>.

</p>

<h3 id="gccgo">Gccgo</h3>

<p>

Due to the alignment of Go's semiannual release schedule with GCC's

annual release schedule,

GCC release 7 contains the Go 1.8.3 version of gccgo.

We expect that the next release, GCC 8, will contain the Go 1.10

version of gccgo.

</p>

<h2 id="runtime">Runtime</h2>

<h3 id="callersframes">Call stacks with inlined frames</h3>

<p>

Users of

<a href="/pkg/runtime#Callers"><code>runtime.Callers</code></a>

should avoid directly inspecting the resulting PC slice and instead use

<a href="/pkg/runtime#CallersFrames"><code>runtime.CallersFrames</code></a>

to get a complete view of the call stack, or

<a href="/pkg/runtime#Caller"><code>runtime.Caller</code></a>

to get information about a single caller.

This is because an individual element of the PC slice cannot account

for inlined frames or other nuances of the call stack.

</p>

<p>

Specifically, code that directly iterates over the PC slice and uses

functions such as

<a href="/pkg/runtime#FuncForPC"><code>runtime.FuncForPC</code></a>

to resolve each PC individually will miss inlined frames.

To get a complete view of the stack, such code should instead use

<code>CallersFrames</code>.

Likewise, code should not assume that the length returned by

<code>Callers</code> is any indication of the call depth.

It should instead count the number of frames returned by

<code>CallersFrames</code>.

</p>

<p>

Code that queries a single caller at a specific depth should use

<code>Caller</code> rather than passing a slice of length 1 to

<code>Callers</code>.

</p>

<p>

<a href="/pkg/runtime#CallersFrames"><code>runtime.CallersFrames</code></a>

has been available since Go 1.7, so code can be updated prior to

upgrading to Go 1.9.

</p>

<h2 id="performance">Performance</h2>

<p>

As always, the changes are so general and varied that precise

statements about performance are difficult to make. Most programs

should run a bit faster, due to speedups in the garbage collector,

better generated code, and optimizations in the core library.

</p>

<h3 id="gc">Garbage Collector</h3>

Library functions that used to trigger stop-the-world garbage

collection now trigger concurrent garbage collection.

Specifically, <a href="/pkg/runtime/#GC"><code>runtime.GC</code></a>,

<a href="/pkg/runtime/debug/#SetGCPercent"><code>debug.SetGCPercent</code></a>,

and

<a href="/pkg/runtime/debug/#FreeOSMemory"><code>debug.FreeOSMemory</code></a>,

now trigger concurrent garbage collection, blocking only the calling

goroutine until the garbage collection is done.

</p>

The

<a href="/pkg/runtime/debug/#SetGCPercent"><code>debug.SetGCPercent</code></a>

function only triggers a garbage collection if one is immediately

necessary because of the new GOGC value.

This makes it possible to adjust GOGC on-the-fly.

</p>

Large object allocation performance is significantly improved in

applications using large (>50GB) heaps containing many large

objects.

</p>

The <a href="/pkg/runtime/#ReadMemStats"><code>runtime.ReadMemStats</code></a>

function now takes less than 100µs even for very large heaps.

</p>

<h2 id="library">Core library</h2>

<h3 id="monotonic-time">Transparent Monotonic Time support</h3>

The <a href="/pkg/time/"><code>time</code></a> package now transparently

tracks monotonic time in each <a href="/pkg/time/#Time"><code>Time</code></a>

value, making computing durations between two <code>Time</code> values

a safe operation in the presence of wall clock adjustments.

See the <a href="/pkg/time/#hdr-Monotonic_Clocks">package docs</a> and

<a href="https://golang.org/design/12914-monotonic">design document</a>

for details.

</p>

<h3 id="math-bits">New bit manipulation package</h3>

Go 1.9 includes a new package,

<a href="/pkg/math/bits/"><code>math/bits</code></a>, with optimized

implementations for manipulating bits. On most architectures,

functions in this package are additionally recognized by the

compiler and treated as intrinsics for additional performance.

</p>

<h3 id="test-helper">Test Helper Functions</h3>

The

new <a href="/pkg/testing/#T.Helper"><code>(*T).Helper</code></a>

and <a href="/pkg/testing/#B.Helper"><code>(*B).Helper</code></a>

methods mark the calling function as a test helper function. When

printing file and line information, that function will be skipped.

This permits writing test helper functions while still having useful

line numbers for users.

</p>

<h3 id="sync-map">Concurrent Map</h3>

The new <a href="/pkg/sync/#Map"><code>Map</code></a> type

in the <a href="/pkg/sync/"><code>sync</code></a> package

is a concurrent map with amortized-constant-time loads, stores, and

deletes. It is safe for multiple goroutines to call a <code>Map</code>'s methods

concurrently.

</p>

<h3 id="pprof-labels">Profiler Labels</h3>

The <a href="/pkg/runtime/pprof">& 8000 lt;code>runtime/pprof</code> package</a>

now supports adding labels to <code>pprof</code> profiler records.

Labels form a key-value map that is used to distinguish calls of the

same function in different contexts when looking at profiles

with the <a href="/cmd/pprof/"><code>pprof</code> command</a>.

The <code>pprof</code> package's

new <a href="/pkg/runtime/pprof/#Do"><code>Do</code> function</a>

runs code associated with some provided labels. Other new functions

in the package help work with labels.

</p>

</dl>

<h3 id="minor_library_changes">Minor changes to the library</h3>

<p>

As always, there are various minor changes and updates to the library,

made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>

in mind.

</p>

<dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>

<dd>

The

ZIP <a href="/pkg/archive/zip/#Writer"><code>Writer</code></a>

now sets the UTF-8 bit in

the <a href="/pkg/archive/zip/#FileHeader.Flags"><code>FileHeader.Flags</code></a>

when appropriate.

</p>

</dl>

<dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>

<dd>

On Linux, Go now calls the <code>getrandom</code> system call

without the <code>GRND_NONBLOCK</code> flag; it will now block

until the kernel has sufficient randomness. On kernels predating

the <code>getrandom</code> system call, Go continues to read

from <code>/dev/urandom</code>.

</p>

</dl>

<dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>

<dd>

On Unix systems the environment

variables <code>SSL_CERT_FILE</code>

and <code>SSL_CERT_DIR</code> can now be used to override the

system default locations for the SSL certificate file and SSL

certificate files directory, respectively.

</p>

<p>The FreeBSD file <code>/usr/local/etc/ssl/cert.pem</code> is

now included in the certificate search path.

</p>

The package now supports excluded domains in name constraints.

In addition to enforcing such constraints,

<a href="/pkg/crypto/x509/#CreateCertificate"><code>CreateCertificate</code></a>

will create certificates with excluded name constraints

if the provided template certificate has the new

field

<a href="/pkg/crypto/x509/#Certificate.ExcludedDNSDomains"><code>ExcludedDNSDomains</code></a>

populated.

</p>

If any SAN extension, including with no DNS names, is present

in the certificate, then the Common Name from

<a href="/pkg/crypto/x509/#Certificate.Subject"><code>Subject</code></a> is ignored.

In previous releases, the code tested only whether DNS-name SANs were

present in a certificate.

</p>

</dl>

<dl id="database/sql"><dt><a href="/pkg/database/sql/">database/sql</a></dt>

<dd>

The package will now use a cached <a href="/pkg/database/sql/#Stmt"><code>Stmt</code></a> if

available in <a href="/pkg/database/sql/#Tx.Stmt"><code>Tx.Stmt</code></a>.

This prevents statements from being re-prepared each time

<a href="/pkg/database/sql/#Tx.Stmt"><code>Tx.Stmt</code></a> is called.

</p>

The package now allows drivers to implement their own argument checkers by implementing

<a href="/pkg/database/sql/driver/#NamedValueChecker"><code>driver.NamedValueChecker</code></a>.

This also allows drivers to support <code>OUTPUT</code> and <code>INOUT</code> parameter types.

<a href="/pkg/database/sql/#Out"><code>Out</code></a> should be used to return output parameters

when supported by the driver.

</p>

<a href="/pkg/database/sql/#Rows.Scan"><code>Rows.Scan</code></a> can now scan user-defined string types.

Previously the package supported scanning into numeric types like <code>type</code> <code>Int</code> <code>int64</code>. It now also supports

scanning into string types like <code>type</code> <code>String</code> <code>string</code>.

</p>

The new <a href="/pkg/database/sql/#DB.Conn"><code>DB.Conn</code></a> method returns the new

<a href="/pkg/database/sql/#Conn"><code>Conn</code></a> type representing an

exclusive connection to the database from the connection pool. All queries run on

a <a href="/pkg/database/sql/#Conn"><code>Conn</code></a> will use the same underlying

connection until <a href="/pkg/database/sql/#Conn.Close"><code>Conn.Close</code></a> is called

to return the connection to the connection pool.

</p>

</dl>

<dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1/">encoding/asn1</a></dt>

<dd>

The new

<a href="/pkg/encoding/asn1/#NullBytes"><code>NullBytes</code></a>

and

<a href="/pkg/encoding/asn1/#NullRawValue"><code>NullRawValue</code></a>

represent the ASN.1 NULL type.

</p>

</dl>

<dl id="encoding/base32"><dt><a href="/pkg/encoding/base32/">encoding/base32</a></dt>

<dd>

The new <a href="/pkg/encoding/base32/#Encoding.WithPadding">Encoding.WithPadding</a>

method adds support for custom padding characters and disabling padding.

</p>

</dl>

<dl id="encoding/csv"><dt><a href="/pkg/encoding/csv/">encoding/csv</a></dt>

<dd>

The new field

<a href="/pkg/encoding/csv/#Reader.ReuseRecord"><code>Reader.ReuseRecord</code></a>

controls whether calls to

may return a slice sharing the backing array of the previous

call's returned slice for improved performance.

</p>

</dl>

<dd>

The sharp flag ('<code>#</code>') is now supported when printing

floating point and complex numbers. It will always print a

decimal point

for <code>%e</code>, <code>%E</code>, <code>%f</code>, <code>%F</code>, <code>%g</code>

and <code>%G</code>; it will not remove trailing zeros

for <code>%g</code> and <code>%G</code>.

</p>

</dl>

<dd>

The package now includes 128-bit FNV-1 and FNV-1a hash support with

<a href="/pkg/hash/fnv/#New128"><code>New128</code></a> and

<a href="/pkg/hash/fnv/#New128a"><code>New128a</code></a>, respectively.

</p>

</dl>

<dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>

<dd>

The package now reports an error if a predefined escaper (one of

"html", "urlquery" and "js") is found in a pipeline and does not match

what the auto-escaper would have decided on its own.

This avoids certain security or correctness issues.

Now use of one of these escapers is always either a no-op or an error.

(The no-op case eases migration from <a href="/pkg/text/template/">text/template</a>.)

</p>

</dl>

<dl id="image"><dt><a href="/pkg/image/">image</a></dt>

<dd>

The <a href="/pkg/image/#Rectangle.Intersect"><code>Rectangle.Intersect</code></a>

method now returns a zero <code>Rectangle</code> when called on

adjacent but non-overlapping rectangles, as documented. In

earlier releases it would incorrectly return an empty but

non-zero <code>Rectangle</code>.

</p>

</dl>

<dl id="image/color"><dt><a href="/pkg/image/color/">image/color</a></dt>

<dd>

The YCbCr to RGBA conversion formula has been tweaked to ensure

that rounding adjustments span the complete [0, 0xffff] RGBA

range.

</p>

</dl>

<dl id="image/png"><dt><a href="/pkg/image/png/">image/png</a></dt>

<dd>

The new <a href="/pkg/image/png/#Encoder.BufferPool"><code>Encoder.BufferPool</code></a>

field allows specifying an <a href="/pkg/image/png/#EncoderBufferPool"><code>EncoderBufferPool</code></a>,

that will be used by the encoder to get temporary <code>EncoderBuffer</code>

buffers when encoding a PNG image.

The use of a <code>BufferPool</code> reduces the number of

memory allocations performed while encoding multiple images.

</p>

The package now supports the decoding of transparent 8-bit

grayscale ("Gray8") images.

</p>

</dl>

<dd>

The new

<a href="/pkg/math/big/#Int.IsInt64"><code>IsInt64</code></a>

and

<a href="/pkg/math/big/#Int.IsUint64"><code>IsUint64</code></a>

methods report whether an <code>Int</code>

may be represented as an <code>int64</code> or <code>uint64</code>

value.

</p>

</dl>

<dl id="mime/multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>

<dd>

The new

<a href="/pkg/mime/multipart/#FileHeader.Size"><code>FileHeader.Size</code></a>

field describes the size of a file in a multipart message.

</p>

</dl>

<dd>

The new

<a href="/pkg/net/#Resolver.StrictErrors"><code>Resolver.StrictErrors</code></a>

provides control over how Go's built-in DNS resolver handles

temporary errors during queries composed of multiple sub-queries,

such as an A+AAAA address lookup.

</p>

The new

<a href="/pkg/net/#Resolver.Dial"><code>Resolver.Dial</code></a>

allows a <code>Resolver</code> to use a custom dial function.

</p>

<a href="/pkg/net/#JoinHostPort"><code>JoinHostPort</code></a> now only places an address in square brackets if the host contains a colon.

In previous releases it would also wrap addresses in square brackets if they contained a percent ('<code>%</code>') sign.

</p>

The new methods

<a href="/pkg/net/#TCPConn.SyscallConn"><code>TCPConn.SyscallConn</code></a>,

<a href="/pkg/net/#IPConn.SyscallConn"><code>IPConn.SyscallConn</code></a>,

<a href="/pkg/net/#UDPConn.SyscallConn"><code>UDPConn.SyscallConn</code></a>,

and

<a href="/pkg/net/#UnixConn.SyscallConn"><code>UnixConn.SyscallConn</code></a>

provide access to the connections' underlying file descriptors.

</p>

It is now safe to call <a href="/pkg/net/#Dial"><code>Dial</code></a> with the address obtained from

<code>(*TCPListener).String()</code> after creating the listener with

<code><a href="/pkg/net/#Listen">Listen</a>("tcp", ":0")</code>.

Previously it failed on some machines with half-configured IPv6 stacks.

</p>

</dl>

<dd>

The <a href="/pkg/net/http/#Cookie.String"><code>Cookie.String</code></a> method, used for

<code>Cookie</code> and <code>Set-Cookie</code> headers, now encloses values in double quotes

if the value contains either a space or a comma.

</p>

<p>Server changes:</p>

<ul>

<a href="/pkg/net/http/#ServeMux"><code>ServeMux</code></a> now ignores ports in the host

header when matching handlers. The host is matched unmodified for <code>CONNECT</code> requests.

</li>

The new <a href="/pkg/net/http/#Server.ServeTLS"><code>Server.ServeTLS</code></a> method wraps

<a href="/pkg/net/http/#Server.Serve"><code>Server.Serve</code></a> with added TLS support.

</li>

<a href="/pkg/net/http/#Server.WriteTimeout"><code>Server.WriteTimeout</code></a>

now applies to HTTP/2 connections and is enforced per-stream.

</li>

HTTP/2 now uses the priority write scheduler by default.

Frames are scheduled by following HTTP/2 priorities as described in

<a href="https://tools.ietf.org/html/rfc7540#section-5.3">RFC 7540 Section 5.3</a>.

</li>

The HTTP handler returned by <a href="/pkg/net/http/#StripPrefix"><code>StripPrefix</code></a>

now calls its provided handler with a modified clone of the original <code>*http.Request</code>.

Any code storing per-request state in maps keyed by <code>*http.Request</code> should

use

<a href="/pkg/net/http/#Request.Context"><code>Request.Context</code></a>,

<a href="/pkg/net/http/#Request.WithContext"><code>Request.WithContext</code></a>,

and

<a href="/pkg/context/#WithValue"><code>context.WithValue</code></a> instead.

</li>

<a href="/pkg/net/http/#LocalAddrContextKey"><code>LocalAddrContextKey</code></a> now contains

the connection's actual network address instead of the interface address used by the listener.

</li>

</ul>

<p>Client & Transport changes:</p>

<ul>

The <a href="/pkg/net/http/#Transport"><code>Transport</code></a>

now supports making requests via SOCKS5 proxy when the URL returned by

<a href="/pkg/net/http/#Transport.Proxy"><code>Transport.Proxy</code></a>

has the scheme <code>socks5</code>.

</li>

</ul>

</dl>

<dd>

The new

<a href="/pkg/net/http/fcgi/#ProcessEnv"><code>ProcessEnv</code></a>

function returns FastCGI environment variables associated with an HTTP request

for which there are no appropriate

<a href="/pkg/net/http/#Request"><code>http.Request</code></a>

fields, such as <code>REMOTE_USER</code>.

</p>

</dl>

<dl id="net/http/httptest"><dt><a href="/pkg/net/http/httptest/">net/http/httptest</a></dt>

<dd>

T 10BC8 he new

<a href="/pkg/net/http/httptest/#Server.Client"><code>Server.Client</code></a>

method returns an HTTP client configured for making requests to the test server.

</p>

<p>

The new

<a href="/pkg/net/http/httptest/#Server.Certificate"><code>Server.Certificate</code></a>

method returns the test server's TLS certificate, if any.

</p>

</dl>

<dl id="net/http/httputil"><dt><a href="/pkg/net/http/httputil/">net/http/httputil</a></dt>

<dd>

The <a href="/pkg/net/http/httputil/#ReverseProxy"><code>ReverseProxy</code></a>

now proxies all HTTP/2 response trailers, even those not declared in the initial response

header. Such undeclared trailers are used by the gRPC protocol.

</p>

</dl>

<dd>

The <code>os</code> package now uses the internal runtime poller

for file I/O.

This reduces the number of threads required for read/write

operations on pipes, and it eliminates races when one goroutine

closes a file while another is using the file for I/O.

</p>

<dd>

On Windows,

is now populated without <code>shell32.dll</code>, improving process start-up time by 1-7 ms.

</p>

</dl>

<dd>

The <code>os/exec</code> package now prevents child processes from being created with

any duplicate environment variables.

If <a href="/pkg/os/exec/#Cmd.Env"><code>Cmd.Env</code></a>

contains duplicate environment keys, only the last

value in the slice for each duplicate key is used.

</p>

</dl>

<dd>

<a href="/pkg/os/user/#Lookup"><code>Lookup</code></a> and

<a href="/pkg/os/user/#LookupId"><code>LookupId</code></a> now

work on Unix systems when <code>CGO_ENABLED=0</code> by reading

the <code>/etc/passwd</code> file.

</p>

<a href="/pkg/os/user/#LookupGroup"><code>LookupGroup</code></a> and

<a href="/pkg/os/user/#LookupGroupId"><code>LookupGroupId</code></a> now

work on Unix systems when <code>CGO_ENABLED=0</code> by reading

the <code>/etc/group</code> file.

</p>

</dl>

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>

<dd>

The new

<a href="/pkg/reflect/#MakeMapWithSize"><code>MakeMapWithSize</code></a>

function creates a map with a capacity hint.

</p>

</dl>

<dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>

<dd>

Tracebacks generated by the runtime and recorded in profiles are

now accurate in the presence of inlining.

To retrieve tracebacks programmatically, applications should use

<a href="/pkg/runtime/#CallersFrames"><code>runtime.CallersFrames</code></a>

rather than directly iterating over the results of

<a href="/pkg/runtime/#Callers"><code>runtime.Callers</code></a>.

</p>

On Windows, Go no longer forces the system timer to run at high

resolution when the program is idle.

This should reduce the impact of Go programs on battery life.

</p>

On FreeBSD, <code>GOMAXPROCS</code> and

<a href="/pkg/runtime/#NumCPU"><code>runtime.NumCPU</code></a>

are now based on the process' CPU mask, rather than the total

number of CPUs.

</p>

The runtime has preliminary support for Android O.

</p>

</dl>

<dl id="runtime/debug"><dt><a href="/pkg/runtime/debug/">runtime/debug</a></dt>

<dd>

Calling

<a href="/pkg/runtime/debug/#SetGCPercent"><code>SetGCPercent</code></a>

with a negative value no longer runs an immediate garbage collection.

</p>

</dl>

<dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>

<dd>

The execution trace now displays mark assist events, which

indicate when an application goroutine is forced to assist

garbage collection because it is allocating too quickly.

</p>

"Sweep" events now encompass the entire process of finding free

space for an allocation, rather than recording each individual

span that is swept.

This reduces allocation latency when tracing allocation-heavy

programs.

The sweep event shows how many bytes were swept and how many

were reclaimed.

</p>

</dl>

<dd>

<a href="/pkg/sync/#Mutex"><code>Mutex</code></a> is now more fair.

</p>

</dl>

<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>

<dd>

The new field

<a href="/pkg/syscall/#Credential.NoSetGroups"><code>Credential.NoSetGroups</code></a>

controls whether Unix systems make a <code>setgroups</code> system call

to set supplementary groups when starting a new process.

</p>

The new field

<a href="/pkg/syscall/#SysProcAttr.AmbientCaps"><code>SysProcAttr.AmbientCaps</code></a>

allows setting ambient capabilities on Linux 4.3+ when creating

a new process.

</p>

On 64-bit x86 Linux, process creation latency has been optimized with

use of <code>CLONE_VFORK</code> and <code>CLONE_VM</code>.

</p>

The new

interface describes some types in the

package that can provide access to their underlying file descriptor

using the new

<a href="/pkg/syscall/#RawConn"><code>RawConn</code></a>

interface.

</p>

</dl>

<dl id="testing/quick"><dt><a href="/pkg/testing/quick/">testing/quick</a></dt>

<dd>

The package now chooses values in the full range when

generating <code>int64</code> and <code>uint64</code> random

numbers; in earlier releases generated values were always

limited to the [-2<sup>62</sup>, 2<sup>62</sup>) range.

</p>

<p>

In previous releases, using a nil

<a href="/pkg/testing/quick/#Config.Rand"><code>Config.Rand</code></a>

value caused a fixed deterministic random number generator to be used.

It now uses a random number generator seeded with the current time.

For the old behavior, set <code>Config.Rand</code> to <code>rand.New(rand.NewSource(0))</code>.

</p>

</dl>

<dl id="text/template"><dt><a href="/pkg/text/template/">text/template</a></dt>

<dd>

The handling of empty blocks, which was broken by a Go 1.8

change that made the result dependent on the order of templates,

has been fixed, restoring the old Go 1.7 behavior.

</p>

</dl>

<dd>

The new methods

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