go/doc/go1.8.html

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<h2 id="introduction">DRAFT RELEASE NOTES - Introduction to Go 1.8</h2>

<p><strong>
Go 1.8 is not yet released. These are work-in-progress
release notes. Go 1.8 is expected to be released in February 2017.
</strong></p>

<p>
The latest Go release, version 1.8, arrives six months after <a href="go1.7">Go 1.7</a>.
Most of its changes are in the implementation of the toolchain, runtime, and libraries.
There is one minor change to the language specification.
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 <a href="#ports">adds support for 32-bit MIPS</a>,
<a href="#compiler">updates the compiler back end</a> to generate more efficient code,
<a href="#gc">reduces GC pauses</a> by eliminating stop-the-world stack rescanning,
<a href="#h2push">adds HTTP/2 Push support</a>,
<a href="#httpshutdown">adds HTTP graceful shutdown</a>,
and <a href="#sort">simplifies sorting slices</a>.
</p>

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

<p>
  When explicitly converting structs, tags are now ignored for structural type identity.
  See <a href="/ref/spec#Conversions">language specification</a> for details.
</p>

<p> <!-- CL 17711 -->
  The language specification now only requires that implementations
  support up to 16-bit exponents in floating-point constants.  This does not affect
  either the “<a href="/cmd/compile/"><code>gc</code></a>” or
  <code>gccgo</code> compilers, both of
  which still support 32-bit exponents.
</p>

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

<p>
Go now supports 32-bit MIPS on Linux for both big-endian
(<code>linux/mips</code>) and little-endian machines
(<code>linux/mipsle</code>).
</p>

<p>
Go now requires DragonFly BSD 4.4.4 or later. <!-- CL 29491 -->
</p>

<p>
The Plan 9 port's networking support is now much more complete
and matches the behavior of Unix and Windows with respect to deadlines
and cancelation.
</p>

<p>
  Go 1.8 now only supports OS X 10.8 or later. This is likely the last
  Go release to continue supporting 10.8. Compiling Go or running
  binaries on older OS X versions is untested.
</p>


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

<p>
There are some instabilities on FreeBSD and NetBSD 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>,
<a href="https://golang.org/issue/16396">issue 16396</a>, and
<a href="https://golang.org/issue/16511">issue 16511</a>.
Any help in solving these issues would be appreciated.
</p>

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

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

<p>
For 64-bit x86 systems, the following instructions have been added:
<code>VBROADCASTSD</code>,
<code>BROADCASTSS</code>,
<code>MOVDDUP</code>,
<code>MOVSHDUP</code>,
<code>MOVSLDUP</code>,
<code>VMOVDDUP</code>,
<code>VMOVSHDUP</code>,
and <code>VMOVSLDUP</code>.</p>

<p>
For 64-bit PPC systems, the common vector scalar instructions have been
added:
<code>LXS</code>,
<code>LXSDX</code>,
<code>LXSI</code>,
<code>LXSIWAX</code>,
<code>LXSIWZX</code>,
<code>LXV</code>,
<code>LXVD2X</code>,
<code>LXVDSX</code>,
<code>LXVW4X</code>,
<code>MFVSR</code>,
<code>MFVSRD</code>,
<code>MFVSRWZ</code>,
<code>MTVSR</code>,
<code>MTVSRD</code>,
<code>MTVSRWA</code>,
<code>MTVSRWZ</code>,
<code>STXS</code>,
<code>STXSDX</code>,
<code>STXSI</code>,
<code>STXSIWX</code>,
<code>STXV</code>,
<code>STXVD2X</code>,
<code>STXVW4X</code>,
<code>XSCV</code>,
<code>XSCVDPSP</code>,
<code>XSCVDPSPN</code>,
<code>XSCVDPSXDS</code>,
<code>XSCVDPSXWS</code>,
<code>XSCVDPUXDS</code>,
<code>XSCVDPUXWS</code>,
<code>XSCVSPDP</code>,
<code>XSCVSPDPN</code>,
<code>XSCVSXDDP</code>,
<code>XSCVSXDSP</code>,
<code>XSCVUXDDP</code>,
<code>XSCVUXDSP</code>,
<code>XSCVX</code>,
<code>XSCVXP</code>,
<code>XVCV</code>,
<code>XVCVDPSP</code>,
<code>XVCVDPSXDS</code>,
<code>XVCVDPSXWS</code>,
<code>XVCVDPUXDS</code>,
<code>XVCVDPUXWS</code>,
<code>XVCVSPDP</code>,
<code>XVCVSPSXDS</code>,
<code>XVCVSPSXWS</code>,
<code>XVCVSPUXDS</code>,
<code>XVCVSPUXWS</code>,
<code>XVCVSXDDP</code>,
<code>XVCVSXDSP</code>,
<code>XVCVSXWDP</code>,
<code>XVCVSXWSP</code>,
<code>XVCVUXDDP</code>,
<code>XVCVUXDSP</code>,
<code>XVCVUXWDP</code>,
<code>XVCVUXWSP</code>,
<code>XVCVX</code>,
<code>XVCVXP</code>,
<code>XXLAND</code>,
<code>XXLANDC</code>,
<code>XXLANDQ</code>,
<code>XXLEQV</code>,
<code>XXLNAND</code>,
<code>XXLNOR</code>,
<code>XXLOR</code>,
<code>XXLORC</code>,
<code>XXLORQ</code>,
<code>XXLXOR</code>,
<code>XXMRG</code>,
<code>XXMRGHW</code>,
<code>XXMRGLW</code>,
<code>XXPERM</code>,
<code>XXPERMDI</code>,
<code>XXSEL</code>,
<code>XXSI</code>,
<code>XXSLDWI</code>,
<code>XXSPLT</code>, and 
<code>XXSPLTW</code>.
</p>

<h3 id="tool_yacc">Yacc</h3>

<p> <!-- CL 27324, CL 27325 -->
The <code>yacc</code> tool (previously available via
“<code>go</code> <code>tool</code> <code>yacc</code>”)
has been removed. As of Go 1.7 it was no longer used by the Go compiler.
It has moved to the “tools” repository and is and is available via
<code>go</code> <code>get</code> <code><a href="https://godoc.org/golang.org/x/tools/cmd/goyacc">golang.org/x/tools/cmd/goyacc</a></code>.
</p>

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

<p>
Go 1.7 introduced a new compiler back end for 64-bit x86 systems.
In Go 1.8, that back end has been developed further and is now used for
all architectures.
</p>

<p>
The new back end, based on
<a href="https://en.wikipedia.org/wiki/Static_single_assignment_form">SSA</a>,
generates more compact, more efficient code
and provides a better platform for optimizations
such as bounds check elimination.
The new back end reduces the CPU time required by
<a href="https://golang.org/test/bench/go1/">our benchmark programs</a> by 20-30%
on 32-bit ARM systems. For 64-bit x86 systems, which already used the SSA backend in
Go 1.7, the gains are a more modest 0-10%. Other architectures will likely
see improvements closer to the 32-bit ARM numbers.
</p>

<p>
  The temporary <code>-ssa=0</code> compiler flag introduced in Go 1.7
  to disable the new backend has been removed in Go 1.8.
</p>

<p>
  In addition to enabling the new compiler back end for all systems,
  Go 1.8 also introduces a new compiler front end. The new compiler
  front end should not be noticeable to users is the foundation for
  future performance work.
</p>

<p>
  The compiler and linker have been optimized and run faster in this
  release than in Go 1.7, although they are still slower than we would
  like and will continue to be optimized in future releases.
  Compared to the previous release, Go 1.8 is
  <a href="https://dave.cheney.net/2016/11/19/go-1-8-toolchain-improvements">about 15% faster</a>.
</p>

<h3 id="cmd_cgo">Cgo</h3>

<p>TODO</p>

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

<p>TODO</p>

<h3 id="cmd_go">Go command</h3>

<p>
The <a href="/cmd/go/"><code>go</code></a> command's basic operation
is unchanged, but there are a number of changes worth noting.
</p>

<p>
  A new
  “<a href="/cmd/go/#hdr-Print_information_for_bug_reports"><code>go</code>
   <code>bug</code></a>” command helps users file bug reports.
</p>

<h3 id="cmd_doc">Go doc</h3>

<p> <!-- CL 25419 -->
  The
  “<a href="/cmd/go/#hdr-Show_documentation_for_package_or_symbol"><code>go</code>
   <code>doc</code></a>” command
  now groups constants and variables with their type,
  following the behavior of
  <a href="/cmd/godoc/"><code>godoc</code></a>.
</p>

<p> <!-- CL 25420 -->
  In order to improve the readability of the <code>doc</code>'s
  output, each summary of the first-level items is guaranteed to
  occupy a single line.
</p>

<p> <!-- CL 31852 -->
  Documentation for interface methods is now only shown when requested
  explicitly.
</p>

<h3 id="plugin">Plugins</h3>

<p>
  Go now supports a “<code>plugin</code>” build mode for generating
  plugins written in Go, and a
  new <a href="/pkg/plugin/"><code>plugin</code></a> package for
  loading such plugins at run time. Plugin support is only currently
  available on Linux and macOS.
</p>

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

<h3 id="liveness">Argument Liveness</h3>

<p>
  <!-- Issue 15843 --> The garbage collector no longer considers
  arguments live throughout the entirety of a function. For more
  information, and for how to force a variable to remain live, see
  the <a href="/pkg/runtime/#KeepAlive"><code>runtime.KeepAlive</code></a>
  function added in Go 1.7.
</p>

<p>
  <i>Updating:</i> Users of finalizers should see the example
  in the <a href="/pkg/runtime/#KeepAlive"><code>KeepAlive</code> documentation</a>
  to see where a call to <code>KeepAlive</code> might be needed.
  </p>

<h3 id="memstats">MemStats Documentation</h3>

<p> <!-- CL 28972 -->
  The runtime's <a href="/pkg/runtime/#MemStats"><code>MemStats</code></a>
  type has been more thoroughly documented.
</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 and
optimizations in the standard library.
</p>

<p>
There have been significant optimizations bringing more than 10% improvements
to implementations in the
TODO TODO:
<a href="/pkg/foo/"><code>foo</code></a>,
<a href="/pkg/bar/"><code>bar</code></a>,
and
<a href="/pkg/quux/"><code>quux</code></a>
packages.
</p>

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

<p>
  Garbage collection pauses should be significantly shorter than they
  were in Go 1.7, often as low as 10 microseconds and usually under 100
  microseconds.
  See the
  <a href="https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md"
     >document on eliminating stop-the-world stack re-scanning</a>
  for details.  More work remains for Go 1.9.
</p>

<h3 id="defer">Defer</h3>

<!-- CL 29656, CL 29656 -->
<p>
  The overhead of <a href="/ref/spec/#Defer_statements">deferred
  function calls</a> has been reduced by half.
</p>

<h3 id="cgoperf">Cgo</h3>

<p>The overhead of calls from Go into C has been reduced by about half.</p>

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

<h3 id="examples">Examples</h3>

<p>
Examples have been added to the documentation across many packages.
</p>

<h3 id="sortslice">Sort</h3>

<p>
The <a href="/pkg/sort/">sort</a> package
now includes a convenience function
<a href="/pkg/sort/#Slice"><code>Slice</code></a> to sort a
slice given a <em>less</em> function.

In many cases this means that writing a new sorter type is not
necessary.
</p>

<p>
Also new are
<a href="/pkg/sort/#SliceStable"><code>SliceStable</code></a> and
<a href="/pkg/sort/#SliceIsSorted"><code>SliceIsSorted</code></a>.
</p>

<h3 id="h2push">HTTP/2 Push</h3>

<p>
The <a href="/pkg/net/http/">net/http</a> package now includes a
mechanism to
send HTTP/2 server pushes from a
<a href="/pkg/net/http/#Handler"><code>Handler</code></a>.
Similar to the existing <code>Flusher</code> and <code>Hijacker</code>
interfaces, an HTTP/2
<a href="/pkg/net/http/#ResponseWriter"><code>ResponseWriter</code></a>
now implements the new
<a href="/pkg/net/http/#Pusher"><code>Pusher</code></a> interface.
</p>

<h3 id="http_shutdown">HTTP Server Graceful Shutdown</h3>

<p> <!-- CL 32329 -->
  The HTTP Server now has support for graceful shutdown using the new
  <a href="/pkg/net/http/#Server.Shutdown"><code>Server.Shutdown</code></a>
  method. The related and more abrupt
  <a href="/pkg/net/http/#Server.Close"><code>Server.Close</code></a>
  is also new.
</p>

<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_tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
  <dd>

    <p> <!-- CL 28471, CL 31440, CL 31441, CL 31444, CL 28418, CL 31439 -->
      The tar implementation corrects many bugs in corner cases of the file format.
      The <a href="/pkg/archive/tar/#Reader"><code>Reader</code></a>
      is now able to process tar files in the PAX format with entries larger than 8GB.
      The <a href="/pkg/archive/tar/#Writer"><code>Writer</code></a>
      no longer produces invalid tar files in some situations involving long pathnames.
    </p>

  </dd>
</dl>

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

    <p> <!-- CL 18274 -->
      The zip <code>Reader</code> now supports modification times in
      the NTFS, UNIX, and Extended Time Stamp metadata fields.
      <!-- CL 30811 -->
      When writing zip files, the Extended Time Stamp field is written
      if TODO: validate implementation.
    </p>

  </dd>
</dl>

<dl id="compress_flate"><dt><a href="/pkg/compress/flate/">compress/flate</a></dt>
  <dd>

    <p> <!-- CL 31640, CL 31174, CL 32149 -->
      There have been some minor fixes to the encoder to improve the
      compression ratio in certain situations. As a result, the exact
      encoded output of <code>DEFLATE</code> may be different from Go 1.7. Since
      DEFLATE is the underlying compression of gzip, png, zlib, and zip,
      those formats may have changed outputs.
    </p>

    <p>
      The encoder, when operating in
      <a href="/pkg/compress/flate/#NoCompression"><code>NoCompression</code></a>
      mode, now produces a consistent output that is not dependent on
      the size of the slices passed to the
      <a href="/pkg/compress/flate/#Writer.Write"><code>Write</code></a>
      method.
    </p>

    <p> <!-- CL 28216 -->
      The decoder, upon encountering an error, now returns any
      buffered data it had uncompressed along with the error.
    </p>

  </dd>
</dl>


<dl id="compress_gzip"><dt><a href="/pkg/compress/gzip/">compress/gzip</a></dt>
  <dd>

    <p>
      The <a href="/pkg/compress/gzip/#Writer"><code>Writer</code></a>
      now encodes a zero <code>MTIME</code> field when
      the <a href="/pkg/compress/gzip/#Header"><code>Header.ModTime</code></a>
      field is the zero value.

      In previous releases of Go, the <code>Writer</code> would encode
      a nonsensical value.

      Similarly,
      the <a href="/pkg/compress/gzip/#Reader"><code>Reader</code></a>
      now updates the <code>Header.ModTime</code> field only if the
      encoded <code>MTIME</code> field is non-zero.
    </p>

  </dd>
</dl>

<dl id="context"><dt><a href="/pkg/context/">context</a></dt>
  <dd>
    <p> <!-- CL 30370 -->
      The <a href="/pkg/context#DeadlineExceeded"><code>DeadlineExceeded</code></a>
      error now implements
      <a href="/pkg/net/#Error"><code>net.Error</code></a>
      and reports true for both the <code>Timeout</code> and
      <code>Temporary</code> methods.
    </p>
  </dd>
</dl>

<dl id="database_sql"><dt><a href="/pkg/database/sql/">database/sql</a></dt>
  <dd>
    <p>
	  The package now supports <code>context.Context</code>. There are new methods
	  ending in <code>Context</code> such as
	  <a href="/pkg/database/sql/#DB.QueryContext"><code>DB.QueryContext</code></a> and
	  <a href="/pkg/database/sql/#DB.PrepareContext"><code>DB.PrepareContext</code></a>
	  that support <code>Context</code>. By using the new <code>Context</code> methods it ensures
	  connections are closed and returned to the connection pool when the
	  request is done. It also enables canceling in-progress queries
	  should the driver support it. Finally, using the <code>Context</code>
	  methods allows the database pool to cancel waiting for the next
	  available connection.
    </p>
    <p>
      The <a href="/pkg/database/sql#IsolationLevel"><code>IsolationLevel</code></a>
	  can now be set when starting a transaction by setting the isolation level
	  on the <code>Context</code> then passing that <code>Context</code> to
	  <a href="/pkg/database/sql#DB.BeginContext"><code>DB.BeginContext</code></a>.
	  An error will be returned if an isolation level is selected that the driver
	  does not support. A read-only attribute may also be set on the transaction
	  with <a href="/pkg/database/sql/#ReadOnlyContext"><code>ReadOnlyContext</code></a>
	</p>
	<p>
      Queries now expose the SQL column type information for drivers that support it.
	  Rows can return <a href="/pkg/database/sql#Rows.ColumnTypes"><code>ColumnTypes</code></a>
	  which can include SQL type information, column type lengths, and the Go type.
    </p>
	<p>
      Multiple result sets are now supported on Rows. After
	  <a href="/pkg/database/sql/#Rows.Next"><code>Rows.Next</code></a> returns false
	  <a href="/pkg/database/sql/#Rows.NextResultSet"><code>Rows.NextResultSet</code></a>
	  may be called to advance to the next result set. The existing <code>Rows</code>
	  should be continued to be used after it advances to the next result set.
    </p>
	<p>
	  <a href="/pkg/database/sql/#NamedParam"><code>NamedParam</code></a> may be used
	  as query arguments. The new function <a href="/pkg/database/sql/#Param"><code>Param</code></a>
	  helps create a <a href="/pkg/database/sql/#NamedParam"><code>NamedParam</code></a>
	  more succinctly.
	<p>
	  Drivers that support the new <a href="/pkg/database/sql/driver/#Pinger"><code>Pinger</code></a>
	  interface can now check if the server is still alive when the the
	  <a href="/pkg/database/sql/#DB.Ping"><code>DB.Ping</code></a>
	  or <a href="/pkg/database/sql/#DB.PingContext"><code>DB.PingContext</code></a>
	  is called.
	</p>
    <p>
	  The new <code>Context</code> query methods work for all drivers, but
	  <code>Context</code> cancelation is not responsive unless the driver has been
	  updated to used them. The other features require driver support in
	  <a href="/pkg/database/sql/driver"><code>database/sql/driver</code></a>.
	  Driver authors should review the new interfaces. Users of existing
	  driver should review the driver documentation to see what
	  it supports and any system specific documentation on each feature.
	</p>
  </dd>
</dl>

<dl id="debug_pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
  <dd>
    <p> <!-- CL 22720, CL 27212, CL 22181, CL 22332, CL 22336, Issue 15345 -->
      TODO: describe Issue 15345 and CL 22181, CL 22332, CL 22336
      <pre>pkg debug/pe, method (*COFFSymbol) FullName(StringTable) (string, error)
pkg debug/pe, method (StringTable) String(uint32) (string, error)
pkg debug/pe, type File struct, COFFSymbols []COFFSymbol
pkg debug/pe, type File struct, StringTable StringTable
pkg debug/pe, type Reloc struct
pkg debug/pe, type Reloc struct, SymbolTableIndex uint32
pkg debug/pe, type Reloc struct, Type uint16
pkg debug/pe, type Reloc struct, VirtualAddress uint32
pkg debug/pe, type Section struct, Relocs []Reloc
pkg debug/pe, type StringTable []uint8</pre>
      </p>
  </dd>
</dl>

<dl id="encoding_base64"><dt><a href="/pkg/encoding/base64/">encoding/base64</a></dt>
  <dd>
    <p> <!-- CL 24964 -->
      The new
      <a href="/pkg/encoding/base64/#Encoding.Strict"><code>Encoding.Strict</code></a>
      method returns an <code>Encoding</code> that causes the decoder
      to return an error when the trailing padding bits are not zero.
    </p>
  </dd>
</dl>

<dl id="encoding_binary"><dt><a href="/pkg/encoding/binary/">encoding/binary</a></dt>
  <dd>
    <p> <!-- CL 28514 -->
      The package now supports boolean values.
      TODO: add docs in encoding/binary package, then link to package docs
      here.
    </p>
  </dd>
</dl>

<dl id="encoding_json"><dt><a href="/pkg/encoding/json/">encoding/json</a></dt>
  <dd>

    <p> <!-- CL 18692  -->
      <a href="/pkg/encoding/json/#UnmarshalTypeError"><code>UnmarshalTypeError</code></a>
      now includes the struct and field name.
    </p>

    <p> <!-- CL 31932 -->
      A <code>nil</code> <a href="/pkg/encoding/json/#Marshaler"><code>Marshaler</code></a>
      now marshals as a JSON "<code>null</code>" value.
    </p>

    <p> <!-- CL 21811 -->
      A <a href="/pkg/encoding/json/#RawMessage"><code>RawMessage</code></a> value now
      marshals the same as its pointer type.
    </p>

    <p> <!-- CL 30371 -->
      Marshal encodes floating-point numbers using the same format as in ES6,
      preferring decimal (not exponential) notation for a wider range of values.
      In particular, all floating-point integers up to 2<sup>64</sup> format the
      same as the equivalent <code>int64</code> representation.
    </p>

    <p> <!-- CL 30944 -->
      Implementations
      of <a href="/pkg/encoding/json/#Unmarshaler"><code>Unmarshaler</code></a>
      are now called with the literal "<code>null</code>" and can
      decide how to handle it.
    </p>

  </dd>
</dl>

<dl id="encoding_pem"><dt><a href="/pkg/encoding/pem/">encoding/pem</a></dt>
  <dd>
    <p> <!-- CL 27391 -->
      <a href="/pkg/encoding/pem/#Decode"><code>Decode</code></a>
      is now strict about the format of the ending line.
    </p>
  </dd>
</dl>

<dl id="expvar"><dt><a href="/pkg/expvar/">expvar</a></dt>
  <dd>
    <p> <!-- CL 30917 -->
      The new methods
      <a href="/pkg/expvar/#Int.Value"><code>Int.Value</code></a>,
      <a href="/pkg/expvar/#String.Value"><code>String.Value</code></a>,
      <a href="/pkg/expvar/#Float.Value"><code>Float.Value</code></a>, and
      <a href="/pkg/expvar/#Func.Value"><code>Func.Value</code></a>
      report the current value of an exported variable.
    </p>

    <p> <!-- CL 24722 -->
      The new
      function <a href="/pkg/expvar/#Handler"><code>Handler</code></a>
      returns the package's HTTP handler, to enable installing it in
      non-standard locations.
      </p>
  </dd>
</dl>


<dl id="image_png"><dt><a href="/pkg/image/png/">image/png</a></dt>
  <dd>
    <p> <!-- CL 32143, CL 32140 -->
      <a href="/pkg/image/png/#Decode"><code>Decode</code></a>
      (and <code>DecodeConfig</code>)
      now supports True Color and grayscale transparency.
    </p>
    <p> <!-- CL 29872 -->
      <a href="/pkg/image/png/#Encoder"><code>Encoder</code></a>
      is now faster and creates smaller output
      when encoding paletted images.
      </p>
  </dd>
</dl>

<dl id="math_rand"><dt><a href="/pkg/math/rand/">math/rand</a></dt>
  <dd>

    <p><!-- CL 27253, CL 33456 -->
      The new <a href="/pkg/math/rand/#Rand.Uint64"><code>Rand.Uint64</code></a>
      method returns <code>uint64</code> values.  The
      new <a href="/pkg/math/rand/#Rand.Source64"><code>Rand.Source64</code></a>
      interface describes sources capable of generating such values
      directly; otherwise the <code>Rand.Uint64</code> method
      constructs a <code>uint64</code> from two calls
      to <code>Rand.Source</code>'s <code>Int63</code> method.
    </p>

  </dd>
</dl>

<dl id="mime_quotedprintable"><dt><a href="/pkg/mime/quotedprintable/">mime/quotedprintable</a></dt>
  <dd>

    <p>
      The
      <a href="/pkg/mime/quotedprintable/#Reader"><code>Reader</code></a>'s
      parsing has been relaxed in two ways to accept
      more input seen in the wild. First, it now accepts
      a <code>=</code> sign even if it's not followed by two hex
      digits. <!-- CL 32174 -->

      Second, it accepts a trailing soft line-break at the end of a
      message. <!-- CL 27530 --> That is, the final byte of the
      message may be a <code>=</code> sign and it will now be ignored.
    </p>

  </dd>
</dl>

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>
  <dd>
    
    <p><!-- CL 30164, CL 33473 -->
      The <a href="/pkg/net/#Conn"><code>Conn</code></a> documentation
      has been updated to clarify expectations of an interface
      implementation. Updates in the <code>net/http</code> packages
      depend on implementations obeying the documentation.
    </p>
    <p><i>Updating:</i> implementations of the <code>Conn</code> interface should verify
      they implement the documented semantics. TODO: https://golang.org/cl/18097
    </p>

    <p>TODO: many other net changes</p>

  </dd>
</dl>

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

    <p>Server changes:</p>
    <ul>
      <li>The server now supports graceful shutdown support, <a href="#http_shutdown">mentioned above</a>.</li>

      <li> <!-- CL 32024 -->
        The <a href="/pkg/net/http/#Server"><code>Server</code></a>
        adds configuration options
        <code>ReadHeaderTimeout</code> and <code>IdleTimeout</code>
        and documents <code>WriteTimeout</code>.
      </li>

      <li> <!-- CL 32014 -->
        <a href="/pkg/net/http/#FileServer"><code>FileServer</code></a>
        and
        <a href="/pkg/net/http/#ServeContent"><code>ServeContent</code></a>
        now support HTTP <code>If-Match</code> conditional requests,
        in addition to the previous <code>If-None-Match</code>
        support.
      </li>

      <li> <!-- CL 27950 -->
        The server now logs bad requests.
      </li>
    </ul>

    <p>
      There are several additions to what a server's <code>Handler</code> can do:
    </p>

    <ul>
      <li><!-- CL 31173 -->
        The <a href="/pkg/context/#Context"><code>Context</code></a>
        returned
        by <a href="/pkg/net/http/#Request.Context"><code>Request.Context</code></a>
        is canceled if the underlying <code>net.Conn</code>
        closes. For instance, if the user closes their browser in the
        middle of a slow request, the <code>Handler</code> can now
        detect that the user is gone. This complements the
        existing <a href="/pkg/net/http/#CloseNotifier"><code>CloseNotifier</code></a>
        support. This functionality requires that the underlying
        <a href="/pkg/net/#Conn"><code>net.Conn</code></a> implements
        <a href="#net">recently-clarified interface documentation</a>.
      </li>

      <li><!-- CL 32479 -->
        To serve trailers known after the header has been written,
        see the new
        <a href="/pkg/net/http/#TrailerPrefix"><code>TrailerPrefix</code></a>
        mechanism.
      </li>

      <li><!-- CL 33099 -->
        A <code>Handler</code> can now abort a response by panicking
        with the error
        <a href="/pkg/net/http/#ErrAbortHandler"><code>ErrAbortHandler</code></a>.
      </li>

      <li><!-- CL 30812 -->
        A <code>Write</code> of zero bytes to a
        <a href="/pkg/net/http/#ResponseWriter"><code>ResponseWriter</code></a>
        is now defined as a
        way to test whether a <code>ResponseWriter</code> has been hijacked:
        if so, the <code>Write</code> returns
        <a href="/pkg/net/http/#ErrHijacked"><code>ErrHijacked</code></a>
        without printing an error
        to the server's error log.
      </li>

    </ul>

    <p>Client &amp; Transport changes:</p>
    <ul>
      <li><!-- CL 28930 -->
        The <a href="/pkg/net/http/#Client"><code>Client</code></a>
        now copies most request headers on redirect. Authentication-specific
        headers are only sent to the same origin or subdomains thereof.
        Cookies are not sent again, unless they were set explicitly.
        TODO: verify this, and link to docs once https://golang.org/issue/18096
        is fixed.
      </li>

      <li><!-- CL 29072 -->
        The <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
        now supports international domain names. Consequently, so do
        <a href="/pkg/net/http/#Get">Get</a> and other helpers.
      </li>

      <li><!-- CL 31733, CL 29852 -->
        The <code>Client</code> now supports 307 and 308 redirects.
        If the redirect requires resending the request body,
        the request must have the new
        <a href="/pkg/net/http/#Request"><code>Request.GetBody</code></a>
        field defined.
        <a href="pkg/net/http/#NewRequest"><code>NewRequest</code></a>
        sets <code>Request.GetBody</code> automatically for common
        body types.
      </li>

      <li><!-- CL 32482 -->
        The <code>Transport</code> now rejects requests for URLs with
        ports containing non-digit characters.
      </li>

      <li><!-- CL 27117 -->
        The <code>Transport</code> will now retry non-idempotent
        requests if no bytes were written before a network failure.
      </li>

      <li><!-- CL 32481 -->
        The
        new <a href="/pkg/net/http/#Transport"><code>Transport.ProxyConnectHeader</code></a>
        allows configuration of header values to send to a proxy
        during a <code>CONNECT</code> request.
      </li>

      <li> <!-- CL 28077 -->
        The <a href="/pkg/net/http/#DefaultTransport"><code>DefaultTransport.Dialer</code></a>
        now enables <code>DualStack</code> ("<a href="https://tools.ietf.org/html/rfc6555">Happy Eyeballs</a>") support,
        to use IPv4 as a backup if it looks like IPv6 might be
        failing.
      </li>
    </ul>

  </dd>
</dl>

<dl id="net_mail"><dt><a href="/pkg/net/mail/">net/mail</a></dt>
  <dd>

    <p> <!-- CL 32176 -->
      Empty quoted strings are once again allowed in the name part of
      an address. That is, Go 1.4 and earlier accepted
      <code>"" &lt;gopher@example.com&gt;</code>,
      but Go 1.5 introduced a bug that rejected this address.
      The address is recognized again.
    </p>

    <p> <!-- CL 31581 -->
      The
      <a href="/pkg/net/mail/#Header.Date"><code>Header.Date</code></a>
      method has always provided a way to parse
      the <code>Date:</code> header.
      A new function
      <a href="/pkg/net/mail/#ParseDate"><code>ParseDate</code></a>
      allows parsing dates found in other
      header lines, such as the <code>Resent-Date:</code> header.
    </p>
    
  </dd>
</dl>

<dl id="net_smtp"><dt><a href="/pkg/net/smtp/">net/smtp</a></dt>
  <dd>

    <p> <!-- CL 33143 -->
      If an implementation of
      the <a href="/pkg/net/smtp/#Auth"><code>Auth</code></a>
      interface's <code>Start</code> method returns an
      empty <code>toServer</code> value, the package no longer sends
      trailing whitespace in the SMTP <code>AUTH</code> command,
      which some servers rejected.
    </p>

  </dd>
</dl>

<dl id="net_url"><dt><a href="/pkg/net/url/">net/url</a></dt>
  <dd>

    <p> <!-- CL 31322 --> The new functions
      <a href="/pkg/net/url/#PathEscape"><code>PathEscape</code></a>
      and
      <a href="/pkg/net/url/#PathUnescape"><code>PathUnescape</code></a>
      are similar to the query escaping and unescaping functions but
      for path elements.</p>

    <p> <!-- CL 28933 --> The new methods
      <a href="/pkg/net/url/#URL.Hostname"><code>URL.Hostname</code></a>
      and
      <a href="/pkg/net/url/#URL.Port"><code>URL.Port</code></a>
      are accessors to the hostname and port fields of a URL
      and deal with the case where the port may or may not be present.
    </p>

    <p> <!-- CL 28343 --> The existing method
      <a href="/pkg/net/url/#URL.ResolveReference"><code>URL.ResolveReference</code></a>
      now properly handles paths with escaped bytes without losing
      the escaping.
    </p>

    <p> <!-- CL 31467 -->
      The <code>URL</code> type now implements
      <a href="/pkg/encoding/#BinaryMarshaler"><code>encoding.BinaryMarshaler</code></a> and
      <a href="/pkg/encoding/#BinaryUnmarshaler"><code>encoding.BinaryUnmarshaler</code></a>,
      making it possible to process URLs in <a href="/pkg/encoding/gob/">gob data</a>.
    </p>

    <p> <!-- CL 29610, CL 31582 -->
      Following RFC 3986,
      <a href="/pkg/net/url/#Parse"><code>Parse</code></a>
      now rejects URLs like <code>this_that:other/thing</code> instead of
      interpreting them as relative paths (<code>this_that</code> is not a valid scheme).
      To force interpretation as a relative path,
      such URLs should be prefixed with <code>"./"</code>.
      The <code>URL.String</code> method now inserts this prefix as needed.
    </p>

  </dd>
</dl>

<dl id="os"><dt><a href="/pkg/os/">os</a></dt>
  <dd>
    <p>
      The new function
      <a href="/pkg/os/#Executable"><code>Executable</code></a> returns
	  the running executable path name.
    </p>
  </dd>
</dl>

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
  <dd>
    <p> <!-- CL 30088 -->
      The new function
      <a href="/pkg/reflect/#Swapper"><code>Swapper</code></a> was
      added to support <a href="#sortslice"><code>sort.Slice</code></a>.
    </p>
  </dd>
</dl>


<dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
  <dd>
    <p> <!-- CL 31210 -->
      The <a href="/pkg/strconv/#Unquote"><code>Unquote</code></a>
      function now strips carriage returns (<code>\r</code>) in
      backquoted raw strings, following the
      <a href="/ref/spec#String_literals">Go language semantics</a>.
    </p>
  </dd>
</dl>

<dl id="time"><dt><a href="/pkg/time/">time</a></dt>
  <dd>

    <p> <!-- CL 20118 --> The new function
      <a href="/pkg/time/#Until"><code>Until</code></a> complements
      the analogous <code>Since</code> function.
    </p>

    <p> <!-- CL 29338 -->
      <a href="/pkg/time/#ParseDuration"><code>ParseDuration</code></a>
      now accepts long fractional parts.
    </p>

    <p> <!-- CL 33429 -->
      <a href="/pkg/time/#Parse"><code>Parse</code></a>
      now rejects dates before the start of a month, such as June 0;
      it already rejected dates beyond the end of the month, such as
      June 31 and July 32.
    </p>

    <p> <!-- CL 33029 -->
      The <code>tzdata</code> database has been updated to version
      2016i for systems that don't already have a local time zone
      database.
    </p>

    <p>
  </dd>
</dl>

<dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
  <dd>
    <p><!-- CL 29970 -->
      The new method
      <a href="/pkg/testing/#T.Name"><code>T.Name</code></a>
      (and <code>B.Name</code>) returns the name of the current
      test or benchmark.
    </p>

    <p><!-- CL 31724 -->
      The new method
      <a href="/pkg/testing/#T.Context"><code>T.Context</code></a>
      (and <code>B.Context</code>) returns
      a <a href="/pkg/context/#Context"><code>Context</code></a> for
      the current running test or benchmark.
    </p>

    <p><!-- CL 32483 -->
      The new function
      <a href="/pkg/testing/#CoverMode"><code>CoverMode</code></a>
      reports the test coverage mode.
    </p>

    <p><!-- CL 32615 -->
      Tests and benchmarks are now marked as failed if the race
      detector is enabled and a data race occurs during execution.
      Previously, individual test cases would appear to pass,
      and only the overall execution of the test binary would fail.
    </p>
   
  </dd>
</dl>

<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
  <dd>
    <p><!-- CL 30935 -->
      <code>SimpleFold</code> now returns its argument unchanged
      if the provided input was an invalid rune.
      Previously, the implementation failed with an index bounds check panic.
    </p>
  </dd>
</dl>