The latest Go release, version 1.6, arrives six months after 1.5. Most of its changes are in the implementation of the language, runtime, and libraries. There are no changes to the language specification. As always, the release maintains the Go 1 promise of compatibility. We expect almost all Go programs to continue to compile and run as before.
The release adds new ports to Linux on 64-bit MIPS and Android on 32-bit x86; defined and enforced rules for sharing Go pointers with C; transparent, automatic support for HTTP/2; and a new mechanism for template reuse.
There are no language changes in this release.
Go 1.6 adds experimental ports to
Linux on 64-bit MIPS (linux/mips64 and linux/mips64le).
These ports support cgo but only with internal linking.
Go 1.6 also adds an experimental port to Android on 32-bit x86 (android/386).
On FreeBSD, Go 1.6 defaults to using clang, not gcc, as the external C compiler.
On Linux on little-endian 64-bit PowerPC (linux/ppc64le),
Go 1.6 now supports cgo with external linking and
is roughly feature complete.
On NaCl, Go 1.5 required SDK version pepper-41. Go 1.6 adds support for later SDK versions.
On 32-bit x86 systems using the -dynlink or -shared compilation modes,
the register CX is now overwritten by certain memory references and should
be avoided in hand-written assembly.
See the assembly documentation for details.
There is one major change to cgo, along with one minor change.
The major change is the definition of rules for sharing Go pointers with C code,
to ensure that such C code can coexist with Go's garbage collector.
Briefly, Go and C may share memory allocated by Go
when a pointer to that memory is passed to C as part of a cgo call,
provided that the memory itself contains no pointers to Go-allocated memory,
and provided that C does not retain the pointer after the call returns.
These rules are checked by the runtime during program execution:
if the runtime detects a violation, it prints a diagnosis and crashes the program.
The checks can be disabled by setting the environment variable
GODEBUG=cgocheck=0, but note that the vast majority of
code identified by the checks is subtly incompatible with garbage collection
in one way or another.
Disabling the checks will typically only lead to more mysterious failure modes.
Fixing the code in question should be strongly preferred
over turning off the checks.
See the cgo documentation for more details.
The minor change is
the addition of explicit C.complexfloat and C.complexdouble types,
separate from Go's complex64 and complex128.
Matching the other numeric types, C's complex types and Go's complex type are
no longer interchangeable.
The compiler toolchain is mostly unchanged. Internally, the most significant change is that the parser is now hand-written instead of generated from yacc.
The compiler, linker, and go command have a new flag -msan,
analogous to -race and only available on linux/amd64,
that enables interoperation with the Clang MemorySanitizer.
Such interoperation is useful mainly for testing a program containing suspect C or C++ code.
The linker has a new option -libgcc to set the expected location
of the C compiler support library when linking cgo code.
The option is only consulted when using -linkmode=internal,
and it may be set to none to disable the use of a support library.
The implementation of build modes started in Go 1.5 has been expanded to more systems.
This release adds support for the c-shared mode on android/386, android/amd64,
android/arm64, linux/386, and linux/arm64;
for the shared mode on linux/386, linux/arm, linux/amd64, and linux/ppc64le;
and for the new pie mode (generating position-independent executables) on
android/386, android/amd64, android/arm, android/arm64, linux/386,
linux/amd64, linux/arm, linux/arm64, and linux/ppc64le.
See the design document for details.
As a reminder, the linker's -X flag changed in Go 1.5.
In Go 1.4 and earlier, it took two arguments, as in
-X importpath.name value
Go 1.5 added an alternative syntax using a single argument
that is itself a name=value pair:
-X importpath.name=value
In Go 1.5 the old syntax was still accepted, after printing a warning suggesting use of the new syntax instead. Go 1.6 continues to accept the old syntax and print the warning. Go 1.7 will remove support for the old syntax.
The release schedules for the GCC and Go projects do not coincide. GCC release 5 contains the Go 1.4 version of gccgo. The next release, GCC 6, will have the Go 1.5 version of gccgo. Due to release scheduling, it is likely that Go 1.6 will not be in a GCC release until GCC 7.
The go command's basic operation
is unchanged, but there are a number of changes worth noting.
Go 1.5 introduced experimental support for vendoring,
enabled by setting the GO15VENDOREXPERIMENT environment variable to 1.
Go 1.6 keeps the vendoring support, no longer considered experimental,
and enables it by default.
It can be disabled explicitly by setting
the GO15VENDOREXPERIMENT environment variable to 0.
Go 1.7 will remove support for the environment variable.
The most likely problem caused by enabling vendoring by default happens
in source trees containing an existing directory named vendor that
does not expect to be interpreted according to new vendoring semantics.
In this case, the simplest fix is to rename the directory to anything other
than vendor and update any affected import paths.
For details about vendoring,
see the documentation for the go command
and the design document.
There is a new build flag, -msan,
that compiles Go with support for the LLVM memory sanitizer.
This is intended mainly for use when linking against C or C++ code
that is being checked with the memory sanitizer.
Go 1.5 introduced the
go doc command,
which allows references to packages using only the package name, as in
go doc http.
In the event of ambiguity, the Go 1.5 behavior was to use the package
with the lexicographically earliest import path.
In Go 1.6, ambiguity is resolved by preferring import paths with
fewer elements, breaking ties using lexicographic comparison.
An important effect of this change is that original copies of packages
are now preferred over vendored copies.
Successful searches also tend to run faster.
The go vet command now diagnoses
passing function or method values as arguments to Printf,
such as when passing f where f() was intended.
As always, the changes are so general and varied that precise statements about performance are difficult to make. Some programs may run faster, some slower. On average the programs in the Go 1 benchmark suite run a few percent faster in Go 1.6 than they did in Go 1.5. The garbage collector's pauses are even lower than in Go 1.5, especially for programs using a large amount of memory.
There have been significant optimizations bringing more than 10% improvements
to implementations of the
compress/bzip2,
compress/gzip,
crypto/aes,
crypto/elliptic,
crypto/ecdsa, and
sort packages.
Go 1.6 adds transparent support in the
net/http package
for the new HTTP/2 protocol.
Go clients and servers will automatically use HTTP/2 as appropriate when using HTTPS.
There is no exported API specific to details of the HTTP/2 protocol handling,
just as there is no exported API specific to HTTP/1.1.
Programs that must disable HTTP/2 can do so by setting
Transport.TLSNextProto (for clients)
or
Server.TLSNextProto (for servers)
to a non-nil, empty map.
Programs that must adjust HTTP/2 protocol-specific details can import and use
golang.org/x/net/http2,
in particular its
ConfigureServer
and
ConfigureTransport
functions.
The runtime has added lightweight, best-effort detection of concurrent misuse of maps. As always, if one goroutine is writing to a map, no other goroutine should be reading or writing the map concurrently. If the runtime detects this condition, it prints a diagnosis and crashes the program. The best way to find out more about the problem is to run the program under the race detector, which will more reliably identify the race and give more detail.
For program-ending panics, the runtime now by default
prints only the stack of the running goroutine,
not all existing goroutines.
Usually only the current goroutine is relevant to a panic,
so omitting the others significantly reduces irrelevant output
in a crash message.
To see the stacks from all goroutines in crash messages, set the environment variable
GOTRACEBACK to all
or call
debug.SetTraceback
before the crash, and rerun the program.
See the runtime documentation for details.
Updating:
Uncaught panics intended to dump the state of the entire program,
such as when a timeout is detected or when explicitly handling a received signal,
should now call debug.SetTraceback("all") before panicking.
Searching for uses of
signal.Notify may help identify such code.
On Windows, Go programs in Go 1.5 and earlier forced
the global Windows timer resolution to 1ms at startup
by calling timeBeginPeriod(1).
Go no longer needs this for good scheduler performance,
and changing the global timer resolution caused problems on some systems,
so the call has been removed.
When using -buildmode=c-archive or
-buildmode=c-shared to build an archive or a shared
library, the handling of signals has changed.
In Go 1.5 the archive or shared library would install a signal handler
for most signals.
In Go 1.6 it will only install a signal handler for the
synchronous signals needed to handle run-time panics in Go code:
SIGBUS, SIGFPE, SIGSEGV.
See the os/signal package for more
details.
The
reflect package has
resolved a long-standing incompatibility
between the gc and gccgo toolchains
regarding embedded unexported struct types containing exported fields.
Code that walks data structures using reflection, especially to implement
serialization in the spirit
of the
encoding/json and
encoding/xml packages,
may need to be updated.
The problem arises when using reflection to walk through
an embedded unexported struct-typed field
into an exported field of that struct.
In this case, reflect had incorrectly reported
the embedded field as exported, by returning an empty Field.PkgPath.
Now it correctly reports the field as unexported
but ignores that fact when evaluating access to exported fields
contained within the struct.
Updating: Typically, code that previously walked over structs and used
f.PkgPath != ""
to exclude inaccessible fields should now use
f.PkgPath != "" && !f.Anonymous
For example, see the changes to the implementations of
encoding/json and
encoding/xml.
In the
sort
package,
the implementation of
Sort
has been rewritten to make about 10% fewer calls to the
Interface's
Less and Swap
methods, with a corresponding overall time savings.
The new algorithm does choose a different ordering than before
for values that compare equal (those pairs for which Less(i, j) and Less(j, i) are false).
Updating:
The definition of Sort makes no guarantee about the final order of equal values,
but the new behavior may still break programs that expect a specific order.
Such programs should either refine their Less implementations
to report the desired order
or should switch to
Stable,
which preserves the original input order
of equal values.
In the text/template package, there are two significant new features to make writing templates easier.
First, it is now possible to trim spaces around template actions, which can make template definitions more readable. A minus sign at the beginning of an action says to trim space before the action, and a minus sign at the end of an action says to trim space after the action. For example, the template
{{23 -}}
<
{{- 45}}
formats as 23<45.
Second, the new {{block}} action,
combined with allowing redefinition of named templates,
provides a simple way to define pieces of a template that
can be replaced in different instantiations.
There is an example
in the text/template package that demonstrates this new feature.
archive/tar package's
implementation corrects many bugs in rare corner cases of the file format.
One visible change is that the
Reader type's
Read method
now presents the content of special file types as being empty,
returning io.EOF immediately.
archive/zip package, the
Reader type now has a
RegisterDecompressor method,
and the
Writer type now has a
RegisterCompressor method,
enabling control over compression options for individual zip files.
These take precedence over the pre-existing global
RegisterDecompressor and
RegisterCompressor functions.
bufio package's
Scanner type now has a
Buffer method,
to specify an initial buffer and maximum buffer size to use during scanning.
This makes it possible, when needed, to scan tokens larger than
MaxScanTokenSize.
Also for the Scanner, the package now defines the
ErrFinalToken error value, for use by
split functions to abort processing or to return a final empty token.
compress/flate package
has deprecated its
ReadError and
WriteError error implementations.
In Go 1.5 they were only rarely returned when an error was encountered;
now they are never returned, although they remain defined for compatibility.
compress/flate,
compress/gzip, and
compress/zlib packages
now report
io.ErrUnexpectedEOF for truncated input streams, instead of
io.EOF.
crypto/cipher package now
overwrites the destination buffer in the event of a GCM decryption failure.
This is to allow the AESNI code to avoid using a temporary buffer.
crypto/tls package
has a variety of minor changes.
It now allows
Listen
to succeed when the
Config
has a nil Certificates, as long as the GetCertificate callback is set,
it adds support for RSA with AES-GCM cipher suites,
and
it adds a
RecordHeaderError
to allow clients (in particular, the net/http package)
to report a better error when attempting a TLS connection to a non-TLS server.
crypto/x509 package
now permits certificates to contain negative serial numbers
(technically an error, but unfortunately common in practice),
and it defines a new
InsecureAlgorithmError
to give a better error message when rejecting a certificate
signed with an insecure algorithm like MD5.
debug/dwarf and
debug/elf packages
together add support for compressed DWARF sections.
User code needs no updating: the sections are decompressed automatically when read.
debug/elf package
adds support for general compressed ELF sections.
User code needs no updating: the sections are decompressed automatically when read.
However, compressed
Sections do not support random access:
they have a nil ReaderAt field.
encoding/asn1 package
now exports
tag and class constants
useful for advanced parsing of ASN.1 structures.
encoding/asn1 package,
Unmarshal now rejects various non-standard integer and length encodings.
encoding/base64 package's
Decoder has been fixed
to process the final bytes of its input. Previously it processed as many four-byte tokens as
possible but ignored the remainder, up to three bytes.
The Decoder therefore now handles inputs in unpadded encodings (like
RawURLEncoding) correctly,
but it also rejects inputs in padded encodings that are truncated or end with invalid bytes,
such as trailing spaces.
encoding/json package
now checks the syntax of a
Number
before marshaling it, requiring that it conforms to the JSON specification for numeric values.
As in previous releases, the zero Number (an empty string) is marshaled as a literal 0 (zero).
encoding/xml package's
Marshal
function now supports a cdata attribute, such as chardata
but encoding its argument in one or more <![CDATA[ ... ]]> tags.
encoding/xml package,
Decoder's
Token method
now reports an error when encountering EOF before seeing all open tags closed,
consistent with its general requirement that tags in the input be properly matched.
To avoid that requirement, use
RawToken.
fmt package now allows
any integer type as an argument to
Printf's * width and precision specification.
In previous releases, the argument to * was required to have type int.
fmt package,
Scanf can now scan hexadecimal strings using %X, as an alias for %x.
Both formats accept any mix of upper- and lower-case hexadecimal.
image
and
image/color packages
add
NYCbCrA
and
NYCbCrA
types, to support Y'CbCr images with non-premultiplied alpha.
io package's
MultiWriter
implementation now implements a WriteString method,
for use by
WriteString.
math/big package,
Int adds
Append
and
Text
methods to give more control over printing.
math/big package,
Float now implements
encoding.TextMarshaler and
encoding.TextUnmarshaler,
allowing it to be serialized in a natural form by the
encoding/json and
encoding/xml packages.
math/big package,
Float's
Append method now supports the special precision argument -1.
As in
strconv.ParseFloat,
precision -1 means to use the smallest number of digits necessary such that
Parse
reading the result into a Float of the same precision
will yield the original value.
math/rand package
adds a
Read
function, and likewise
Rand adds a
Read method.
These make it easier to generate pseudorandom test data.
Note that, like the rest of the package,
these should not be used in cryptographic settings;
for such purposes, use the crypto/rand package instead.
net package's
ParseMAC function now accepts 20-byte IP-over-InfiniBand (IPoIB) link-layer addresses.
net package,
there have been a few changes to DNS lookups.
First, the
DNSError error implementation now implements
Error,
and in particular its new
IsTemporary
method returns true for DNS server errors.
Second, DNS lookup functions such as
LookupAddr
now return rooted domain names (with a trailing dot)
on Plan 9 and Windows, to match the behavior of Go on Unix systems.
net/http package has
a number of minor additions beyond the HTTP/2 support already discussed.
First, the
FileServer now sorts its generated directory listings by file name.
Second, the
ServeFile function now refuses to serve a result
if the request's URL path contains “..” (dot-dot) as a path element.
Programs should typically use FileServer and
Dir
instead of calling ServeFile directly.
Programs that need to serve file content in response to requests for URLs containing dot-dot can
still call ServeContent.
Third, the
Client now allows user code to set the
Expect: 100-continue header (see
Transport.ExpectContinueTimeout).
Fourth, there are
five new error codes:
StatusPreconditionRequired (428),
StatusTooManyRequests (429),
StatusRequestHeaderFieldsTooLarge (431), and
StatusNetworkAuthenticationRequired (511) from RFC 6585,
as well as the recently-approved
StatusUnavailableForLegalReasons (451).
Fifth, the implementation and documentation of
CloseNotifier
has been substantially changed.
The Hijacker
interface now works correctly on connections that have previously
been used with CloseNotifier.
The documentation now describes when CloseNotifier
is expected to work.
net/http package,
there are a few changes related to the handling of a
Request data structure with its Method field set to the empty string.
An empty Method field has always been documented as an alias for "GET"
and it remains so.
However, Go 1.6 fixes a few routines that did not treat an empty
Method the same as an explicit "GET".
Most notably, in previous releases
Client followed redirects only with
Method set explicitly to "GET";
in Go 1.6 Client also follows redirects for the empty Method.
Finally,
NewRequest accepts a method argument that has not been
documented as allowed to be empty.
In past releases, passing an empty method argument resulted
in a Request with an empty Method field.
In Go 1.6, the resulting Request always has an initialized
Method field: if its argument is an empty string, NewRequest
sets the Method field in the returned Request to "GET".
net/http/httptest package's
ResponseRecorder now initializes a default Content-Type header
using the same content-sniffing algorithm as in
http.Server.
net/url package's
Parse is now stricter and more spec-compliant regarding the parsing
of host names.
For example, spaces in the host name are no longer accepted.
net/url package,
the Error type now implements
net.Error.
os package's
IsExist,
IsNotExist,
and
IsPermission
now return correct results when inquiring about an
SyscallError.
os.Stdout
or os.Stderr (more precisely, an os.File
opened for file descriptor 1 or 2) fails due to a broken pipe error,
the program will raise a SIGPIPE signal.
By default this will cause the program to exit; this may be changed by
calling the
os/signal
Notify function
for syscall.SIGPIPE.
A write to a broken pipe on a file descriptor other 1 or 2 will simply
return syscall.EPIPE (possibly wrapped in
os.PathError
and/or os.SyscallError)
to the caller.
The old behavior of raising an uncatchable SIGPIPE signal
after 10 consecutive writes to a broken pipe no longer occurs.
os/exec package,
Cmd's
Output method continues to return an
ExitError when a command exits with an unsuccessful status.
If standard error would otherwise have been discarded,
the returned ExitError now holds a prefix and suffix
(currently 32 kB) of the failed command's standard error output,
for debugging or for inclusion in error messages.
The ExitError's
String
method does not show the captured standard error;
programs must retrieve it from the data structure
separately.
path/filepath package's
Join function now correctly handles the case when the base is a relative drive path.
For example, Join(`c:`, `a`) now
returns `c:a` instead of `c:\a` as in past releases.
This may affect code that expects the incorrect result.
regexp package,
the
Regexp type has always been safe for use by
concurrent goroutines.
It uses a sync.Mutex to protect
a cache of scratch spaces used during regular expression searches.
Some high-concurrency servers using the same Regexp from many goroutines
have seen degraded performance due to contention on that mutex.
To help such servers, Regexp now has a
Copy method,
which makes a copy of a Regexp that shares most of the structure
of the original but has its own scratch space cache.
Two goroutines can use different copies of a Regexp
without mutex contention.
A copy does have additional space overhead, so Copy
should only be used when contention has been observed.
strconv package adds
IsGraphic,
similar to IsPrint.
It also adds
QuoteToGraphic,
QuoteRuneToGraphic,
AppendQuoteToGraphic,
and
AppendQuoteRuneToGraphic,
analogous to
QuoteToASCII,
QuoteRuneToASCII,
and so on.
The ASCII family escapes all space characters except ASCII space (U+0020).
In contrast, the Graphic family does not escape any Unicode space characters (category Zs).
testing package,
when a test calls
t.Parallel,
that test is paused until all non-parallel tests complete, and then
that test continues execution with all other parallel tests.
Go 1.6 changes the time reported for such a test:
previously the time counted only the parallel execution,
but now it also counts the time from the start of testing
until the call to t.Parallel.
text/template package
contains two minor changes, in addition to the major changes
described above.
First, it adds a new
ExecError type
returned for any error during
Execute
that does not originate in a Write to the underlying writer.
Callers can distinguish template usage errors from I/O errors by checking for
ExecError.
Second, the
Funcs method
now checks that the names used as keys in the
FuncMap
are identifiers that can appear in a template function invocation.
If not, Funcs panics.
time package's
Parse function has always rejected any day of month larger than 31,
such as January 32.
In Go 1.6, Parse now also rejects February 29 in non-leap years,
February 30, February 31, April 31, June 31, September 31, and November 31.