+NOTE: This is a DRAFT of the Go 1.7 release notes, prepared for the Go 1.7 beta. +Go 1.7 has NOT yet been released. +By our regular schedule, it is expected some time in August 2016. + + +
+ ++The latest Go release, version 1.7, arrives six months after 1.6. +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 promise of compatibility. +We expect almost all Go programs to continue to compile and run as before. +
+ ++The release adds a port to IBM LinuxOne; +updates the x86-64 compiler back end to generate more efficient code; +includes the context package, promoted from the +x/net subrepository +and now used throughout the standard library; +and adds support in the testing package for +creating hierarchies of tests and benchmarks. +The release also removes the ability to disable +the vendoring changes started in Go 1.5: +vendoring is now an official part of the Go toolchain. +
+ +
+There is one tiny language change in this release.
+The section on terminating statements
+clarifies that to determine whether a statement list ends in a terminating statement,
+the “final non-empty statement” is considered the end,
+matching the existing behavior of the gc and gccgo compiler toolchains.
+In earlier releases the definition referred only to the “final statement,”
+leaving the effect of trailing empty statements at the least unclear.
+The go/types
+package has been updated to match the gc and gccgo compiler toolchains
+in this respect.
+
+Go 1.7 adds an experimental port to Linux on z Systems (linux/s390x)
+and the beginning of a port to Plan 9 on ARM (plan9/arm).
+
+The experimental ports to Linux on 64-bit MIPS (linux/mips64 and linux/mips64le)
+added in Go 1.6 now have full support for cgo and external linking.
+
+The experimental port to Linux on big-endian 64-bit PowerPC (linux/ppc64)
+now requires the POWER8 architecture or later.
+TODO: Pending ppc64le change for cgo.
+
+The OpenBSD port now requires OpenBSD 5.6 or later, for access to the getentropy(2) system call. +
+ +
+For 64-bit ARM systems, the vector register names have been
+corrected to V0 through V31;
+previous releases incorrectly referred to them as V32 through V63.
+
+For 64-bit x86 systems, the following instructions have been added:
+PCMPESTRI,
+RORXL,
+RORXQ,
+VINSERTI128,
+VPADDD,
+VPADDQ,
+VPALIGNR,
+VPBLENDD,
+VPERM2F128,
+VPERM2I128,
+VPOR,
+VPSHUFB,
+VPSHUFD,
+VPSLLD,
+VPSLLDQ,
+VPSLLQ,
+VPSRLD,
+VPSRLDQ,
+and
+VPSRLQ.
+
+This release includes a new code generation back end for 64-bit x86 systems, +following a proposal from 2015 +that has been under development since then. +The new back end, based on +SSA, +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 our benchmark programs by 5-35%. +
+ +
+For this release, the new back end can be disabled by passing
+-ssa=0 to the compiler.
+If you find that your program compiles or runs successfully
+only with the new back end disabled, please
+file a bug report.
+
+The format of exported metadata written by the compiler in package archives has changed: +the old textual format has been replaced by a more compact binary format. +This results in somewhat smaller package archives and fixes a few +long-standing corner case bugs. +
+ +
+For this release, the new export format can be disabled by passing
+-newexport=0 to the compiler.
+If you find that your program compiles or runs successfully
+only with the new export format disabled, please
+file a bug report.
+
+The linker's -X option no longer supports the unusual two-argument form
+-X name value,
+as announced in the Go 1.6 release
+and in warnings printed by the linker.
+Use -X name=value instead.
+
+The compiler and linker have been optimized and run significantly faster in this release than in Go 1.6, +although they are still slower than we would like and will continue to be optimized in future releases. +
+ ++Due to changes across the compiler toolchain and standard library, +binaries built with this release should typically be smaller than binaries +built with Go 1.6, +sometimes by as much as 20-30%. +
+ ++Packages using cgo may now include +Fortran source files (in addition to C, C++, Objective C, and SWIG), +although the Go bindings must still use C language APIs. +
+ +
+Go bindings may now use a new helper function C.CBytes.
+In contrast to C.CString, which takes a Go string
+and returns a *C.byte (a C char*),
+C.CBytes takes a Go []byte
+and returns an unsafe.Pointer (a C void*).
+
+Packages and binaries built using cgo have in past releases
+produced different output on each build,
+due to the embedding of temporary directory names.
+When using this release with
+new enough versions of GCC or Clang
+(those that support the -fdebug-prefix-map option),
+those builds should finally be deterministic.
+
+Due to the alignment of Go's semiannual release schedule with GCC's annual release schedule, +GCC release 6 contains the Go 1.6.1 version of gccgo. +The next release, GCC 7, will likely have the Go 1.8 version of gccgo. +
+ +
+The go command's basic operation
+is unchanged, but there are a number of changes worth noting.
+
+This release removes support for the GO15VENDOREXPERIMENT environment variable,
+as announced in the Go 1.6 release.
+Vendoring support
+is now a standard feature of the go command and toolchain.
+
+The Package data structure made available to
+“go list” now includes a
+StaleReason field explaining why a particular package
+is or is not considered stale (in need of rebuilding).
+This field is available to the -f or -json
+options and is useful for understanding why a target is being rebuilt.
+
+The “go get” command now supports
+import paths referring to git.openstack.org.
+
+This release adds experimental, minimal support for building programs using
+binary-only packages,
+packages distributed in binary form
+without the corresponding source code.
+This feature is needed in some commercial settings
+but is not intended to be fully integrated into the rest of the toolchain.
+For example, tools that assume access to complete source code
+will not work with such packages, and there are no plans to support
+such packages in the “go get” command.
+
+The “go doc” command
+now groups constructors with the type they construct,
+following godoc.
+
+The “go vet” command
+has more accurate analysis in its -copylock and -printf checks,
+and a new -tests check that checks the name and signature of likely test functions.
+To avoid confusion with the new -tests check, the old, unadvertised
+-test option has been removed; it was equivalent to -all -shadow.
+
+The new subcommand “go tool dist list”
+prints all supported operating system/architecture pairs.
+
+The “go tool trace” command,
+introduced in Go 1.5,
+has been refined in various ways.
+
+First, collecting traces is significantly more efficient than in past releases. +In this release, the typical execution-time overhead of collecting a trace is about 25%; +in past releases it was at least 400%. +Second, trace files now include file and line number information, +making them more self-contained and making the +original executable optional when running the trace tool. +Third, the trace tool now breaks up large traces to avoid limits +in the browser-based viewer. +
+ ++Although the trace file format has changed in this release, +the Go 1.7 tools can still read traces from earlier releases. +
+ ++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 core library. +On x86-64 systems, many programs will run significantly faster, +due to improvements in generated code brought by the +new compiler back end. +As noted above, in our own benchmarks, +the code generation changes alone typically reduce program CPU time by 5-35%. +
+ +
+
+There have been significant optimizations bringing more than 10% improvements
+to implementations in the
+crypto/sha1,
+crypto/sha256,
+encoding/binary,
+fmt,
+hash/adler32,
+hash/crc32,
+hash/crc64,
+image/color,
+math/big,
+strconv,
+strings,
+unicode,
+and
+unicode/utf16
+packages.
+
+Go 1.7 moves the golang.org/x/net/context package
+into the standard library as context.
+This allows the use of contexts for cancellation, timeouts, and passing
+request-scoped data in other standard library packages,
+including
+net,
+net/http,
+and
+os/exec,
+as noted below.
+
+TODO: Example here. +
+ ++For more information about contexts, see the +package documentation +and the Go blog post +“Go Concurrent Patterns: Context.” +
+ +
+The testing package now supports the definition
+of tests with subtests and benchmarks with sub-benchmarks.
+
+TODO: Where is the documentation for this? +
+ ++TODO: Example here. +
+ +
+All panics started by the runtime now use panic values
+that implement both the builtin error,
+and
+runtime.Error,
+as
+required by the language specification.
+
+The new function
+KeepAlive
+provides an explicit mechanism for declaring
+that an allocated object is currently reachable,
+typically to delay the execution of an associated finalizer.
+
+The new function
+CallersFrames
+translates a PC slice obtained from
+Callers
+into a sequence of frames corresponding to the call stack.
+This new API should be preferred instead of direct use of
+FuncForPC,
+because the frame sequence can more accurately describe
+call stacks with inlined function calls.
+
+The new function
+SetCgoTraceback
+facilitates tighter integration between Go and C code executing
+in the same process called using cgo.
+
+On 32-bit systems, the runtime can now use memory allocated +by the operating system anywhere in the address space, +eliminating the +“memory allocated by OS not in usable range” failure +common in some environments. +
+ +
+On Windows, Go programs in Go 1.5 and earlier forced
+the global Windows timer resolution to 1ms at startup
+by calling timeBeginPeriod(1).
+Changing the global timer resolution caused problems on some systems,
+and testing suggested that the call was not needed for good scheduler performance,
+so Go 1.6 removed the call.
+Go 1.7 brings the call back: under some workloads the call
+is still needed for good scheduler performance.
+
+As always, there are various minor changes and updates to the library, +made with the Go 1 promise of compatibility +in mind. +
+ + + +
+In previous releases of Go, if
+Reader's
+Peek method
+were asked for more bytes than fit in the underlying buffer,
+it would return an empty slice and the error ErrBufferFull.
+Now it returns the entire underlying buffer, still accompanied by the error ErrBufferFull.
+
+The new functions
+ContainsAny and
+ContainsRune
+have been added for symmetry with
+the strings package.
+
+In previous releases of Go, if
+Reader's
+Read method
+were asked for zero bytes with no data remaining, it would
+return a count of 0 and no error.
+Now it returns a count of 0 and the error
+io.EOF .
+
+The
+Reader type has a new method
+Reset to allow reuse of a Reader.
+
+As noted above,
+there are significant performance optimizations throughout the package.
+Decompression speed is improved by about 10%,
+while compression speed for DefaultCompression is roughly doubled.
+
+In addition to those general improvements,
+the
+BestSpeed
+compressor has been replaced entirely and uses an
+algorithm similar to Snappy,
+resulting in about a 2.5X speed increase,
+although the output can be 5-10% larger than with the previous algorithm.
+
+There is also a new compression level
+HuffmanOnly
+that applies Huffman but not Lempel-Ziv encoding.
+Forgoing Lempel-Ziv encoding means that
+HuffmanOnly runs about 3X faster than the new BestSpeed
+but at the cost of producing compressed outputs that are 20-40% larger than those
+generated by the new BestSpeed.
+
+TODO: Describe Config.DynamicRecordSizingDisabled or whatever replaces it. +
+ +
+The TLS client now has optional, limited support for server-initiated renegotiation,
+enabled by setting the
+Config's
+Renegotiation field.
+This is needed for connecting to many Microsoft Azure servers.
+
+The errors returned by the package now consistently begin with a
+tls: prefix.
+In past releases, some errors used a crypto/tls: prefix,
+some used a tls: prefix, and some had no prefix at all.
+
+When generating self-signed certificates, the package no longer sets the +“Authority Key Identifier” field by default. +
+
+The new function
+SystemCertPool
+provides access to the entire system certificate pool if available.
+There is also a new associated error type
+SystemRootsError.
+
+The
+Reader type's new
+SeekPC method and the
+Data type's new
+Ranges method
+help to find the compilation unit to pass to a
+LineReader
+and to identify the specific function for a given program counter.
+
+The new
+R_390 relocation type
+and its many predefined constants
+support the S390 port.
+
+The ASN.1 decoder now rejects non-minimal integer encodings. +This may cause the package to reject some invalid but formerly accepted ASN.1 data. +
+
+The
+Encoder's new
+SetIndent method
+sets the indentation parameters for JSON encoding,
+like in the top-level
+Indent function.
+
+The
+Encoder's new
+SetEscapeHTML method
+controls whether the
+&, <, and >
+characters in quoted strings should be escaped as
+\u0026, \u003c, and \u003e,
+respectively.
+As in previous releases, the encoder defaults to applying this escaping,
+to avoid certain problems that can arise when embedding JSON in HTML.
+
+In earlier versions of Go, this package only supported encoding and decoding
+maps using keys with string types.
+Go 1.7 adds support for maps using keys with integer types:
+the encoding uses a quoted decimal representation as the JSON key.
+Go 1.7 also adds support for encoding maps using non-string keys that implement
+MarshalJSON
+(see
+Marshaler)
+or
+MarshalText
+(see
+encoding.TextMarshaler)
+methods,
+as well as support for decoding maps using non-string keys that implement
+UnmarshalJSON
+(see
+Unmarshaler)
+or
+UnmarshalText
+(see
+encoding.TextUnmarshaler)
+methods.
+These methods are ignored for keys with string types in order to preserve
+the encoding and decoding used in earlier versions of Go.
+
+When encoding a slice of typed bytes,
+Marshal
+now generates an array of elements encoded using
+that byte type's
+MarshalJSON
+or
+MarshalText
+method if present,
+only falling back to the default base64-encoded string data if neither method is available.
+Earlier versions of Go accept both the original base64-encoded string encoding
+and the array encoding (assuming the byte type also implements
+UnmarshalJSON
+or
+UnmarshalText
+as appropriate),
+so this change should be semantically backwards compatible with earlier versions of Go,
+even though it does change the chosen encoding.
+
+To implement the go command's new support for binary-only packages
+and for Fortran code in cgo-based packages,
+the
+Package type
+adds new fields BinaryOnly, CgoFFLAGS, and FFiles.
+
+To support the corresponding change in go test described above,
+Example struct adds a Unordered field
+indicating whether the example may generate its output lines in any order.
+
+The package adds new constants
+SeekStart, SeekCurrent, and SeekEnd,
+for use with
+Seeker
+implementations.
+These constants are preferred over os.SEEK_SET, os.SEEK_CUR, and os.SEEK_END,
+but the latter will be preserved for compatibility.
+
+The
+Float type adds
+GobEncode and
+GobDecode methods,
+so that values of type Float can now be encoded and decoded using the
+encoding/gob
+package.
+
+The
+Writer
+implementation now emits each multipart section's header sorted by key.
+Previously, iteration over a map caused the section header to use a
+non-deterministic order.
+
+As part of the introduction of context, the
+Dialer type has a new method
+DialContext, like
+Dial but adding the
+context.Context
+for the dial operation.
+The context is intended to obsolete the Dialer's
+Cancel and Deadline fields,
+but the implementation continues to respect them,
+for backwards compatibility.
+
+The
+IP type's
+String method has changed its result for invalid IP addresses.
+In past releases, if an IP byte slice had length other than 0, 4, or 16, String
+returned "?".
+Go 1.7 adds the hexadecimal encoding of the bytes, as in "?12ab".
+
+The pure Go name resolution
+implementation now respects nsswtch.conf's
+stated preference for the priority of DNS lookups compared to
+local file (that is, /etc/hosts) lookups.
+
+ResponseWriter's
+documentation now makes clear that beginning to write the response
+may prevent future reads on the request body.
+For maximal compatibility, implementations are encouraged to
+read the request body completely before writing any part of the response.
+
+As part of the introduction of context, the
+Request has a new methods
+Context, to retrieve the associated context, and
+WithContext, to construct a copy of Request
+with a modified context.
+
+In the
+Server implementation,
+Serve records in the request context
+both the underlying *Server using the key ServerContextKey
+and the local address on which the request was received (a
+Addr) using the key LocalAddrContextKey.
+For example, the address on which a request received is
+req.Context().Value(http.LocalAddrContextKey).(net.Addr).
+
+The server implementation now
+pads response codes less than 100 to three digits
+as required by the protocol,
+so that w.WriteHeader(5) uses the HTTP response
+status 005, not just 5.
+
+In the client, the
+Transport implementation passes the request context
+to any dial operation connecting to the remote server.
+If a custom dialer is needed, the new Transport field
+Dialer is preferred over the existing Dial field,
+because the former can accept a context.
+
+The
+Transport also adds fields
+IdleConnTimeout,
+MaxIdleConns,
+and
+MaxResponseHeaderBytes
+to help control client resources consumed
+by idle or chatty servers.
+
+A
+Client's configured CheckRedirect function can now
+return ErrUseLastResponse to indicate that the
+most recent redirect response should be returned as the
+result of the HTTP request.
+That response is now available to the CheckRedirect function
+as req.Response.
+
+Since Go 1, the default behavior of the HTTP client is
+to request server-side compression
+using the Accept-Encoding request header
+and then to uncompress the response body transparently,
+and this behavior is adjustable using the
+Transport's DisableCompression field.
+In Go 1.7, to aid the implementation of HTTP proxies, the
+Response's new
+Uncompressed field reports whether
+this transparent uncompression took place.
+
+DetectContentType
+adds support for a few new audio and video content types.
+
+The
+Handler
+adds a new field
+Stderr
+that allows redirection of the child process's
+standard error away from the host process's
+standard error.
+
+The new function
+NewRequest
+prepares a new
+http.Request
+suitable for passing to an
+http.Handler during a test.
+
+The
+ResponseRecorder's new
+Result method
+returns the recorded
+http.Response.
+Tests that need to check the response's headers or trailers
+should call Result and inspect the response fields
+instead of accessing
+ResponseRecorder's HeaderMap directly.
+
+The
+ReverseProxy implementation now responds with “502 Bad Gateway”
+when it cannot reach a back end; in earlier releases it responded with “500 Internal Server Error.”
+
+Both
+ClientConn and
+ServerConn have been documented as deprecated.
+They are low-level, old, and unused by Go's current HTTP stack
+and will no longer be updated.
+Programs should use
+http.Client,
+http.Transport,
+and
+http.Server
+instead.
+
+The runtime trace HTTP handler, installed to handle the path /debug/pprof/trace,
+now accepts a fractional number in its seconds query parameter,
+allowing collection of traces for intervals smaller than one second.
+This is especially useful on busy servers.
+
+The address parser now allows unescaped UTF-8 text in addresses
+following RFC 6532,
+but it does not apply any normalization to the result.
+For compatibility with older mail parsers,
+the address encoder, namely
+Address's
+String method,
+continues to escape all UTF-8 text following RFC 5322,
+
+The
+URL's
+new ForceQuery field
+records whether the URL must have a query string,
+in order to distinguish URLs without query strings (like /search)
+from URLs with empty query strings (like /search?).
+
+The
+File
+type adds a new
+Size
+method, so that File implements the new
+SizedReaderAt method.
+
+IsExists now returns true for syscall.ENOTEMPTY,
+on systems where that error exists.
+
+On Windows,
+Remove now removes read-only files when possible,
+making the implementation behave as on
+non-Windows systems.
+
+As part of the introduction of context,
+the new constructor
+CommandContext
+is like
+Command but includes a context that can be used to cancel the command execution.
+
+The
+Current
+function is now implemented even when cgo is not available.
+
+The new
+Group type,
+along with the lookup functions
+LookupGroup and
+LookupGroupId
+and the new field GroupIds in the User struct,
+provides access to system-specific user group information.
+
+Although
+Value's
+Field method has always been documented to panic
+if the given field number i is out of range, it has instead
+silently returned a zero
+Value.
+Go 1.7 changes the method to behave as documented.
+
+The new
+StructOf
+function constructs a struct type at run time.
+It completes the set of type constructors, joining
+ArrayOf,
+ChanOf,
+FuncOf,
+MapOf,
+PtrTo,
+and
+SliceOf.
+
+StructTag's
+new method
+Lookup
+is like
+Get
+but distinguishes the tag not containing the given key
+from the tag associating an empty string with the given key.
+
+In previous releases of Go, if
+Reader's
+Read method
+were asked for zero bytes with no data remaining, it would
+return a count of 0 and no error.
+Now it returns a count of 0 and the error
+io.EOF .
+
+The
+Reader type has a new method
+Reset to allow reuse of a Reader.
+
+TODO: Describe whatever the final state is after golang.org/issue/15813 is resolved. +
+
+Duration's
+time.Duration.String method now reports the zero duration as "0s", not "0".
+ParseDuration continues to accept both forms.
+
+The method call time.Local.String() now returns "Local" on all systems;
+in earlier releases, it returned an empty string on Windows.
+
+The time zone database in
+$GOROOT/lib/time has been updated
+to IANA release 2016d.
+This fallback database is only used when the system time zone database
+cannot be found, for example on Windows.
+The Windows time zone abbreviation list has also been updated.
+
+On Linux, the
+SysProcAttr struct
+(as used in
+os/exec.Cmd's SysProcAttr field)
+has a new Unshare field.
+If the field is nonzero, the child process created by
+ForkExec
+(as used in exec.Cmd's Run method)
+will call the
+unshare(2)
+system call before executing the new program.
+