Go 1.7 Draft Release notes

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 .

It is important to note that both BestSpeed and HuffmanOnly produce a compressed output that is RFC 1951 compliant. In other words, any valid DEFLATE decompressor will continue to be able to decompress these outputs.

Lastly, there is a minor change to the decompressor's implementation of io.Reader . In previous versions, the decompressor deferred reporting io.EOF until exactly no more bytes could be read. Now, it reports io.EOF more eagerly when reading the last set of bytes.

The TLS implementation sends the first few data packets on each connection using small record sizes, gradually increasing to the TLS maximum record size. This heuristic reduces the amount of data that must be received before the first packet can be decrypted, improving communication latency over low-bandwidth networks. Setting Config 's DynamicRecordSizingDisabled field to true forces the behavior of Go 1.6 and earlier, where packets are as large as possible from the start of the connection.

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 the MarshalText (see encoding.TextMarshaler ) method, as well as support for decoding maps using non-string keys that implement the UnmarshalText (see encoding.TextUnmarshaler ) method. 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, 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 Goname 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, 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 DialContext is preferred over the existing Dial field, to allow the transport to supply 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 decompress 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 decompression 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? ).

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, 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.

The Method and NumMethod methods of Type and Value no longer return or count unexported methods.

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 .

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.