Introduction to Go 1.2

RED TEXT IS FROM THE 1.1 DOC AND NEEDS TO BE UPDATED. (It is here for formatting and style reference.)

Since the release of Go version 1.1 in April, 2013, the release schedule has been shortened to make the release process more efficient. This release, Go version 1.2 or Go 1.2 for short, arrives roughly six months after 1.1, while 1.1 took over a year to appear after 1.0. Because of the shorter time scale, 1.2 is a smaller delta than the step from 1.0 to 1.1, but it still has some significant developments, including a better scheduler and one new language feature. Of course, Go 1.2 keeps the promise of compatibility. The overwhelming majority of programs built with Go 1.1 (or 1.0 for that matter) will run without any changes whatsoever when moved to 1.2, although the introduction of one restriction to a corner of the language may expose already-incorrect code (see the discussion of the use of nil).

Changes to the language

In the interest of firming up the specification, one corner case has been clarified, with consequences for programs. There is also one new language feature.

Use of nil

The language now specifies that, for safety reasons, certain uses of nil pointers are guaranteed to trigger a run-time panic. For instance, in Go 1.0, given code like

type T struct {
    X [1<<24]byte
    Field int32
}

func main() {
    var x *T
    ...
}

the nil pointer x could be used to access memory incorrectly: the expression x.Field could access memory at address 1<<24. To prevent such unsafe behavior, in Go 1.2 the compilers now guarantee that any indirection through a nil pointer, such as illustrated here but also in nil pointers to arrays, nil interface values, nil slices, and so on, will either panic or return a correct, safe non-nil value. In short, any expression that explicitly or implicitly requires evaluation of a nil address is an error. The implementation may inject extra tests into the compiled program to enforce this behavior.

Further details are in the design document.

Updating: Most code that depended on the old behavior is erroneous and will fail when run. Such programs will need to be updated by hand.

Three-index slices

Go 1.2 adds the ability to specify the capacity as well as the length when using a slicing operation on an existing array or slice. A slicing operation creates a new slice by describing a contiguous section of an already-created array or slice:

var array [10]int
slice := array[2:4]

The capacity of the slice is the maximum number of elements that the slice may hold, even after reslicing; it reflects the size of the underlying array. In this example, the capacity of the slice variable is 8.

Go 1.2 adds new syntax to allow a slicing operation to specify the capacity as well as the length. A second colon introduces the capacity value, which must be less than or equal to the capacity of the source slice or array, adjusted for the origin. For instance,

slice = array[2:4:6]

sets the slice to have the same length as in the earlier example but its capacity is now only 4 elements (6-2). It is impossible to use this new slice value to access the last two elements of the original array.

In this three-index notation, a missing first index ([:i:j]) defaults to zero but the other two indices must always be specified explicitly. It is possible that future releases of Go may introduce default values for these indices.

Further details are in the design document.

Updating: This is a backwards-compatible change that affects no existing programs.

Changes to the implementations and tools

• runtime: preemption of goroutines at function entry (CL 12371043).
• go/build: support including C++ code with cgo (CL 8248043).

Godoc moved to the go.tools subrepository

A binary is still included with the distribution, but the source code for the godoc command has moved to the go.tools subrepository. The core of the program has been split into a library, while the command itself is in a separate directory. The move allows the code to be updated easily and the separation into a library and command makes it easier to construct custom binaries for local sites and different deployment methods.

Updating: Since godoc was not part of the library, no client code depends on the godoc sources and no updating is required.

The binary distributions available from

$ go get code.google.com/p/go.tools/cmd/godoc

TODO: choose which to call out The various routines to scan textual input in the bufio package, ReadBytes, ReadString and particularly ReadLine, are needlessly complex to use for simple purposes. In Go 1.1, a new type, Scanner, has been added to make it easier to do simple tasks such as read the input as a sequence of lines or space-delimited words. It simplifies the problem by terminating the scan on problematic input such as pathologically long lines, and having a simple default: line-oriented input, with each line stripped of its terminator. Here is code to reproduce the input a line at a time:

Updating: To correct breakage caused by the new struct field, go fix will rewrite code to add tags for these types. More generally, go vet will identify composite literals that should be revised to use field tags.

• Breaking change: archive/tar,archive/zip: fix os.FileInfo implementation to provide base name only (CL 13118043).
• encoding: new package defining generic encoding interfaces (CL 12541051).
• fmt: indexed access to arguments in Printf etc. (CL 9680043).
• sync/atomic: add Swap functions (CL 12670045).
• text/template: add comparison functions (CL 13091045).
• text/template: allow {{"{{"}}else if ... {{"}}"}} to simplify if chains (CL 13327043).

Minor changes to the library

The following list summarizes a number of minor changes to the library, mostly additions. See the relevant package documentation for more information about each change.

• The archive/zip package adds the DataOffset accessor to return the offset of a file's (possibly compressed) data within the archive.
• The bufio package adds Reset methods to Reader and Writer. These methods allow the Readers and Writers to be re-used on new input and output readers and writers, saving allocation overhead.
• The compress/bzip2 can now decompress concatenated archives.
• The compress/flate package adds a Reset method on the Writer, allowing compression of one file to start with another's dictionary.
• compress/gzip: add Reset method on Writer (CL 13435043).
• The container/heap package adds a Fix method to provide a more efficient way to update an item's position in the heap.
• The container/list package adds the MoveBefore and MoveAfter methods, which implement the obvious rearrangement.
• The crypto/cipher package adds the a new GCM mode (Galois Counter Mode), which is almost always used with AES encryption.
• The crypto/md5 package adds a new Sum function to simplify hashing without sacrificing performance.
• Similarly, the crypto/sha1 package adds a new Sum function.
• Also, the crypto/sha256 package adds Sum256 and Sum224 functions.
• Finally, the crypto/sha512 package adds Sum512 and Sum384 functions.
• The crypto/x509 package adds support for reading and writing arbitrary extensions.
• The crypto/tls package adds support for TLS 1.1, 1.2 and AES-GCM.
• The database/sql package adds a SetMaxOpenConns method on DB to limit the number of open connections to the database.
• The encoding/csv package now always allows trailing commas on fields.
• The encoding/gob package now supports the generic encoding interfaces of the encoding package described above.
• The encoding/json package now will always escape ampersands as "\u0026" when printing strings. It will now accept but correct invalid UTF-8 in Marshal (such input was previously rejected). Finally, it now supports the generic encoding interfaces of the encoding package described above.
• The encoding/xml package now allows attributes stored in pointers to be marshaled. It also supports the generic encoding interfaces of the encoding package described above through the new Marshaler, Unmarshaler, and related MarshalerAttr and UnmarshalerAttr interfaces.
• The flag package now has a Getter interface to allow the value of a flag to be retrieved. Due to the Go 1 compatibility guidelines, this method cannot be added to the existing Value interface, but all the existing standard flag types implement it. The package also now exports the CommandLine flag set, which holds the flags from the command line.
• The go/build package adds the AllTags field to the Package type, to make it easier to process build tags.
• The image/draw package now exports an interface, Drawer, that wraps the standard Draw method. The Porter-Duff operators now implement this interface, in effect binding an operation to the draw operator rather than providing it explicitly. Given a paletted image as its destination, the new FloydSteinberg implementation of the Drawer interface will use the Floyd-Steinberg error diffusion algorithm to draw the image. To create palettes suitable for such processing, the new Quantizer interface represents implementations of quantization algorithms that choose a palette given a full-color image. There are no implementations of this interface in the library.
• The image/gif package can now create GIF files using the new Encode and EncodeAll functions. Their options argument allows specification of an image Quantizer to use; if it is nil, the generated GIF will use the Plan9 color map (palette) defined in the new image/color/palette package. The options also specify a Drawer to use to create the output image; if it is nil, Floyd-Steinberg error diffusion is used.
• TheCopy method of the io package now prioritizes its arguments differently. If one argument implements WriterTo and the other implements iReaderFrom, Copy will now invoke WriterTo to do the work, so that less intermediate buffering is required in general.
• net: new build tag netgo for building a pure Go net package (CL 7100050).
• net/http: don't allow sending invalid cookie lines (CL 12204043).
• net/http: allow ReadResponse with nil *Request parameter (CL 9821043).
• net/http: allow responses to HEAD requests, detect type and length (CL 12583043).
• The runtime package relaxes the constraints on finalizer functions in SetFinalizer: the actual argument can now be any type that is assignable to the formal type of the function, as is the case for any normal function call in Go.
• The sort package has a new Stable function that implements stable sorting. It is less efficient than the normal sort algorithm, however.
• The strings package adds an IndexByte function for consistency with the bytes package.
• syscall: implemented Sendfile for Darwin, added Syscall9 for Darwin/amd64 (CL 10980043).
• The testing package now exports theTB interface. It records the methods in common with the T and B types, to make it easier to share code between tests and benchmarks. Also, the AllocsPerRun function now quantizes the return value to an integer (although it still has type float64), to round off any error caused by initialization and make the result more repeatable.
• The text/template package now automatically dereferences pointer values when evaluating the arguments to "escape" functions such as "html", to bring the behavior of such functions in agreement with that of other printing functions such as "printf".
• In the time package, the Parse function and Format method now handle time zone offsets with seconds, such as in the historical date "1871-01-01T05:33:02+00:34:08". Also, pattern matching in the formats for those routines is stricter: a non-lowercase letter must now follow the standard words such as "Jan" and "Mon".
• The unicode package adds In, a nicer-to-use but equivalent version of the original IsOneOf, to see whether a character is a member of a Unicode category.