package bisect

import "internal/bisect"

Package bisect can be used by compilers and other programs to serve as a target for the bisect debugging tool. See golang.org/x/tools/cmd/bisect for details about using the tool.

To be a bisect target, allowing bisect to help determine which of a set of independent changes provokes a failure, a program needs to:

  1. Define a way to accept a change pattern on its command line or in its environment. The most common mechanism is a command-line flag. The pattern can be passed to New to create a Matcher, the compiled form of a pattern.

  2. Assign each change a unique ID. One possibility is to use a sequence number, but the most common mechanism is to hash some kind of identifying information like the file and line number where the change might be applied. Hash hashes its arguments to compute an ID.

  3. Enable each change that the pattern says should be enabled. The Matcher.ShouldEnable method answers this question for a given change ID.

  4. Print a report identifying each change that the pattern says should be printed. The Matcher.ShouldPrint method answers this question for a given change ID. The report consists of one more lines on standard error or standard output that contain a “match marker”. Marker returns the match marker for a given ID. When bisect reports a change as causing the failure, it identifies the change by printing the report lines with the match marker removed.

Example Usage

A program starts by defining how it receives the pattern. In this example, we will assume a flag. The next step is to compile the pattern: 

m, err := bisect.New(patternFlag)
if err != nil {
	log.Fatal(err)
}

Then, each time a potential change is considered, the program computes a change ID by hashing identifying information (source file and line, in this case) and then calls m.ShouldPrint and m.ShouldEnable to decide whether to print and enable the change, respectively. The two can return different values depending on whether bisect is trying to find a minimal set of changes to disable or to enable to provoke the failure.

It is usually helpful to write a helper function that accepts the identifying information and then takes care of hashing, printing, and reporting whether the identified change should be enabled. For example, a helper for changes identified by a file and line number would be: 

func ShouldEnable(file string, line int) {
	h := bisect.Hash(file, line)
	if m.ShouldPrint(h) {
		fmt.Fprintf(os.Stderr, "%v %s:%d\n", bisect.Marker(h), file, line)
	}
	return m.ShouldEnable(h)
}

Finally, note that New returns a nil Matcher when there is no pattern, meaning that the target is not running under bisect at all, so all changes should be enabled and none should be printed. In that common case, the computation of the hash can be avoided entirely by checking for m == nil first: 

func ShouldEnable(file string, line int) bool {
	if m == nil {
		return true
	}
	h := bisect.Hash(file, line)
	if m.ShouldPrint(h) {
		fmt.Fprintf(os.Stderr, "%v %s:%d\n", bisect.Marker(h), file, line)
	}
	return m.ShouldEnable(h)
}

When the identifying information is expensive to format, this code can call Matcher.MarkerOnly to find out whether short report lines containing only the marker are permitted for a given run. (Bisect permits such lines when it is still exploring the space of possible changes and will not be showing the output to the user.) If so, the client can choose to print only the marker: 

func ShouldEnable(file string, line int) bool {
	if m == nil {
		return true
	}
	h := bisect.Hash(file, line)
	if m.ShouldPrint(h) {
		if m.MarkerOnly() {
			bisect.PrintMarker(os.Stderr, h)
		} else {
			fmt.Fprintf(os.Stderr, "%v %s:%d\n", bisect.Marker(h), file, line)
		}
	}
	return m.ShouldEnable(h)
}

This specific helper – deciding whether to enable a change identified by file and line number and printing about the change when necessary – is provided by the Matcher.FileLine method.

Another common usage is deciding whether to make a change in a function based on the caller's stack, to identify the specific calling contexts that the change breaks. The Matcher.Stack method takes care of obtaining the stack, printing it when necessary, and reporting whether to enable the change based on that stack.

Pattern Syntax

Patterns are generated by the bisect tool and interpreted by New. Users should not have to understand the patterns except when debugging a target's bisect support or debugging the bisect tool itself.

The pattern syntax selecting a change is a sequence of bit strings separated by + and - operators. Each bit string denotes the set of changes with IDs ending in those bits, + is set addition, - is set subtraction, and the expression is evaluated in the usual left-to-right order. The special binary number “y” denotes the set of all changes, standing in for the empty bit string. In the expression, all the + operators must appear before all the - operators. A leading + adds to an empty set. A leading - subtracts from the set of all possible suffixes.

For example:

In the syntaxes described so far, the pattern specifies the changes to enable and report. If a pattern is prefixed by a “!”, the meaning changes: the pattern specifies the changes to DISABLE and report. This mode of operation is needed when a program passes with all changes enabled but fails with no changes enabled. In this case, bisect searches for minimal sets of changes to disable. Put another way, the leading “!” inverts the result from Matcher.ShouldEnable but does not invert the result from Matcher.ShouldPrint.

As a convenience for manual debugging, “n” is an alias for “!y”, meaning to disable and report all changes.

Finally, a leading “v” in the pattern indicates that the reports will be shown to the user of bisect to describe the changes involved in a failure. At the API level, the leading “v” causes [Matcher.Visible] to return true. See the next section for details.

Match Reports

The target program must enable only those changed matched by the pattern, and it must print a match report for each such change. A match report consists of one or more lines of text that will be printed by the bisect tool to describe a change implicated in causing a failure. Each line in the report for a given change must contain a match marker with that change ID, as returned by Marker. The markers are elided when displaying the lines to the user.

A match marker has the form “[bisect-match 0x1234]” where 0x1234 is the change ID in hexadecimal. An alternate form is “[bisect-match 010101]”, giving the change ID in binary.

When [Matcher.Visible] returns false, the match reports are only being processed by bisect to learn the set of enabled changes, not shown to the user, meaning that each report can be a match marker on a line by itself, eliding the usual textual description. When the textual description is expensive to compute, checking [Matcher.Visible] can help the avoid that expense in most runs.

Index

Functions

func AppendMarker

func AppendMarker(dst []byte, id uint64) []byte

AppendMarker is like Marker but appends the marker to dst.

func CutMarker

func CutMarker(line string) (short string, id uint64, ok bool)

CutMarker finds the first match marker in line and removes it, returning the shortened line (with the marker removed), the ID from the match marker, and whether a marker was found at all. If there is no marker, CutMarker returns line, 0, false.

func Hash

func Hash(data ...any) uint64

Hash computes a hash of the data arguments, each of which must be of type string, byte, int, uint, int32, uint32, int64, uint64, uintptr, or a slice of one of those types.

func Marker

func Marker(id uint64) string

Marker returns the match marker text to use on any line reporting details about a match of the given ID. It always returns the hexadecimal format.

func PrintMarker

func PrintMarker(w Writer, h uint64) error

PrintMarker prints to w a one-line report containing only the marker for h. It is appropriate to use when Matcher.ShouldPrint and Matcher.MarkerOnly both return true.

Types

type Matcher

type Matcher struct {
	// contains filtered or unexported fields
}

A Matcher is the parsed, compiled form of a PATTERN string. The nil *Matcher is valid: it has all changes enabled but none reported.

func New

func New(pattern string) (*Matcher, error)

New creates and returns a new Matcher implementing the given pattern. The pattern syntax is defined in the package doc comment.

In addition to the pattern syntax syntax, New("") returns nil, nil. The nil *Matcher is valid for use: it returns true from ShouldEnable and false from ShouldPrint for all changes. Callers can avoid calling Hash, Matcher.ShouldEnable, and Matcher.ShouldPrint entirely when they recognize the nil Matcher.

func (*Matcher) FileLine

func (m *Matcher) FileLine(w Writer, file string, line int) bool

FileLine reports whether the change identified by file and line should be enabled. If the change should be printed, FileLine prints a one-line report to w.

func (*Matcher) MarkerOnly

func (m *Matcher) MarkerOnly() bool

MarkerOnly reports whether it is okay to print only the marker for a given change, omitting the identifying information. MarkerOnly returns true when bisect is using the printed reports only for an intermediate search step, not for showing to users.

func (*Matcher) ShouldEnable

func (m *Matcher) ShouldEnable(id uint64) bool

ShouldEnable reports whether the change with the given id should be enabled.

func (*Matcher) ShouldPrint

func (m *Matcher) ShouldPrint(id uint64) bool

ShouldPrint reports whether to print identifying information about the change with the given id.

func (*Matcher) Stack

func (m *Matcher) Stack(w Writer) bool

MatchStack assigns the current call stack a change ID. If the stack should be printed, MatchStack prints it. Then MatchStack reports whether a change at the current call stack should be enabled.

type Writer

type Writer interface {
	Write([]byte) (int, error)
}

Writer is the same interface as io.Writer. It is duplicated here to avoid importing io.