Overview¶

Package bisect can be used by compilers and other programsto serve as a target for the bisect debugging tool.Seegolang.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 independentchanges 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 toNew to create aMatcher, 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 informationlike 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.TheMatcher.ShouldEnable method answers this question for a given change ID.

4. Print a report identifying each change that the pattern says should be printed.TheMatcher.ShouldPrint method answers this question for a given change ID.The report consists of one more lines on standard error or standard outputthat 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 changeby 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 computesa change ID by hashing identifying information (source file and line, in this case)and then calls m.ShouldPrint and m.ShouldEnable to decide whether toprint and enable the change, respectively. The two can return different valuesdepending on whether bisect is trying to find a minimal set of changes todisable or to enable to provoke the failure.

It is usually helpful to write a helper function that accepts the identifying informationand then takes care of hashing, printing, and reporting whether the identified changeshould be enabled. For example, a helper for changes identified by a file and line numberwould 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 entirelyby 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 callMatcher.MarkerOnly to find out whether short report lines containing only themarker are permitted for a given run. (Bisect permits such lines when it isstill exploring the space of possible changes and will not be showing theoutput 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 byfile and line number and printing about the change when necessary – isprovided by theMatcher.FileLine method.

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

Pattern Syntax¶

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

The pattern syntax selecting a change is a sequence of bit stringsseparated by + and - operators. Each bit string denotes the set ofchanges 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 allpossible suffixes.

For example:

• “01+10” and “+01+10” both denote the set of changeswith IDs ending with the bits 01 or 10.

• “01+10-1001” denotes the set of changes with IDsending with the bits 01 or 10, but excluding those ending in 1001.

• “-01-1000” and “y-01-1000 both denote the set of all changeswith IDs not ending in 01 nor 1000.

• “0+1-01+001” is not a valid pattern, because all the + operators do notappear before all the - operators.

In the syntaxes described so far, the pattern specifies the changes toenable and report. If a pattern is prefixed by a “!”, the meaningchanges: the pattern specifies the changes to DISABLE and report. Thismode of operation is needed when a program passes with all changesenabled but fails with no changes enabled. In this case, bisectsearches for minimal sets of changes to disable.Put another way, the leading “!” inverts the result fromMatcher.ShouldEnablebut does not invert the result fromMatcher.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 shownto 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 matchedby 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 beprinted by the bisect tool to describe a change implicated in causinga failure. Each line in the report for a given change must contain amatch marker with that change ID, as returned byMarker.The markers are elided when displaying the lines to the user.

A match marker has the form “[bisect-match 0x1234]” where0x1234 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 onlybeing processed by bisect to learn the set of enabled changes,not shown to the user, meaning that each report can be a matchmarker 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 expensein most runs.

Index¶

• func AppendMarker(dst []byte, id uint64) []byte
• func CutMarker(line string) (short string, id uint64, ok bool)
• func Hash(data ...any) uint64
• func Marker(id uint64) string
• func PrintMarker(w Writer, h uint64) error
• type Matcher
• • func New(pattern string) (*Matcher, error)
• • func (m *Matcher) FileLine(w Writer, file string, line int) bool
  • func (m *Matcher) MarkerOnly() bool
  • func (m *Matcher) ShouldEnable(id uint64) bool
  • func (m *Matcher) ShouldPrint(id uint64) bool
  • func (m *Matcher) Stack(w Writer) bool
• type Writer

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