Package bisect

Overview ▾

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.Enable] method answers this question for a given change ID.

4. Report each change that the pattern says should be reported. The [Matcher.Report] 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 those 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.ShouldEnable and m.ShouldReport to decide whether to enable and report the change, respectively:

for each change {
	h := bisect.Hash(file, line)
	if m.ShouldEnable(h) {
		enableChange()
	}
	if m.ShouldReport(h) {
		log.Printf("%v %s:%d", bisect.Marker(h), file, line)
	}
}

Note that the two return different values when bisect is searching for a minimal set of changes to disable to provoke a failure.

Finally, note that New returns a nil Matcher when there is no pattern, meaning that the target is not running under bisect at all. In that common case, the computation of the hash can be avoided entirely by checking for m == nil first:

for each change {
	if m == nil {
		enableChange()
	} else {
		h := bisect.Hash(file, line)
		if m.ShouldEnable(h) {
			enableChange()
		}
		if m.ShouldReport(h) {
			log.Printf("%v %s:%d", bisect.Marker(h), file, line)
		}
	}
}

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:

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

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

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

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

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.ShouldReport.

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 ▾

Package files

bisect.go