internal/bisect/bisect.go

// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// 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.Verbose] 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.Verbose] 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.Verbose] can help the avoid that expense
// in most runs.
package bisect

// 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 ShouldReport for all changes. Callers can avoid calling
// [Hash], [Matcher.ShouldEnable], and [Matcher.ShouldPrint] entirely
// when they recognize the nil Matcher.
func New(pattern string) (*Matcher, error) {
	if pattern == "" {
		return nil, nil
	}

	m := new(Matcher)

	// Allow multiple v, so that “bisect cmd vPATTERN” can force verbose all the time.
	p := pattern
	for len(p) > 0 && p[0] == 'v' {
		m.verbose = true
		p = p[1:]
		if p == "" {
			return nil, &parseError{"invalid pattern syntax: " + pattern}
		}
	}

	// Allow multiple !, each negating the last, so that “bisect cmd !PATTERN” works
	// even when bisect chooses to add its own !.
	m.enable = true
	for len(p) > 0 && p[0] == '!' {
		m.enable = !m.enable
		p = p[1:]
		if p == "" {
			return nil, &parseError{"invalid pattern syntax: " + pattern}
		}
	}

	if p == "n" {
		// n is an alias for !y.
		m.enable = !m.enable
		p = "y"
	}

	// Parse actual pattern syntax.
	result := true
	bits := uint64(0)
	start := 0
	wid := 1 // 1-bit (binary); sometimes 4-bit (hex)
	for i := 0; i <= len(p); i++ {
		// Imagine a trailing - at the end of the pattern to flush final suffix
		c := byte('-')
		if i < len(p) {
			c = p[i]
		}
		if i == start && wid == 1 && c == 'x' { // leading x for hex
			start = i + 1
			wid = 4
			continue
		}
		switch c {
		default:
			return nil, &parseError{"invalid pattern syntax: " + pattern}
		case '2', '3', '4', '5', '6', '7', '8', '9':
			if wid != 4 {
				return nil, &parseError{"invalid pattern syntax: " + pattern}
			}
			fallthrough
		case '0', '1':
			bits <<= wid
			bits |= uint64(c - '0')
		case 'a', 'b', 'c', 'd', 'e', 'f', 'A', 'B', 'C', 'D', 'E', 'F':
			if wid != 4 {
				return nil, &parseError{"invalid pattern syntax: " + pattern}
			}
			bits <<= 4
			bits |= uint64(c&^0x20 - 'A' + 10)
		case 'y':
			if i+1 < len(p) && (p[i+1] == '0' || p[i+1] == '1') {
				return nil, &parseError{"invalid pattern syntax: " + pattern}
			}
			bits = 0
		case '+', '-':
			if c == '+' && result == false {
				// Have already seen a -. Should be - from here on.
				return nil, &parseError{"invalid pattern syntax (+ after -): " + pattern}
			}
			if i > 0 {
				n := (i - start) * wid
				if n > 64 {
					return nil, &parseError{"pattern bits too long: " + pattern}
				}
				if n <= 0 {
					return nil, &parseError{"invalid pattern syntax: " + pattern}
				}
				if p[start] == 'y' {
					n = 0
				}
				mask := uint64(1)<<n - 1
				m.list = append(m.list, cond{mask, bits, result})
			} else if c == '-' {
				// leading - subtracts from complete set
				m.list = append(m.list, cond{0, 0, true})
			}
			bits = 0
			result = c == '+'
			start = i + 1
			wid = 1
		}
	}
	return m, nil
}

// A Matcher is the parsed, compiled form of a PATTERN string.
// The nil *Matcher is valid: it has all changes enabled but none reported.
type Matcher struct {
	verbose bool
	enable  bool   // when true, list is for “enable and report” (when false, “disable and report”)
	list    []cond // conditions; later ones win over earlier ones
}

// A cond is a single condition in the matcher.
// Given an input id, if id&mask == bits, return the result.
type cond struct {
	mask   uint64
	bits   uint64
	result bool
}

// Verbose reports whether the reports will be shown to users
// and need to include a human-readable change description.
// If not, the target can print just the Marker on a line by itself
// and perhaps save some computation.
func (m *Matcher) Verbose() bool {
	return m.verbose
}

// ShouldEnable reports whether the change with the given id should be enabled.
func (m *Matcher) ShouldEnable(id uint64) bool {
	if m == nil {
		return true
	}
	// Don't use slices.Backward here (no imports).
	for i := len(m.list) - 1; i >= 0; i-- {
		c := &m.list[i]
		if id&c.mask == c.bits {
			return c.result == m.enable
		}
	}
	return false == m.enable
}

// ShouldReport reports whether the change with the given id should be reported.
func (m *Matcher) ShouldReport(id uint64) bool {
	if m == nil {
		return false
	}
	// Don't use slices.Backward here (no imports).
	for i := len(m.list) - 1; i >= 0; i-- {
		c := &m.list[i]
		if id&c.mask == c.bits {
			return c.result
		}
	}
	return false
}

// 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 Marker(id uint64) string {
	return string(AppendMarker(nil, id))
}

// AppendMarker is like [Marker] but appends the marker to dst.
func AppendMarker(dst []byte, id uint64) []byte {
	const prefix = "[bisect-match 0x"
	var buf [len(prefix) + 16 + 1]byte
	copy(buf[:], prefix)
	for i := range 16 {
		buf[len(prefix)+i] = "0123456789abcdef"[id>>60]
		id <<= 4
	}
	buf[len(prefix)+16] = ']'
	return append(dst, buf[:]...)
}

// 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 CutMarker(line string) (short string, id uint64, ok bool) {
	// Find first instance of prefix.
	prefix := "[bisect-match "
	i := 0
	for ; ; i++ {
		if i >= len(line)-len(prefix) {
			return line, 0, false
		}
		if line[i] == '[' && line[i:i+len(prefix)] == prefix {
			break
		}
	}

	// Scan to ].
	j := i + len(prefix)
	for j < len(line) && line[j] != ']' {
		j++
	}
	if j >= len(line) {
		return line, 0, false
	}

	// Parse id.
	idstr := line[i+len(prefix) : j]
	if len(idstr) >= 3 && idstr[:2] == "0x" {
		// parse hex
		if len(idstr) > 2+16 { // max 0x + 16 digits
			return line, 0, false
		}
		for i := 2; i < len(idstr); i++ {
			id <<= 4
			switch c := idstr[i]; {
			case '0' <= c && c <= '9':
				id |= uint64(c - '0')
			case 'a' <= c && c <= 'f':
				id |= uint64(c - 'a' + 10)
			case 'A' <= c && c <= 'F':
				id |= uint64(c - 'A' + 10)
			}
		}
	} else {
		if idstr == "" || len(idstr) > 64 { // min 1 digit, max 64 digits
			return line, 0, false
		}
		// parse binary
		for i := 0; i < len(idstr); i++ {
			id <<= 1
			switch c := idstr[i]; c {
			default:
				return line, 0, false
			case '0', '1':
				id |= uint64(c - '0')
			}
		}
	}

	// Construct shortened line.
	// Remove at most one space from around the marker,
	// so that "foo [marker] bar" shortens to "foo bar".
	j++ // skip ]
	if i > 0 && line[i-1] == ' ' {
		i--
	} else if j < len(line) && line[j] == ' ' {
		j++
	}
	short = line[:i] + line[j:]
	return short, id, true
}

// 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 Hash(data ...any) uint64 {
	h := offset64
	for _, v := range data {
		switch v := v.(type) {
		default:
			// Note: Not printing the type, because reflect.ValueOf(v)
			// would make the interfaces prepared by the caller escape
			// and therefore allocate. This way, Hash(file, line) runs
			// without any allocation. It should be clear from the
			// source code calling Hash what the bad argument was.
			panic("bisect.Hash: unexpected argument type")
		case string:
			h = fnvString(h, v)
		case byte:
			h = fnv(h, v)
		case int:
			h = fnvUint64(h, uint64(v))
		case uint:
			h = fnvUint64(h, uint64(v))
		case int32:
			h = fnvUint32(h, uint32(v))
		case uint32:
			h = fnvUint32(h, v)
		case int64:
			h = fnvUint64(h, uint64(v))
		case uint64:
			h = fnvUint64(h, v)
		case uintptr:
			h = fnvUint64(h, uint64(v))
		case []string:
			for _, x := range v {
				h = fnvString(h, x)
			}
		case []byte:
			for _, x := range v {
				h = fnv(h, x)
			}
		case []int:
			for _, x := range v {
				h = fnvUint64(h, uint64(x))
			}
		case []uint:
			for _, x := range v {
				h = fnvUint64(h, uint64(x))
			}
		case []int32:
			for _, x := range v {
				h = fnvUint32(h, uint32(x))
			}
		case []uint32:
			for _, x := range v {
				h = fnvUint32(h, x)
			}
		case []int64:
			for _, x := range v {
				h = fnvUint64(h, uint64(x))
			}
		case []uint64:
			for _, x := range v {
				h = fnvUint64(h, x)
			}
		case []uintptr:
			for _, x := range v {
				h = fnvUint64(h, uint64(x))
			}
		}
	}
	return h
}

// Trivial error implementation, here to avoid importing errors.

type parseError struct{ text string }

func (e *parseError) Error() string { return e.text }

// FNV-1a implementation. See Go's hash/fnv/fnv.go.
// Copied here for simplicity (can handle uints directly)
// and to avoid the dependency.

const (
	offset64 uint64 = 14695981039346656037
	prime64  uint64 = 1099511628211
)

func fnv(h uint64, x byte) uint64 {
	h ^= uint64(x)
	h *= prime64
	return h
}

func fnvString(h uint64, x string) uint64 {
	for i := 0; i < len(x); i++ {
		h ^= uint64(x[i])
		h *= prime64
	}
	return h
}

func fnvUint64(h uint64, x uint64) uint64 {
	for range 8 {
		h ^= uint64(x & 0xFF)
		x >>= 8
		h *= prime64
	}
	return h
}

func fnvUint32(h uint64, x uint32) uint64 {
	for range 4 {
		h ^= uint64(x & 0xFF)
		x >>= 8
		h *= prime64
	}
	return h
}