mirror of https://go.googlesource.com/go
779 lines
23 KiB
Go
779 lines
23 KiB
Go
// Copyright 2023 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package bisect can be used by compilers and other programs
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// to serve as a target for the bisect debugging tool.
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// See [golang.org/x/tools/cmd/bisect] for details about using the tool.
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//
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// To be a bisect target, allowing bisect to help determine which of a set of independent
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// changes provokes a failure, a program needs to:
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//
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// 1. Define a way to accept a change pattern on its command line or in its environment.
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// The most common mechanism is a command-line flag.
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// The pattern can be passed to [New] to create a [Matcher], the compiled form of a pattern.
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//
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// 2. Assign each change a unique ID. One possibility is to use a sequence number,
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// but the most common mechanism is to hash some kind of identifying information
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// like the file and line number where the change might be applied.
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// [Hash] hashes its arguments to compute an ID.
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//
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// 3. Enable each change that the pattern says should be enabled.
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// The [Matcher.ShouldEnable] method answers this question for a given change ID.
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//
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// 4. Print a report identifying each change that the pattern says should be printed.
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// The [Matcher.ShouldPrint] method answers this question for a given change ID.
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// The report consists of one more lines on standard error or standard output
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// that contain a “match marker”. [Marker] returns the match marker for a given ID.
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// When bisect reports a change as causing the failure, it identifies the change
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// by printing the report lines with the match marker removed.
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//
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// # Example Usage
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//
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// A program starts by defining how it receives the pattern. In this example, we will assume a flag.
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// The next step is to compile the pattern:
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//
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// m, err := bisect.New(patternFlag)
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// if err != nil {
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// log.Fatal(err)
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// }
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//
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// Then, each time a potential change is considered, the program computes
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// a change ID by hashing identifying information (source file and line, in this case)
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// and then calls m.ShouldPrint and m.ShouldEnable to decide whether to
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// print and enable the change, respectively. The two can return different values
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// depending on whether bisect is trying to find a minimal set of changes to
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// disable or to enable to provoke the failure.
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//
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// It is usually helpful to write a helper function that accepts the identifying information
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// and then takes care of hashing, printing, and reporting whether the identified change
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// should be enabled. For example, a helper for changes identified by a file and line number
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// would be:
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//
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// func ShouldEnable(file string, line int) {
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// h := bisect.Hash(file, line)
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// if m.ShouldPrint(h) {
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// fmt.Fprintf(os.Stderr, "%v %s:%d\n", bisect.Marker(h), file, line)
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// }
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// return m.ShouldEnable(h)
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// }
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//
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// Finally, note that New returns a nil Matcher when there is no pattern,
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// meaning that the target is not running under bisect at all,
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// so all changes should be enabled and none should be printed.
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// In that common case, the computation of the hash can be avoided entirely
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// by checking for m == nil first:
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//
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// func ShouldEnable(file string, line int) bool {
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// if m == nil {
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// return true
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// }
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// h := bisect.Hash(file, line)
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// if m.ShouldPrint(h) {
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// fmt.Fprintf(os.Stderr, "%v %s:%d\n", bisect.Marker(h), file, line)
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// }
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// return m.ShouldEnable(h)
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// }
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//
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// When the identifying information is expensive to format, this code can call
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// [Matcher.MarkerOnly] to find out whether short report lines containing only the
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// marker are permitted for a given run. (Bisect permits such lines when it is
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// still exploring the space of possible changes and will not be showing the
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// output to the user.) If so, the client can choose to print only the marker:
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//
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// func ShouldEnable(file string, line int) bool {
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// if m == nil {
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// return true
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// }
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// h := bisect.Hash(file, line)
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// if m.ShouldPrint(h) {
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// if m.MarkerOnly() {
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// bisect.PrintMarker(os.Stderr, h)
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// } else {
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// fmt.Fprintf(os.Stderr, "%v %s:%d\n", bisect.Marker(h), file, line)
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// }
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// }
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// return m.ShouldEnable(h)
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// }
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//
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// This specific helper – deciding whether to enable a change identified by
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// file and line number and printing about the change when necessary – is
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// provided by the [Matcher.FileLine] method.
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//
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// Another common usage is deciding whether to make a change in a function
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// based on the caller's stack, to identify the specific calling contexts that the
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// change breaks. The [Matcher.Stack] method takes care of obtaining the stack,
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// printing it when necessary, and reporting whether to enable the change
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// based on that stack.
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//
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// # Pattern Syntax
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//
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// Patterns are generated by the bisect tool and interpreted by [New].
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// Users should not have to understand the patterns except when
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// debugging a target's bisect support or debugging the bisect tool itself.
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//
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// The pattern syntax selecting a change is a sequence of bit strings
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// separated by + and - operators. Each bit string denotes the set of
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// changes with IDs ending in those bits, + is set addition, - is set subtraction,
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// and the expression is evaluated in the usual left-to-right order.
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// The special binary number “y” denotes the set of all changes,
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// standing in for the empty bit string.
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// In the expression, all the + operators must appear before all the - operators.
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// A leading + adds to an empty set. A leading - subtracts from the set of all
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// possible suffixes.
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//
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// For example:
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//
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// - “01+10” and “+01+10” both denote the set of changes
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// with IDs ending with the bits 01 or 10.
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//
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// - “01+10-1001” denotes the set of changes with IDs
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// ending with the bits 01 or 10, but excluding those ending in 1001.
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//
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// - “-01-1000” and “y-01-1000 both denote the set of all changes
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// with IDs not ending in 01 nor 1000.
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//
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// - “0+1-01+001” is not a valid pattern, because all the + operators do not
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// appear before all the - operators.
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//
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// In the syntaxes described so far, the pattern specifies the changes to
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// enable and report. If a pattern is prefixed by a “!”, the meaning
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// changes: the pattern specifies the changes to DISABLE and report. This
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// mode of operation is needed when a program passes with all changes
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// enabled but fails with no changes enabled. In this case, bisect
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// searches for minimal sets of changes to disable.
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// Put another way, the leading “!” inverts the result from [Matcher.ShouldEnable]
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// but does not invert the result from [Matcher.ShouldPrint].
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//
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// As a convenience for manual debugging, “n” is an alias for “!y”,
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// meaning to disable and report all changes.
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//
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// Finally, a leading “v” in the pattern indicates that the reports will be shown
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// to the user of bisect to describe the changes involved in a failure.
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// At the API level, the leading “v” causes [Matcher.Visible] to return true.
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// See the next section for details.
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//
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// # Match Reports
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//
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// The target program must enable only those changed matched
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// by the pattern, and it must print a match report for each such change.
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// A match report consists of one or more lines of text that will be
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// printed by the bisect tool to describe a change implicated in causing
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// a failure. Each line in the report for a given change must contain a
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// match marker with that change ID, as returned by [Marker].
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// The markers are elided when displaying the lines to the user.
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//
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// A match marker has the form “[bisect-match 0x1234]” where
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// 0x1234 is the change ID in hexadecimal.
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// An alternate form is “[bisect-match 010101]”, giving the change ID in binary.
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//
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// When [Matcher.Visible] returns false, the match reports are only
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// being processed by bisect to learn the set of enabled changes,
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// not shown to the user, meaning that each report can be a match
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// marker on a line by itself, eliding the usual textual description.
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// When the textual description is expensive to compute,
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// checking [Matcher.Visible] can help the avoid that expense
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// in most runs.
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package bisect
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import (
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"runtime"
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"sync"
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"sync/atomic"
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)
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// New creates and returns a new Matcher implementing the given pattern.
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// The pattern syntax is defined in the package doc comment.
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//
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// In addition to the pattern syntax syntax, New("") returns nil, nil.
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// The nil *Matcher is valid for use: it returns true from ShouldEnable
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// and false from ShouldPrint for all changes. Callers can avoid calling
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// [Hash], [Matcher.ShouldEnable], and [Matcher.ShouldPrint] entirely
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// when they recognize the nil Matcher.
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func New(pattern string) (*Matcher, error) {
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if pattern == "" {
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return nil, nil
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}
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m := new(Matcher)
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p := pattern
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// Special case for leading 'q' so that 'qn' quietly disables, e.g. fmahash=qn to disable fma
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// Any instance of 'v' disables 'q'.
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if len(p) > 0 && p[0] == 'q' {
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m.quiet = true
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p = p[1:]
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if p == "" {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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}
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// Allow multiple v, so that “bisect cmd vPATTERN” can force verbose all the time.
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for len(p) > 0 && p[0] == 'v' {
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m.verbose = true
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m.quiet = false
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p = p[1:]
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if p == "" {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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}
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// Allow multiple !, each negating the last, so that “bisect cmd !PATTERN” works
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// even when bisect chooses to add its own !.
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m.enable = true
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for len(p) > 0 && p[0] == '!' {
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m.enable = !m.enable
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p = p[1:]
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if p == "" {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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}
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if p == "n" {
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// n is an alias for !y.
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m.enable = !m.enable
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p = "y"
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}
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// Parse actual pattern syntax.
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result := true
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bits := uint64(0)
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start := 0
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wid := 1 // 1-bit (binary); sometimes 4-bit (hex)
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for i := 0; i <= len(p); i++ {
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// Imagine a trailing - at the end of the pattern to flush final suffix
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c := byte('-')
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if i < len(p) {
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c = p[i]
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}
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if i == start && wid == 1 && c == 'x' { // leading x for hex
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start = i + 1
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wid = 4
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continue
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}
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switch c {
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default:
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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case '2', '3', '4', '5', '6', '7', '8', '9':
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if wid != 4 {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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fallthrough
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case '0', '1':
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bits <<= wid
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bits |= uint64(c - '0')
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case 'a', 'b', 'c', 'd', 'e', 'f', 'A', 'B', 'C', 'D', 'E', 'F':
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if wid != 4 {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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bits <<= 4
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bits |= uint64(c&^0x20 - 'A' + 10)
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case 'y':
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if i+1 < len(p) && (p[i+1] == '0' || p[i+1] == '1') {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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bits = 0
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case '+', '-':
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if c == '+' && result == false {
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// Have already seen a -. Should be - from here on.
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return nil, &parseError{"invalid pattern syntax (+ after -): " + pattern}
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}
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if i > 0 {
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n := (i - start) * wid
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if n > 64 {
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return nil, &parseError{"pattern bits too long: " + pattern}
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}
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if n <= 0 {
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return nil, &parseError{"invalid pattern syntax: " + pattern}
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}
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if p[start] == 'y' {
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n = 0
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}
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mask := uint64(1)<<n - 1
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m.list = append(m.list, cond{mask, bits, result})
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} else if c == '-' {
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// leading - subtracts from complete set
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m.list = append(m.list, cond{0, 0, true})
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}
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bits = 0
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result = c == '+'
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start = i + 1
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wid = 1
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}
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}
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return m, nil
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}
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// A Matcher is the parsed, compiled form of a PATTERN string.
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// The nil *Matcher is valid: it has all changes enabled but none reported.
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type Matcher struct {
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verbose bool // annotate reporting with human-helpful information
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quiet bool // disables all reporting. reset if verbose is true. use case is -d=fmahash=qn
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enable bool // when true, list is for “enable and report” (when false, “disable and report”)
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list []cond // conditions; later ones win over earlier ones
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dedup atomic.Pointer[dedup]
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}
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// A cond is a single condition in the matcher.
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// Given an input id, if id&mask == bits, return the result.
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type cond struct {
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mask uint64
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bits uint64
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result bool
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}
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// MarkerOnly reports whether it is okay to print only the marker for
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// a given change, omitting the identifying information.
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// MarkerOnly returns true when bisect is using the printed reports
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// only for an intermediate search step, not for showing to users.
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func (m *Matcher) MarkerOnly() bool {
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return !m.verbose
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}
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// ShouldEnable reports whether the change with the given id should be enabled.
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func (m *Matcher) ShouldEnable(id uint64) bool {
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if m == nil {
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return true
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}
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return m.matchResult(id) == m.enable
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}
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// ShouldPrint reports whether to print identifying information about the change with the given id.
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func (m *Matcher) ShouldPrint(id uint64) bool {
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if m == nil || m.quiet {
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return false
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}
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return m.matchResult(id)
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}
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// matchResult returns the result from the first condition that matches id.
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func (m *Matcher) matchResult(id uint64) bool {
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for i := len(m.list) - 1; i >= 0; i-- {
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c := &m.list[i]
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if id&c.mask == c.bits {
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return c.result
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}
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}
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return false
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}
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// FileLine reports whether the change identified by file and line should be enabled.
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// If the change should be printed, FileLine prints a one-line report to w.
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func (m *Matcher) FileLine(w Writer, file string, line int) bool {
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if m == nil {
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return true
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}
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return m.fileLine(w, file, line)
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}
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// fileLine does the real work for FileLine.
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// This lets FileLine's body handle m == nil and potentially be inlined.
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func (m *Matcher) fileLine(w Writer, file string, line int) bool {
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h := Hash(file, line)
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if m.ShouldPrint(h) {
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if m.MarkerOnly() {
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PrintMarker(w, h)
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} else {
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printFileLine(w, h, file, line)
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}
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}
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return m.ShouldEnable(h)
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}
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// printFileLine prints a non-marker-only report for file:line to w.
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func printFileLine(w Writer, h uint64, file string, line int) error {
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const markerLen = 40 // overestimate
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b := make([]byte, 0, markerLen+len(file)+24)
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b = AppendMarker(b, h)
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b = appendFileLine(b, file, line)
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b = append(b, '\n')
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_, err := w.Write(b)
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return err
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}
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// appendFileLine appends file:line to dst, returning the extended slice.
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func appendFileLine(dst []byte, file string, line int) []byte {
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dst = append(dst, file...)
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dst = append(dst, ':')
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u := uint(line)
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if line < 0 {
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dst = append(dst, '-')
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u = -u
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}
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var buf [24]byte
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i := len(buf)
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for i == len(buf) || u > 0 {
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i--
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buf[i] = '0' + byte(u%10)
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u /= 10
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}
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dst = append(dst, buf[i:]...)
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return dst
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}
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// MatchStack assigns the current call stack a change ID.
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// If the stack should be printed, MatchStack prints it.
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// Then MatchStack reports whether a change at the current call stack should be enabled.
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func (m *Matcher) Stack(w Writer) bool {
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if m == nil {
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return true
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}
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return m.stack(w)
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}
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// stack does the real work for Stack.
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// This lets stack's body handle m == nil and potentially be inlined.
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func (m *Matcher) stack(w Writer) bool {
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const maxStack = 16
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var stk [maxStack]uintptr
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n := runtime.Callers(2, stk[:])
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// caller #2 is not for printing; need it to normalize PCs if ASLR.
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if n <= 1 {
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return false
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}
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base := stk[0]
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// normalize PCs
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for i := range stk[:n] {
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stk[i] -= base
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}
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h := Hash(stk[:n])
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if m.ShouldPrint(h) {
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var d *dedup
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for {
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d = m.dedup.Load()
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if d != nil {
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break
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}
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d = new(dedup)
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if m.dedup.CompareAndSwap(nil, d) {
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break
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}
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}
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if m.MarkerOnly() {
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if !d.seenLossy(h) {
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PrintMarker(w, h)
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}
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} else {
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if !d.seen(h) {
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// Restore PCs in stack for printing
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for i := range stk[:n] {
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stk[i] += base
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}
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printStack(w, h, stk[1:n])
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}
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}
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}
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return m.ShouldEnable(h)
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}
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// Writer is the same interface as io.Writer.
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// It is duplicated here to avoid importing io.
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type Writer interface {
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Write([]byte) (int, error)
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}
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// PrintMarker prints to w a one-line report containing only the marker for h.
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// It is appropriate to use when [Matcher.ShouldPrint] and [Matcher.MarkerOnly] both return true.
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func PrintMarker(w Writer, h uint64) error {
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var buf [50]byte
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b := AppendMarker(buf[:0], h)
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b = append(b, '\n')
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_, err := w.Write(b)
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return err
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}
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// printStack prints to w a multi-line report containing a formatting of the call stack stk,
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// with each line preceded by the marker for h.
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func printStack(w Writer, h uint64, stk []uintptr) error {
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buf := make([]byte, 0, 2048)
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|
||
var prefixBuf [100]byte
|
||
prefix := AppendMarker(prefixBuf[:0], h)
|
||
|
||
frames := runtime.CallersFrames(stk)
|
||
for {
|
||
f, more := frames.Next()
|
||
buf = append(buf, prefix...)
|
||
buf = append(buf, f.Function...)
|
||
buf = append(buf, "()\n"...)
|
||
buf = append(buf, prefix...)
|
||
buf = append(buf, '\t')
|
||
buf = appendFileLine(buf, f.File, f.Line)
|
||
buf = append(buf, '\n')
|
||
if !more {
|
||
break
|
||
}
|
||
}
|
||
buf = append(buf, prefix...)
|
||
buf = append(buf, '\n')
|
||
_, err := w.Write(buf)
|
||
return err
|
||
}
|
||
|
||
// 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 := 0; i < 16; i++ {
|
||
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.
|
||
|
||
// parseError is a trivial error implementation,
|
||
// defined 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 integers more directly)
|
||
// and to avoid importing hash/fnv.
|
||
|
||
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 i := 0; i < 8; i++ {
|
||
h ^= x & 0xFF
|
||
x >>= 8
|
||
h *= prime64
|
||
}
|
||
return h
|
||
}
|
||
|
||
func fnvUint32(h uint64, x uint32) uint64 {
|
||
for i := 0; i < 4; i++ {
|
||
h ^= uint64(x & 0xFF)
|
||
x >>= 8
|
||
h *= prime64
|
||
}
|
||
return h
|
||
}
|
||
|
||
// A dedup is a deduplicator for call stacks, so that we only print
|
||
// a report for new call stacks, not for call stacks we've already
|
||
// reported.
|
||
//
|
||
// It has two modes: an approximate but lock-free mode that
|
||
// may still emit some duplicates, and a precise mode that uses
|
||
// a lock and never emits duplicates.
|
||
type dedup struct {
|
||
// 128-entry 4-way, lossy cache for seenLossy
|
||
recent [128][4]uint64
|
||
|
||
// complete history for seen
|
||
mu sync.Mutex
|
||
m map[uint64]bool
|
||
}
|
||
|
||
// seen records that h has now been seen and reports whether it was seen before.
|
||
// When seen returns false, the caller is expected to print a report for h.
|
||
func (d *dedup) seen(h uint64) bool {
|
||
d.mu.Lock()
|
||
if d.m == nil {
|
||
d.m = make(map[uint64]bool)
|
||
}
|
||
seen := d.m[h]
|
||
d.m[h] = true
|
||
d.mu.Unlock()
|
||
return seen
|
||
}
|
||
|
||
// seenLossy is a variant of seen that avoids a lock by using a cache of recently seen hashes.
|
||
// Each cache entry is N-way set-associative: h can appear in any of the slots.
|
||
// If h does not appear in any of them, then it is inserted into a random slot,
|
||
// overwriting whatever was there before.
|
||
func (d *dedup) seenLossy(h uint64) bool {
|
||
cache := &d.recent[uint(h)%uint(len(d.recent))]
|
||
for i := 0; i < len(cache); i++ {
|
||
if atomic.LoadUint64(&cache[i]) == h {
|
||
return true
|
||
}
|
||
}
|
||
|
||
// Compute index in set to evict as hash of current set.
|
||
ch := offset64
|
||
for _, x := range cache {
|
||
ch = fnvUint64(ch, x)
|
||
}
|
||
atomic.StoreUint64(&cache[uint(ch)%uint(len(cache))], h)
|
||
return false
|
||
}
|