// Copyright 2025 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.

//go:build linux || darwin

// The stacks command finds all gopls stack traces reported by
// telemetry in the past 7 days, and reports their associated GitHub
// issue, creating new issues as needed.
//
// The association of stacks with GitHub issues (labelled
// gopls/telemetry-wins) is represented in two different ways by the
// body (first comment) of the issue:
//
//  1. Each distinct stack is identified by an ID, 6-digit base64
//     string such as "TwtkSg". If a stack's ID appears anywhere
//     within the issue body, the stack is associated with the issue.
//
//     Some problems are highly deterministic, resulting in many
//     field reports of the exact same stack. For such problems, a
//     single ID in the issue body suffices to record the
//     association. But most problems are exhibited in a variety of
//     ways, leading to multiple field reports of similar but
//     distinct stacks. Hence the following way to associate stacks
//     with issues.
//
//  2. Each GitHub issue body may start with a code block of this form:
//
//     ```
//     #!stacks
//     "runtime.sigpanic" && "golang.hover:+170"
//     ```
//
//     The first line indicates the purpose of the block; the
//     remainder is a predicate that matches stacks.
//     It is an expression defined by this grammar:
//
//     >  expr = "string literal"
//     >       | ( expr )
//     >       | ! expr
//     >       | expr && expr
//     >       | expr || expr
//
//     Each string literal must match complete words in the text.
//     The predicate is evaluated separately for each configuration,
//     against text consisting of the stack followed by one line of the form
//
//     GOTOOLCHAIN=go1.23.0 GOOS=darwin GOARCH=arm64 golang.org/x/tools/gopls@v0.22.0
//
//     As an example of literal matching, "fu+12" matches "x:fu+12 "
//     but not "fu:123" or "snafu+12"; it is not a regular expression.
//     The other productions are boolean operations.
//
//     The stacks command gathers all such predicates out of the
//     labelled issues and evaluates each one against each new stack.
//     If the predicate for an issue matches, the issue is considered
//     to have "claimed" the stack: the stack command appends a
//     comment containing the new (variant) stack to the issue, and
//     appends the stack's ID to the last line of the issue body.
//
//     It is an error if two issues' predicates attempt to claim the
//     same stack.
package main

import (
	"bytes"
	"context"
	"encoding/base64"
	"encoding/json"
	"flag"
	"fmt"
	"go/ast"
	"go/parser"
	"go/token"
	"hash/fnv"
	"io"
	"log"
	"net/http"
	"net/url"
	"os"
	"os/exec"
	"path"
	"path/filepath"
	"regexp"
	"runtime"
	"slices"
	"sort"
	"strconv"
	"strings"
	"time"
	"unicode"

	"golang.org/x/mod/semver"
	"golang.org/x/sys/unix"
	"golang.org/x/telemetry"
	"golang.org/x/telemetry/internal/browser"
)

// flags
var (
	programFlag = flag.String("program", "golang.org/x/tools/gopls", "Package path of program to process")

	daysFlag = flag.Int("days", 7, "number of previous days of telemetry data to read")

	dryRun = flag.Bool("n", false, "dry run, avoid updating issues")

	verbose = flag.Bool("v", false, "verbose")
)

// ProgramConfig is the configuration for processing reports for a specific
// program.
type ProgramConfig struct {
	// Program is the package path of the program to process.
	Program string

	// IncludeClient indicates that stack Info should include gopls/client metadata.
	IncludeClient bool

	// SearchLabel is the GitHub label used to find all existing reports.
	SearchLabel string

	// NewIssuePrefix is the package prefix to apply to new issue titles.
	NewIssuePrefix string

	// NewIssueLabels are the labels to apply to new issues.
	NewIssueLabels []string

	// MatchSymbolPrefix is the prefix of "interesting" symbol names.
	//
	// A given stack will be "blamed" on the deepest symbol in the stack that:
	// 1. Matches MatchSymbolPrefix
	// 2. Is an exported function or any method on an exported Type.
	// 3. Does _not_ match IgnoreSymbolContains.
	MatchSymbolPrefix string

	// IgnoreSymbolContains are "uninteresting" symbol substrings. e.g.,
	// logging packages.
	IgnoreSymbolContains []string

	// Repository is the repository where the issues should be created, for example: "golang/go"
	Repository string
}

var programs = map[string]ProgramConfig{
	"golang.org/x/tools/gopls": {
		Program:        "golang.org/x/tools/gopls",
		IncludeClient:  true,
		SearchLabel:    "gopls/telemetry-wins",
		NewIssuePrefix: "x/tools/gopls",
		NewIssueLabels: []string{
			"gopls",
			"Tools",
			"gopls/telemetry-wins",
			"NeedsInvestigation",
		},
		MatchSymbolPrefix: "golang.org/x/tools/gopls/",
		IgnoreSymbolContains: []string{
			"internal/util/bug.",
			"internal/bug.", // former name in gopls/0.14.2
		},
		Repository: "golang/go",
	},
	"cmd/compile": {
		Program:        "cmd/compile",
		SearchLabel:    "compiler/telemetry-wins",
		NewIssuePrefix: "cmd/compile",
		NewIssueLabels: []string{
			"compiler/runtime",
			"compiler/telemetry-wins",
			"NeedsInvestigation",
		},
		MatchSymbolPrefix: "cmd/compile",
		IgnoreSymbolContains: []string{
			// Various "fatal" wrappers.
			"Fatal", // base.Fatal*, ssa.Value.Fatal*, etc.
			"cmd/compile/internal/base.Assert",
			"cmd/compile/internal/noder.assert",
			"cmd/compile/internal/ssa.Compile.func1", // basically a Fatalf wrapper.
			// Panic recovery.
			"cmd/compile/internal/types2.(*Checker).handleBailout",
			"cmd/compile/internal/gc.handlePanic",
		},
		Repository: "golang/go",
	},
	"github.com/go-delve/delve/cmd/dlv": {
		Program:        "github.com/go-delve/delve/cmd/dlv",
		IncludeClient:  false,
		SearchLabel:    "delve/telemetry-wins",
		NewIssuePrefix: "telemetry report",
		NewIssueLabels: []string{
			"delve/telemetry-wins",
		},
		MatchSymbolPrefix: "github.com/go-delve/delve",
		IgnoreSymbolContains: []string{
			"service/dap.(*Session).recoverPanic",
			"rpccommon.newInternalError",
			"rpccommon.(*ServerImpl).serveJSONCodec",
		},
		Repository: "go-delve/delve",
	},
}

func main() {
	log.SetFlags(0)
	log.SetPrefix("stacks: ")
	flag.Parse()

	var ghclient *githubClient

	// Read GitHub authentication token from $HOME/.stacks.token.
	//
	// You can create one using the flow at: GitHub > You > Settings >
	// Developer Settings > Personal Access Tokens > Fine-grained tokens >
	// Generate New Token.  Generate the token on behalf of golang/go
	// with R/W access to "Issues".
	// The token is typically of the form "github_pat_XXX", with 82 hex digits.
	// Save it in the file, with mode 0400.
	//
	// For security, secret tokens should be read from files, not
	// command-line flags or environment variables.
	{
		home, err := os.UserHomeDir()
		if err != nil {
			log.Fatal(err)
		}
		tokenFile := filepath.Join(home, ".stacks.token")
		content, err := os.ReadFile(tokenFile)
		if err != nil {
			log.Fatalf("cannot read GitHub authentication token: %v", err)
		}
		ghclient = &githubClient{authToken: string(bytes.TrimSpace(content))}
	}

	pcfg, ok := programs[*programFlag]
	if !ok {
		log.Fatalf("unknown -program %s", *programFlag)
	}

	// Read all recent telemetry reports.
	stacks, distinctStacks, stackToURL, err := readReports(pcfg, *daysFlag)
	if err != nil {
		log.Fatalf("Error reading reports: %v", err)
	}

	issues, err := readIssues(ghclient, pcfg)
	if err != nil {
		log.Fatalf("Error reading issues: %v", err)
	}

	// Map stacks to existing issues (if any).
	claimedBy := claimStacks(issues, stacks)

	// Update existing issues that claimed new stacks.
	updateIssues(ghclient, pcfg.Repository, issues, stacks, stackToURL)

	// For each stack, show existing issue or create a new one.
	// Aggregate stack IDs by issue summary.
	var (
		// Both vars map the summary line to the stack count.
		existingIssues = make(map[string]int64)
		newIssues      = make(map[string]int64)

		allStacks, corruptStacks int64 // count of all stacks, and those labeled "memory-corruption"
		corruptCauses            int64 // same, but counts issues not stacks
	)
	for stack, counts := range stacks {
		id := stackID(stack)

		var total int64 // counts of this stack
		for _, count := range counts {
			total += count
		}
		allStacks += total

		if issue, ok := claimedBy[id]; ok {
			// existing issue, already updated above, just store
			// the summary.
			state := issue.State
			if issue.State == "closed" && issue.StateReason == "completed" {
				state = "completed"
			}
			summary := fmt.Sprintf("#%d: %s [%s]",
				issue.Number, issue.Title, state)
			if state == "completed" && issue.Milestone != nil {
				summary += " milestone " + strings.TrimPrefix(issue.Milestone.Title, "gopls/")
			}
			existingIssues[summary] += total

			if slices.ContainsFunc(issue.Labels, func(label Label) bool {
				return strings.HasSuffix(label.Name, "/memory-corruption") // e.g. gopls/memory-corruption
			}) {
				corruptStacks += total
				corruptCauses++
			}

		} else {
			// new issue, need to create GitHub issue and store
			// summary.
			title := newIssue(pcfg, stack, id, stackToURL[stack], counts)
			summary := fmt.Sprintf("%s: %s [%s]", id, title, "new")
			newIssues[summary] += total
		}
	}

	fmt.Printf("\nFound %d distinct stacks in last %v days:\n", distinctStacks, *daysFlag)
	print := func(caption string, issues map[string]int64) {
		// Print items in descending frequency.
		keys := keySlice(issues)
		sort.Slice(keys, func(i, j int) bool {
			return issues[keys[i]] > issues[keys[j]]
		})
		fmt.Printf("%s issues:\n", caption)
		for _, summary := range keys {
			count := issues[summary]
			// Show closed issues in "white".
			if isTerminal(os.Stdout) && (strings.Contains(summary, "[closed]") || strings.Contains(summary, "[completed]")) {
				// ESC + "[" + n + "m" => change color to n
				// (37 = white, 0 = default)
				summary = "\x1B[37m" + summary + "\x1B[0m"
			}
			fmt.Printf("%s (n=%d)\n", summary, count)
		}
		if len(keys) == 0 {
			fmt.Println(" (none)")
		}
	}
	print("\nExisting", existingIssues)
	print("\nNew", newIssues)

	fmt.Printf("\nMemory corruption: %d/%d (%.2g%%) of reports, %d/%d (%.2g%%) of causes\n",
		corruptStacks, allStacks, float64(corruptStacks)*100/float64(allStacks),
		corruptCauses, len(stacks), float64(corruptCauses)*100/float64(len(stacks)))
}

// Info is used as a key for de-duping and aggregating.
// Do not add detail about particular records (e.g. data, telemetry URL).
type Info struct {
	Program        string // "golang.org/x/tools/gopls"
	ProgramVersion string // "v0.16.1"
	GoVersion      string // "go1.23"
	GOOS, GOARCH   string
	GoplsClient    string // e.g. "vscode" (only set if Program == "golang.org/x/tools/gopls")
}

func (info Info) String() string {
	s := fmt.Sprintf("%s@%s %s %s/%s",
		info.Program, info.ProgramVersion,
		info.GoVersion, info.GOOS, info.GOARCH)
	if info.GoplsClient != "" {
		s += " gopls.client=" + info.GoplsClient
	}
	return s
}

// readReports downloads telemetry stack reports for a program from the
// specified number of most recent days.
//
// stacks is a map of stack text to program metadata to stack+metadata report
// count.
// distinctStacks is the number of distinct stacks across all reports.
// stackToURL maps the stack text to the oldest telemetry JSON report it was
// included in.
func readReports(pcfg ProgramConfig, days int) (stacks map[string]map[Info]int64, distinctStacks int, stackToURL map[string]string, err error) {
	log.Println("Reading reports...")

	stacks = make(map[string]map[Info]int64)
	stackToURL = make(map[string]string)

	t := time.Now()
	for i := range days {
		date := t.Add(-time.Duration(i+1) * 24 * time.Hour).Format(time.DateOnly)

		url := fmt.Sprintf("https://telemetry.go.dev/data/%s", date)
		resp, err := http.Get(url)
		if err != nil {
			return nil, 0, nil, fmt.Errorf("error on GET %s: %v", url, err)
		}
		defer resp.Body.Close()
		if resp.StatusCode != 200 {
			return nil, 0, nil, fmt.Errorf("GET %s returned %d %s", url, resp.StatusCode, resp.Status)
		}

		dec := json.NewDecoder(resp.Body)
		for {
			var report telemetry.Report
			if err := dec.Decode(&report); err != nil {
				if err == io.EOF {
					break
				}
				return nil, 0, nil, fmt.Errorf("error decoding report: %v", err)
			}
			for _, prog := range report.Programs {
				if prog.Program != pcfg.Program {
					continue
				}
				if len(prog.Stacks) == 0 {
					continue
				}
				// Ignore @devel versions as they correspond to
				// ephemeral (and often numerous) variations of
				// the program as we work on a fix to a bug.
				if prog.Version == "devel" {
					continue
				}

				// Include applicable client names (e.g. vscode, eglot) for gopls.
				var clientSuffix string
				if pcfg.IncludeClient {
					var clients []string
					for key := range prog.Counters {
						if client, ok := strings.CutPrefix(key, "gopls/client:"); ok {
							clients = append(clients, client)
						}
					}
					sort.Strings(clients)
					if len(clients) > 0 {
						clientSuffix = strings.Join(clients, ",")
					}
				}

				info := Info{
					Program:        prog.Program,
					ProgramVersion: prog.Version,
					GoVersion:      prog.GoVersion,
					GOOS:           prog.GOOS,
					GOARCH:         prog.GOARCH,
					GoplsClient:    clientSuffix,
				}
				for stack, count := range prog.Stacks {
					counts := stacks[stack]
					if counts == nil {
						counts = make(map[Info]int64)
						stacks[stack] = counts
					}
					counts[info] += count
					stackToURL[stack] = url
				}
				distinctStacks += len(prog.Stacks)
			}
		}
	}

	return stacks, distinctStacks, stackToURL, nil
}

// readIssues returns all existing issues for the given program and parses any
// predicates.
func readIssues(cli *githubClient, pcfg ProgramConfig) ([]*Issue, error) {
	log.Println("Reading issues...")

	// Query GitHub for all existing GitHub issues with the report label.
	issues, err := cli.searchIssues(pcfg.Repository, pcfg.SearchLabel)
	if err != nil {
		// TODO(jba): return error instead of dying, or doc.
		log.Fatalf("GitHub issues label %q search failed: %v", pcfg.SearchLabel, err)
	}

	// Extract and validate predicate expressions in ```#!stacks...``` code blocks.
	// See the package doc comment for the grammar.
	for _, issue := range issues {
		block := findPredicateBlock(issue.Body)
		if block != "" {
			pred, err := parsePredicate(block)
			if err != nil {
				log.Printf("invalid predicate in issue #%d: %v\n<<%s>>",
					issue.Number, err, block)
				continue
			}
			issue.predicate = block
			issue.matches = pred
		}
	}

	return issues, nil
}

// parsePredicate parses a predicate expression, returning a function that evaluates
// the predicate on a stack.
// The expression must match this grammar:
//
//	expr = "string literal"
//	     | ( expr )
//	     | ! expr
//	     | expr && expr
//	     | expr || expr
//
// The value of a string literal is whether it is a substring of the stack, respecting word boundaries.
// That is, a literal L behaves like the regular expression \bL'\b, where L' is L with
// regexp metacharacters quoted.
func parsePredicate(s string) (func(string) bool, error) {
	expr, err := parser.ParseExpr(s)
	if err != nil {
		return nil, fmt.Errorf("parse error: %w", err)
	}

	// Cache compiled regexps since we need them more than once.
	literalRegexps := make(map[*ast.BasicLit]*regexp.Regexp)

	// Check for errors in the predicate so we can report them now,
	// ensuring that evaluation is error-free.
	var validate func(ast.Expr) error
	validate = func(e ast.Expr) error {
		switch e := e.(type) {
		case *ast.UnaryExpr:
			if e.Op != token.NOT {
				return fmt.Errorf("invalid op: %s", e.Op)
			}
			return validate(e.X)

		case *ast.BinaryExpr:
			if e.Op != token.LAND && e.Op != token.LOR {
				return fmt.Errorf("invalid op: %s", e.Op)
			}
			if err := validate(e.X); err != nil {
				return err
			}
			return validate(e.Y)

		case *ast.ParenExpr:
			return validate(e.X)

		case *ast.BasicLit:
			if e.Kind != token.STRING {
				return fmt.Errorf("invalid literal (%s)", e.Kind)
			}
			lit, err := strconv.Unquote(e.Value)
			if err != nil {
				return err
			}
			// The end of the literal (usually "symbol",
			// "pkg.symbol", or "pkg.symbol:+1") must
			// match a word boundary. However, the start
			// of the literal need not: an input line such
			// as "domain.name/dir/pkg.symbol:+1" should
			// match literal "pkg.symbol", but the slash
			// is not a word boundary (witness:
			// https://go.dev/play/p/w-8ev_VUBSq).
			//
			// It may match multiple words if it contains
			// non-word runes like whitespace.
			//
			// The constructed regular expression is always valid.
			literalRegexps[e] = regexp.MustCompile(regexp.QuoteMeta(lit) + `\b`)

		default:
			return fmt.Errorf("syntax error (%T)", e)
		}
		return nil
	}
	if err := validate(expr); err != nil {
		return nil, err
	}

	return func(stack string) bool {
		var eval func(ast.Expr) bool
		eval = func(e ast.Expr) bool {
			switch e := e.(type) {
			case *ast.UnaryExpr:
				return !eval(e.X)

			case *ast.BinaryExpr:
				if e.Op == token.LAND {
					return eval(e.X) && eval(e.Y)
				} else {
					return eval(e.X) || eval(e.Y)
				}

			case *ast.ParenExpr:
				return eval(e.X)

			case *ast.BasicLit:
				return literalRegexps[e].MatchString(stack)
			}
			panic("unreachable")
		}
		return eval(expr)
	}, nil
}

// claimStacks maps each stack ID to its issue (if any).
//
// It returns a map of stack text to the issue that claimed it.
//
// An issue can claim a stack two ways:
//
//  1. if the issue body contains the ID of the stack. Matching
//     is a little loose but base64 will rarely produce words
//     that appear in the body by chance.
//
//  2. if the issue body contains a ```#!stacks``` predicate that
//     matches the stack together with at least one of its configurations.
//
// We log an error if two different issues attempt to claim
// the same stack.
func claimStacks(issues []*Issue, stacks map[string]map[Info]int64) map[string]*Issue {
	log.Println("Processing claims...")

	// This is O(new stacks x existing issues x stack configurations).
	claimedBy := make(map[string]*Issue)
	claimType := make(map[string]bool) // records whether claim was due to predicate (true) or ID (false) in issue body
	for stack, counts := range stacks {
		id := stackID(stack)

		for _, issue := range issues {
			byPredicate := false
			if strings.Contains(issue.Body, id) {
				// nop
			} else if issue.matches != nil && matchesAnyConfig(issue.matches, stack, counts) {
				byPredicate = true
				if false {
					log.Printf("predicate %s matches stack %s", issue.predicate, id)
				}
			} else {
				continue
			}

			if prev := claimedBy[id]; prev != nil && prev != issue {
				fmt.Printf("stack %s is claimed by issues #%d (%s) and #%d (%s):%s\n\n",
					id,
					// Indicate how each claim was made.
					prev.Number, cond(claimType[id], "predicate", "hash"),
					issue.Number, cond(byPredicate, "predicate", "hash"),
					strings.ReplaceAll("\n"+stack, "\n", "\n- "))
				continue
			}
			if false {
				log.Printf("stack %s claimed by issue #%d",
					id, issue.Number)
			}
			claimedBy[id] = issue
			claimType[id] = byPredicate
			if byPredicate {
				// The stack ID matched the predicate but was not
				// found in the issue body, so this is a new stack.
				issue.newStacks = append(issue.newStacks, stack)
			}
		}
	}

	return claimedBy
}

// matchesAnyConfig reports whether match accepts the stack in at least one of
// the configurations in counts.
func matchesAnyConfig(match func(string) bool, stack string, counts map[Info]int64) bool {
	for info := range counts {
		text := fmt.Sprintf("%s\nGOTOOLCHAIN=%s GOOS=%s GOARCH=%s %s@%s",
			stack, info.GoVersion, info.GOOS, info.GOARCH, info.Program, info.ProgramVersion)
		if match(text) {
			return true
		}
	}
	return false
}

// updateIssues updates existing issues that claimed new stacks by predicate.
func updateIssues(cli *githubClient, repo string, issues []*Issue, stacks map[string]map[Info]int64, stackToURL map[string]string) {
	log.Println("Updating issues...")

	for _, issue := range issues {
		if len(issue.newStacks) == 0 {
			continue
		}

		// Add a comment to the existing issue listing all its new stacks.
		// (Save the ID of each stack for the second step.)
		//
		// We record the predicate expression at the time of
		// the match, as the predicate in the issue body will
		// evolve as users edit it, so we need to leave a
		// record of its history.
		comment := new(bytes.Buffer)
		if issue.predicate != "" {
			fmt.Fprintf(comment, "\n\nFound new stacks for predicate:\n")
			fmt.Fprintf(comment, "```\n#!stacks\n%s\n```\n\n", strings.TrimSpace(issue.predicate))
		}
		var newStackIDs []string
		for _, stack := range issue.newStacks {
			id := stackID(stack)
			newStackIDs = append(newStackIDs, id)
			writeStackComment(comment, stack, id, stackToURL[stack], stacks[stack])
		}

		if err := cli.addIssueComment(repo, issue.Number, comment.String()); err != nil {
			log.Println(err)
			continue
		}

		// Append to the "Dups: ID ..." list on last line of issue body.
		body := strings.TrimSpace(issue.Body)
		lastLineStart := strings.LastIndexByte(body, '\n') + 1
		lastLine := body[lastLineStart:]
		if !strings.HasPrefix(lastLine, "Dups:") {
			body += "\nDups:"
		}
		body += " " + strings.Join(newStackIDs, " ")

		update := updateIssue{number: issue.Number, Body: body}
		if shouldReopen(issue, stacks) {
			update.State = "open"
			update.StateReason = "reopened"
		}
		if err := cli.updateIssue(repo, update); err != nil {
			log.Printf("added comment to issue #%d but failed to update: %v",
				issue.Number, err)
			continue
		}

		log.Printf("added stacks %s to issue #%d (%s)", newStackIDs, issue.Number, issue.Title)
	}
}

// An issue should be re-opened if it was closed as fixed, and at least one of the
// new stacks happened since the version containing the fix.
func shouldReopen(issue *Issue, stacks map[string]map[Info]int64) bool {
	if !issue.isFixed() {
		return false
	}
	if *verbose {
		log.Printf("shouldReopen %q", issue.Title)
	}
	issueProgram, issueVersion, ok := parseMilestone(issue.Milestone)
	if !ok {
		if *verbose {
			log.Printf("unparsable milestone %#v", issue.Milestone)
		}
		return false
	}

	matchProgram := func(infoProg string) bool {
		switch issueProgram {
		case "gopls", "dlv":
			return path.Base(infoProg) == issueProgram
		case "go":
			// At present, we only care about compiler stacks.
			// Issues should have milestones like "Go1.24".
			return infoProg == "cmd/compile"
		default:
			return false
		}
	}

	for _, stack := range issue.newStacks {
		for info := range stacks[stack] {
			if *verbose {
				log.Printf("\tmatchProgram %v (%q %q) semver.Compare %v", matchProgram(info.Program), issueProgram, info.Program, semver.Compare(semVer(info.ProgramVersion), issueVersion))
			}
			if matchProgram(info.Program) && semver.Compare(semVer(info.ProgramVersion), issueVersion) >= 0 {
				log.Printf("reopening issue #%d: purportedly fixed in %s@%s, but found a new stack from version %s",
					issue.Number, issueProgram, issueVersion, info.ProgramVersion)
				return true
			}
		}
	}
	return false
}

// An issue is fixed if it was closed because it was completed.
func (i *Issue) isFixed() bool {
	return i.State == "closed" && i.StateReason == "completed"
}

// parseMilestone parses a the title of a GitHub milestone.
// If  it is in the format PROGRAM/VERSION (for example, "gopls/v0.17.0"),
// then it returns PROGRAM and VERSION.
// If it is in the format Go1.X, then it returns "go" as the program and
// "v1.X" or "v1.X.0" as the version.
// Otherwise, the last return value is false.
func parseMilestone(m *Milestone) (program, version string, ok bool) {
	if m == nil {
		return "", "", false
	}
	if strings.HasPrefix(m.Title, "Go") {
		v := semVer(m.Title)
		if !semver.IsValid(v) {
			return "", "", false
		}
		return "go", v, true
	}
	program, version, ok = cutLast(m.Title, "/")
	if !ok || program == "" || version == "" || version[0] != 'v' {
		return "", "", false
	}
	return program, version, true
}

// semVer returns a semantic version for its argument, which may already be
// a semantic version, or may be a Go version.
//
//	 v1.2.3 => v1.2.3
//		go1.24 => v1.24
//		Go1.23.5 => v1.23.5
//	 goHome => vHome
//
// It returns "", false if the go version is in the wrong format.
func semVer(v string) string {
	if strings.HasPrefix(v, "go") || strings.HasPrefix(v, "Go") {
		return "v" + v[2:]
	}
	return v
}

// stackID returns a 32-bit identifier for a stack
// suitable for use in GitHub issue titles.
func stackID(stack string) string {
	// Encode it using base64 (6 bytes) for brevity,
	// as a single issue's body might contain multiple IDs
	// if separate issues with same cause were manually de-duped,
	// e.g. "AAAAAA, BBBBBB"
	//
	// https://hbfs.wordpress.com/2012/03/30/finding-collisions:
	// the chance of a collision is 1 - exp(-n(n-1)/2d) where n
	// is the number of items and d is the number of distinct values.
	// So, even with n=10^4 telemetry-reported stacks each identified
	// by a uint32 (d=2^32), we have a 1% chance of a collision,
	// which is plenty good enough.
	h := fnv.New32()
	io.WriteString(h, stack)
	return base64.URLEncoding.EncodeToString(h.Sum(nil))[:6]
}

// newIssue creates a browser tab with a populated GitHub "New issue"
// form for the specified stack. (The triage person is expected to
// manually de-dup the issue before deciding whether to submit the form.)
//
// It returns the title.
func newIssue(pcfg ProgramConfig, stack, id, jsonURL string, counts map[Info]int64) string {
	// Use a heuristic to find a suitable symbol to blame in the title: the
	// first public function or method of a public type, in
	// MatchSymbolPrefix, to appear in the stack trace. We can always
	// refine it later.
	//
	// TODO(adonovan): include in the issue a source snippet ±5
	// lines around the PC in this symbol.
	var symbol string
outer:
	for _, line := range strings.Split(stack, "\n") {
		for _, s := range pcfg.IgnoreSymbolContains {
			if strings.Contains(line, s) {
				continue outer // not interesting
			}
		}
		// Look for:
		//   pcfg.MatchSymbolPrefix/.../pkg.Func
		//   pcfg.MatchSymbolPrefix/.../pkg.Type.method
		//   pcfg.MatchSymbolPrefix/.../pkg.(*Type).method
		if _, rest, ok := strings.Cut(line, pcfg.MatchSymbolPrefix); ok {
			if i := strings.IndexByte(rest, '.'); i >= 0 {
				rest = rest[i+1:]
				rest = strings.TrimPrefix(rest, "(*")
				if rest != "" && 'A' <= rest[0] && rest[0] <= 'Z' {
					rest, _, _ = strings.Cut(rest, ":")
					symbol = " " + rest
					break
				}
			}
		}
	}

	// Populate the form (title, body, label)
	title := fmt.Sprintf("%s: bug in %s", pcfg.NewIssuePrefix, symbol)

	body := new(bytes.Buffer)

	// Add a placeholder ```#!stacks``` block since this is a new issue.
	body.WriteString("```" + `
#!stacks
"<insert predicate here>"
` + "```\n")
	fmt.Fprintf(body, "Issue created by [stacks](https://pkg.go.dev/golang.org/x/telemetry/cmd/stacks).\n\n")

	writeStackComment(body, stack, id, jsonURL, counts)

	labels := strings.Join(pcfg.NewIssueLabels, ",")

	// Report it. The user will interactively finish the task,
	// since they will typically de-dup it without even creating a new issue
	// by expanding the #!stacks predicate of an existing issue.
	if !browser.Open("https://github.com/" + pcfg.Repository + "/issues/new?labels=" + labels + "&title=" + url.QueryEscape(title) + "&body=" + url.QueryEscape(body.String())) {
		log.Print("Please file a new issue at golang.org/issue/new using this template:\n\n")
		log.Printf("Title: %s\n", title)
		log.Printf("Labels: %s\n", labels)
		log.Printf("Body: %s\n", body)
	}

	return title
}

// writeStackComment writes a stack in Markdown form, for a new GitHub
// issue or new comment on an existing one.
func writeStackComment(body *bytes.Buffer, stack, id string, jsonURL string, counts map[Info]int64) {
	if len(counts) == 0 {
		panic("no counts")
	}
	var info Info // pick an arbitrary key
	for info = range counts {
		break
	}

	fmt.Fprintf(body, "This stack `%s` was [reported by telemetry](%s):\n\n",
		id, jsonURL)

	// Read the mapping from symbols to file/line.
	pclntab, err := readPCLineTable(info, defaultStacksDir)
	if err != nil {
		log.Fatal(err)
	}

	// Parse the stack and get the symbol names out.
	for frame := range strings.SplitSeq(stack, "\n") {
		if url := frameURL(pclntab, info, frame); url != "" {
			fmt.Fprintf(body, "- [`%s`](%s)\n", frame, url)
		} else {
			fmt.Fprintf(body, "- `%s`\n", frame)
		}
	}

	// Add counts, gopls version, and platform info.
	// This isn't very precise but should provide clues.
	fmt.Fprintf(body, "```\n")
	for info, count := range counts {
		fmt.Fprintf(body, "%s (%d)\n", info, count)
	}
	fmt.Fprintf(body, "```\n\n")

	// Give hints for disassembling the right executable.
	// Beware that go install behaves differently for cross-compiles;
	// the command below was chosen to work for simple and cross builds.
	// TODO(adonovan): simplify when go.dev/issue/44469 is resolved.
	fmt.Fprintf(body, "Use this command to reproduce the executable:\n")
	fmt.Fprintf(body, "`(HOME=$(mktemp -d); GOOS=%s GOARCH=%s GOTOOLCHAIN=%s go install %s@%s && find $HOME/go/bin -type f)`\n",
		info.GOOS,
		info.GOARCH,
		info.GoVersion,
		info.Program, info.ProgramVersion)

	fmt.Fprintf(body, "To disassemble: `go tool objdump exe | less`\n\n")
}

// frameURL returns the CodeSearch URL for the stack frame, if known.
func frameURL(pclntab map[string]FileLine, info Info, frame string) string {
	// e.g. "golang.org/x/tools/gopls/foo.(*Type).Method.inlined.func3:+5,+0x123",
	symbol, offset, ok := strings.Cut(frame, ":")
	if !ok {
		// Not a symbol (perhaps stack counter title: "gopls/bug"?)
		return ""
	}

again:
	fileline, ok := pclntab[symbol]
	if !ok {
		// objdump reports ELF symbol names, which in
		// rare cases may be the Go symbols of
		// runtime.CallersFrames mangled by (e.g.) the
		// addition of .abi0 suffix; see
		// https://github.com/golang/go/issues/69390#issuecomment-2343795920
		// So this should not be a hard error.
		if symbol != "runtime.goexit" {
			log.Printf("no pclntab info for symbol: %s", symbol)

			// This can also happen for inlined symbols, e.g.
			//  golang.org/x/tools/gopls/foo.(*bar).wiz.func1.1
			// Such symbol names, chosen by cmd/compile/internal/ir.closureName,
			// appear in the pclntab and thus the backtrace,
			// but not in the ELF symbol table, and thus not
			// in the output of "go tool objdump".
			//
			// Many such symbols are formed by appending a gensym
			// counter ".%d" to a real symbol. If the line offset
			// is absolute (e.g. "foo.func1.1:=123"), then we only
			// need the filename, which is the same as that of the
			// enclosing function ("foo.func1").
			// So try stripping off the suffix in that case.
			if before, after, ok := cutLast(symbol, "."); ok && strings.Contains(offset, "=") {
				if _, err := strconv.Atoi(after); err == nil { // have "fn.%d"
					symbol = before
					log.Printf("trying without numeric suffix: %s", symbol)
					goto again
				}
			}
		}

		return ""
	}

	if offset == "" {
		log.Fatalf("missing line offset: %s", frame)
	}
	if unicode.IsDigit(rune(offset[0])) {
		// Fix gopls/v0.14.2 legacy syntax ":%d" -> ":+%d".
		offset = "+" + offset
	}

	// CL 664175 (Apr 2025) changed the format
	// to include relative PCs ("+%d,+0x%x") too.
	// We can discard the PC part here.
	if before, _, ok := strings.Cut(offset, ",+"); ok {
		offset = before
	}

	offsetNum, err := strconv.Atoi(offset[1:])
	if err != nil {
		log.Fatalf("invalid line offset: %s", frame)
	}
	linenum := fileline.line
	switch offset[0] {
	case '-':
		linenum -= offsetNum
	case '+':
		linenum += offsetNum
	case '=':
		linenum = offsetNum
	}

	// Construct CodeSearch URL.

	// std module?
	firstSegment, _, _ := strings.Cut(fileline.file, "/")
	if !strings.Contains(firstSegment, ".") {
		// (First segment is a dir beneath GOROOT/src, not a module domain name.)
		return fmt.Sprintf("https://cs.opensource.google/go/go/+/%s:src/%s;l=%d",
			info.GoVersion, fileline.file, linenum)
	}

	// x/tools repo (tools or gopls module)?
	if rest, ok := strings.CutPrefix(fileline.file, "golang.org/x/tools"); ok {
		switch rest[0] {
		case '/':
			// "golang.org/x/tools/gopls" -> "gopls"
			rest = rest[1:]
		case '@':
			// "golang.org/x/tools@version/dir/file.go" -> "dir/file.go"
			rest = rest[strings.Index(rest, "/")+1:]
		}

		return fmt.Sprintf("https://cs.opensource.google/go/x/tools/+/%s:%s;l=%d",
			"gopls/"+info.ProgramVersion, rest, linenum)
	}

	// other x/ module dependency?
	// e.g. golang.org/x/sync@v0.8.0/errgroup/errgroup.go
	if rest, ok := strings.CutPrefix(fileline.file, "golang.org/x/"); ok {
		if modVer, filename, ok := strings.Cut(rest, "/"); ok {
			if mod, version, ok := strings.Cut(modVer, "@"); ok {
				return fmt.Sprintf("https://cs.opensource.google/go/x/%s/+/%s:%s;l=%d",
					mod, version, filename, linenum)
			}
		}
	}

	// Delve
	const delveRepo = "github.com/go-delve/delve/"
	if strings.HasPrefix(fileline.file, delveRepo) {
		filename := fileline.file[len(delveRepo):]
		return fmt.Sprintf("https://%sblob/%s/%s#L%d", delveRepo, info.ProgramVersion, filename, linenum)

	}

	log.Printf("no CodeSearch URL for %q (%s:%d)",
		symbol, fileline.file, linenum)
	return ""
}

// -- GitHub client --

// A githubClient interacts with GitHub.
// During testing, updates to GitHub are saved in changes instead of being applied.
// Reads from GitHub occur normally.
type githubClient struct {
	authToken     string // mandatory GitHub authentication token (for R/W issues access)
	divertChanges bool   // divert attempted GitHub changes to the changes field instead of executing them
	changes       []any  // slice of (addIssueComment | updateIssueBody)
}

func (cli *githubClient) takeChanges() []any {
	r := cli.changes
	cli.changes = nil
	return r
}

// addIssueComment is a change for creating a comment on an issue.
type addIssueComment struct {
	number  int
	comment string
}

// updateIssue is a change for modifying an existing issue.
// It includes the issue number and the fields that can be updated on a GitHub issue.
// A JSON-marshaled updateIssue can be used as the body of the update request sent to GitHub.
// See https://docs.github.com/en/rest/issues/issues?apiVersion=2022-11-28#update-an-issue.
type updateIssue struct {
	number      int    // issue number; must be unexported
	Body        string `json:"body,omitempty"`
	State       string `json:"state,omitempty"`        // "open" or "closed"
	StateReason string `json:"state_reason,omitempty"` // "completed", "not_planned", "reopened"
}

// -- GitHub search --

// searchIssues queries the GitHub issue tracker.
func (cli *githubClient) searchIssues(repo, label string) ([]*Issue, error) {
	label = url.QueryEscape(label)

	// Slurp all issues with the telemetry label.
	//
	// The pagination link headers have an annoying format, but ultimately
	// are just ?page=1, ?page=2, etc with no extra state. So just keep
	// trying new pages until we get no more results.
	//
	// NOTE: With this scheme, GitHub clearly has no protection against
	// race conditions, so presumably we could get duplicate issues or miss
	// issues across pages.

	getPage := func(page int) ([]*Issue, error) {
		url := fmt.Sprintf("https://api.github.com/repos/%s/issues?state=all&labels=%s&per_page=100&page=%d", repo, label, page)
		req, err := http.NewRequest("GET", url, nil)
		if err != nil {
			return nil, err
		}
		req.Header.Add("Authorization", "Bearer "+cli.authToken)
		resp, err := http.DefaultClient.Do(req)
		if err != nil {
			return nil, err
		}
		defer resp.Body.Close()
		if resp.StatusCode != http.StatusOK {
			body, _ := io.ReadAll(resp.Body)
			return nil, fmt.Errorf("search query %s failed: %s (body: %s)", url, resp.Status, body)
		}
		var r []*Issue
		if err := json.NewDecoder(resp.Body).Decode(&r); err != nil {
			return nil, err
		}

		return r, nil
	}

	var results []*Issue
	for page := 1; ; page++ {
		r, err := getPage(page)
		if err != nil {
			return nil, err
		}
		if len(r) == 0 {
			// No more results.
			break
		}

		results = append(results, r...)
	}

	return results, nil
}

// updateIssue updates the numbered issue.
func (cli *githubClient) updateIssue(repo string, update updateIssue) error {
	if cli.divertChanges {
		cli.changes = append(cli.changes, update)
		return nil
	}

	data, err := json.Marshal(update)
	if err != nil {
		return err
	}

	url := fmt.Sprintf("https://api.github.com/repos/%s/issues/%d", repo, update.number)
	if err := cli.requestChange("PATCH", url, data, http.StatusOK); err != nil {
		return fmt.Errorf("updating issue: %v", err)
	}
	return nil
}

// addIssueComment adds a markdown comment to the numbered issue.
func (cli *githubClient) addIssueComment(repo string, number int, comment string) error {
	if cli.divertChanges {
		cli.changes = append(cli.changes, addIssueComment{number, comment})
		return nil
	}

	// https://docs.github.com/en/rest/issues/comments#create-an-issue-comment
	var payload struct {
		Body string `json:"body"`
	}
	payload.Body = comment
	data, err := json.Marshal(payload)
	if err != nil {
		return err
	}

	url := fmt.Sprintf("https://api.github.com/repos/%s/issues/%d/comments", repo, number)
	if err := cli.requestChange("POST", url, data, http.StatusCreated); err != nil {
		return fmt.Errorf("creating issue comment: %v", err)
	}
	return nil
}

// requestChange sends a request to url using method, which may change the state at the server.
// The data is sent as the request body, and wantStatus is the expected response status code.
func (cli *githubClient) requestChange(method, url string, data []byte, wantStatus int) error {
	if *dryRun {
		log.Printf("DRY RUN: %s %s", method, url)
		return nil
	}
	req, err := http.NewRequest(method, url, bytes.NewReader(data))
	if err != nil {
		return err
	}
	req.Header.Add("Authorization", "Bearer "+cli.authToken)
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode != wantStatus {
		body, _ := io.ReadAll(resp.Body)
		return fmt.Errorf("request failed: %s (body: %s)", resp.Status, body)
	}
	return nil
}

// See https://docs.github.com/en/rest/issues/issues?apiVersion=2022-11-28#list-repository-issues.

type Issue struct {
	Number      int
	HTMLURL     string `json:"html_url"`
	Title       string
	State       string
	StateReason string `json:"state_reason"`
	User        *User
	CreatedAt   time.Time `json:"created_at"`
	Body        string    // in Markdown format
	Labels      []Label   `json:"labels"`
	Milestone   *Milestone

	// Set by readIssues.
	predicate string            // predicate source expression
	matches   func(string) bool // reports whether predicate matches stack text

	// Set by claimIssues.
	newStacks []string // new stacks to add to existing issue (comments and IDs)
}

type Label struct {
	Name string
}

func (issue *Issue) String() string { return fmt.Sprintf("#%d", issue.Number) }

type User struct {
	Login   string
	HTMLURL string `json:"html_url"`
}

type Milestone struct {
	Title string
}

// -- pclntab --

type FileLine struct {
	file string // "module@version/dir/file.go" or path relative to $GOROOT/src
	line int
}

const defaultStacksDir = "/tmp/stacks-cache"

// readPCLineTable builds the gopls executable specified by info,
// reads its PC-to-line-number table, and returns the file/line of
// each TEXT symbol.
//
// stacksDir is a semi-durable temp directory (i.e. lasts for at least a few
// hours) to hold recent sources and executables.
func readPCLineTable(info Info, stacksDir string) (map[string]FileLine, error) {
	// Telemetry records always have a valid GoVersion field,
	// but internal unit tests may not. Reject them, otherwise
	// they'll use GOTOOLCHAIN="" and will fail to build
	// (for example) older versions of gopls that compile-time
	// assert the sizeof(token.FileSet), which changed in go1.25.
	if info.GoVersion == "" {
		panic("readPCLineTable: missing GoVersion")
	}

	// The stacks dir will be a semi-durable temp directory
	// (i.e. lasts for at least hours) holding source trees
	// and executables we have built recently.
	//
	// Each subdir will hold a specific revision.
	if err := os.MkdirAll(stacksDir, 0777); err != nil {
		return nil, fmt.Errorf("can't create stacks dir: %v", err)
	}

	// When building a subrepo tool, we must clone the source of the
	// subrepo, and run go build from that checkout.
	//
	// When building a main repo tool, no need to clone or change
	// directories. GOTOOLCHAIN is sufficient to fetch and build the
	// appropriate version.
	var buildDir string // cwd for "go build", and output directory for exe
	switch info.Program {
	case "golang.org/x/tools/gopls":
		// Fetch the source for the tools repo,
		// shallow-cloning just the desired revision.
		// (Skip if it's already cloned.)
		revDir := filepath.Join(stacksDir, info.ProgramVersion)
		if !fileExists(filepath.Join(revDir, "go.mod")) {
			// We check for presence of the go.mod file,
			// not just the directory itself, as the /tmp reaper
			// often removes stale files before removing their directories.
			// Remove those stale directories now.
			_ = os.RemoveAll(revDir) // ignore errors

			// TODO(prattmic): Consider using ProgramConfig
			// configuration if we add more configurations.
			log.Printf("cloning tools@gopls/%s", info.ProgramVersion)
			if err := shallowClone(revDir, "https://go.googlesource.com/tools", "gopls/"+info.ProgramVersion); err != nil {
				_ = os.RemoveAll(revDir) // ignore errors
				return nil, fmt.Errorf("clone: %v", err)
			}
		}

		// gopls is in its own module, we must build from there.
		buildDir = filepath.Join(revDir, "gopls")

	case "cmd/compile":
		// Nothing to do, GOTOOLCHAIN is sufficient.

		// Set output directory (and avoid any dependence on
		// the current directory, which may be a Go module
		// directory containing a go.mod.)
		//
		// The exe name is unambiguous so we don't need to use a subdir.
		buildDir = stacksDir

	case "github.com/go-delve/delve/cmd/dlv":
		revDir := filepath.Join(stacksDir, "delve@"+info.ProgramVersion)
		if !fileExists(filepath.Join(revDir, "go.mod")) {
			_ = os.RemoveAll(revDir)
			log.Printf("cloning github.com/go-delve/delve@%s", info.ProgramVersion)
			if err := shallowClone(revDir, "https://github.com/go-delve/delve", info.ProgramVersion); err != nil {
				_ = os.RemoveAll(revDir)
				return nil, fmt.Errorf("clone: %v", err)
			}
		}
		buildDir = revDir

	default:
		return nil, fmt.Errorf("don't know how to build unknown program %s", info.Program)
	}

	if !strings.HasPrefix(buildDir, stacksDir) {
		log.Fatalf("buildDir %q is not within stack temp dir %q", buildDir, stacksDir)
	}

	// Build the executable with the correct GOTOOLCHAIN, GOOS, GOARCH.
	// (The buildDir implies the ProgramVersion, if relevant.)
	// Use -trimpath for normalized file names.
	// (Skip if it's already built.)
	exe := filepath.Join(buildDir,
		fmt.Sprintf("exe-%s-%s.%s-%s",
			strings.ReplaceAll(info.Program, "/", "_"),
			info.GoVersion,
			info.GOOS,
			info.GOARCH))
	if !fileExists(exe) {
		log.Printf("building %s@%s with %s for %s/%s",
			info.Program, info.ProgramVersion, info.GoVersion, info.GOOS, info.GOARCH)

		cmd := exec.Command("go", "build", "-trimpath", "-o", exe, info.Program)
		cmd.Stderr = os.Stderr
		cmd.Dir = buildDir
		cmd.Env = append(os.Environ(),
			"GOTOOLCHAIN="+info.GoVersion,
			"GOEXPERIMENT=", // Don't forward GOEXPERIMENT from current environment since the GOTOOLCHAIN selected might not support the same experiments.
			"GOOS="+info.GOOS,
			"GOARCH="+info.GOARCH,
		)
		if err := cmd.Run(); err != nil {
			return nil, fmt.Errorf("building: %v (rm -fr %s?)", err, stacksDir)
		}
	}

	// Read pclntab of executable.
	cmd := exec.Command("go", "tool", "objdump", exe)
	cmd.Stdout = new(strings.Builder)
	cmd.Stderr = os.Stderr
	cmd.Env = append(os.Environ(),
		"GOTOOLCHAIN="+info.GoVersion,
		"GOEXPERIMENT=", // Don't forward GOEXPERIMENT from current environment since the GOTOOLCHAIN selected might not support the same experiments.
		"GOOS="+info.GOOS,
		"GOARCH="+info.GOARCH,
	)
	if err := cmd.Run(); err != nil {
		return nil, fmt.Errorf("reading pclntab %v", err)
	}
	pclntab := make(map[string]FileLine)
	lines := strings.Split(fmt.Sprint(cmd.Stdout), "\n")
	for i, line := range lines {
		// Each function is of this form:
		//
		// TEXT symbol(SB) filename
		//    basename.go:line instruction
		//    ...
		if !strings.HasPrefix(line, "TEXT ") {
			continue
		}
		fields := strings.Fields(line)
		if len(fields) != 3 {
			continue // symbol without file (e.g. go:buildid)
		}

		symbol := strings.TrimSuffix(fields[1], "(SB)")

		filename := fields[2]

		_, line, ok := strings.Cut(strings.Fields(lines[i+1])[0], ":")
		if !ok {
			return nil, fmt.Errorf("can't parse 'basename.go:line' from first instruction of %s:\n%s",
				symbol, line)
		}
		linenum, err := strconv.Atoi(line)
		if err != nil {
			return nil, fmt.Errorf("can't parse line number of %s: %s", symbol, line)
		}
		pclntab[symbol] = FileLine{filename, linenum}
	}

	return pclntab, nil
}

// shallowClone performs a shallow clone of repo into dir at the given
// 'commitish' ref (any commit reference understood by git).
//
// The directory dir must not already exist.
func shallowClone(dir, repo, commitish string) error {
	if err := os.Mkdir(dir, 0750); err != nil {
		return fmt.Errorf("creating dir for %s: %v", repo, err)
	}

	// Set a timeout for git fetch. If this proves flaky, it can be removed.
	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Minute)
	defer cancel()

	// Use a shallow fetch to download just the relevant commit.
	shInit := fmt.Sprintf("git init && git fetch --depth=1 %q %q && git checkout FETCH_HEAD", repo, commitish)
	initCmd := exec.CommandContext(ctx, "/bin/sh", "-c", shInit)
	initCmd.Dir = dir
	if output, err := initCmd.CombinedOutput(); err != nil {
		return fmt.Errorf("checking out %s: %v\n%s", repo, err, output)
	}
	return nil
}

func fileExists(filename string) bool {
	_, err := os.Stat(filename)
	return err == nil
}

// findPredicateBlock returns the content (sans "#!stacks") of the
// code block at the start of the issue body.
// Logic plundered from x/build/cmd/watchflakes/github.go.
func findPredicateBlock(body string) string {
	// Extract ```-fenced or indented code block at start of issue description (body).
	body = strings.ReplaceAll(body, "\r\n", "\n")
	lines := strings.SplitAfter(body, "\n")
	for len(lines) > 0 && strings.TrimSpace(lines[0]) == "" {
		lines = lines[1:]
	}
	text := ""
	// A code quotation is bracketed by sequence of 3+ backticks.
	// (More than 3 are permitted so that one can quote 3 backticks.)
	if len(lines) > 0 && strings.HasPrefix(lines[0], "```") {
		marker := lines[0]
		n := 0
		for n < len(marker) && marker[n] == '`' {
			n++
		}
		marker = marker[:n]
		i := 1
		for i := 1; i < len(lines); i++ {
			if strings.HasPrefix(lines[i], marker) && strings.TrimSpace(strings.TrimLeft(lines[i], "`")) == "" {
				text = strings.Join(lines[1:i], "")
				break
			}
		}
		if i < len(lines) {
		}
	} else if strings.HasPrefix(lines[0], "\t") || strings.HasPrefix(lines[0], "    ") {
		i := 1
		for i < len(lines) && (strings.HasPrefix(lines[i], "\t") || strings.HasPrefix(lines[i], "    ")) {
			i++
		}
		text = strings.Join(lines[:i], "")
	}

	// Must start with #!stacks so we're sure it is for us.
	hdr, rest, _ := strings.Cut(text, "\n")
	hdr = strings.TrimSpace(hdr)
	if hdr != "#!stacks" {
		return ""
	}
	return rest
}

// isTerminal reports whether file is a terminal,
// avoiding a dependency on golang.org/x/term.
func isTerminal(file *os.File) bool {
	// Hardwire the constants to avoid the need for build tags.
	// The values here are good for our dev machines.
	switch runtime.GOOS {
	case "darwin":
		const TIOCGETA = 0x40487413 // from unix.TIOCGETA
		_, err := unix.IoctlGetTermios(int(file.Fd()), TIOCGETA)
		return err == nil
	case "linux":
		const TCGETS = 0x5401 // from unix.TCGETS
		_, err := unix.IoctlGetTermios(int(file.Fd()), TCGETS)
		return err == nil
	}
	panic("unreachable")
}

// keySlice returns the keys of the map M, like slices.Collect(maps.Keys(m)).
// (Copied from x/tools/gopls/internal/moremaps.)
func keySlice[M ~map[K]V, K comparable, V any](m M) []K {
	r := make([]K, 0, len(m))
	for k := range m {
		r = append(r, k)
	}
	return r
}

// cutLast is the "last" analogue of [strings.Cut].
// (Copied from x/tools/gopls/internal.morestrings.)
func cutLast(s, sep string) (before, after string, ok bool) {
	if i := strings.LastIndex(s, sep); i >= 0 {
		return s[:i], s[i+len(sep):], true
	}
	return s, "", false
}

func cond[T any](cond bool, t, f T) T {
	if cond {
		return t
	} else {
		return f
	}
}