open-code-review/internal/diff/git.go

// SPDX-License-Identifier: Apache-2.0
// Copyright 2026 alibaba/open-code-review Contributors

package diff

import (
	"bytes"
	"context"
	"fmt"
	"net/url"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"unicode/utf8"

	"github.com/bmatcuk/doublestar/v4"

	"github.com/alibaba/open-code-review/internal/gitcmd"
	"github.com/alibaba/open-code-review/internal/model"
)

// DiffContextLines defines the number of context lines around each changed hunk.
const DiffContextLines = 3

// providerDirIgnoreDirs: directory prefixes to always exclude from diff results.
var providerDirIgnoreDirs = []string{
	".idea/",
	".vscode/",
	".svn/",
	".git/",
	"vendor/",
	"node_modules/",
	"target/",
	".happypack/",
	".cachefile/",
	"_packages/",
	"rpm/",
	"pkgs/",
}

// Mode defines how the diff is retrieved.
type Mode int

const (
	ModeWorkspace Mode = iota // current workspace (staged + unstaged + untracked)
	ModeCommit                // single commit vs its parent
	ModeRange                 // merge-base(from,to)..to
)

// Provider retrieves and parse git diffs from a repository.
type Provider struct {
	repoDir string
	mode    Mode
	runner  *gitcmd.Runner

	// Range mode parameters
	from, to string // from/to refs for range comparison

	// Commit mode parameter
	commit string // single commit hash/ref

	mergeBase string // cached common ancestor for range mode
}

// NewProvider creates a Provider for range mode: from..to (via merge-base).
func NewProvider(repoDir, from, to string, runner *gitcmd.Runner) *Provider {
	return &Provider{
		repoDir: repoDir,
		mode:    ModeRange,
		from:    from,
		to:      to,
		runner:  runner,
	}
}

// NewCommitProvider creates a Provider for commit mode: show changes introduced by a single commit.
func NewCommitProvider(repoDir, commit string, runner *gitcmd.Runner) *Provider {
	return &Provider{
		repoDir: repoDir,
		mode:    ModeCommit,
		commit:  commit,
		runner:  runner,
	}
}

// NewWorkspaceProvider creates a Provider for workspace mode (current uncommitted changes).
func NewWorkspaceProvider(repoDir string, runner *gitcmd.Runner) *Provider {
	return &Provider{
		repoDir: repoDir,
		mode:    ModeWorkspace,
		runner:  runner,
	}
}

// InputResolution carries this run's frozen, immutable commit endpoints, per the
// run-manifest input-mode matrix. An empty field means "not applicable or not
// resolvable" — a root commit and a merge commit have no single comparison base,
// an unborn workspace has no HEAD, and a workspace has no immutable head — and a
// caller must never treat an empty value as a real endpoint or fabricate one.
// ExactRange is populated only when both a unique base and a head resolve.
type InputResolution struct {
	ResolvedBase string
	ResolvedHead string
	ExactRange   string
}

// ResolveInput freezes this run's commit endpoints by asking git, following the
// input-mode matrix:
//
//   - range:     base = merge-base(from,to); head = the commit `to` resolves to;
//     exact_range = base..head only when both resolve.
//   - commit:    head = the commit resolved from `commit`; base is the first
//     parent used by the diff, and exact_range = first-parent..head. A root
//     commit has no base or exact range.
//   - workspace: base = current HEAD when the repository has one (empty on an
//     unborn repository); head and range stay empty (a workspace has no immutable
//     head).
//
// Commit mode follows the same first-parent comparison used by GetDiff. Root
// commits have no parent, so only their resolved head is available.
//
// It runs read-only git queries and never returns an error: an unresolvable
// endpoint is reported as an empty field, never a fabricated SHA.
func (p *Provider) ResolveInput(ctx context.Context) InputResolution {
	switch p.mode {
	case ModeRange:
		base := p.MergeBase(ctx)
		head := p.resolveCommit(ctx, p.to)
		r := InputResolution{ResolvedBase: base, ResolvedHead: head}
		if base != "" && head != "" {
			r.ExactRange = base + ".." + head
		}
		return r
	case ModeCommit:
		head := p.resolveCommit(ctx, p.commit)
		r := InputResolution{ResolvedHead: head}
		if parents := p.commitParents(ctx, p.commit); len(parents) > 0 && head != "" {
			// GetDiff renders merge commits against their first parent, so the
			// manifest must record that same concrete comparison base.
			r.ResolvedBase = parents[0]
			r.ExactRange = parents[0] + ".." + head
		}
		return r
	case ModeWorkspace:
		// base = current HEAD if the repository has one; an unborn repository has
		// no HEAD, so this stays empty rather than fabricating a base.
		return InputResolution{ResolvedBase: p.resolveCommit(ctx, "HEAD")}
	default:
		return InputResolution{}
	}
}

// IsRangeMode returns true when comparing two refs.
func (p *Provider) IsRangeMode() bool {
	return p.mode == ModeRange
}

// IsCommitMode returns true when analyzing a single commit.
func (p *Provider) IsCommitMode() bool {
	return p.mode == ModeCommit
}

// MergeBase returns the computed merge-base commit hash for range mode.
func (p *Provider) MergeBase(ctx context.Context) string {
	if p.mode != ModeRange || p.mergeBase != "" {
		return p.mergeBase
	}
	p.mergeBase = p.computeMergeBase(ctx, p.from, p.to)
	return p.mergeBase
}

// GetDiff returns all changes as parsed model.Diff structs.
func (p *Provider) GetDiff(ctx context.Context) ([]model.Diff, error) {
	var combined strings.Builder

	switch p.mode {
	case ModeRange:
		base := p.MergeBase(ctx)
		if base == "" {
			return nil, fmt.Errorf("cannot find merge-base between %s and %s", p.from, p.to)
		}
		out, stderr, err := p.runGitSplit(ctx, "-c", "core.quotepath=false", "diff", "--no-ext-diff", "--no-textconv", "--find-renames", "--src-prefix=a/", "--dst-prefix=b/", "--no-color", "-U"+fmt.Sprint(DiffContextLines), "--end-of-options", base, p.to, "--")
		if err != nil {
			return nil, gitFailure("git diff", stderr, err)
		}
		combined.WriteString(out)

	case ModeCommit:
		// --diff-merges=first-parent: for merge commits, plain `git show`
		// emits a combined diff ("diff --cc"), which ParseDiffText cannot
		// parse — the commit would silently yield zero reviewable diffs.
		// Diffs against the first parent instead, in regular unified format.
		out, stderr, err := p.runGitSplit(ctx, "-c", "core.quotepath=false", "show", "--no-ext-diff", "--no-textconv", "--find-renames", "--src-prefix=a/", "--dst-prefix=b/", "--no-color", "--diff-merges=first-parent", "-U"+fmt.Sprint(DiffContextLines), "--end-of-options", p.commit)
		if err != nil {
			return nil, gitFailure("git show", stderr, err)
		}
		combined.WriteString(out)

	case ModeWorkspace:
		// The stderr returned here is the fallback's (`git diff --staged`), and
		// that is the one worth surfacing. Reaching the fallback at all means
		// `git diff HEAD` already failed, and in the case the fallback exists
		// for — a repository with no commits — it failed with "bad revision
		// 'HEAD'", which is expected rather than diagnostic. Only the second
		// failure describes what actually blocked the review.
		tracked, stderr, err := p.workspaceTrackedDiff(ctx)
		if err != nil {
			return nil, gitFailure("workspace tracked diff", stderr, err)
		}
		combined.WriteString(tracked)

		untracked, err := p.untrackedFileDiffs(ctx)
		if err != nil {
			return nil, fmt.Errorf("untracked file diff failed: %w", err)
		}
		for _, ud := range untracked {
			combined.WriteString(ud)
			combined.WriteString("\n\n")
		}
	}

	var ref string
	switch p.mode {
	case ModeRange:
		ref = p.to
	case ModeCommit:
		ref = p.commit
	}

	diffs, err := ParseDiffText(ctx, combined.String(), p.repoDir, ref, p.runner)
	if err != nil {
		return nil, err
	}
	return p.filterDiffs(diffs), nil
}

// loadGitignorePatterns reads and parses .gitignore patterns from the repo root.
func (p *Provider) loadGitignorePatterns() []string {
	data, err := os.ReadFile(filepath.Join(p.repoDir, ".gitignore"))
	if err != nil {
		return nil
	}
	var patterns []string
	for line := range strings.SplitSeq(string(data), "\n") {
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		patterns = append(patterns, line)
	}
	return patterns
}

// isPathExcluded returns true when the given relative file path should be skipped
// based on hardcoded dir rules or .gitignore patterns.
//
// Patterns are resolved the way git resolves them: in file order, with the LAST
// matching pattern deciding, and a leading "!" inverting that pattern's verdict.
// Order matters because the "allow list" idiom (ignore everything with `*`, then
// re-include with `!` lines — github/gitignore ships one per language) is only
// correct under last-match-wins. Treating negations as unmatchable made every
// file in such a repository look excluded, so a review silently covered nothing.
func (p *Provider) isPathExcluded(relPath string, gitignorePatterns []string) bool {
	// Hardcoded directory prefix checks. These are an unconditional blocklist:
	// a .gitignore negation cannot re-admit .git/ or node_modules/.
	for _, prefix := range providerDirIgnoreDirs {
		dirPart := strings.TrimSuffix(prefix, "/")
		if relPath == dirPart || strings.HasPrefix(relPath, prefix) {
			return true
		}
	}

	excluded := false
	for _, pat := range gitignorePatterns {
		body, negated := strings.CutPrefix(pat, "!")
		if body == "" {
			continue
		}

		// Directory-only patterns (trailing "/") apply to directories, never to
		// files. Git uses a negated one such as `!*/` to keep descending into
		// subdirectories, not to re-admit the files inside them — honouring it
		// here would readmit everything below the root.
		if negated && strings.HasSuffix(body, "/") {
			continue
		}

		if matchGitignoreBody(relPath, body) {
			excluded = !negated
		}
	}
	return excluded
}

// matchGitignorePattern checks if relPath matches a single .gitignore pattern.
//
// Polarity is not this function's concern: a negated pattern reports false, so
// callers testing one pattern in isolation still read it as "does this exclude
// the path". Ordered resolution across a whole pattern list, where negations do
// carry meaning, lives in isPathExcluded.
func matchGitignorePattern(relPath, pat string) bool {
	if strings.HasPrefix(pat, "!") {
		return false
	}
	return matchGitignoreBody(relPath, pat)
}

// matchGitignoreBody reports whether relPath matches a single pattern body —
// the pattern with any leading "!" already stripped.
func matchGitignoreBody(relPath, body string) bool {
	// Directory-only patterns (trailing /)
	if pattern, ok := strings.CutSuffix(body, "/"); ok {
		return matchGitignoreDirectory(relPath, pattern)
	}

	// A leading "/" anchors the pattern to the repository root rather than
	// making it a path pattern; "/.golangci.yml" addresses the root file.
	anchored := false
	if trimmed, ok := strings.CutPrefix(body, "/"); ok {
		body, anchored = trimmed, true
	}

	// "**" is not expressible with filepath.Match, so patterns containing it go
	// through doublestar, which implements gitignore's globstar semantics.
	if strings.Contains(body, "**") {
		matched, err := doublestar.Match(body, relPath)
		return err == nil && matched
	}

	// Patterns without / match basename — unless anchored, where the pattern
	// addresses that name at the root only.
	if !strings.Contains(body, "/") {
		target := filepath.Base(relPath)
		if anchored {
			target = relPath
		}
		matched, _ := filepath.Match(body, target)
		return matched
	}

	// Patterns with / match against the full relative path
	if matched, _ := filepath.Match(body, relPath); matched {
		return true
	}
	// Also try matching against suffix of path, but not for anchored patterns:
	// "/docs/api.md" names one file, not any path ending that way.
	//
	// The leading "/" makes the suffix start on a path component: without it
	// "src/main.go" also matches "othersrc/main.go", because the tail of
	// "othersrc" completes the pattern.
	if !anchored && strings.HasSuffix(relPath, "/"+body) {
		return true
	}

	return false
}

func matchGitignoreDirectory(relPath, pattern string) bool {
	pattern, anchored := strings.CutPrefix(pattern, "/")
	if pattern == "" {
		return false
	}

	lastSlash := strings.LastIndex(relPath, "/")
	if lastSlash < 0 {
		return false
	}

	components := strings.Split(relPath[:lastSlash], "/")
	// Slash-containing patterns are relative to the .gitignore location;
	// slashless patterns match a directory name at any depth.
	matchFullPath := anchored || strings.Contains(pattern, "/")
	for i, component := range components {
		candidate := component
		if matchFullPath {
			candidate = strings.Join(components[:i+1], "/")
		}
		matched, err := doublestar.Match(pattern, candidate)
		if err == nil && matched {
			return true
		}
	}
	return false
}

// filterDiffs removes diffs whose file paths are excluded.
func (p *Provider) filterDiffs(diffs []model.Diff) []model.Diff {
	patterns := p.loadGitignorePatterns()
	var result []model.Diff
	for _, d := range diffs {
		path := d.NewPath
		if path == "/dev/null" {
			path = d.OldPath
		}
		if !p.isPathExcluded(path, patterns) {
			result = append(result, d)
		}
	}
	return result
}

// ---- Internal helpers ----

func (p *Provider) computeMergeBase(ctx context.Context, from, to string) string {
	out, err := p.runGit(ctx, "merge-base", "--end-of-options", from, to)
	if err != nil {
		return ""
	}
	return strings.TrimSpace(out)
}

// RemoteIdentity returns a stable, credential-free identity string for the
// repository's "origin" remote, suitable for hashing into the run manifest's
// repository.identity_sha256. It reads the configured origin URL and canonicalizes
// it — dropping any embedded userinfo, query and fragment (so credentials never
// leak), lowercasing the host while keeping any port, and trimming a trailing
// ".git"/"/" — so the same repository yields the same identity regardless of how
// it was cloned. It returns "" when there is no origin remote, or when origin is
// a local-filesystem remote (the caller then omits repository identity).
func (p *Provider) RemoteIdentity(ctx context.Context) string {
	out, err := p.runGit(ctx, "remote", "get-url", "origin")
	if err != nil {
		return ""
	}
	return canonicalRemote(firstLine(out))
}

// canonicalRemote reduces a git remote URL to a stable, credential-free identity
// string for hashing into repository.identity_sha256, so the same repository
// yields the same identity regardless of transport or embedded credentials.
//
// Network remotes canonicalize to "host[:port]/path": the host is lowercased and
// any port is KEPT (two remotes differing only in port are distinct endpoints),
// while the path preserves case and loses a trailing ".git"/"/".
//
// Local remotes (file://, absolute/relative filesystem paths, Windows drive
// paths, UNC shares) have no stable network identity and no credentials to
// strip; they canonicalize to "" so the caller omits repository identity, the
// same behavior as a missing origin.
//
// An empty or unrecognizable input yields "".
func canonicalRemote(raw string) string {
	s := strings.TrimSpace(raw)
	if s == "" {
		return ""
	}
	// Drop query (?…) and fragment (#…): never part of repository identity.
	if i := strings.IndexAny(s, "?#"); i >= 0 {
		s = s[:i]
	}
	// Local remotes carry no stable network identity (see doc comment). Detect
	// them before the scp split so a Windows "C:\…" path is not mistaken for a
	// "host:path" with host "c".
	if isLocalRemote(s) {
		return ""
	}
	// scheme://[user[:pass]@]host[:port]/path. url.Host is "host[:port]" and
	// never includes userinfo, so credentials drop out and the port is kept.
	if strings.Contains(s, "://") {
		if u, err := url.Parse(s); err == nil && u.Scheme != "" && u.Host != "" {
			return joinHostPath(strings.ToLower(u.Host), u.Path)
		}
		return ""
	}
	// scp-like: [user@]host:path. The userinfo "@" lives in the host segment,
	// which ends at the FIRST ":"; split there first so any "@" inside the path
	// is preserved rather than truncated as if it were userinfo.
	colon := strings.IndexByte(s, ':')
	if colon < 0 {
		return ""
	}
	hostSeg, path := s[:colon], s[colon+1:]
	if at := strings.LastIndexByte(hostSeg, '@'); at >= 0 {
		hostSeg = hostSeg[at+1:]
	}
	host := strings.ToLower(hostSeg)
	if host == "" {
		return ""
	}
	return joinHostPath(host, path)
}

// joinHostPath assembles the canonical "host[/path]" form, trimming a leading
// "/" and a trailing ".git"/"/" from the path while preserving its case.
func joinHostPath(host, path string) string {
	path = strings.TrimPrefix(path, "/")
	path = strings.TrimSuffix(path, "/")
	path = strings.TrimSuffix(path, ".git")
	path = strings.TrimSuffix(path, "/")
	if path == "" {
		return host
	}
	return host + "/" + path
}

// isLocalRemote reports whether a remote URL points at the local filesystem
// rather than a network host: a file:// URL, a POSIX absolute/relative/home
// path, a Windows drive path (X:\ or X:/), or a UNC share (\\server\share).
func isLocalRemote(s string) bool {
	switch {
	case strings.HasPrefix(s, "file://"):
		return true
	case strings.HasPrefix(s, "/"), strings.HasPrefix(s, "~"):
		return true
	case strings.HasPrefix(s, "./"), strings.HasPrefix(s, "../"), s == ".", s == "..":
		return true
	case strings.HasPrefix(s, `\\`): // UNC \\server\share
		return true
	}
	// Windows drive path: X:\ or X:/. Require a separator after the colon so a
	// single-letter scp host ("c:path") is not misread — real hosts have a dot.
	if len(s) >= 3 && isASCIILetter(s[0]) && s[1] == ':' && (s[2] == '\\' || s[2] == '/') {
		return true
	}
	return false
}

func isASCIILetter(b byte) bool {
	return (b >= 'a' && b <= 'z') || (b >= 'A' && b <= 'Z')
}

// resolveCommit returns the immutable commit SHA a ref points at, or "" when the
// ref does not resolve to a commit (e.g. an unborn HEAD, or a bad ref). The
// ^{commit} peel collapses a tag or tree ref to its commit; --verify --quiet
// makes an unresolvable ref exit non-zero silently rather than printing an error.
func (p *Provider) resolveCommit(ctx context.Context, ref string) string {
	out, err := p.runGit(ctx, "rev-parse", "--verify", "--quiet", "--end-of-options", ref+"^{commit}")
	if err != nil {
		return ""
	}
	return firstLine(out)
}

// commitParents returns the parent commit SHAs of ref: zero for a root commit,
// one for an ordinary commit, and 2+ for a merge. It uses `rev-list --parents -n
// 1`, whose single line is "<commit> <parent1> <parent2>…" — the leading commit
// token is dropped, the rest are the parents. `rev-list` (unlike `rev-parse`)
// does not echo --end-of-options, so the marker stays safe against a ref that
// looks like an option. An error yields nil so the caller treats it as "no
// unique base".
func (p *Provider) commitParents(ctx context.Context, ref string) []string {
	out, err := p.runGit(ctx, "rev-list", "--parents", "-n", "1", "--end-of-options", ref)
	if err != nil {
		return nil
	}
	fields := strings.Fields(firstLine(out))
	if len(fields) <= 1 {
		return nil // root commit (only the commit itself, no parents) or empty
	}
	return fields[1:]
}

// firstLine returns the first non-empty trimmed line of git output, so a stray
// trailing newline or an unexpected second line never pollutes a resolved SHA.
func firstLine(out string) string {
	for _, line := range strings.Split(out, "\n") {
		if s := strings.TrimSpace(line); s != "" {
			return s
		}
	}
	return ""
}

// workspaceTrackedDiff returns the tracked-file diff, git's stderr, and the
// failure if there was one. Stderr is returned separately so the caller can
// quote it without risking diff content in the error; see runGitSplit.
func (p *Provider) workspaceTrackedDiff(ctx context.Context) (string, string, error) {
	out, _, err := p.runGitSplit(ctx, "-c", "core.quotepath=false", "diff", "--no-ext-diff", "--no-textconv", "--find-renames", "--src-prefix=a/", "--dst-prefix=b/", "--no-color", "-U"+fmt.Sprint(DiffContextLines), "--end-of-options", "HEAD", "--")
	if err == nil && out != "" {
		return out, "", nil
	}
	if ctx.Err() != nil {
		return "", "", ctx.Err()
	}
	// Fall back to the staged diff when `git diff HEAD` errored or was empty. This is
	// not redundant with the call above: in a repository with no commits yet there is no
	// HEAD, so `git diff HEAD` fails with "bad revision 'HEAD'", but `git diff --staged`
	// still surfaces staged changes by diffing the index against the empty tree — the only
	// way to review a workspace before its first commit.
	return p.runGitSplit(ctx, "-c", "core.quotepath=false", "diff", "--no-ext-diff", "--no-textconv", "--find-renames", "--src-prefix=a/", "--dst-prefix=b/", "--no-color", "-U"+fmt.Sprint(DiffContextLines), "--staged", "--")
}

func (p *Provider) untrackedFileDiffs(ctx context.Context) ([]string, error) {
	files, err := p.untrackedFilesList(ctx)
	if err != nil {
		return nil, err
	}

	var results []string
	for _, f := range files {
		content, rerr := readWorkspaceFileForDiff(p.repoDir, f)
		if rerr != nil {
			continue
		}

		lineCount := bytes.Count(content, []byte{'\n'})
		if len(content) > 0 && content[len(content)-1] != '\n' {
			lineCount++
		}

		var sb strings.Builder
		sb.WriteString(fmt.Sprintf("diff --git a/%s b/%s\n", f, f))
		sb.WriteString("--- /dev/null\n")
		sb.WriteString(fmt.Sprintf("+++ b/%s\n", f))
		sb.WriteString(fmt.Sprintf("@@ -0,0 +1,%d @@\n", lineCount))

		lines := bytes.Split(content, []byte{'\n'})
		if len(lines) > 0 && len(lines[len(lines)-1]) == 0 {
			lines = lines[:len(lines)-1]
		}
		for _, line := range lines {
			sb.WriteByte('+')
			sb.Write(line)
			sb.WriteByte('\n')
		}
		results = append(results, sb.String())
	}
	return results, nil
}

func (p *Provider) untrackedFilesList(ctx context.Context) ([]string, error) {
	out, err := p.runGit(ctx, "-c", "core.quotepath=false", "ls-files", "--others", "--exclude-standard")
	if err != nil || out == "" {
		return nil, nil
	}
	patterns := p.loadGitignorePatterns()
	var files []string
	for _, line := range strings.Split(strings.TrimSpace(out), "\n") {
		line = strings.TrimSpace(line)
		if line == "" {
			continue
		}
		if !p.isPathExcluded(line, patterns) {
			files = append(files, line)
		}
	}
	return files, nil
}

func (p *Provider) runGit(ctx context.Context, args ...string) (string, error) {
	if p.runner != nil {
		return p.runner.Run(ctx, p.repoDir, args...)
	}
	cmd := exec.CommandContext(ctx, "git", args...)
	cmd.Dir = p.repoDir
	out, err := cmd.CombinedOutput()
	return string(out), err
}

// runGitSplit runs git with stdout and stderr kept apart, for the callers that
// quote the failure back to the user.
//
// runGit's combined output cannot be quoted safely. Git writes the diff to
// stdout and its diagnosis to stderr, so a command killed mid-write leaves a
// tail made entirely of diff content — repository source, and whatever that
// source happens to contain — with no diagnosis anywhere in it. That string
// does not stay local: reviewResultError hands it to span.RecordError, so it
// reaches whatever telemetry backend is configured. classifyItemError guards
// the run manifest against raw error text for the same reason.
//
// Stderr carries the diagnosis by construction, since die() writes there, so
// quoting it alone loses nothing a reader wants and cannot carry diff.
//
// Mirrors runGitGrep in internal/tool/code_search.go, cancellation guard
// included: a killed process reports the signal rather than the reason, and
// the reason is what the caller needs.
func (p *Provider) runGitSplit(ctx context.Context, args ...string) (string, string, error) {
	if p.runner != nil {
		stdout, stderr, err := p.runner.RunSplit(ctx, p.repoDir, args...)
		if ctx.Err() != nil && err != nil {
			return "", "", ctx.Err()
		}
		return stdout, stderr, err
	}

	cmd := exec.CommandContext(ctx, "git", args...)
	cmd.Dir = p.repoDir

	var stdout, stderr bytes.Buffer
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr

	err := cmd.Run()
	if ctx.Err() != nil && err != nil && cmd.ProcessState != nil && cmd.ProcessState.ExitCode() == -1 {
		return "", "", ctx.Err()
	}
	return stdout.String(), stderr.String(), err
}

// gitDiagLimit bounds how much of git's stderr is quoted back in an error.
// Git's own diagnosis is a line or two, so the ceiling is a backstop for the
// pathological case — a flood of warnings ahead of the fatal — rather than the
// common one.
const gitDiagLimit = 2000

// gitFailure builds an error that carries git's own message.
//
// Every diff-producing caller used to drop git's output on the floor, so a
// failure surfaced as a bare "git show failed: exit status 129" — true, and
// useless. Diagnosing one then meant asking the reporter to re-run the command
// by hand to see what git actually said (#972). The exit status alone cannot
// distinguish an unsupported option from a bad revision or a permission error.
//
// Callers pass stderr, never runGit's combined output; see runGitSplit for why.
func gitFailure(op, stderr string, err error) error {
	diag := strings.TrimSpace(stderr)
	if diag == "" {
		return fmt.Errorf("%s failed: %w", op, err)
	}
	if len(diag) > gitDiagLimit {
		// Keep the tail: die() exits the process, so the fatal git ends on is
		// the last thing it writes, behind any warnings that preceded it.
		diag = diag[len(diag)-gitDiagLimit:]
		// Cutting by bytes can land mid-rune. Git speaks the user's locale,
		// so this is not hypothetical — #972 came from a Japanese-language
		// Windows install. Drop the partial leading rune rather than emit
		// invalid UTF-8.
		for len(diag) > 0 && !utf8.RuneStart(diag[0]) {
			diag = diag[1:]
		}
		diag = "..." + diag
	}
	return fmt.Errorf("%s failed: %w: %s", op, err, diag)
}