Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 3316 insertions, 0 deletions

diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
new file mode 100644
index 0000000..422e83c
--- /dev/null
+++ b/src/cmd/go/internal/work/exec.go
@@ -0,0 +1,3316 @@
+// Copyright 2011 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.
+
+// Action graph execution.
+
+package work
+
+import (
+	"bytes"
+	"cmd/go/internal/fsys"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	exec "internal/execabs"
+	"internal/lazyregexp"
+	"io"
+	"io/fs"
+	"log"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+)
+
+// actionList returns the list of actions in the dag rooted at root
+// as visited in a depth-first post-order traversal.
+func actionList(root *Action) []*Action {
+	seen := map[*Action]bool{}
+	all := []*Action{}
+	var walk func(*Action)
+	walk = func(a *Action) {
+		if seen[a] {
+			return
+		}
+		seen[a] = true
+		for _, a1 := range a.Deps {
+			walk(a1)
+		}
+		all = append(all, a)
+	}
+	walk(root)
+	return all
+}
+
+// do runs the action graph rooted at root.
+func (b *Builder) Do(ctx context.Context, root *Action) {
+	ctx, span := trace.StartSpan(ctx, "exec.Builder.Do ("+root.Mode+" "+root.Target+")")
+	defer span.Done()
+
+	if !b.IsCmdList {
+		// If we're doing real work, take time at the end to trim the cache.
+		c := cache.Default()
+		defer c.Trim()
+	}
+
+	// Build list of all actions, assigning depth-first post-order priority.
+	// The original implementation here was a true queue
+	// (using a channel) but it had the effect of getting
+	// distracted by low-level leaf actions to the detriment
+	// of completing higher-level actions. The order of
+	// work does not matter much to overall execution time,
+	// but when running "go test std" it is nice to see each test
+	// results as soon as possible. The priorities assigned
+	// ensure that, all else being equal, the execution prefers
+	// to do what it would have done first in a simple depth-first
+	// dependency order traversal.
+	all := actionList(root)
+	for i, a := range all {
+		a.priority = i
+	}
+
+	// Write action graph, without timing information, in case we fail and exit early.
+	writeActionGraph := func() {
+		if file := cfg.DebugActiongraph; file != "" {
+			if strings.HasSuffix(file, ".go") {
+				// Do not overwrite Go source code in:
+				//	go build -debug-actiongraph x.go
+				base.Fatalf("go: refusing to write action graph to %v\n", file)
+			}
+			js := actionGraphJSON(root)
+			if err := os.WriteFile(file, []byte(js), 0666); err != nil {
+				fmt.Fprintf(os.Stderr, "go: writing action graph: %v\n", err)
+				base.SetExitStatus(1)
+			}
+		}
+	}
+	writeActionGraph()
+
+	b.readySema = make(chan bool, len(all))
+
+	// Initialize per-action execution state.
+	for _, a := range all {
+		for _, a1 := range a.Deps {
+			a1.triggers = append(a1.triggers, a)
+		}
+		a.pending = len(a.Deps)
+		if a.pending == 0 {
+			b.ready.push(a)
+			b.readySema <- true
+		}
+	}
+
+	// Handle runs a single action and takes care of triggering
+	// any actions that are runnable as a result.
+	handle := func(ctx context.Context, a *Action) {
+		if a.json != nil {
+			a.json.TimeStart = time.Now()
+		}
+		var err error
+		if a.Func != nil && (!a.Failed || a.IgnoreFail) {
+			// TODO(matloob): Better action descriptions
+			desc := "Executing action "
+			if a.Package != nil {
+				desc += "(" + a.Mode + " " + a.Package.Desc() + ")"
+			}
+			ctx, span := trace.StartSpan(ctx, desc)
+			a.traceSpan = span
+			for _, d := range a.Deps {
+				trace.Flow(ctx, d.traceSpan, a.traceSpan)
+			}
+			err = a.Func(b, ctx, a)
+			span.Done()
+		}
+		if a.json != nil {
+			a.json.TimeDone = time.Now()
+		}
+
+		// The actions run in parallel but all the updates to the
+		// shared work state are serialized through b.exec.
+		b.exec.Lock()
+		defer b.exec.Unlock()
+
+		if err != nil {
+			if err == errPrintedOutput {
+				base.SetExitStatus(2)
+			} else {
+				base.Errorf("%s", err)
+			}
+			a.Failed = true
+		}
+
+		for _, a0 := range a.triggers {
+			if a.Failed {
+				a0.Failed = true
+			}
+			if a0.pending--; a0.pending == 0 {
+				b.ready.push(a0)
+				b.readySema <- true
+			}
+		}
+
+		if a == root {
+			close(b.readySema)
+		}
+	}
+
+	var wg sync.WaitGroup
+
+	// Kick off goroutines according to parallelism.
+	// If we are using the -n flag (just printing commands)
+	// drop the parallelism to 1, both to make the output
+	// deterministic and because there is no real work anyway.
+	par := cfg.BuildP
+	if cfg.BuildN {
+		par = 1
+	}
+	for i := 0; i < par; i++ {
+		wg.Add(1)
+		go func() {
+			ctx := trace.StartGoroutine(ctx)
+			defer wg.Done()
+			for {
+				select {
+				case _, ok := <-b.readySema:
+					if !ok {
+						return
+					}
+					// Receiving a value from b.readySema entitles
+					// us to take from the ready queue.
+					b.exec.Lock()
+					a := b.ready.pop()
+					b.exec.Unlock()
+					handle(ctx, a)
+				case <-base.Interrupted:
+					base.SetExitStatus(1)
+					return
+				}
+			}
+		}()
+	}
+
+	wg.Wait()
+
+	// Write action graph again, this time with timing information.
+	writeActionGraph()
+}
+
+// buildActionID computes the action ID for a build action.
+func (b *Builder) buildActionID(a *Action) cache.ActionID {
+	p := a.Package
+	h := cache.NewHash("build " + p.ImportPath)
+
+	// Configuration independent of compiler toolchain.
+	// Note: buildmode has already been accounted for in buildGcflags
+	// and should not be inserted explicitly. Most buildmodes use the
+	// same compiler settings and can reuse each other's results.
+	// If not, the reason is already recorded in buildGcflags.
+	fmt.Fprintf(h, "compile\n")
+	// Only include the package directory if it may affect the output.
+	// We trim workspace paths for all packages when -trimpath is set.
+	// The compiler hides the exact value of $GOROOT
+	// when building things in GOROOT.
+	// Assume b.WorkDir is being trimmed properly.
+	// When -trimpath is used with a package built from the module cache,
+	// use the module path and version instead of the directory.
+	if !p.Goroot && !cfg.BuildTrimpath && !strings.HasPrefix(p.Dir, b.WorkDir) {
+		fmt.Fprintf(h, "dir %s\n", p.Dir)
+	} else if cfg.BuildTrimpath && p.Module != nil {
+		fmt.Fprintf(h, "module %s@%s\n", p.Module.Path, p.Module.Version)
+	}
+	if p.Module != nil {
+		fmt.Fprintf(h, "go %s\n", p.Module.GoVersion)
+	}
+	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
+	fmt.Fprintf(h, "import %q\n", p.ImportPath)
+	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
+	if p.Internal.ForceLibrary {
+		fmt.Fprintf(h, "forcelibrary\n")
+	}
+	if len(p.CgoFiles)+len(p.SwigFiles) > 0 {
+		fmt.Fprintf(h, "cgo %q\n", b.toolID("cgo"))
+		cppflags, cflags, cxxflags, fflags, ldflags, _ := b.CFlags(p)
+		fmt.Fprintf(h, "CC=%q %q %q %q\n", b.ccExe(), cppflags, cflags, ldflags)
+		if len(p.CXXFiles)+len(p.SwigFiles) > 0 {
+			fmt.Fprintf(h, "CXX=%q %q\n", b.cxxExe(), cxxflags)
+		}
+		if len(p.FFiles) > 0 {
+			fmt.Fprintf(h, "FC=%q %q\n", b.fcExe(), fflags)
+		}
+		// TODO(rsc): Should we include the SWIG version or Fortran/GCC/G++/Objective-C compiler versions?
+	}
+	if p.Internal.CoverMode != "" {
+		fmt.Fprintf(h, "cover %q %q\n", p.Internal.CoverMode, b.toolID("cover"))
+	}
+	fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo)
+
+	// Configuration specific to compiler toolchain.
+	switch cfg.BuildToolchainName {
+	default:
+		base.Fatalf("buildActionID: unknown build toolchain %q", cfg.BuildToolchainName)
+	case "gc":
+		fmt.Fprintf(h, "compile %s %q %q\n", b.toolID("compile"), forcedGcflags, p.Internal.Gcflags)
+		if len(p.SFiles) > 0 {
+			fmt.Fprintf(h, "asm %q %q %q\n", b.toolID("asm"), forcedAsmflags, p.Internal.Asmflags)
+		}
+
+		// GOARM, GOMIPS, etc.
+		key, val := cfg.GetArchEnv()
+		fmt.Fprintf(h, "%s=%s\n", key, val)
+
+		// TODO(rsc): Convince compiler team not to add more magic environment variables,
+		// or perhaps restrict the environment variables passed to subprocesses.
+		// Because these are clumsy, undocumented special-case hacks
+		// for debugging the compiler, they are not settable using 'go env -w',
+		// and so here we use os.Getenv, not cfg.Getenv.
+		magic := []string{
+			"GOCLOBBERDEADHASH",
+			"GOSSAFUNC",
+			"GO_SSA_PHI_LOC_CUTOFF",
+			"GOSSAHASH",
+		}
+		for _, env := range magic {
+			if x := os.Getenv(env); x != "" {
+				fmt.Fprintf(h, "magic %s=%s\n", env, x)
+			}
+		}
+		if os.Getenv("GOSSAHASH") != "" {
+			for i := 0; ; i++ {
+				env := fmt.Sprintf("GOSSAHASH%d", i)
+				x := os.Getenv(env)
+				if x == "" {
+					break
+				}
+				fmt.Fprintf(h, "magic %s=%s\n", env, x)
+			}
+		}
+		if os.Getenv("GSHS_LOGFILE") != "" {
+			// Clumsy hack. Compiler writes to this log file,
+			// so do not allow use of cache at all.
+			// We will still write to the cache but it will be
+			// essentially unfindable.
+			fmt.Fprintf(h, "nocache %d\n", time.Now().UnixNano())
+		}
+
+	case "gccgo":
+		id, err := b.gccgoToolID(BuildToolchain.compiler(), "go")
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		fmt.Fprintf(h, "compile %s %q %q\n", id, forcedGccgoflags, p.Internal.Gccgoflags)
+		fmt.Fprintf(h, "pkgpath %s\n", gccgoPkgpath(p))
+		fmt.Fprintf(h, "ar %q\n", BuildToolchain.(gccgoToolchain).ar())
+		if len(p.SFiles) > 0 {
+			id, _ = b.gccgoToolID(BuildToolchain.compiler(), "assembler-with-cpp")
+			// Ignore error; different assembler versions
+			// are unlikely to make any difference anyhow.
+			fmt.Fprintf(h, "asm %q\n", id)
+		}
+	}
+
+	// Input files.
+	inputFiles := str.StringList(
+		p.GoFiles,
+		p.CgoFiles,
+		p.CFiles,
+		p.CXXFiles,
+		p.FFiles,
+		p.MFiles,
+		p.HFiles,
+		p.SFiles,
+		p.SysoFiles,
+		p.SwigFiles,
+		p.SwigCXXFiles,
+		p.EmbedFiles,
+	)
+	for _, file := range inputFiles {
+		fmt.Fprintf(h, "file %s %s\n", file, b.fileHash(filepath.Join(p.Dir, file)))
+	}
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 != nil {
+			fmt.Fprintf(h, "import %s %s\n", p1.ImportPath, contentID(a1.buildID))
+		}
+	}
+
+	return h.Sum()
+}
+
+// needCgoHdr reports whether the actions triggered by this one
+// expect to be able to access the cgo-generated header file.
+func (b *Builder) needCgoHdr(a *Action) bool {
+	// If this build triggers a header install, run cgo to get the header.
+	if !b.IsCmdList && (a.Package.UsesCgo() || a.Package.UsesSwig()) && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
+		for _, t1 := range a.triggers {
+			if t1.Mode == "install header" {
+				return true
+			}
+		}
+		for _, t1 := range a.triggers {
+			for _, t2 := range t1.triggers {
+				if t2.Mode == "install header" {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+// allowedVersion reports whether the version v is an allowed version of go
+// (one that we can compile).
+// v is known to be of the form "1.23".
+func allowedVersion(v string) bool {
+	// Special case: no requirement.
+	if v == "" {
+		return true
+	}
+	// Special case "1.0" means "go1", which is OK.
+	if v == "1.0" {
+		return true
+	}
+	// Otherwise look through release tags of form "go1.23" for one that matches.
+	for _, tag := range cfg.BuildContext.ReleaseTags {
+		if strings.HasPrefix(tag, "go") && tag[2:] == v {
+			return true
+		}
+	}
+	return false
+}
+
+const (
+	needBuild uint32 = 1 << iota
+	needCgoHdr
+	needVet
+	needCompiledGoFiles
+	needStale
+)
+
+// build is the action for building a single package.
+// Note that any new influence on this logic must be reported in b.buildActionID above as well.
+func (b *Builder) build(ctx context.Context, a *Action) (err error) {
+	p := a.Package
+
+	bit := func(x uint32, b bool) uint32 {
+		if b {
+			return x
+		}
+		return 0
+	}
+
+	cachedBuild := false
+	need := bit(needBuild, !b.IsCmdList && a.needBuild || b.NeedExport) |
+		bit(needCgoHdr, b.needCgoHdr(a)) |
+		bit(needVet, a.needVet) |
+		bit(needCompiledGoFiles, b.NeedCompiledGoFiles)
+
+	if !p.BinaryOnly {
+		if b.useCache(a, b.buildActionID(a), p.Target) {
+			// We found the main output in the cache.
+			// If we don't need any other outputs, we can stop.
+			// Otherwise, we need to write files to a.Objdir (needVet, needCgoHdr).
+			// Remember that we might have them in cache
+			// and check again after we create a.Objdir.
+			cachedBuild = true
+			a.output = []byte{} // start saving output in case we miss any cache results
+			need &^= needBuild
+			if b.NeedExport {
+				p.Export = a.built
+				p.BuildID = a.buildID
+			}
+			if need&needCompiledGoFiles != 0 {
+				if err := b.loadCachedSrcFiles(a); err == nil {
+					need &^= needCompiledGoFiles
+				}
+			}
+		}
+
+		// Source files might be cached, even if the full action is not
+		// (e.g., go list -compiled -find).
+		if !cachedBuild && need&needCompiledGoFiles != 0 {
+			if err := b.loadCachedSrcFiles(a); err == nil {
+				need &^= needCompiledGoFiles
+			}
+		}
+
+		if need == 0 {
+			return nil
+		}
+		defer b.flushOutput(a)
+	}
+
+	defer func() {
+		if err != nil && err != errPrintedOutput {
+			err = fmt.Errorf("go build %s: %v", a.Package.ImportPath, err)
+		}
+		if err != nil && b.IsCmdList && b.NeedError && p.Error == nil {
+			p.Error = &load.PackageError{Err: err}
+		}
+	}()
+	if cfg.BuildN {
+		// In -n mode, print a banner between packages.
+		// The banner is five lines so that when changes to
+		// different sections of the bootstrap script have to
+		// be merged, the banners give patch something
+		// to use to find its context.
+		b.Print("\n#\n# " + a.Package.ImportPath + "\n#\n\n")
+	}
+
+	if cfg.BuildV {
+		b.Print(a.Package.ImportPath + "\n")
+	}
+
+	if a.Package.BinaryOnly {
+		p.Stale = true
+		p.StaleReason = "binary-only packages are no longer supported"
+		if b.IsCmdList {
+			return nil
+		}
+		return errors.New("binary-only packages are no longer supported")
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+	objdir := a.Objdir
+
+	// Load cached cgo header, but only if we're skipping the main build (cachedBuild==true).
+	if cachedBuild && need&needCgoHdr != 0 {
+		if err := b.loadCachedCgoHdr(a); err == nil {
+			need &^= needCgoHdr
+		}
+	}
+
+	// Load cached vet config, but only if that's all we have left
+	// (need == needVet, not testing just the one bit).
+	// If we are going to do a full build anyway,
+	// we're going to regenerate the files below anyway.
+	if need == needVet {
+		if err := b.loadCachedVet(a); err == nil {
+			need &^= needVet
+		}
+	}
+	if need == 0 {
+		return nil
+	}
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	gofiles := str.StringList(a.Package.GoFiles)
+	cgofiles := str.StringList(a.Package.CgoFiles)
+	cfiles := str.StringList(a.Package.CFiles)
+	sfiles := str.StringList(a.Package.SFiles)
+	cxxfiles := str.StringList(a.Package.CXXFiles)
+	var objects, cgoObjects, pcCFLAGS, pcLDFLAGS []string
+
+	if a.Package.UsesCgo() || a.Package.UsesSwig() {
+		if pcCFLAGS, pcLDFLAGS, err = b.getPkgConfigFlags(a.Package); err != nil {
+			return
+		}
+	}
+
+	// Compute overlays for .c/.cc/.h/etc. and if there are any overlays
+	// put correct contents of all those files in the objdir, to ensure
+	// the correct headers are included. nonGoOverlay is the overlay that
+	// points from nongo files to the copied files in objdir.
+	nonGoFileLists := [][]string{a.Package.CFiles, a.Package.SFiles, a.Package.CXXFiles, a.Package.HFiles, a.Package.FFiles}
+OverlayLoop:
+	for _, fs := range nonGoFileLists {
+		for _, f := range fs {
+			if _, ok := fsys.OverlayPath(mkAbs(p.Dir, f)); ok {
+				a.nonGoOverlay = make(map[string]string)
+				break OverlayLoop
+			}
+		}
+	}
+	if a.nonGoOverlay != nil {
+		for _, fs := range nonGoFileLists {
+			for i := range fs {
+				from := mkAbs(p.Dir, fs[i])
+				opath, _ := fsys.OverlayPath(from)
+				dst := objdir + filepath.Base(fs[i])
+				if err := b.copyFile(dst, opath, 0666, false); err != nil {
+					return err
+				}
+				a.nonGoOverlay[from] = dst
+			}
+		}
+	}
+
+	// Run SWIG on each .swig and .swigcxx file.
+	// Each run will generate two files, a .go file and a .c or .cxx file.
+	// The .go file will use import "C" and is to be processed by cgo.
+	if a.Package.UsesSwig() {
+		outGo, outC, outCXX, err := b.swig(a, a.Package, objdir, pcCFLAGS)
+		if err != nil {
+			return err
+		}
+		cgofiles = append(cgofiles, outGo...)
+		cfiles = append(cfiles, outC...)
+		cxxfiles = append(cxxfiles, outCXX...)
+	}
+
+	// If we're doing coverage, preprocess the .go files and put them in the work directory
+	if a.Package.Internal.CoverMode != "" {
+		for i, file := range str.StringList(gofiles, cgofiles) {
+			var sourceFile string
+			var coverFile string
+			var key string
+			if strings.HasSuffix(file, ".cgo1.go") {
+				// cgo files have absolute paths
+				base := filepath.Base(file)
+				sourceFile = file
+				coverFile = objdir + base
+				key = strings.TrimSuffix(base, ".cgo1.go") + ".go"
+			} else {
+				sourceFile = filepath.Join(a.Package.Dir, file)
+				coverFile = objdir + file
+				key = file
+			}
+			coverFile = strings.TrimSuffix(coverFile, ".go") + ".cover.go"
+			cover := a.Package.Internal.CoverVars[key]
+			if cover == nil || base.IsTestFile(file) {
+				// Not covering this file.
+				continue
+			}
+			if err := b.cover(a, coverFile, sourceFile, cover.Var); err != nil {
+				return err
+			}
+			if i < len(gofiles) {
+				gofiles[i] = coverFile
+			} else {
+				cgofiles[i-len(gofiles)] = coverFile
+			}
+		}
+	}
+
+	// Run cgo.
+	if a.Package.UsesCgo() || a.Package.UsesSwig() {
+		// In a package using cgo, cgo compiles the C, C++ and assembly files with gcc.
+		// There is one exception: runtime/cgo's job is to bridge the
+		// cgo and non-cgo worlds, so it necessarily has files in both.
+		// In that case gcc only gets the gcc_* files.
+		var gccfiles []string
+		gccfiles = append(gccfiles, cfiles...)
+		cfiles = nil
+		if a.Package.Standard && a.Package.ImportPath == "runtime/cgo" {
+			filter := func(files, nongcc, gcc []string) ([]string, []string) {
+				for _, f := range files {
+					if strings.HasPrefix(f, "gcc_") {
+						gcc = append(gcc, f)
+					} else {
+						nongcc = append(nongcc, f)
+					}
+				}
+				return nongcc, gcc
+			}
+			sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
+		} else {
+			for _, sfile := range sfiles {
+				data, err := os.ReadFile(filepath.Join(a.Package.Dir, sfile))
+				if err == nil {
+					if bytes.HasPrefix(data, []byte("TEXT")) || bytes.Contains(data, []byte("\nTEXT")) ||
+						bytes.HasPrefix(data, []byte("DATA")) || bytes.Contains(data, []byte("\nDATA")) ||
+						bytes.HasPrefix(data, []byte("GLOBL")) || bytes.Contains(data, []byte("\nGLOBL")) {
+						return fmt.Errorf("package using cgo has Go assembly file %s", sfile)
+					}
+				}
+			}
+			gccfiles = append(gccfiles, sfiles...)
+			sfiles = nil
+		}
+
+		outGo, outObj, err := b.cgo(a, base.Tool("cgo"), objdir, pcCFLAGS, pcLDFLAGS, mkAbsFiles(a.Package.Dir, cgofiles), gccfiles, cxxfiles, a.Package.MFiles, a.Package.FFiles)
+		if err != nil {
+			return err
+		}
+		if cfg.BuildToolchainName == "gccgo" {
+			cgoObjects = append(cgoObjects, a.Objdir+"_cgo_flags")
+		}
+		cgoObjects = append(cgoObjects, outObj...)
+		gofiles = append(gofiles, outGo...)
+
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared":
+			b.cacheCgoHdr(a)
+		}
+	}
+
+	var srcfiles []string // .go and non-.go
+	srcfiles = append(srcfiles, gofiles...)
+	srcfiles = append(srcfiles, sfiles...)
+	srcfiles = append(srcfiles, cfiles...)
+	srcfiles = append(srcfiles, cxxfiles...)
+	b.cacheSrcFiles(a, srcfiles)
+
+	// Running cgo generated the cgo header.
+	need &^= needCgoHdr
+
+	// Sanity check only, since Package.load already checked as well.
+	if len(gofiles) == 0 {
+		return &load.NoGoError{Package: a.Package}
+	}
+
+	// Prepare Go vet config if needed.
+	if need&needVet != 0 {
+		buildVetConfig(a, srcfiles)
+		need &^= needVet
+	}
+	if need&needCompiledGoFiles != 0 {
+		if err := b.loadCachedSrcFiles(a); err != nil {
+			return fmt.Errorf("loading compiled Go files from cache: %w", err)
+		}
+		need &^= needCompiledGoFiles
+	}
+	if need == 0 {
+		// Nothing left to do.
+		return nil
+	}
+
+	// Collect symbol ABI requirements from assembly.
+	symabis, err := BuildToolchain.symabis(b, a, sfiles)
+	if err != nil {
+		return err
+	}
+
+	// Prepare Go import config.
+	// We start it off with a comment so it can't be empty, so icfg.Bytes() below is never nil.
+	// It should never be empty anyway, but there have been bugs in the past that resulted
+	// in empty configs, which then unfortunately turn into "no config passed to compiler",
+	// and the compiler falls back to looking in pkg itself, which mostly works,
+	// except when it doesn't.
+	var icfg bytes.Buffer
+	fmt.Fprintf(&icfg, "# import config\n")
+	for i, raw := range a.Package.Internal.RawImports {
+		final := a.Package.Imports[i]
+		if final != raw {
+			fmt.Fprintf(&icfg, "importmap %s=%s\n", raw, final)
+		}
+	}
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil || p1.ImportPath == "" || a1.built == "" {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
+	}
+
+	// Prepare Go embed config if needed.
+	// Unlike the import config, it's okay for the embed config to be empty.
+	var embedcfg []byte
+	if len(p.Internal.Embed) > 0 {
+		var embed struct {
+			Patterns map[string][]string
+			Files    map[string]string
+		}
+		embed.Patterns = p.Internal.Embed
+		embed.Files = make(map[string]string)
+		for _, file := range p.EmbedFiles {
+			embed.Files[file] = filepath.Join(p.Dir, file)
+		}
+		js, err := json.MarshalIndent(&embed, "", "\t")
+		if err != nil {
+			return fmt.Errorf("marshal embedcfg: %v", err)
+		}
+		embedcfg = js
+	}
+
+	if p.Internal.BuildInfo != "" && cfg.ModulesEnabled {
+		if err := b.writeFile(objdir+"_gomod_.go", modload.ModInfoProg(p.Internal.BuildInfo, cfg.BuildToolchainName == "gccgo")); err != nil {
+			return err
+		}
+		gofiles = append(gofiles, objdir+"_gomod_.go")
+	}
+
+	// Compile Go.
+	objpkg := objdir + "_pkg_.a"
+	ofile, out, err := BuildToolchain.gc(b, a, objpkg, icfg.Bytes(), embedcfg, symabis, len(sfiles) > 0, gofiles)
+	if len(out) > 0 {
+		output := b.processOutput(out)
+		if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
+			output += "note: module requires Go " + p.Module.GoVersion + "\n"
+		}
+		b.showOutput(a, a.Package.Dir, a.Package.Desc(), output)
+		if err != nil {
+			return errPrintedOutput
+		}
+	}
+	if err != nil {
+		if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
+			b.showOutput(a, a.Package.Dir, a.Package.Desc(), "note: module requires Go "+p.Module.GoVersion+"\n")
+		}
+		return err
+	}
+	if ofile != objpkg {
+		objects = append(objects, ofile)
+	}
+
+	// Copy .h files named for goos or goarch or goos_goarch
+	// to names using GOOS and GOARCH.
+	// For example, defs_linux_amd64.h becomes defs_GOOS_GOARCH.h.
+	_goos_goarch := "_" + cfg.Goos + "_" + cfg.Goarch
+	_goos := "_" + cfg.Goos
+	_goarch := "_" + cfg.Goarch
+	for _, file := range a.Package.HFiles {
+		name, ext := fileExtSplit(file)
+		switch {
+		case strings.HasSuffix(name, _goos_goarch):
+			targ := file[:len(name)-len(_goos_goarch)] + "_GOOS_GOARCH." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(a.Package.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		case strings.HasSuffix(name, _goarch):
+			targ := file[:len(name)-len(_goarch)] + "_GOARCH." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(a.Package.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		case strings.HasSuffix(name, _goos):
+			targ := file[:len(name)-len(_goos)] + "_GOOS." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(a.Package.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		}
+	}
+
+	for _, file := range cfiles {
+		out := file[:len(file)-len(".c")] + ".o"
+		if err := BuildToolchain.cc(b, a, objdir+out, file); err != nil {
+			return err
+		}
+		objects = append(objects, out)
+	}
+
+	// Assemble .s files.
+	if len(sfiles) > 0 {
+		ofiles, err := BuildToolchain.asm(b, a, sfiles)
+		if err != nil {
+			return err
+		}
+		objects = append(objects, ofiles...)
+	}
+
+	// For gccgo on ELF systems, we write the build ID as an assembler file.
+	// This lets us set the SHF_EXCLUDE flag.
+	// This is read by readGccgoArchive in cmd/internal/buildid/buildid.go.
+	if a.buildID != "" && cfg.BuildToolchainName == "gccgo" {
+		switch cfg.Goos {
+		case "aix", "android", "dragonfly", "freebsd", "illumos", "linux", "netbsd", "openbsd", "solaris":
+			asmfile, err := b.gccgoBuildIDFile(a)
+			if err != nil {
+				return err
+			}
+			ofiles, err := BuildToolchain.asm(b, a, []string{asmfile})
+			if err != nil {
+				return err
+			}
+			objects = append(objects, ofiles...)
+		}
+	}
+
+	// NOTE(rsc): On Windows, it is critically important that the
+	// gcc-compiled objects (cgoObjects) be listed after the ordinary
+	// objects in the archive. I do not know why this is.
+	// https://golang.org/issue/2601
+	objects = append(objects, cgoObjects...)
+
+	// Add system object files.
+	for _, syso := range a.Package.SysoFiles {
+		objects = append(objects, filepath.Join(a.Package.Dir, syso))
+	}
+
+	// Pack into archive in objdir directory.
+	// If the Go compiler wrote an archive, we only need to add the
+	// object files for non-Go sources to the archive.
+	// If the Go compiler wrote an archive and the package is entirely
+	// Go sources, there is no pack to execute at all.
+	if len(objects) > 0 {
+		if err := BuildToolchain.pack(b, a, objpkg, objects); err != nil {
+			return err
+		}
+	}
+
+	if err := b.updateBuildID(a, objpkg, true); err != nil {
+		return err
+	}
+
+	a.built = objpkg
+	return nil
+}
+
+func (b *Builder) cacheObjdirFile(a *Action, c *cache.Cache, name string) error {
+	f, err := os.Open(a.Objdir + name)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	_, _, err = c.Put(cache.Subkey(a.actionID, name), f)
+	return err
+}
+
+func (b *Builder) findCachedObjdirFile(a *Action, c *cache.Cache, name string) (string, error) {
+	file, _, err := c.GetFile(cache.Subkey(a.actionID, name))
+	if err != nil {
+		return "", fmt.Errorf("loading cached file %s: %w", name, err)
+	}
+	return file, nil
+}
+
+func (b *Builder) loadCachedObjdirFile(a *Action, c *cache.Cache, name string) error {
+	cached, err := b.findCachedObjdirFile(a, c, name)
+	if err != nil {
+		return err
+	}
+	return b.copyFile(a.Objdir+name, cached, 0666, true)
+}
+
+func (b *Builder) cacheCgoHdr(a *Action) {
+	c := cache.Default()
+	b.cacheObjdirFile(a, c, "_cgo_install.h")
+}
+
+func (b *Builder) loadCachedCgoHdr(a *Action) error {
+	c := cache.Default()
+	return b.loadCachedObjdirFile(a, c, "_cgo_install.h")
+}
+
+func (b *Builder) cacheSrcFiles(a *Action, srcfiles []string) {
+	c := cache.Default()
+	var buf bytes.Buffer
+	for _, file := range srcfiles {
+		if !strings.HasPrefix(file, a.Objdir) {
+			// not generated
+			buf.WriteString("./")
+			buf.WriteString(file)
+			buf.WriteString("\n")
+			continue
+		}
+		name := file[len(a.Objdir):]
+		buf.WriteString(name)
+		buf.WriteString("\n")
+		if err := b.cacheObjdirFile(a, c, name); err != nil {
+			return
+		}
+	}
+	c.PutBytes(cache.Subkey(a.actionID, "srcfiles"), buf.Bytes())
+}
+
+func (b *Builder) loadCachedVet(a *Action) error {
+	c := cache.Default()
+	list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles"))
+	if err != nil {
+		return fmt.Errorf("reading srcfiles list: %w", err)
+	}
+	var srcfiles []string
+	for _, name := range strings.Split(string(list), "\n") {
+		if name == "" { // end of list
+			continue
+		}
+		if strings.HasPrefix(name, "./") {
+			srcfiles = append(srcfiles, name[2:])
+			continue
+		}
+		if err := b.loadCachedObjdirFile(a, c, name); err != nil {
+			return err
+		}
+		srcfiles = append(srcfiles, a.Objdir+name)
+	}
+	buildVetConfig(a, srcfiles)
+	return nil
+}
+
+func (b *Builder) loadCachedSrcFiles(a *Action) error {
+	c := cache.Default()
+	list, _, err := c.GetBytes(cache.Subkey(a.actionID, "srcfiles"))
+	if err != nil {
+		return fmt.Errorf("reading srcfiles list: %w", err)
+	}
+	var files []string
+	for _, name := range strings.Split(string(list), "\n") {
+		if name == "" { // end of list
+			continue
+		}
+		if strings.HasPrefix(name, "./") {
+			files = append(files, name[len("./"):])
+			continue
+		}
+		file, err := b.findCachedObjdirFile(a, c, name)
+		if err != nil {
+			return fmt.Errorf("finding %s: %w", name, err)
+		}
+		files = append(files, file)
+	}
+	a.Package.CompiledGoFiles = files
+	return nil
+}
+
+// vetConfig is the configuration passed to vet describing a single package.
+type vetConfig struct {
+	ID           string   // package ID (example: "fmt [fmt.test]")
+	Compiler     string   // compiler name (gc, gccgo)
+	Dir          string   // directory containing package
+	ImportPath   string   // canonical import path ("package path")
+	GoFiles      []string // absolute paths to package source files
+	NonGoFiles   []string // absolute paths to package non-Go files
+	IgnoredFiles []string // absolute paths to ignored source files
+
+	ImportMap   map[string]string // map import path in source code to package path
+	PackageFile map[string]string // map package path to .a file with export data
+	Standard    map[string]bool   // map package path to whether it's in the standard library
+	PackageVetx map[string]string // map package path to vetx data from earlier vet run
+	VetxOnly    bool              // only compute vetx data; don't report detected problems
+	VetxOutput  string            // write vetx data to this output file
+
+	SucceedOnTypecheckFailure bool // awful hack; see #18395 and below
+}
+
+func buildVetConfig(a *Action, srcfiles []string) {
+	// Classify files based on .go extension.
+	// srcfiles does not include raw cgo files.
+	var gofiles, nongofiles []string
+	for _, name := range srcfiles {
+		if strings.HasSuffix(name, ".go") {
+			gofiles = append(gofiles, name)
+		} else {
+			nongofiles = append(nongofiles, name)
+		}
+	}
+
+	ignored := str.StringList(a.Package.IgnoredGoFiles, a.Package.IgnoredOtherFiles)
+
+	// Pass list of absolute paths to vet,
+	// so that vet's error messages will use absolute paths,
+	// so that we can reformat them relative to the directory
+	// in which the go command is invoked.
+	vcfg := &vetConfig{
+		ID:           a.Package.ImportPath,
+		Compiler:     cfg.BuildToolchainName,
+		Dir:          a.Package.Dir,
+		GoFiles:      mkAbsFiles(a.Package.Dir, gofiles),
+		NonGoFiles:   mkAbsFiles(a.Package.Dir, nongofiles),
+		IgnoredFiles: mkAbsFiles(a.Package.Dir, ignored),
+		ImportPath:   a.Package.ImportPath,
+		ImportMap:    make(map[string]string),
+		PackageFile:  make(map[string]string),
+		Standard:     make(map[string]bool),
+	}
+	a.vetCfg = vcfg
+	for i, raw := range a.Package.Internal.RawImports {
+		final := a.Package.Imports[i]
+		vcfg.ImportMap[raw] = final
+	}
+
+	// Compute the list of mapped imports in the vet config
+	// so that we can add any missing mappings below.
+	vcfgMapped := make(map[string]bool)
+	for _, p := range vcfg.ImportMap {
+		vcfgMapped[p] = true
+	}
+
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil || p1.ImportPath == "" {
+			continue
+		}
+		// Add import mapping if needed
+		// (for imports like "runtime/cgo" that appear only in generated code).
+		if !vcfgMapped[p1.ImportPath] {
+			vcfg.ImportMap[p1.ImportPath] = p1.ImportPath
+		}
+		if a1.built != "" {
+			vcfg.PackageFile[p1.ImportPath] = a1.built
+		}
+		if p1.Standard {
+			vcfg.Standard[p1.ImportPath] = true
+		}
+	}
+}
+
+// VetTool is the path to an alternate vet tool binary.
+// The caller is expected to set it (if needed) before executing any vet actions.
+var VetTool string
+
+// VetFlags are the default flags to pass to vet.
+// The caller is expected to set them before executing any vet actions.
+var VetFlags []string
+
+// VetExplicit records whether the vet flags were set explicitly on the command line.
+var VetExplicit bool
+
+func (b *Builder) vet(ctx context.Context, a *Action) error {
+	// a.Deps[0] is the build of the package being vetted.
+	// a.Deps[1] is the build of the "fmt" package.
+
+	a.Failed = false // vet of dependency may have failed but we can still succeed
+
+	if a.Deps[0].Failed {
+		// The build of the package has failed. Skip vet check.
+		// Vet could return export data for non-typecheck errors,
+		// but we ignore it because the package cannot be compiled.
+		return nil
+	}
+
+	vcfg := a.Deps[0].vetCfg
+	if vcfg == nil {
+		// Vet config should only be missing if the build failed.
+		return fmt.Errorf("vet config not found")
+	}
+
+	vcfg.VetxOnly = a.VetxOnly
+	vcfg.VetxOutput = a.Objdir + "vet.out"
+	vcfg.PackageVetx = make(map[string]string)
+
+	h := cache.NewHash("vet " + a.Package.ImportPath)
+	fmt.Fprintf(h, "vet %q\n", b.toolID("vet"))
+
+	vetFlags := VetFlags
+
+	// In GOROOT, we enable all the vet tests during 'go test',
+	// not just the high-confidence subset. This gets us extra
+	// checking for the standard library (at some compliance cost)
+	// and helps us gain experience about how well the checks
+	// work, to help decide which should be turned on by default.
+	// The command-line still wins.
+	//
+	// Note that this flag change applies even when running vet as
+	// a dependency of vetting a package outside std.
+	// (Otherwise we'd have to introduce a whole separate
+	// space of "vet fmt as a dependency of a std top-level vet"
+	// versus "vet fmt as a dependency of a non-std top-level vet".)
+	// This is OK as long as the packages that are farther down the
+	// dependency tree turn on *more* analysis, as here.
+	// (The unsafeptr check does not write any facts for use by
+	// later vet runs, nor does unreachable.)
+	if a.Package.Goroot && !VetExplicit && VetTool == "" {
+		// Turn off -unsafeptr checks.
+		// There's too much unsafe.Pointer code
+		// that vet doesn't like in low-level packages
+		// like runtime, sync, and reflect.
+		// Note that $GOROOT/src/buildall.bash
+		// does the same for the misc-compile trybots
+		// and should be updated if these flags are
+		// changed here.
+		vetFlags = []string{"-unsafeptr=false"}
+
+		// Also turn off -unreachable checks during go test.
+		// During testing it is very common to make changes
+		// like hard-coded forced returns or panics that make
+		// code unreachable. It's unreasonable to insist on files
+		// not having any unreachable code during "go test".
+		// (buildall.bash still runs with -unreachable enabled
+		// for the overall whole-tree scan.)
+		if cfg.CmdName == "test" {
+			vetFlags = append(vetFlags, "-unreachable=false")
+		}
+	}
+
+	// Note: We could decide that vet should compute export data for
+	// all analyses, in which case we don't need to include the flags here.
+	// But that would mean that if an analysis causes problems like
+	// unexpected crashes there would be no way to turn it off.
+	// It seems better to let the flags disable export analysis too.
+	fmt.Fprintf(h, "vetflags %q\n", vetFlags)
+
+	fmt.Fprintf(h, "pkg %q\n", a.Deps[0].actionID)
+	for _, a1 := range a.Deps {
+		if a1.Mode == "vet" && a1.built != "" {
+			fmt.Fprintf(h, "vetout %q %s\n", a1.Package.ImportPath, b.fileHash(a1.built))
+			vcfg.PackageVetx[a1.Package.ImportPath] = a1.built
+		}
+	}
+	key := cache.ActionID(h.Sum())
+
+	if vcfg.VetxOnly && !cfg.BuildA {
+		c := cache.Default()
+		if file, _, err := c.GetFile(key); err == nil {
+			a.built = file
+			return nil
+		}
+	}
+
+	js, err := json.MarshalIndent(vcfg, "", "\t")
+	if err != nil {
+		return fmt.Errorf("internal error marshaling vet config: %v", err)
+	}
+	js = append(js, '\n')
+	if err := b.writeFile(a.Objdir+"vet.cfg", js); err != nil {
+		return err
+	}
+
+	// TODO(rsc): Why do we pass $GCCGO to go vet?
+	env := b.cCompilerEnv()
+	if cfg.BuildToolchainName == "gccgo" {
+		env = append(env, "GCCGO="+BuildToolchain.compiler())
+	}
+
+	p := a.Package
+	tool := VetTool
+	if tool == "" {
+		tool = base.Tool("vet")
+	}
+	runErr := b.run(a, p.Dir, p.ImportPath, env, cfg.BuildToolexec, tool, vetFlags, a.Objdir+"vet.cfg")
+
+	// If vet wrote export data, save it for input to future vets.
+	if f, err := os.Open(vcfg.VetxOutput); err == nil {
+		a.built = vcfg.VetxOutput
+		cache.Default().Put(key, f)
+		f.Close()
+	}
+
+	return runErr
+}
+
+// linkActionID computes the action ID for a link action.
+func (b *Builder) linkActionID(a *Action) cache.ActionID {
+	p := a.Package
+	h := cache.NewHash("link " + p.ImportPath)
+
+	// Toolchain-independent configuration.
+	fmt.Fprintf(h, "link\n")
+	fmt.Fprintf(h, "buildmode %s goos %s goarch %s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
+	fmt.Fprintf(h, "import %q\n", p.ImportPath)
+	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
+
+	// Toolchain-dependent configuration, shared with b.linkSharedActionID.
+	b.printLinkerConfig(h, p)
+
+	// Input files.
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 != nil {
+			if a1.built != "" || a1.buildID != "" {
+				buildID := a1.buildID
+				if buildID == "" {
+					buildID = b.buildID(a1.built)
+				}
+				fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(buildID))
+			}
+			// Because we put package main's full action ID into the binary's build ID,
+			// we must also put the full action ID into the binary's action ID hash.
+			if p1.Name == "main" {
+				fmt.Fprintf(h, "packagemain %s\n", a1.buildID)
+			}
+			if p1.Shlib != "" {
+				fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
+			}
+		}
+	}
+
+	return h.Sum()
+}
+
+// printLinkerConfig prints the linker config into the hash h,
+// as part of the computation of a linker-related action ID.
+func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
+	switch cfg.BuildToolchainName {
+	default:
+		base.Fatalf("linkActionID: unknown toolchain %q", cfg.BuildToolchainName)
+
+	case "gc":
+		fmt.Fprintf(h, "link %s %q %s\n", b.toolID("link"), forcedLdflags, ldBuildmode)
+		if p != nil {
+			fmt.Fprintf(h, "linkflags %q\n", p.Internal.Ldflags)
+		}
+
+		// GOARM, GOMIPS, etc.
+		key, val := cfg.GetArchEnv()
+		fmt.Fprintf(h, "%s=%s\n", key, val)
+
+		// The linker writes source file paths that say GOROOT_FINAL, but
+		// only if -trimpath is not specified (see ld() in gc.go).
+		gorootFinal := cfg.GOROOT_FINAL
+		if cfg.BuildTrimpath {
+			gorootFinal = trimPathGoRootFinal
+		}
+		fmt.Fprintf(h, "GOROOT=%s\n", gorootFinal)
+
+		// GO_EXTLINK_ENABLED controls whether the external linker is used.
+		fmt.Fprintf(h, "GO_EXTLINK_ENABLED=%s\n", cfg.Getenv("GO_EXTLINK_ENABLED"))
+
+		// TODO(rsc): Do cgo settings and flags need to be included?
+		// Or external linker settings and flags?
+
+	case "gccgo":
+		id, err := b.gccgoToolID(BuildToolchain.linker(), "go")
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		fmt.Fprintf(h, "link %s %s\n", id, ldBuildmode)
+		// TODO(iant): Should probably include cgo flags here.
+	}
+}
+
+// link is the action for linking a single command.
+// Note that any new influence on this logic must be reported in b.linkActionID above as well.
+func (b *Builder) link(ctx context.Context, a *Action) (err error) {
+	if b.useCache(a, b.linkActionID(a), a.Package.Target) || b.IsCmdList {
+		return nil
+	}
+	defer b.flushOutput(a)
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	if err := BuildToolchain.ld(b, a, a.Target, importcfg, a.Deps[0].built); err != nil {
+		return err
+	}
+
+	// Update the binary with the final build ID.
+	// But if OmitDebug is set, don't rewrite the binary, because we set OmitDebug
+	// on binaries that we are going to run and then delete.
+	// There's no point in doing work on such a binary.
+	// Worse, opening the binary for write here makes it
+	// essentially impossible to safely fork+exec due to a fundamental
+	// incompatibility between ETXTBSY and threads on modern Unix systems.
+	// See golang.org/issue/22220.
+	// We still call updateBuildID to update a.buildID, which is important
+	// for test result caching, but passing rewrite=false (final arg)
+	// means we don't actually rewrite the binary, nor store the
+	// result into the cache. That's probably a net win:
+	// less cache space wasted on large binaries we are not likely to
+	// need again. (On the other hand it does make repeated go test slower.)
+	// It also makes repeated go run slower, which is a win in itself:
+	// we don't want people to treat go run like a scripting environment.
+	if err := b.updateBuildID(a, a.Target, !a.Package.Internal.OmitDebug); err != nil {
+		return err
+	}
+
+	a.built = a.Target
+	return nil
+}
+
+func (b *Builder) writeLinkImportcfg(a *Action, file string) error {
+	// Prepare Go import cfg.
+	var icfg bytes.Buffer
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
+		if p1.Shlib != "" {
+			fmt.Fprintf(&icfg, "packageshlib %s=%s\n", p1.ImportPath, p1.Shlib)
+		}
+	}
+	return b.writeFile(file, icfg.Bytes())
+}
+
+// PkgconfigCmd returns a pkg-config binary name
+// defaultPkgConfig is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) PkgconfigCmd() string {
+	return envList("PKG_CONFIG", cfg.DefaultPkgConfig)[0]
+}
+
+// splitPkgConfigOutput parses the pkg-config output into a slice of
+// flags. This implements the algorithm from pkgconf/libpkgconf/argvsplit.c.
+func splitPkgConfigOutput(out []byte) ([]string, error) {
+	if len(out) == 0 {
+		return nil, nil
+	}
+	var flags []string
+	flag := make([]byte, 0, len(out))
+	escaped := false
+	quote := byte(0)
+
+	for _, c := range out {
+		if escaped {
+			if quote != 0 {
+				switch c {
+				case '$', '`', '"', '\\':
+				default:
+					flag = append(flag, '\\')
+				}
+				flag = append(flag, c)
+			} else {
+				flag = append(flag, c)
+			}
+			escaped = false
+		} else if quote != 0 {
+			if c == quote {
+				quote = 0
+			} else {
+				switch c {
+				case '\\':
+					escaped = true
+				default:
+					flag = append(flag, c)
+				}
+			}
+		} else if strings.IndexByte(" \t\n\v\f\r", c) < 0 {
+			switch c {
+			case '\\':
+				escaped = true
+			case '\'', '"':
+				quote = c
+			default:
+				flag = append(flag, c)
+			}
+		} else if len(flag) != 0 {
+			flags = append(flags, string(flag))
+			flag = flag[:0]
+		}
+	}
+	if escaped {
+		return nil, errors.New("broken character escaping in pkgconf output ")
+	}
+	if quote != 0 {
+		return nil, errors.New("unterminated quoted string in pkgconf output ")
+	} else if len(flag) != 0 {
+		flags = append(flags, string(flag))
+	}
+
+	return flags, nil
+}
+
+// Calls pkg-config if needed and returns the cflags/ldflags needed to build the package.
+func (b *Builder) getPkgConfigFlags(p *load.Package) (cflags, ldflags []string, err error) {
+	if pcargs := p.CgoPkgConfig; len(pcargs) > 0 {
+		// pkg-config permits arguments to appear anywhere in
+		// the command line. Move them all to the front, before --.
+		var pcflags []string
+		var pkgs []string
+		for _, pcarg := range pcargs {
+			if pcarg == "--" {
+				// We're going to add our own "--" argument.
+			} else if strings.HasPrefix(pcarg, "--") {
+				pcflags = append(pcflags, pcarg)
+			} else {
+				pkgs = append(pkgs, pcarg)
+			}
+		}
+		for _, pkg := range pkgs {
+			if !load.SafeArg(pkg) {
+				return nil, nil, fmt.Errorf("invalid pkg-config package name: %s", pkg)
+			}
+		}
+		var out []byte
+		out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--cflags", pcflags, "--", pkgs)
+		if err != nil {
+			b.showOutput(nil, p.Dir, b.PkgconfigCmd()+" --cflags "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out))
+			b.Print(err.Error() + "\n")
+			return nil, nil, errPrintedOutput
+		}
+		if len(out) > 0 {
+			cflags, err = splitPkgConfigOutput(out)
+			if err != nil {
+				return nil, nil, err
+			}
+			if err := checkCompilerFlags("CFLAGS", "pkg-config --cflags", cflags); err != nil {
+				return nil, nil, err
+			}
+		}
+		out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--libs", pcflags, "--", pkgs)
+		if err != nil {
+			b.showOutput(nil, p.Dir, b.PkgconfigCmd()+" --libs "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out))
+			b.Print(err.Error() + "\n")
+			return nil, nil, errPrintedOutput
+		}
+		if len(out) > 0 {
+			ldflags = strings.Fields(string(out))
+			if err := checkLinkerFlags("LDFLAGS", "pkg-config --libs", ldflags); err != nil {
+				return nil, nil, err
+			}
+		}
+	}
+
+	return
+}
+
+func (b *Builder) installShlibname(ctx context.Context, a *Action) error {
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	// TODO: BuildN
+	a1 := a.Deps[0]
+	err := os.WriteFile(a.Target, []byte(filepath.Base(a1.Target)+"\n"), 0666)
+	if err != nil {
+		return err
+	}
+	if cfg.BuildX {
+		b.Showcmd("", "echo '%s' > %s # internal", filepath.Base(a1.Target), a.Target)
+	}
+	return nil
+}
+
+func (b *Builder) linkSharedActionID(a *Action) cache.ActionID {
+	h := cache.NewHash("linkShared")
+
+	// Toolchain-independent configuration.
+	fmt.Fprintf(h, "linkShared\n")
+	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
+
+	// Toolchain-dependent configuration, shared with b.linkActionID.
+	b.printLinkerConfig(h, nil)
+
+	// Input files.
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if a1.built == "" {
+			continue
+		}
+		if p1 != nil {
+			fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
+			if p1.Shlib != "" {
+				fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
+			}
+		}
+	}
+	// Files named on command line are special.
+	for _, a1 := range a.Deps[0].Deps {
+		p1 := a1.Package
+		fmt.Fprintf(h, "top %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
+	}
+
+	return h.Sum()
+}
+
+func (b *Builder) linkShared(ctx context.Context, a *Action) (err error) {
+	if b.useCache(a, b.linkSharedActionID(a), a.Target) || b.IsCmdList {
+		return nil
+	}
+	defer b.flushOutput(a)
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
+	// TODO(rsc): There is a missing updateBuildID here,
+	// but we have to decide where to store the build ID in these files.
+	a.built = a.Target
+	return BuildToolchain.ldShared(b, a, a.Deps[0].Deps, a.Target, importcfg, a.Deps)
+}
+
+// BuildInstallFunc is the action for installing a single package or executable.
+func BuildInstallFunc(b *Builder, ctx context.Context, a *Action) (err error) {
+	defer func() {
+		if err != nil && err != errPrintedOutput {
+			// a.Package == nil is possible for the go install -buildmode=shared
+			// action that installs libmangledname.so, which corresponds to
+			// a list of packages, not just one.
+			sep, path := "", ""
+			if a.Package != nil {
+				sep, path = " ", a.Package.ImportPath
+			}
+			err = fmt.Errorf("go %s%s%s: %v", cfg.CmdName, sep, path, err)
+		}
+	}()
+
+	a1 := a.Deps[0]
+	a.buildID = a1.buildID
+	if a.json != nil {
+		a.json.BuildID = a.buildID
+	}
+
+	// If we are using the eventual install target as an up-to-date
+	// cached copy of the thing we built, then there's no need to
+	// copy it into itself (and that would probably fail anyway).
+	// In this case a1.built == a.Target because a1.built == p.Target,
+	// so the built target is not in the a1.Objdir tree that b.cleanup(a1) removes.
+	if a1.built == a.Target {
+		a.built = a.Target
+		if !a.buggyInstall {
+			b.cleanup(a1)
+		}
+		// Whether we're smart enough to avoid a complete rebuild
+		// depends on exactly what the staleness and rebuild algorithms
+		// are, as well as potentially the state of the Go build cache.
+		// We don't really want users to be able to infer (or worse start depending on)
+		// those details from whether the modification time changes during
+		// "go install", so do a best-effort update of the file times to make it
+		// look like we rewrote a.Target even if we did not. Updating the mtime
+		// may also help other mtime-based systems that depend on our
+		// previous mtime updates that happened more often.
+		// This is still not perfect - we ignore the error result, and if the file was
+		// unwritable for some reason then pretending to have written it is also
+		// confusing - but it's probably better than not doing the mtime update.
+		//
+		// But don't do that for the special case where building an executable
+		// with -linkshared implicitly installs all its dependent libraries.
+		// We want to hide that awful detail as much as possible, so don't
+		// advertise it by touching the mtimes (usually the libraries are up
+		// to date).
+		if !a.buggyInstall && !b.IsCmdList {
+			if cfg.BuildN {
+				b.Showcmd("", "touch %s", a.Target)
+			} else if err := allowInstall(a); err == nil {
+				now := time.Now()
+				os.Chtimes(a.Target, now, now)
+			}
+		}
+		return nil
+	}
+
+	// If we're building for go list -export,
+	// never install anything; just keep the cache reference.
+	if b.IsCmdList {
+		a.built = a1.built
+		return nil
+	}
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	perm := fs.FileMode(0666)
+	if a1.Mode == "link" {
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared", "plugin":
+		default:
+			perm = 0777
+		}
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	if !a.buggyInstall {
+		defer b.cleanup(a1)
+	}
+
+	return b.moveOrCopyFile(a.Target, a1.built, perm, false)
+}
+
+// allowInstall returns a non-nil error if this invocation of the go command is
+// allowed to install a.Target.
+//
+// (The build of cmd/go running under its own test is forbidden from installing
+// to its original GOROOT.)
+var allowInstall = func(*Action) error { return nil }
+
+// cleanup removes a's object dir to keep the amount of
+// on-disk garbage down in a large build. On an operating system
+// with aggressive buffering, cleaning incrementally like
+// this keeps the intermediate objects from hitting the disk.
+func (b *Builder) cleanup(a *Action) {
+	if !cfg.BuildWork {
+		if cfg.BuildX {
+			// Don't say we are removing the directory if
+			// we never created it.
+			if _, err := os.Stat(a.Objdir); err == nil || cfg.BuildN {
+				b.Showcmd("", "rm -r %s", a.Objdir)
+			}
+		}
+		os.RemoveAll(a.Objdir)
+	}
+}
+
+// moveOrCopyFile is like 'mv src dst' or 'cp src dst'.
+func (b *Builder) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) error {
+	if cfg.BuildN {
+		b.Showcmd("", "mv %s %s", src, dst)
+		return nil
+	}
+
+	// If we can update the mode and rename to the dst, do it.
+	// Otherwise fall back to standard copy.
+
+	// If the source is in the build cache, we need to copy it.
+	if strings.HasPrefix(src, cache.DefaultDir()) {
+		return b.copyFile(dst, src, perm, force)
+	}
+
+	// On Windows, always copy the file, so that we respect the NTFS
+	// permissions of the parent folder. https://golang.org/issue/22343.
+	// What matters here is not cfg.Goos (the system we are building
+	// for) but runtime.GOOS (the system we are building on).
+	if runtime.GOOS == "windows" {
+		return b.copyFile(dst, src, perm, force)
+	}
+
+	// If the destination directory has the group sticky bit set,
+	// we have to copy the file to retain the correct permissions.
+	// https://golang.org/issue/18878
+	if fi, err := os.Stat(filepath.Dir(dst)); err == nil {
+		if fi.IsDir() && (fi.Mode()&fs.ModeSetgid) != 0 {
+			return b.copyFile(dst, src, perm, force)
+		}
+	}
+
+	// The perm argument is meant to be adjusted according to umask,
+	// but we don't know what the umask is.
+	// Create a dummy file to find out.
+	// This avoids build tags and works even on systems like Plan 9
+	// where the file mask computation incorporates other information.
+	mode := perm
+	f, err := os.OpenFile(filepath.Clean(dst)+"-go-tmp-umask", os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
+	if err == nil {
+		fi, err := f.Stat()
+		if err == nil {
+			mode = fi.Mode() & 0777
+		}
+		name := f.Name()
+		f.Close()
+		os.Remove(name)
+	}
+
+	if err := os.Chmod(src, mode); err == nil {
+		if err := os.Rename(src, dst); err == nil {
+			if cfg.BuildX {
+				b.Showcmd("", "mv %s %s", src, dst)
+			}
+			return nil
+		}
+	}
+
+	return b.copyFile(dst, src, perm, force)
+}
+
+// copyFile is like 'cp src dst'.
+func (b *Builder) copyFile(dst, src string, perm fs.FileMode, force bool) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cp %s %s", src, dst)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	sf, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer sf.Close()
+
+	// Be careful about removing/overwriting dst.
+	// Do not remove/overwrite if dst exists and is a directory
+	// or a non-empty non-object file.
+	if fi, err := os.Stat(dst); err == nil {
+		if fi.IsDir() {
+			return fmt.Errorf("build output %q already exists and is a directory", dst)
+		}
+		if !force && fi.Mode().IsRegular() && fi.Size() != 0 && !isObject(dst) {
+			return fmt.Errorf("build output %q already exists and is not an object file", dst)
+		}
+	}
+
+	// On Windows, remove lingering ~ file from last attempt.
+	if base.ToolIsWindows {
+		if _, err := os.Stat(dst + "~"); err == nil {
+			os.Remove(dst + "~")
+		}
+	}
+
+	mayberemovefile(dst)
+	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil && base.ToolIsWindows {
+		// Windows does not allow deletion of a binary file
+		// while it is executing. Try to move it out of the way.
+		// If the move fails, which is likely, we'll try again the
+		// next time we do an install of this binary.
+		if err := os.Rename(dst, dst+"~"); err == nil {
+			os.Remove(dst + "~")
+		}
+		df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	}
+	if err != nil {
+		return fmt.Errorf("copying %s: %w", src, err) // err should already refer to dst
+	}
+
+	_, err = io.Copy(df, sf)
+	df.Close()
+	if err != nil {
+		mayberemovefile(dst)
+		return fmt.Errorf("copying %s to %s: %v", src, dst, err)
+	}
+	return nil
+}
+
+// writeFile writes the text to file.
+func (b *Builder) writeFile(file string, text []byte) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cat >%s << 'EOF' # internal\n%sEOF", file, text)
+	}
+	if cfg.BuildN {
+		return nil
+	}
+	return os.WriteFile(file, text, 0666)
+}
+
+// Install the cgo export header file, if there is one.
+func (b *Builder) installHeader(ctx context.Context, a *Action) error {
+	src := a.Objdir + "_cgo_install.h"
+	if _, err := os.Stat(src); os.IsNotExist(err) {
+		// If the file does not exist, there are no exported
+		// functions, and we do not install anything.
+		// TODO(rsc): Once we know that caching is rebuilding
+		// at the right times (not missing rebuilds), here we should
+		// probably delete the installed header, if any.
+		if cfg.BuildX {
+			b.Showcmd("", "# %s not created", src)
+		}
+		return nil
+	}
+
+	if err := allowInstall(a); err != nil {
+		return err
+	}
+
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	return b.moveOrCopyFile(a.Target, src, 0666, true)
+}
+
+// cover runs, in effect,
+//	go tool cover -mode=b.coverMode -var="varName" -o dst.go src.go
+func (b *Builder) cover(a *Action, dst, src string, varName string) error {
+	return b.run(a, a.Objdir, "cover "+a.Package.ImportPath, nil,
+		cfg.BuildToolexec,
+		base.Tool("cover"),
+		"-mode", a.Package.Internal.CoverMode,
+		"-var", varName,
+		"-o", dst,
+		src)
+}
+
+var objectMagic = [][]byte{
+	{'!', '<', 'a', 'r', 'c', 'h', '>', '\n'}, // Package archive
+	{'<', 'b', 'i', 'g', 'a', 'f', '>', '\n'}, // Package AIX big archive
+	{'\x7F', 'E', 'L', 'F'},                   // ELF
+	{0xFE, 0xED, 0xFA, 0xCE},                  // Mach-O big-endian 32-bit
+	{0xFE, 0xED, 0xFA, 0xCF},                  // Mach-O big-endian 64-bit
+	{0xCE, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 32-bit
+	{0xCF, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 64-bit
+	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00},      // PE (Windows) as generated by 6l/8l and gcc
+	{0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
+	{0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
+	{0x00, 0x00, 0x06, 0x47},                  // Plan 9 arm
+	{0x00, 0x61, 0x73, 0x6D},                  // WASM
+	{0x01, 0xDF},                              // XCOFF 32bit
+	{0x01, 0xF7},                              // XCOFF 64bit
+}
+
+func isObject(s string) bool {
+	f, err := os.Open(s)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	buf := make([]byte, 64)
+	io.ReadFull(f, buf)
+	for _, magic := range objectMagic {
+		if bytes.HasPrefix(buf, magic) {
+			return true
+		}
+	}
+	return false
+}
+
+// mayberemovefile removes a file only if it is a regular file
+// When running as a user with sufficient privileges, we may delete
+// even device files, for example, which is not intended.
+func mayberemovefile(s string) {
+	if fi, err := os.Lstat(s); err == nil && !fi.Mode().IsRegular() {
+		return
+	}
+	os.Remove(s)
+}
+
+// fmtcmd formats a command in the manner of fmt.Sprintf but also:
+//
+//	If dir is non-empty and the script is not in dir right now,
+//	fmtcmd inserts "cd dir\n" before the command.
+//
+//	fmtcmd replaces the value of b.WorkDir with $WORK.
+//	fmtcmd replaces the value of goroot with $GOROOT.
+//	fmtcmd replaces the value of b.gobin with $GOBIN.
+//
+//	fmtcmd replaces the name of the current directory with dot (.)
+//	but only when it is at the beginning of a space-separated token.
+//
+func (b *Builder) fmtcmd(dir string, format string, args ...interface{}) string {
+	cmd := fmt.Sprintf(format, args...)
+	if dir != "" && dir != "/" {
+		dot := " ."
+		if dir[len(dir)-1] == filepath.Separator {
+			dot += string(filepath.Separator)
+		}
+		cmd = strings.ReplaceAll(" "+cmd, " "+dir, dot)[1:]
+		if b.scriptDir != dir {
+			b.scriptDir = dir
+			cmd = "cd " + dir + "\n" + cmd
+		}
+	}
+	if b.WorkDir != "" {
+		cmd = strings.ReplaceAll(cmd, b.WorkDir, "$WORK")
+		escaped := strconv.Quote(b.WorkDir)
+		escaped = escaped[1 : len(escaped)-1] // strip quote characters
+		if escaped != b.WorkDir {
+			cmd = strings.ReplaceAll(cmd, escaped, "$WORK")
+		}
+	}
+	return cmd
+}
+
+// showcmd prints the given command to standard output
+// for the implementation of -n or -x.
+func (b *Builder) Showcmd(dir string, format string, args ...interface{}) {
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(b.fmtcmd(dir, format, args...) + "\n")
+}
+
+// showOutput prints "# desc" followed by the given output.
+// The output is expected to contain references to 'dir', usually
+// the source directory for the package that has failed to build.
+// showOutput rewrites mentions of dir with a relative path to dir
+// when the relative path is shorter. This is usually more pleasant.
+// For example, if fmt doesn't compile and we are in src/html,
+// the output is
+//
+//	$ go build
+//	# fmt
+//	../fmt/print.go:1090: undefined: asdf
+//	$
+//
+// instead of
+//
+//	$ go build
+//	# fmt
+//	/usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
+//	$
+//
+// showOutput also replaces references to the work directory with $WORK.
+//
+// If a is not nil and a.output is not nil, showOutput appends to that slice instead of
+// printing to b.Print.
+//
+func (b *Builder) showOutput(a *Action, dir, desc, out string) {
+	prefix := "# " + desc
+	suffix := "\n" + out
+	if reldir := base.ShortPath(dir); reldir != dir {
+		suffix = strings.ReplaceAll(suffix, " "+dir, " "+reldir)
+		suffix = strings.ReplaceAll(suffix, "\n"+dir, "\n"+reldir)
+	}
+	suffix = strings.ReplaceAll(suffix, " "+b.WorkDir, " $WORK")
+
+	if a != nil && a.output != nil {
+		a.output = append(a.output, prefix...)
+		a.output = append(a.output, suffix...)
+		return
+	}
+
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(prefix, suffix)
+}
+
+// errPrintedOutput is a special error indicating that a command failed
+// but that it generated output as well, and that output has already
+// been printed, so there's no point showing 'exit status 1' or whatever
+// the wait status was. The main executor, builder.do, knows not to
+// print this error.
+var errPrintedOutput = errors.New("already printed output - no need to show error")
+
+var cgoLine = lazyregexp.New(`\[[^\[\]]+\.(cgo1|cover)\.go:[0-9]+(:[0-9]+)?\]`)
+var cgoTypeSigRe = lazyregexp.New(`\b_C2?(type|func|var|macro)_\B`)
+
+// run runs the command given by cmdline in the directory dir.
+// If the command fails, run prints information about the failure
+// and returns a non-nil error.
+func (b *Builder) run(a *Action, dir string, desc string, env []string, cmdargs ...interface{}) error {
+	out, err := b.runOut(a, dir, env, cmdargs...)
+	if len(out) > 0 {
+		if desc == "" {
+			desc = b.fmtcmd(dir, "%s", strings.Join(str.StringList(cmdargs...), " "))
+		}
+		b.showOutput(a, dir, desc, b.processOutput(out))
+		if err != nil {
+			err = errPrintedOutput
+		}
+	}
+	return err
+}
+
+// processOutput prepares the output of runOut to be output to the console.
+func (b *Builder) processOutput(out []byte) string {
+	if out[len(out)-1] != '\n' {
+		out = append(out, '\n')
+	}
+	messages := string(out)
+	// Fix up output referring to cgo-generated code to be more readable.
+	// Replace x.go:19[/tmp/.../x.cgo1.go:18] with x.go:19.
+	// Replace *[100]_Ctype_foo with *[100]C.foo.
+	// If we're using -x, assume we're debugging and want the full dump, so disable the rewrite.
+	if !cfg.BuildX && cgoLine.MatchString(messages) {
+		messages = cgoLine.ReplaceAllString(messages, "")
+		messages = cgoTypeSigRe.ReplaceAllString(messages, "C.")
+	}
+	return messages
+}
+
+// runOut runs the command given by cmdline in the directory dir.
+// It returns the command output and any errors that occurred.
+// It accumulates execution time in a.
+func (b *Builder) runOut(a *Action, dir string, env []string, cmdargs ...interface{}) ([]byte, error) {
+	cmdline := str.StringList(cmdargs...)
+
+	for _, arg := range cmdline {
+		// GNU binutils commands, including gcc and gccgo, interpret an argument
+		// @foo anywhere in the command line (even following --) as meaning
+		// "read and insert arguments from the file named foo."
+		// Don't say anything that might be misinterpreted that way.
+		if strings.HasPrefix(arg, "@") {
+			return nil, fmt.Errorf("invalid command-line argument %s in command: %s", arg, joinUnambiguously(cmdline))
+		}
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		var envcmdline string
+		for _, e := range env {
+			if j := strings.IndexByte(e, '='); j != -1 {
+				if strings.ContainsRune(e[j+1:], '\'') {
+					envcmdline += fmt.Sprintf("%s=%q", e[:j], e[j+1:])
+				} else {
+					envcmdline += fmt.Sprintf("%s='%s'", e[:j], e[j+1:])
+				}
+				envcmdline += " "
+			}
+		}
+		envcmdline += joinUnambiguously(cmdline)
+		b.Showcmd(dir, "%s", envcmdline)
+		if cfg.BuildN {
+			return nil, nil
+		}
+	}
+
+	var buf bytes.Buffer
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	if cmd.Path != "" {
+		cmd.Args[0] = cmd.Path
+	}
+	cmd.Stdout = &buf
+	cmd.Stderr = &buf
+	cleanup := passLongArgsInResponseFiles(cmd)
+	defer cleanup()
+	cmd.Dir = dir
+	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
+
+	// Add the TOOLEXEC_IMPORTPATH environment variable for -toolexec tools.
+	// It doesn't really matter if -toolexec isn't being used.
+	if a != nil && a.Package != nil {
+		cmd.Env = append(cmd.Env, "TOOLEXEC_IMPORTPATH="+a.Package.ImportPath)
+	}
+
+	cmd.Env = append(cmd.Env, env...)
+	start := time.Now()
+	err := cmd.Run()
+	if a != nil && a.json != nil {
+		aj := a.json
+		aj.Cmd = append(aj.Cmd, joinUnambiguously(cmdline))
+		aj.CmdReal += time.Since(start)
+		if ps := cmd.ProcessState; ps != nil {
+			aj.CmdUser += ps.UserTime()
+			aj.CmdSys += ps.SystemTime()
+		}
+	}
+
+	// err can be something like 'exit status 1'.
+	// Add information about what program was running.
+	// Note that if buf.Bytes() is non-empty, the caller usually
+	// shows buf.Bytes() and does not print err at all, so the
+	// prefix here does not make most output any more verbose.
+	if err != nil {
+		err = errors.New(cmdline[0] + ": " + err.Error())
+	}
+	return buf.Bytes(), err
+}
+
+// joinUnambiguously prints the slice, quoting where necessary to make the
+// output unambiguous.
+// TODO: See issue 5279. The printing of commands needs a complete redo.
+func joinUnambiguously(a []string) string {
+	var buf bytes.Buffer
+	for i, s := range a {
+		if i > 0 {
+			buf.WriteByte(' ')
+		}
+		q := strconv.Quote(s)
+		// A gccgo command line can contain -( and -).
+		// Make sure we quote them since they are special to the shell.
+		// The trimpath argument can also contain > (part of =>) and ;. Quote those too.
+		if s == "" || strings.ContainsAny(s, " ()>;") || len(q) > len(s)+2 {
+			buf.WriteString(q)
+		} else {
+			buf.WriteString(s)
+		}
+	}
+	return buf.String()
+}
+
+// cCompilerEnv returns environment variables to set when running the
+// C compiler. This is needed to disable escape codes in clang error
+// messages that confuse tools like cgo.
+func (b *Builder) cCompilerEnv() []string {
+	return []string{"TERM=dumb"}
+}
+
+// mkdir makes the named directory.
+func (b *Builder) Mkdir(dir string) error {
+	// Make Mkdir(a.Objdir) a no-op instead of an error when a.Objdir == "".
+	if dir == "" {
+		return nil
+	}
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	// We can be a little aggressive about being
+	// sure directories exist. Skip repeated calls.
+	if b.mkdirCache[dir] {
+		return nil
+	}
+	b.mkdirCache[dir] = true
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "mkdir -p %s", dir)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		return err
+	}
+	return nil
+}
+
+// symlink creates a symlink newname -> oldname.
+func (b *Builder) Symlink(oldname, newname string) error {
+	// It's not an error to try to recreate an existing symlink.
+	if link, err := os.Readlink(newname); err == nil && link == oldname {
+		return nil
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "ln -s %s %s", oldname, newname)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+	return os.Symlink(oldname, newname)
+}
+
+// mkAbs returns an absolute path corresponding to
+// evaluating f in the directory dir.
+// We always pass absolute paths of source files so that
+// the error messages will include the full path to a file
+// in need of attention.
+func mkAbs(dir, f string) string {
+	// Leave absolute paths alone.
+	// Also, during -n mode we use the pseudo-directory $WORK
+	// instead of creating an actual work directory that won't be used.
+	// Leave paths beginning with $WORK alone too.
+	if filepath.IsAbs(f) || strings.HasPrefix(f, "$WORK") {
+		return f
+	}
+	return filepath.Join(dir, f)
+}
+
+type toolchain interface {
+	// gc runs the compiler in a specific directory on a set of files
+	// and returns the name of the generated output file.
+	gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error)
+	// cc runs the toolchain's C compiler in a directory on a C file
+	// to produce an output file.
+	cc(b *Builder, a *Action, ofile, cfile string) error
+	// asm runs the assembler in a specific directory on specific files
+	// and returns a list of named output files.
+	asm(b *Builder, a *Action, sfiles []string) ([]string, error)
+	// symabis scans the symbol ABIs from sfiles and returns the
+	// path to the output symbol ABIs file, or "" if none.
+	symabis(b *Builder, a *Action, sfiles []string) (string, error)
+	// pack runs the archive packer in a specific directory to create
+	// an archive from a set of object files.
+	// typically it is run in the object directory.
+	pack(b *Builder, a *Action, afile string, ofiles []string) error
+	// ld runs the linker to create an executable starting at mainpkg.
+	ld(b *Builder, root *Action, out, importcfg, mainpkg string) error
+	// ldShared runs the linker to create a shared library containing the pkgs built by toplevelactions
+	ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error
+
+	compiler() string
+	linker() string
+}
+
+type noToolchain struct{}
+
+func noCompiler() error {
+	log.Fatalf("unknown compiler %q", cfg.BuildContext.Compiler)
+	return nil
+}
+
+func (noToolchain) compiler() string {
+	noCompiler()
+	return ""
+}
+
+func (noToolchain) linker() string {
+	noCompiler()
+	return ""
+}
+
+func (noToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error) {
+	return "", nil, noCompiler()
+}
+
+func (noToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	return nil, noCompiler()
+}
+
+func (noToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	return "", noCompiler()
+}
+
+func (noToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	return noCompiler()
+}
+
+func (noToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	return noCompiler()
+}
+
+func (noToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	return noCompiler()
+}
+
+func (noToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	return noCompiler()
+}
+
+// gcc runs the gcc C compiler to create an object from a single C file.
+func (b *Builder) gcc(a *Action, p *load.Package, workdir, out string, flags []string, cfile string) error {
+	return b.ccompile(a, p, out, flags, cfile, b.GccCmd(p.Dir, workdir))
+}
+
+// gxx runs the g++ C++ compiler to create an object from a single C++ file.
+func (b *Builder) gxx(a *Action, p *load.Package, workdir, out string, flags []string, cxxfile string) error {
+	return b.ccompile(a, p, out, flags, cxxfile, b.GxxCmd(p.Dir, workdir))
+}
+
+// gfortran runs the gfortran Fortran compiler to create an object from a single Fortran file.
+func (b *Builder) gfortran(a *Action, p *load.Package, workdir, out string, flags []string, ffile string) error {
+	return b.ccompile(a, p, out, flags, ffile, b.gfortranCmd(p.Dir, workdir))
+}
+
+// ccompile runs the given C or C++ compiler and creates an object from a single source file.
+func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []string, file string, compiler []string) error {
+	file = mkAbs(p.Dir, file)
+	desc := p.ImportPath
+	outfile = mkAbs(p.Dir, outfile)
+
+	// Elide source directory paths if -trimpath or GOROOT_FINAL is set.
+	// This is needed for source files (e.g., a .c file in a package directory).
+	// TODO(golang.org/issue/36072): cgo also generates files with #line
+	// directives pointing to the source directory. It should not generate those
+	// when -trimpath is enabled.
+	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		if cfg.BuildTrimpath {
+			// Keep in sync with Action.trimpath.
+			// The trimmed paths are a little different, but we need to trim in the
+			// same situations.
+			var from, toPath string
+			if m := p.Module; m != nil {
+				from = m.Dir
+				toPath = m.Path + "@" + m.Version
+			} else {
+				from = p.Dir
+				toPath = p.ImportPath
+			}
+			// -fdebug-prefix-map requires an absolute "to" path (or it joins the path
+			// with the working directory). Pick something that makes sense for the
+			// target platform.
+			var to string
+			if cfg.BuildContext.GOOS == "windows" {
+				to = filepath.Join(`\\_\_`, toPath)
+			} else {
+				to = filepath.Join("/_", toPath)
+			}
+			flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+from+"="+to)
+		} else if p.Goroot && cfg.GOROOT_FINAL != cfg.GOROOT {
+			flags = append(flags[:len(flags):len(flags)], "-fdebug-prefix-map="+cfg.GOROOT+"="+cfg.GOROOT_FINAL)
+		}
+	}
+
+	overlayPath := file
+	if p, ok := a.nonGoOverlay[overlayPath]; ok {
+		overlayPath = p
+	}
+	output, err := b.runOut(a, filepath.Dir(overlayPath), b.cCompilerEnv(), compiler, flags, "-o", outfile, "-c", filepath.Base(overlayPath))
+	if len(output) > 0 {
+		// On FreeBSD 11, when we pass -g to clang 3.8 it
+		// invokes its internal assembler with -dwarf-version=2.
+		// When it sees .section .note.GNU-stack, it warns
+		// "DWARF2 only supports one section per compilation unit".
+		// This warning makes no sense, since the section is empty,
+		// but it confuses people.
+		// We work around the problem by detecting the warning
+		// and dropping -g and trying again.
+		if bytes.Contains(output, []byte("DWARF2 only supports one section per compilation unit")) {
+			newFlags := make([]string, 0, len(flags))
+			for _, f := range flags {
+				if !strings.HasPrefix(f, "-g") {
+					newFlags = append(newFlags, f)
+				}
+			}
+			if len(newFlags) < len(flags) {
+				return b.ccompile(a, p, outfile, newFlags, file, compiler)
+			}
+		}
+
+		b.showOutput(a, p.Dir, desc, b.processOutput(output))
+		if err != nil {
+			err = errPrintedOutput
+		} else if os.Getenv("GO_BUILDER_NAME") != "" {
+			return errors.New("C compiler warning promoted to error on Go builders")
+		}
+	}
+	return err
+}
+
+// gccld runs the gcc linker to create an executable from a set of object files.
+func (b *Builder) gccld(a *Action, p *load.Package, objdir, outfile string, flags []string, objs []string) error {
+	var cmd []string
+	if len(p.CXXFiles) > 0 || len(p.SwigCXXFiles) > 0 {
+		cmd = b.GxxCmd(p.Dir, objdir)
+	} else {
+		cmd = b.GccCmd(p.Dir, objdir)
+	}
+
+	cmdargs := []interface{}{cmd, "-o", outfile, objs, flags}
+	dir := p.Dir
+	out, err := b.runOut(a, base.Cwd, b.cCompilerEnv(), cmdargs...)
+
+	if len(out) > 0 {
+		// Filter out useless linker warnings caused by bugs outside Go.
+		// See also cmd/link/internal/ld's hostlink method.
+		var save [][]byte
+		var skipLines int
+		for _, line := range bytes.SplitAfter(out, []byte("\n")) {
+			// golang.org/issue/26073 - Apple Xcode bug
+			if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
+				continue
+			}
+
+			if skipLines > 0 {
+				skipLines--
+				continue
+			}
+
+			// Remove duplicate main symbol with runtime/cgo on AIX.
+			// With runtime/cgo, two main are available:
+			// One is generated by cgo tool with {return 0;}.
+			// The other one is the main calling runtime.rt0_go
+			// in runtime/cgo.
+			// The second can't be used by cgo programs because
+			// runtime.rt0_go is unknown to them.
+			// Therefore, we let ld remove this main version
+			// and used the cgo generated one.
+			if p.ImportPath == "runtime/cgo" && bytes.Contains(line, []byte("ld: 0711-224 WARNING: Duplicate symbol: .main")) {
+				skipLines = 1
+				continue
+			}
+
+			save = append(save, line)
+		}
+		out = bytes.Join(save, nil)
+		if len(out) > 0 {
+			b.showOutput(nil, dir, p.ImportPath, b.processOutput(out))
+			if err != nil {
+				err = errPrintedOutput
+			}
+		}
+	}
+	return err
+}
+
+// Grab these before main helpfully overwrites them.
+var (
+	origCC  = cfg.Getenv("CC")
+	origCXX = cfg.Getenv("CXX")
+)
+
+// gccCmd returns a gcc command line prefix
+// defaultCC is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) GccCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.ccExe(), incdir, workdir)
+}
+
+// gxxCmd returns a g++ command line prefix
+// defaultCXX is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) GxxCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.cxxExe(), incdir, workdir)
+}
+
+// gfortranCmd returns a gfortran command line prefix.
+func (b *Builder) gfortranCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.fcExe(), incdir, workdir)
+}
+
+// ccExe returns the CC compiler setting without all the extra flags we add implicitly.
+func (b *Builder) ccExe() []string {
+	return b.compilerExe(origCC, cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+}
+
+// cxxExe returns the CXX compiler setting without all the extra flags we add implicitly.
+func (b *Builder) cxxExe() []string {
+	return b.compilerExe(origCXX, cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+}
+
+// fcExe returns the FC compiler setting without all the extra flags we add implicitly.
+func (b *Builder) fcExe() []string {
+	return b.compilerExe(cfg.Getenv("FC"), "gfortran")
+}
+
+// compilerExe returns the compiler to use given an
+// environment variable setting (the value not the name)
+// and a default. The resulting slice is usually just the name
+// of the compiler but can have additional arguments if they
+// were present in the environment value.
+// For example if CC="gcc -DGOPHER" then the result is ["gcc", "-DGOPHER"].
+func (b *Builder) compilerExe(envValue string, def string) []string {
+	compiler := strings.Fields(envValue)
+	if len(compiler) == 0 {
+		compiler = strings.Fields(def)
+	}
+	return compiler
+}
+
+// compilerCmd returns a command line prefix for the given environment
+// variable and using the default command when the variable is empty.
+func (b *Builder) compilerCmd(compiler []string, incdir, workdir string) []string {
+	// NOTE: env.go's mkEnv knows that the first three
+	// strings returned are "gcc", "-I", incdir (and cuts them off).
+	a := []string{compiler[0], "-I", incdir}
+	a = append(a, compiler[1:]...)
+
+	// Definitely want -fPIC but on Windows gcc complains
+	// "-fPIC ignored for target (all code is position independent)"
+	if cfg.Goos != "windows" {
+		a = append(a, "-fPIC")
+	}
+	a = append(a, b.gccArchArgs()...)
+	// gcc-4.5 and beyond require explicit "-pthread" flag
+	// for multithreading with pthread library.
+	if cfg.BuildContext.CgoEnabled {
+		switch cfg.Goos {
+		case "windows":
+			a = append(a, "-mthreads")
+		default:
+			a = append(a, "-pthread")
+		}
+	}
+
+	if cfg.Goos == "aix" {
+		// mcmodel=large must always be enabled to allow large TOC.
+		a = append(a, "-mcmodel=large")
+	}
+
+	// disable ASCII art in clang errors, if possible
+	if b.gccSupportsFlag(compiler, "-fno-caret-diagnostics") {
+		a = append(a, "-fno-caret-diagnostics")
+	}
+	// clang is too smart about command-line arguments
+	if b.gccSupportsFlag(compiler, "-Qunused-arguments") {
+		a = append(a, "-Qunused-arguments")
+	}
+
+	// disable word wrapping in error messages
+	a = append(a, "-fmessage-length=0")
+
+	// Tell gcc not to include the work directory in object files.
+	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		if workdir == "" {
+			workdir = b.WorkDir
+		}
+		workdir = strings.TrimSuffix(workdir, string(filepath.Separator))
+		a = append(a, "-fdebug-prefix-map="+workdir+"=/tmp/go-build")
+	}
+
+	// Tell gcc not to include flags in object files, which defeats the
+	// point of -fdebug-prefix-map above.
+	if b.gccSupportsFlag(compiler, "-gno-record-gcc-switches") {
+		a = append(a, "-gno-record-gcc-switches")
+	}
+
+	// On OS X, some of the compilers behave as if -fno-common
+	// is always set, and the Mach-O linker in 6l/8l assumes this.
+	// See https://golang.org/issue/3253.
+	if cfg.Goos == "darwin" || cfg.Goos == "ios" {
+		a = append(a, "-fno-common")
+	}
+
+	return a
+}
+
+// gccNoPie returns the flag to use to request non-PIE. On systems
+// with PIE (position independent executables) enabled by default,
+// -no-pie must be passed when doing a partial link with -Wl,-r.
+// But -no-pie is not supported by all compilers, and clang spells it -nopie.
+func (b *Builder) gccNoPie(linker []string) string {
+	if b.gccSupportsFlag(linker, "-no-pie") {
+		return "-no-pie"
+	}
+	if b.gccSupportsFlag(linker, "-nopie") {
+		return "-nopie"
+	}
+	return ""
+}
+
+// gccSupportsFlag checks to see if the compiler supports a flag.
+func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
+	key := [2]string{compiler[0], flag}
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	if b, ok := b.flagCache[key]; ok {
+		return b
+	}
+	if b.flagCache == nil {
+		b.flagCache = make(map[[2]string]bool)
+	}
+
+	tmp := os.DevNull
+	if runtime.GOOS == "windows" {
+		f, err := os.CreateTemp(b.WorkDir, "")
+		if err != nil {
+			return false
+		}
+		f.Close()
+		tmp = f.Name()
+		defer os.Remove(tmp)
+	}
+
+	// We used to write an empty C file, but that gets complicated with
+	// go build -n. We tried using a file that does not exist, but that
+	// fails on systems with GCC version 4.2.1; that is the last GPLv2
+	// version of GCC, so some systems have frozen on it.
+	// Now we pass an empty file on stdin, which should work at least for
+	// GCC and clang.
+	cmdArgs := str.StringList(compiler, flag, "-c", "-x", "c", "-", "-o", tmp)
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
+		if cfg.BuildN {
+			return false
+		}
+	}
+	cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
+	cmd.Dir = b.WorkDir
+	cmd.Env = base.AppendPWD(os.Environ(), cmd.Dir)
+	cmd.Env = append(cmd.Env, "LC_ALL=C")
+	out, _ := cmd.CombinedOutput()
+	// GCC says "unrecognized command line option".
+	// clang says "unknown argument".
+	// Older versions of GCC say "unrecognised debug output level".
+	// For -fsplit-stack GCC says "'-fsplit-stack' is not supported".
+	supported := !bytes.Contains(out, []byte("unrecognized")) &&
+		!bytes.Contains(out, []byte("unknown")) &&
+		!bytes.Contains(out, []byte("unrecognised")) &&
+		!bytes.Contains(out, []byte("is not supported"))
+	b.flagCache[key] = supported
+	return supported
+}
+
+// gccArchArgs returns arguments to pass to gcc based on the architecture.
+func (b *Builder) gccArchArgs() []string {
+	switch cfg.Goarch {
+	case "386":
+		return []string{"-m32"}
+	case "amd64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "x86_64", "-m64"}
+		}
+		return []string{"-m64"}
+	case "arm64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "arm64"}
+		}
+	case "arm":
+		return []string{"-marm"} // not thumb
+	case "s390x":
+		return []string{"-m64", "-march=z196"}
+	case "mips64", "mips64le":
+		return []string{"-mabi=64"}
+	case "mips", "mipsle":
+		return []string{"-mabi=32", "-march=mips32"}
+	case "ppc64":
+		if cfg.Goos == "aix" {
+			return []string{"-maix64"}
+		}
+	}
+	return nil
+}
+
+// envList returns the value of the given environment variable broken
+// into fields, using the default value when the variable is empty.
+func envList(key, def string) []string {
+	v := cfg.Getenv(key)
+	if v == "" {
+		v = def
+	}
+	return strings.Fields(v)
+}
+
+// CFlags returns the flags to use when invoking the C, C++ or Fortran compilers, or cgo.
+func (b *Builder) CFlags(p *load.Package) (cppflags, cflags, cxxflags, fflags, ldflags []string, err error) {
+	defaults := "-g -O2"
+
+	if cppflags, err = buildFlags("CPPFLAGS", "", p.CgoCPPFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if cflags, err = buildFlags("CFLAGS", defaults, p.CgoCFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if cxxflags, err = buildFlags("CXXFLAGS", defaults, p.CgoCXXFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if fflags, err = buildFlags("FFLAGS", defaults, p.CgoFFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if ldflags, err = buildFlags("LDFLAGS", defaults, p.CgoLDFLAGS, checkLinkerFlags); err != nil {
+		return
+	}
+
+	return
+}
+
+func buildFlags(name, defaults string, fromPackage []string, check func(string, string, []string) error) ([]string, error) {
+	if err := check(name, "#cgo "+name, fromPackage); err != nil {
+		return nil, err
+	}
+	return str.StringList(envList("CGO_"+name, defaults), fromPackage), nil
+}
+
+var cgoRe = lazyregexp.New(`[/\\:]`)
+
+func (b *Builder) cgo(a *Action, cgoExe, objdir string, pcCFLAGS, pcLDFLAGS, cgofiles, gccfiles, gxxfiles, mfiles, ffiles []string) (outGo, outObj []string, err error) {
+	p := a.Package
+	cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, cgoFFLAGS, cgoLDFLAGS, err := b.CFlags(p)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	cgoCPPFLAGS = append(cgoCPPFLAGS, pcCFLAGS...)
+	cgoLDFLAGS = append(cgoLDFLAGS, pcLDFLAGS...)
+	// If we are compiling Objective-C code, then we need to link against libobjc
+	if len(mfiles) > 0 {
+		cgoLDFLAGS = append(cgoLDFLAGS, "-lobjc")
+	}
+
+	// Likewise for Fortran, except there are many Fortran compilers.
+	// Support gfortran out of the box and let others pass the correct link options
+	// via CGO_LDFLAGS
+	if len(ffiles) > 0 {
+		fc := cfg.Getenv("FC")
+		if fc == "" {
+			fc = "gfortran"
+		}
+		if strings.Contains(fc, "gfortran") {
+			cgoLDFLAGS = append(cgoLDFLAGS, "-lgfortran")
+		}
+	}
+
+	if cfg.BuildMSan {
+		cgoCFLAGS = append([]string{"-fsanitize=memory"}, cgoCFLAGS...)
+		cgoLDFLAGS = append([]string{"-fsanitize=memory"}, cgoLDFLAGS...)
+	}
+
+	// Allows including _cgo_export.h, as well as the user's .h files,
+	// from .[ch] files in the package.
+	cgoCPPFLAGS = append(cgoCPPFLAGS, "-I", objdir)
+
+	// cgo
+	// TODO: CGO_FLAGS?
+	gofiles := []string{objdir + "_cgo_gotypes.go"}
+	cfiles := []string{"_cgo_export.c"}
+	for _, fn := range cgofiles {
+		f := strings.TrimSuffix(filepath.Base(fn), ".go")
+		gofiles = append(gofiles, objdir+f+".cgo1.go")
+		cfiles = append(cfiles, f+".cgo2.c")
+	}
+
+	// TODO: make cgo not depend on $GOARCH?
+
+	cgoflags := []string{}
+	if p.Standard && p.ImportPath == "runtime/cgo" {
+		cgoflags = append(cgoflags, "-import_runtime_cgo=false")
+	}
+	if p.Standard && (p.ImportPath == "runtime/race" || p.ImportPath == "runtime/msan" || p.ImportPath == "runtime/cgo") {
+		cgoflags = append(cgoflags, "-import_syscall=false")
+	}
+
+	// Update $CGO_LDFLAGS with p.CgoLDFLAGS.
+	// These flags are recorded in the generated _cgo_gotypes.go file
+	// using //go:cgo_ldflag directives, the compiler records them in the
+	// object file for the package, and then the Go linker passes them
+	// along to the host linker. At this point in the code, cgoLDFLAGS
+	// consists of the original $CGO_LDFLAGS (unchecked) and all the
+	// flags put together from source code (checked).
+	cgoenv := b.cCompilerEnv()
+	if len(cgoLDFLAGS) > 0 {
+		flags := make([]string, len(cgoLDFLAGS))
+		for i, f := range cgoLDFLAGS {
+			flags[i] = strconv.Quote(f)
+		}
+		cgoenv = append(cgoenv, "CGO_LDFLAGS="+strings.Join(flags, " "))
+	}
+
+	if cfg.BuildToolchainName == "gccgo" {
+		if b.gccSupportsFlag([]string{BuildToolchain.compiler()}, "-fsplit-stack") {
+			cgoCFLAGS = append(cgoCFLAGS, "-fsplit-stack")
+		}
+		cgoflags = append(cgoflags, "-gccgo")
+		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+			cgoflags = append(cgoflags, "-gccgopkgpath="+pkgpath)
+		}
+	}
+
+	switch cfg.BuildBuildmode {
+	case "c-archive", "c-shared":
+		// Tell cgo that if there are any exported functions
+		// it should generate a header file that C code can
+		// #include.
+		cgoflags = append(cgoflags, "-exportheader="+objdir+"_cgo_install.h")
+	}
+
+	execdir := p.Dir
+
+	// Rewrite overlaid paths in cgo files.
+	// cgo adds //line and #line pragmas in generated files with these paths.
+	var trimpath []string
+	for i := range cgofiles {
+		path := mkAbs(p.Dir, cgofiles[i])
+		if opath, ok := fsys.OverlayPath(path); ok {
+			cgofiles[i] = opath
+			trimpath = append(trimpath, opath+"=>"+path)
+		}
+	}
+	if len(trimpath) > 0 {
+		cgoflags = append(cgoflags, "-trimpath", strings.Join(trimpath, ";"))
+	}
+
+	if err := b.run(a, execdir, p.ImportPath, cgoenv, cfg.BuildToolexec, cgoExe, "-objdir", objdir, "-importpath", p.ImportPath, cgoflags, "--", cgoCPPFLAGS, cgoCFLAGS, cgofiles); err != nil {
+		return nil, nil, err
+	}
+	outGo = append(outGo, gofiles...)
+
+	// Use sequential object file names to keep them distinct
+	// and short enough to fit in the .a header file name slots.
+	// We no longer collect them all into _all.o, and we'd like
+	// tools to see both the .o suffix and unique names, so
+	// we need to make them short enough not to be truncated
+	// in the final archive.
+	oseq := 0
+	nextOfile := func() string {
+		oseq++
+		return objdir + fmt.Sprintf("_x%03d.o", oseq)
+	}
+
+	// gcc
+	cflags := str.StringList(cgoCPPFLAGS, cgoCFLAGS)
+	for _, cfile := range cfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, objdir+cfile); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	for _, file := range gccfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	cxxflags := str.StringList(cgoCPPFLAGS, cgoCXXFLAGS)
+	for _, file := range gxxfiles {
+		ofile := nextOfile()
+		if err := b.gxx(a, p, a.Objdir, ofile, cxxflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	for _, file := range mfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	fflags := str.StringList(cgoCPPFLAGS, cgoFFLAGS)
+	for _, file := range ffiles {
+		ofile := nextOfile()
+		if err := b.gfortran(a, p, a.Objdir, ofile, fflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	switch cfg.BuildToolchainName {
+	case "gc":
+		importGo := objdir + "_cgo_import.go"
+		if err := b.dynimport(a, p, objdir, importGo, cgoExe, cflags, cgoLDFLAGS, outObj); err != nil {
+			return nil, nil, err
+		}
+		outGo = append(outGo, importGo)
+
+	case "gccgo":
+		defunC := objdir + "_cgo_defun.c"
+		defunObj := objdir + "_cgo_defun.o"
+		if err := BuildToolchain.cc(b, a, defunObj, defunC); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, defunObj)
+
+	default:
+		noCompiler()
+	}
+
+	// Double check the //go:cgo_ldflag comments in the generated files.
+	// The compiler only permits such comments in files whose base name
+	// starts with "_cgo_". Make sure that the comments in those files
+	// are safe. This is a backstop against people somehow smuggling
+	// such a comment into a file generated by cgo.
+	if cfg.BuildToolchainName == "gc" && !cfg.BuildN {
+		var flags []string
+		for _, f := range outGo {
+			if !strings.HasPrefix(filepath.Base(f), "_cgo_") {
+				continue
+			}
+
+			src, err := os.ReadFile(f)
+			if err != nil {
+				return nil, nil, err
+			}
+
+			const cgoLdflag = "//go:cgo_ldflag"
+			idx := bytes.Index(src, []byte(cgoLdflag))
+			for idx >= 0 {
+				// We are looking at //go:cgo_ldflag.
+				// Find start of line.
+				start := bytes.LastIndex(src[:idx], []byte("\n"))
+				if start == -1 {
+					start = 0
+				}
+
+				// Find end of line.
+				end := bytes.Index(src[idx:], []byte("\n"))
+				if end == -1 {
+					end = len(src)
+				} else {
+					end += idx
+				}
+
+				// Check for first line comment in line.
+				// We don't worry about /* */ comments,
+				// which normally won't appear in files
+				// generated by cgo.
+				commentStart := bytes.Index(src[start:], []byte("//"))
+				commentStart += start
+				// If that line comment is //go:cgo_ldflag,
+				// it's a match.
+				if bytes.HasPrefix(src[commentStart:], []byte(cgoLdflag)) {
+					// Pull out the flag, and unquote it.
+					// This is what the compiler does.
+					flag := string(src[idx+len(cgoLdflag) : end])
+					flag = strings.TrimSpace(flag)
+					flag = strings.Trim(flag, `"`)
+					flags = append(flags, flag)
+				}
+				src = src[end:]
+				idx = bytes.Index(src, []byte(cgoLdflag))
+			}
+		}
+
+		// We expect to find the contents of cgoLDFLAGS in flags.
+		if len(cgoLDFLAGS) > 0 {
+		outer:
+			for i := range flags {
+				for j, f := range cgoLDFLAGS {
+					if f != flags[i+j] {
+						continue outer
+					}
+				}
+				flags = append(flags[:i], flags[i+len(cgoLDFLAGS):]...)
+				break
+			}
+		}
+
+		if err := checkLinkerFlags("LDFLAGS", "go:cgo_ldflag", flags); err != nil {
+			return nil, nil, err
+		}
+	}
+
+	return outGo, outObj, nil
+}
+
+// dynimport creates a Go source file named importGo containing
+// //go:cgo_import_dynamic directives for each symbol or library
+// dynamically imported by the object files outObj.
+func (b *Builder) dynimport(a *Action, p *load.Package, objdir, importGo, cgoExe string, cflags, cgoLDFLAGS, outObj []string) error {
+	cfile := objdir + "_cgo_main.c"
+	ofile := objdir + "_cgo_main.o"
+	if err := b.gcc(a, p, objdir, ofile, cflags, cfile); err != nil {
+		return err
+	}
+
+	linkobj := str.StringList(ofile, outObj, mkAbsFiles(p.Dir, p.SysoFiles))
+	dynobj := objdir + "_cgo_.o"
+
+	// we need to use -pie for Linux/ARM to get accurate imported sym
+	ldflags := cgoLDFLAGS
+	if (cfg.Goarch == "arm" && cfg.Goos == "linux") || cfg.Goos == "android" {
+		// -static -pie doesn't make sense, and causes link errors.
+		// Issue 26197.
+		n := make([]string, 0, len(ldflags))
+		for _, flag := range ldflags {
+			if flag != "-static" {
+				n = append(n, flag)
+			}
+		}
+		ldflags = append(n, "-pie")
+	}
+	if err := b.gccld(a, p, objdir, dynobj, ldflags, linkobj); err != nil {
+		return err
+	}
+
+	// cgo -dynimport
+	var cgoflags []string
+	if p.Standard && p.ImportPath == "runtime/cgo" {
+		cgoflags = []string{"-dynlinker"} // record path to dynamic linker
+	}
+	return b.run(a, base.Cwd, p.ImportPath, b.cCompilerEnv(), cfg.BuildToolexec, cgoExe, "-dynpackage", p.Name, "-dynimport", dynobj, "-dynout", importGo, cgoflags)
+}
+
+// Run SWIG on all SWIG input files.
+// TODO: Don't build a shared library, once SWIG emits the necessary
+// pragmas for external linking.
+func (b *Builder) swig(a *Action, p *load.Package, objdir string, pcCFLAGS []string) (outGo, outC, outCXX []string, err error) {
+	if err := b.swigVersionCheck(); err != nil {
+		return nil, nil, nil, err
+	}
+
+	intgosize, err := b.swigIntSize(objdir)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	for _, f := range p.SwigFiles {
+		goFile, cFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, false, intgosize)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		if goFile != "" {
+			outGo = append(outGo, goFile)
+		}
+		if cFile != "" {
+			outC = append(outC, cFile)
+		}
+	}
+	for _, f := range p.SwigCXXFiles {
+		goFile, cxxFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, true, intgosize)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		if goFile != "" {
+			outGo = append(outGo, goFile)
+		}
+		if cxxFile != "" {
+			outCXX = append(outCXX, cxxFile)
+		}
+	}
+	return outGo, outC, outCXX, nil
+}
+
+// Make sure SWIG is new enough.
+var (
+	swigCheckOnce sync.Once
+	swigCheck     error
+)
+
+func (b *Builder) swigDoVersionCheck() error {
+	out, err := b.runOut(nil, "", nil, "swig", "-version")
+	if err != nil {
+		return err
+	}
+	re := regexp.MustCompile(`[vV]ersion +([\d]+)([.][\d]+)?([.][\d]+)?`)
+	matches := re.FindSubmatch(out)
+	if matches == nil {
+		// Can't find version number; hope for the best.
+		return nil
+	}
+
+	major, err := strconv.Atoi(string(matches[1]))
+	if err != nil {
+		// Can't find version number; hope for the best.
+		return nil
+	}
+	const errmsg = "must have SWIG version >= 3.0.6"
+	if major < 3 {
+		return errors.New(errmsg)
+	}
+	if major > 3 {
+		// 4.0 or later
+		return nil
+	}
+
+	// We have SWIG version 3.x.
+	if len(matches[2]) > 0 {
+		minor, err := strconv.Atoi(string(matches[2][1:]))
+		if err != nil {
+			return nil
+		}
+		if minor > 0 {
+			// 3.1 or later
+			return nil
+		}
+	}
+
+	// We have SWIG version 3.0.x.
+	if len(matches[3]) > 0 {
+		patch, err := strconv.Atoi(string(matches[3][1:]))
+		if err != nil {
+			return nil
+		}
+		if patch < 6 {
+			// Before 3.0.6.
+			return errors.New(errmsg)
+		}
+	}
+
+	return nil
+}
+
+func (b *Builder) swigVersionCheck() error {
+	swigCheckOnce.Do(func() {
+		swigCheck = b.swigDoVersionCheck()
+	})
+	return swigCheck
+}
+
+// Find the value to pass for the -intgosize option to swig.
+var (
+	swigIntSizeOnce  sync.Once
+	swigIntSize      string
+	swigIntSizeError error
+)
+
+// This code fails to build if sizeof(int) <= 32
+const swigIntSizeCode = `
+package main
+const i int = 1 << 32
+`
+
+// Determine the size of int on the target system for the -intgosize option
+// of swig >= 2.0.9. Run only once.
+func (b *Builder) swigDoIntSize(objdir string) (intsize string, err error) {
+	if cfg.BuildN {
+		return "$INTBITS", nil
+	}
+	src := filepath.Join(b.WorkDir, "swig_intsize.go")
+	if err = os.WriteFile(src, []byte(swigIntSizeCode), 0666); err != nil {
+		return
+	}
+	srcs := []string{src}
+
+	p := load.GoFilesPackage(context.TODO(), srcs)
+
+	if _, _, e := BuildToolchain.gc(b, &Action{Mode: "swigDoIntSize", Package: p, Objdir: objdir}, "", nil, nil, "", false, srcs); e != nil {
+		return "32", nil
+	}
+	return "64", nil
+}
+
+// Determine the size of int on the target system for the -intgosize option
+// of swig >= 2.0.9.
+func (b *Builder) swigIntSize(objdir string) (intsize string, err error) {
+	swigIntSizeOnce.Do(func() {
+		swigIntSize, swigIntSizeError = b.swigDoIntSize(objdir)
+	})
+	return swigIntSize, swigIntSizeError
+}
+
+// Run SWIG on one SWIG input file.
+func (b *Builder) swigOne(a *Action, p *load.Package, file, objdir string, pcCFLAGS []string, cxx bool, intgosize string) (outGo, outC string, err error) {
+	cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, _, _, err := b.CFlags(p)
+	if err != nil {
+		return "", "", err
+	}
+
+	var cflags []string
+	if cxx {
+		cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCXXFLAGS)
+	} else {
+		cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCFLAGS)
+	}
+
+	n := 5 // length of ".swig"
+	if cxx {
+		n = 8 // length of ".swigcxx"
+	}
+	base := file[:len(file)-n]
+	goFile := base + ".go"
+	gccBase := base + "_wrap."
+	gccExt := "c"
+	if cxx {
+		gccExt = "cxx"
+	}
+
+	gccgo := cfg.BuildToolchainName == "gccgo"
+
+	// swig
+	args := []string{
+		"-go",
+		"-cgo",
+		"-intgosize", intgosize,
+		"-module", base,
+		"-o", objdir + gccBase + gccExt,
+		"-outdir", objdir,
+	}
+
+	for _, f := range cflags {
+		if len(f) > 3 && f[:2] == "-I" {
+			args = append(args, f)
+		}
+	}
+
+	if gccgo {
+		args = append(args, "-gccgo")
+		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+			args = append(args, "-go-pkgpath", pkgpath)
+		}
+	}
+	if cxx {
+		args = append(args, "-c++")
+	}
+
+	out, err := b.runOut(a, p.Dir, nil, "swig", args, file)
+	if err != nil {
+		if len(out) > 0 {
+			if bytes.Contains(out, []byte("-intgosize")) || bytes.Contains(out, []byte("-cgo")) {
+				return "", "", errors.New("must have SWIG version >= 3.0.6")
+			}
+			b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig error
+			return "", "", errPrintedOutput
+		}
+		return "", "", err
+	}
+	if len(out) > 0 {
+		b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig warning
+	}
+
+	// If the input was x.swig, the output is x.go in the objdir.
+	// But there might be an x.go in the original dir too, and if it
+	// uses cgo as well, cgo will be processing both and will
+	// translate both into x.cgo1.go in the objdir, overwriting one.
+	// Rename x.go to _x_swig.go to avoid this problem.
+	// We ignore files in the original dir that begin with underscore
+	// so _x_swig.go cannot conflict with an original file we were
+	// going to compile.
+	goFile = objdir + goFile
+	newGoFile := objdir + "_" + base + "_swig.go"
+	if err := os.Rename(goFile, newGoFile); err != nil {
+		return "", "", err
+	}
+	return newGoFile, objdir + gccBase + gccExt, nil
+}
+
+// disableBuildID adjusts a linker command line to avoid creating a
+// build ID when creating an object file rather than an executable or
+// shared library. Some systems, such as Ubuntu, always add
+// --build-id to every link, but we don't want a build ID when we are
+// producing an object file. On some of those system a plain -r (not
+// -Wl,-r) will turn off --build-id, but clang 3.0 doesn't support a
+// plain -r. I don't know how to turn off --build-id when using clang
+// other than passing a trailing --build-id=none. So that is what we
+// do, but only on systems likely to support it, which is to say,
+// systems that normally use gold or the GNU linker.
+func (b *Builder) disableBuildID(ldflags []string) []string {
+	switch cfg.Goos {
+	case "android", "dragonfly", "linux", "netbsd":
+		ldflags = append(ldflags, "-Wl,--build-id=none")
+	}
+	return ldflags
+}
+
+// mkAbsFiles converts files into a list of absolute files,
+// assuming they were originally relative to dir,
+// and returns that new list.
+func mkAbsFiles(dir string, files []string) []string {
+	abs := make([]string, len(files))
+	for i, f := range files {
+		if !filepath.IsAbs(f) {
+			f = filepath.Join(dir, f)
+		}
+		abs[i] = f
+	}
+	return abs
+}
+
+// passLongArgsInResponseFiles modifies cmd such that, for
+// certain programs, long arguments are passed in "response files", a
+// file on disk with the arguments, with one arg per line. An actual
+// argument starting with '@' means that the rest of the argument is
+// a filename of arguments to expand.
+//
+// See issues 18468 (Windows) and 37768 (Darwin).
+func passLongArgsInResponseFiles(cmd *exec.Cmd) (cleanup func()) {
+	cleanup = func() {} // no cleanup by default
+
+	var argLen int
+	for _, arg := range cmd.Args {
+		argLen += len(arg)
+	}
+
+	// If we're not approaching 32KB of args, just pass args normally.
+	// (use 30KB instead to be conservative; not sure how accounting is done)
+	if !useResponseFile(cmd.Path, argLen) {
+		return
+	}
+
+	tf, err := os.CreateTemp("", "args")
+	if err != nil {
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	cleanup = func() { os.Remove(tf.Name()) }
+	var buf bytes.Buffer
+	for _, arg := range cmd.Args[1:] {
+		fmt.Fprintf(&buf, "%s\n", encodeArg(arg))
+	}
+	if _, err := tf.Write(buf.Bytes()); err != nil {
+		tf.Close()
+		cleanup()
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	if err := tf.Close(); err != nil {
+		cleanup()
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	cmd.Args = []string{cmd.Args[0], "@" + tf.Name()}
+	return cleanup
+}
+
+// Windows has a limit of 32 KB arguments. To be conservative and not worry
+// about whether that includes spaces or not, just use 30 KB. Darwin's limit is
+// less clear. The OS claims 256KB, but we've seen failures with arglen as
+// small as 50KB.
+const ArgLengthForResponseFile = (30 << 10)
+
+func useResponseFile(path string, argLen int) bool {
+	// Unless the program uses objabi.Flagparse, which understands
+	// response files, don't use response files.
+	// TODO: do we need more commands? asm? cgo? For now, no.
+	prog := strings.TrimSuffix(filepath.Base(path), ".exe")
+	switch prog {
+	case "compile", "link":
+	default:
+		return false
+	}
+
+	if argLen > ArgLengthForResponseFile {
+		return true
+	}
+
+	// On the Go build system, use response files about 10% of the
+	// time, just to exercise this codepath.
+	isBuilder := os.Getenv("GO_BUILDER_NAME") != ""
+	if isBuilder && rand.Intn(10) == 0 {
+		return true
+	}
+
+	return false
+}
+
+// encodeArg encodes an argument for response file writing.
+func encodeArg(arg string) string {
+	// If there aren't any characters we need to reencode, fastpath out.
+	if !strings.ContainsAny(arg, "\\\n") {
+		return arg
+	}
+	var b strings.Builder
+	for _, r := range arg {
+		switch r {
+		case '\\':
+			b.WriteByte('\\')
+			b.WriteByte('\\')
+		case '\n':
+			b.WriteByte('\\')
+			b.WriteByte('n')
+		default:
+			b.WriteRune(r)
+		}
+	}
+	return b.String()
+}