cmd/go: record both build ID and content ID in archives and binaries

The content ID will be needed for content-based staleness
determination. It is defined as the SHA256 hash of the file
in which it appears, with occurrences of the build+content IDs
changed to zeros during the hashing operation.

Storing the content ID in the archives is a little tricky
but it means that later builds need not rehash the archives
each time they are referenced, so under the assumption
that each package is imported at least once after being
compiled, hashing at build time is a win. (Also the whole
file is more likely to be in cache at build time,
since we just wrote it.)

In my unscientific tests, the time for "go build -a std cmd"
rises from about 14.3s to 14.5s on my laptop, or under 2%.

Change-Id: Ia3d4dc657d003e8295631f73363868bd92ebf96a
Reviewed-on: https://go-review.googlesource.com/69054Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/deps.go

@@ -332,6 +332,7 @@ var builddeps = map[string][]string{
 		"cmd/go/internal/str",  // cmd/go/internal/work
 		"cmd/internal/buildid", // cmd/go/internal/work
 		"container/heap",       // cmd/go/internal/work
+		"crypto/sha256",        // cmd/go/internal/work
 		"debug/elf",            // cmd/go/internal/work
 		"errors",               // cmd/go/internal/work
 		"flag",                 // cmd/go/internal/work

src/cmd/go/internal/load/pkg.go

@@ -1118,6 +1118,12 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 		// For binary-only package, use build ID from supplied package binary.
 		buildID, err := buildid.ReadFile(p.Target)
 		if err == nil {
+			// The stored build ID used to be "<actionID>".
+			// Now it is "<actionID>.<contentID>".
+			// For now at least, we want only the <actionID> part here.
+			if i := strings.Index(buildID, "."); i >= 0 {
+				buildID = buildID[:i]
+			}
 			p.Internal.BuildID = buildID
 		}
 	} else {
@@ -1215,6 +1221,9 @@ func PackageList(roots []*Package) []*Package {
 // at the named pkgs (command-line arguments).
 func ComputeStale(pkgs ...*Package) {
 	for _, p := range PackageList(pkgs) {
+		if p.Internal.BuildID == "" {
+			computeBuildID(p)
+		}
 		p.Stale, p.StaleReason = isStale(p)
 	}
 }
@@ -1541,6 +1550,12 @@ func isStale(p *Package) (bool, string) {
 	// two versions of Go compiling a single GOPATH.
 	// See issue 8290 and issue 10702.
 	targetBuildID, err := buildid.ReadFile(p.Target)
+	// The build ID used to be "<actionID>".
+	// Now we've started writing "<actionID>.<contentID>".
+	// Ignore contentID for now and record only "<actionID>" here.
+	if i := strings.Index(targetBuildID, "."); i >= 0 {
+		targetBuildID = targetBuildID[:i]
+	}
 	if err == nil && targetBuildID != p.Internal.BuildID {
 		return true, "build ID mismatch"
 	}

src/cmd/go/internal/work/build.go

@@ -8,6 +8,7 @@ import (
 	"bufio"
 	"bytes"
 	"container/heap"
+	"crypto/sha256"
 	"debug/elf"
 	"encoding/json"
 	"errors"
@@ -684,6 +685,7 @@ type Action struct {
 	Args       []string                      // additional args for runProgram
 
 	triggers []*Action // inverse of deps
+	buildID  string
 
 	// Generated files, directories.
 	Objdir string // directory for intermediate objects
@@ -1466,6 +1468,18 @@ func (b *Builder) build(a *Action) (err error) {
 		}
 	}
 
+	// We want to keep the action ID available for consultation later,
+	// but we'll append to it the SHA256 of the file (without this ID included).
+	// We don't know the SHA256 yet, so make one up to find and replace
+	// later. Becuase the action ID is a hash of the inputs to this built,
+	// the chance of SHA256(actionID) occurring elsewhere in the result
+	// of the build is essentially zero, at least in 2017.
+	actionID := a.Package.Internal.BuildID
+	if actionID == "" {
+		return fmt.Errorf("missing action ID")
+	}
+	a.buildID = actionID + "." + fmt.Sprintf("%x", sha256.Sum256([]byte(actionID)))
+
 	var gofiles, cgofiles, objdirCgofiles, cfiles, sfiles, cxxfiles, objects, cgoObjects, pcCFLAGS, pcLDFLAGS []string
 
 	gofiles = append(gofiles, a.Package.GoFiles...)
@@ -1682,6 +1696,10 @@ func (b *Builder) build(a *Action) (err error) {
 		}
 	}
 
+	if err := b.updateBuildID(a, actionID, objpkg); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -1699,11 +1717,65 @@ func (b *Builder) link(a *Action) (err error) {
 		}
 	}
 
+	actionID := a.Package.Internal.BuildID
+	if actionID == "" {
+		return fmt.Errorf("missing action ID")
+	}
+	a.buildID = actionID + "." + fmt.Sprintf("%x", sha256.Sum256([]byte(actionID)))
+
 	objpkg := a.Objdir + "_pkg_.a"
 	if err := BuildToolchain.ld(b, a, a.Target, importcfg, objpkg); err != nil {
 		return err
 	}
 
+	if err := b.updateBuildID(a, actionID, a.Target); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (b *Builder) updateBuildID(a *Action, actionID, target string) error {
+	if cfg.BuildX || cfg.BuildN {
+		b.Showcmd("", "%s # internal", joinUnambiguously(str.StringList(base.Tool("buildid"), "-w", target)))
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	// Find occurrences of old ID and compute new content-based ID.
+	r, err := os.Open(target)
+	if err != nil {
+		return err
+	}
+	matches, hash, err := buildid.FindAndHash(r, a.buildID, 0)
+	r.Close()
+	if err != nil {
+		return err
+	}
+	newID := fmt.Sprintf("%s.%x", actionID, hash)
+	if len(newID) != len(a.buildID) {
+		return fmt.Errorf("internal error: build ID length mismatch %d+1+%d != %d", len(actionID), len(hash)*2, len(a.buildID))
+	}
+
+	// Replace with new content-based ID.
+	a.buildID = newID
+	if len(matches) == 0 {
+		// Assume the user specified -buildid= to override what we were going to choose.
+		return nil
+	}
+	w, err := os.OpenFile(target, os.O_WRONLY, 0)
+	if err != nil {
+		return err
+	}
+	err = buildid.Rewrite(w, matches, newID)
+	if err != nil {
+		w.Close()
+		return err
+	}
+	if err := w.Close(); err != nil {
+		return err
+	}
 	return nil
 }
 
@@ -2451,8 +2523,8 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg []byte, a
 	if cfg.BuildContext.InstallSuffix != "" {
 		gcargs = append(gcargs, "-installsuffix", cfg.BuildContext.InstallSuffix)
 	}
-	if p.Internal.BuildID != "" {
-		gcargs = append(gcargs, "-buildid", p.Internal.BuildID)
+	if a.buildID != "" {
+		gcargs = append(gcargs, "-buildid", a.buildID)
 	}
 	platform := cfg.Goos + "/" + cfg.Goarch
 	if p.Internal.OmitDebug || platform == "nacl/amd64p32" || platform == "darwin/arm" || platform == "darwin/arm64" || cfg.Goos == "plan9" {
@@ -2758,7 +2830,7 @@ func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string)
 	// Store BuildID inside toolchain binaries as a unique identifier of the
 	// tool being run, for use by content-based staleness determination.
 	if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {
-		ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.Package.Internal.BuildID)
+		ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.buildID)
 	}
 
 	// If the user has not specified the -extld option, then specify the
@@ -2772,8 +2844,8 @@ func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string)
 		compiler = envList("CC", cfg.DefaultCC)
 	}
 	ldflags = append(ldflags, "-buildmode="+ldBuildmode)
-	if root.Package.Internal.BuildID != "" {
-		ldflags = append(ldflags, "-buildid="+root.Package.Internal.BuildID)
+	if root.buildID != "" {
+		ldflags = append(ldflags, "-buildid="+root.buildID)
 	}
 	ldflags = append(ldflags, cfg.BuildLdflags...)
 	ldflags = setextld(ldflags, compiler)