runtime/coverage: runtime routines to emit coverage data

This patch fleshes out the runtime support for emitting coverage data
at the end of a run of an instrumented binary. Data is emitted in the
form of a pair of files, a meta-out-file and counter-data-outfile,
each written to the dir GOCOVERDIR. The meta-out-file is emitted only
if required; no need to emit again if an existing meta-data file with
the same hash and length is present.

Updates #51430.

Change-Id: I59d20a4b8c05910c933ee29527972f8e401b1685
Reviewed-on: https://go-review.googlesource.com/c/go/+/355451
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Than McIntosh <thanm@google.com>

src/cmd/go/internal/work/gc.go

@@ -32,6 +32,7 @@ const trimPathGoRootFinal string = "$GOROOT"
 var runtimePackages = map[string]struct{}{
 	"internal/abi":            struct{}{},
 	"internal/bytealg":        struct{}{},
+	"internal/coverage/rtcov": struct{}{},
 	"internal/cpu":            struct{}{},
 	"internal/goarch":         struct{}{},
 	"internal/goos":           struct{}{},
@@ -39,7 +40,6 @@ var runtimePackages = map[string]struct{}{
 	"runtime/internal/atomic": struct{}{},
 	"runtime/internal/math":   struct{}{},
 	"runtime/internal/sys":    struct{}{},
-	"runtime/internal/syscall": struct{}{},
 }
 
 // The Go toolchain.

src/go/build/deps_test.go

@@ -1,4 +1,4 @@
-// Copyright 2012 The Go Authors. All rights reserved.
+// Copyright 2022 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.
 
@@ -584,6 +584,14 @@ var depsRules = `
     FMT, internal/coverage, os,
     path/filepath, regexp, sort, strconv
     < internal/coverage/pods;
+
+    FMT, bufio, crypto/md5, encoding/binary, runtime/debug,
+    internal/coverage, internal/coverage/cmerge,
+    internal/coverage/cformat, internal/coverage/calloc,
+    internal/coverage/decodecounter, internal/coverage/decodemeta,
+    internal/coverage/encodecounter, internal/coverage/encodemeta,
+    internal/coverage/pods, os, path/filepath, reflect, time, unsafe
+    < runtime/coverage;
 `
 
 // listStdPkgs returns the same list of packages as "go list std".

src/internal/coverage/rtcov/rtcov.go

@@ -23,3 +23,12 @@ type CovMetaBlob struct {
 	CounterMode        uint8 // coverage.CounterMode
 	CounterGranularity uint8 // coverage.CounterGranularity
 }
+
+// CovCounterBlob is a container for encapsulating a counter section
+// (BSS variable) for an instrumented Go module. Here "counters"
+// points to the counter payload and "len" is the number of uint32
+// entries in the section.
+type CovCounterBlob struct {
+	Counters *uint32
+	Len      uint64
+}

src/runtime/coverage/dummy.s

@@ -0,0 +1,8 @@
+// Copyright 2022 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.
+
+// The runtime package uses //go:linkname to push a few functions into this
+// package but we still need a .s file so the Go tool does not pass -complete
+// to 'go tool compile' so the latter does not complain about Go functions
+// with no bodies.

src/runtime/coverage/emit.go

@@ -0,0 +1,611 @@
+// Copyright 2022 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.
+
+package coverage
+
+import (
+	"crypto/md5"
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/encodecounter"
+	"internal/coverage/encodemeta"
+	"internal/coverage/rtcov"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"time"
+	"unsafe"
+)
+
+// This file contains functions that support the writing of data files
+// emitted at the end of code coverage testing runs, from instrumented
+// executables.
+
+// getCovMetaList returns a list of meta-data blobs registered
+// for the currently executing instrumented program. It is defined in the
+// runtime.
+func getCovMetaList() []rtcov.CovMetaBlob
+
+// getCovCounterList returns a list of counter-data blobs registered
+// for the currently executing instrumented program. It is defined in the
+// runtime.
+func getCovCounterList() []rtcov.CovCounterBlob
+
+// getCovPkgMap returns a map storing the remapped package IDs for
+// hard-coded runtime packages (see internal/coverage/pkgid.go for
+// more on why hard-coded package IDs are needed). This function
+// is defined in the runtime.
+func getCovPkgMap() map[int]int
+
+// emitState holds useful state information during the emit process.
+//
+// When an instrumented program finishes execution and starts the
+// process of writing out coverage data, it's possible that an
+// existing meta-data file already exists in the output directory. In
+// this case openOutputFiles() below will leave the 'mf' field below
+// as nil. If a new meta-data file is needed, field 'mfname' will be
+// the final desired path of the meta file, 'mftmp' will be a
+// temporary file, and 'mf' will be an open os.File pointer for
+// 'mftmp'. The meta-data file payload will be written to 'mf', the
+// temp file will be then closed and renamed (from 'mftmp' to
+// 'mfname'), so as to insure that the meta-data file is created
+// atomically; we want this so that things work smoothly in cases
+// where there are several instances of a given instrumented program
+// all terminating at the same time and trying to create meta-data
+// files simultaneously.
+//
+// For counter data files there is less chance of a collision, hence
+// the openOutputFiles() stores the counter data file in 'cfname' and
+// then places the *io.File into 'cf'.
+type emitState struct {
+	mfname string   // path of final meta-data output file
+	mftmp  string   // path to meta-data temp file (if needed)
+	mf     *os.File // open os.File for meta-data temp file
+	cfname string   // path of final counter data file
+	cftmp  string   // path to counter data temp file
+	cf     *os.File // open os.File for counter data file
+	outdir string   // output directory
+
+	// List of meta-data symbols obtained from the runtime
+	metalist []rtcov.CovMetaBlob
+
+	// List of counter-data symbols obtained from the runtime
+	counterlist []rtcov.CovCounterBlob
+
+	// Table to use for remapping hard-coded pkg ids.
+	pkgmap map[int]int
+
+	// emit debug trace output
+	debug bool
+}
+
+var (
+	// finalHash is computed at init time from the list of meta-data
+	// symbols registered during init. It is used both for writing the
+	// meta-data file and counter-data files.
+	finalHash [16]byte
+	// Set to true when we've computed finalHash + finalMetaLen.
+	finalHashComputed bool
+	// Total meta-data length.
+	finalMetaLen uint64
+	// Records whether we've already attempted to write meta-data.
+	metaDataEmitAttempted bool
+	// Counter mode for this instrumented program run.
+	cmode coverage.CounterMode
+	// Counter granularity for this instrumented program run.
+	cgran coverage.CounterGranularity
+	// Cached value of GOCOVERDIR environment variable.
+	goCoverDir string
+	// Copy of os.Args made at init time, converted into map format.
+	capturedOsArgs map[string]string
+	// Flag used in tests to signal that coverage data already written.
+	covProfileAlreadyEmitted bool
+)
+
+// fileType is used to select between counter-data files and
+// meta-data files.
+type fileType int
+
+const (
+	noFile = 1 << iota
+	metaDataFile
+	counterDataFile
+)
+
+// emitMetaData emits the meta-data output file for this coverage run.
+// This entry point is intended to be invoked by the compiler from
+// an instrumented program's main package init func.
+func emitMetaData() {
+	if covProfileAlreadyEmitted {
+		return
+	}
+	ml, err := prepareForMetaEmit()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: coverage meta-data prep failed: %v\n", err)
+		if os.Getenv("GOCOVERDEBUG") != "" {
+			panic("meta-data write failure")
+		}
+	}
+	if len(ml) == 0 {
+		fmt.Fprintf(os.Stderr, "program not built with -cover\n")
+		return
+	}
+
+	goCoverDir = os.Getenv("GOCOVERDIR")
+	if goCoverDir == "" {
+		fmt.Fprintf(os.Stderr, "warning: GOCOVERDIR not set, no coverage data emitted\n")
+		return
+	}
+
+	if err := emitMetaDataToDirectory(goCoverDir, ml); err != nil {
+		fmt.Fprintf(os.Stderr, "error: coverage meta-data emit failed: %v\n", err)
+		if os.Getenv("GOCOVERDEBUG") != "" {
+			panic("meta-data write failure")
+		}
+	}
+}
+
+func modeClash(m coverage.CounterMode) bool {
+	if m == coverage.CtrModeRegOnly || m == coverage.CtrModeTestMain {
+		return false
+	}
+	if cmode == coverage.CtrModeInvalid {
+		cmode = m
+		return false
+	}
+	return cmode != m
+}
+
+func granClash(g coverage.CounterGranularity) bool {
+	if cgran == coverage.CtrGranularityInvalid {
+		cgran = g
+		return false
+	}
+	return cgran != g
+}
+
+// prepareForMetaEmit performs preparatory steps needed prior to
+// emitting a meta-data file, notably computing a final hash of
+// all meta-data blobs and capturing os args.
+func prepareForMetaEmit() ([]rtcov.CovMetaBlob, error) {
+	// Ask the runtime for the list of coverage meta-data symbols.
+	ml := getCovMetaList()
+
+	// In the normal case (go build -o prog.exe ... ; ./prog.exe)
+	// len(ml) will always be non-zero, but we check here since at
+	// some point this function will be reachable via user-callable
+	// APIs (for example, to write out coverage data from a server
+	// program that doesn't ever call os.Exit).
+	if len(ml) == 0 {
+		return nil, nil
+	}
+
+	s := &emitState{
+		metalist: ml,
+		debug:    os.Getenv("GOCOVERDEBUG") != "",
+	}
+
+	// Capture os.Args() now so as to avoid issues if args
+	// are rewritten during program execution.
+	capturedOsArgs = captureOsArgs()
+
+	if s.debug {
+		fmt.Fprintf(os.Stderr, "=+= GOCOVERDIR is %s\n", os.Getenv("GOCOVERDIR"))
+		fmt.Fprintf(os.Stderr, "=+= contents of covmetalist:\n")
+		for k, b := range ml {
+			fmt.Fprintf(os.Stderr, "=+= slot: %d path: %s ", k, b.PkgPath)
+			if b.PkgID != -1 {
+				fmt.Fprintf(os.Stderr, " hcid: %d", b.PkgID)
+			}
+			fmt.Fprintf(os.Stderr, "\n")
+		}
+		pm := getCovPkgMap()
+		fmt.Fprintf(os.Stderr, "=+= remap table:\n")
+		for from, to := range pm {
+			fmt.Fprintf(os.Stderr, "=+= from %d to %d\n",
+				uint32(from), uint32(to))
+		}
+	}
+
+	h := md5.New()
+	tlen := uint64(unsafe.Sizeof(coverage.MetaFileHeader{}))
+	for _, entry := range ml {
+		if _, err := h.Write(entry.Hash[:]); err != nil {
+			return nil, err
+		}
+		tlen += uint64(entry.Len)
+		ecm := coverage.CounterMode(entry.CounterMode)
+		if modeClash(ecm) {
+			return nil, fmt.Errorf("coverage counter mode clash: package %s uses mode=%d, but package %s uses mode=%s\n", ml[0].PkgPath, cmode, entry.PkgPath, ecm)
+		}
+		ecg := coverage.CounterGranularity(entry.CounterGranularity)
+		if granClash(ecg) {
+			return nil, fmt.Errorf("coverage counter granularity clash: package %s uses gran=%d, but package %s uses gran=%s\n", ml[0].PkgPath, cgran, entry.PkgPath, ecg)
+		}
+	}
+
+	// Hash mode and granularity as well.
+	h.Write([]byte(cmode.String()))
+	h.Write([]byte(cgran.String()))
+
+	// Compute final digest.
+	fh := h.Sum(nil)
+	copy(finalHash[:], fh)
+	finalHashComputed = true
+	finalMetaLen = tlen
+
+	return ml, nil
+}
+
+// emitMetaData emits the meta-data output file to the specified
+// directory, returning an error if something went wrong.
+func emitMetaDataToDirectory(outdir string, ml []rtcov.CovMetaBlob) error {
+	ml, err := prepareForMetaEmit()
+	if err != nil {
+		return err
+	}
+	if len(ml) == 0 {
+		return nil
+	}
+
+	metaDataEmitAttempted = true
+
+	s := &emitState{
+		metalist: ml,
+		debug:    os.Getenv("GOCOVERDEBUG") != "",
+		outdir:   outdir,
+	}
+
+	// Open output files.
+	if err := s.openOutputFiles(finalHash, finalMetaLen, metaDataFile); err != nil {
+		return err
+	}
+
+	// Emit meta-data file only if needed (may already be present).
+	if s.needMetaDataFile() {
+		if err := s.emitMetaDataFile(finalHash, finalMetaLen); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// emitCounterData emits the counter data output file for this coverage run.
+// This entry point is intended to be invoked by the runtime when an
+// instrumented program is terminating or calling os.Exit().
+func emitCounterData() {
+	if goCoverDir == "" || !finalHashComputed || covProfileAlreadyEmitted {
+		return
+	}
+	if err := emitCounterDataToDirectory(goCoverDir); err != nil {
+		fmt.Fprintf(os.Stderr, "error: coverage counter data emit failed: %v\n", err)
+		if os.Getenv("GOCOVERDEBUG") != "" {
+			panic("counter-data write failure")
+		}
+	}
+}
+
+// emitMetaData emits the counter-data output file for this coverage run.
+func emitCounterDataToDirectory(outdir string) error {
+	// Ask the runtime for the list of coverage counter symbols.
+	cl := getCovCounterList()
+	if len(cl) == 0 {
+		// no work to do here.
+		return nil
+	}
+
+	if !finalHashComputed {
+		return fmt.Errorf("error: meta-data not available (binary not built with -cover?)")
+	}
+
+	// Ask the runtime for the list of coverage counter symbols.
+	pm := getCovPkgMap()
+	s := &emitState{
+		counterlist: cl,
+		pkgmap:      pm,
+		outdir:      outdir,
+		debug:       os.Getenv("GOCOVERDEBUG") != "",
+	}
+
+	// Open output file.
+	if err := s.openOutputFiles(finalHash, finalMetaLen, counterDataFile); err != nil {
+		return err
+	}
+	if s.cf == nil {
+		return fmt.Errorf("counter data output file open failed (no additional info")
+	}
+
+	// Emit counter data file.
+	if err := s.emitCounterDataFile(finalHash, s.cf); err != nil {
+		return err
+	}
+	if err := s.cf.Close(); err != nil {
+		return fmt.Errorf("closing counter data file: %v", err)
+	}
+
+	// Counter file has now been closed. Rename the temp to the
+	// final desired path.
+	if err := os.Rename(s.cftmp, s.cfname); err != nil {
+		return fmt.Errorf("writing %s: rename from %s failed: %v\n", s.cfname, s.cftmp, err)
+	}
+
+	return nil
+}
+
+// openMetaFile determines whether we need to emit a meta-data output
+// file, or whether we can reuse the existing file in the coverage out
+// dir. It updates mfname/mftmp/mf fields in 's', returning an error
+// if something went wrong. See the comment on the emitState type
+// definition above for more on how file opening is managed.
+func (s *emitState) openMetaFile(metaHash [16]byte, metaLen uint64) error {
+
+	// Open meta-outfile for reading to see if it exists.
+	fn := fmt.Sprintf("%s.%x", coverage.MetaFilePref, metaHash)
+	s.mfname = filepath.Join(s.outdir, fn)
+	fi, err := os.Stat(s.mfname)
+	if err != nil || fi.Size() != int64(metaLen) {
+		// We need a new meta-file.
+		tname := "tmp." + fn + fmt.Sprintf("%d", time.Now().UnixNano())
+		s.mftmp = filepath.Join(s.outdir, tname)
+		s.mf, err = os.Create(s.mftmp)
+		if err != nil {
+			return fmt.Errorf("creating meta-data file %s: %v", s.mftmp, err)
+		}
+	}
+	return nil
+}
+
+// openCounterFile opens an output file for the counter data portion
+// of a test coverage run. If updates the 'cfname' and 'cf' fields in
+// 's', returning an error if something went wrong.
+func (s *emitState) openCounterFile(metaHash [16]byte) error {
+	processID := os.Getpid()
+	fn := fmt.Sprintf(coverage.CounterFileTempl, coverage.CounterFilePref, metaHash, processID, time.Now().UnixNano())
+	s.cfname = filepath.Join(s.outdir, fn)
+	s.cftmp = filepath.Join(s.outdir, "tmp."+fn)
+	var err error
+	s.cf, err = os.Create(s.cftmp)
+	if err != nil {
+		return fmt.Errorf("creating counter data file %s: %v", s.cftmp, err)
+	}
+	return nil
+}
+
+// openOutputFiles opens output files in preparation for emitting
+// coverage data. In the case of the meta-data file, openOutputFiles
+// may determine that we can reuse an existing meta-data file in the
+// outdir, in which case it will leave the 'mf' field in the state
+// struct as nil. If a new meta-file is needed, the field 'mfname'
+// will be the final desired path of the meta file, 'mftmp' will be a
+// temporary file, and 'mf' will be an open os.File pointer for
+// 'mftmp'. The idea is that the client/caller will write content into
+// 'mf', close it, and then rename 'mftmp' to 'mfname'. This function
+// also opens the counter data output file, setting 'cf' and 'cfname'
+// in the state struct.
+func (s *emitState) openOutputFiles(metaHash [16]byte, metaLen uint64, which fileType) error {
+	fi, err := os.Stat(s.outdir)
+	if err != nil {
+		return fmt.Errorf("output directory %q inaccessible (err: %v); no coverage data written", s.outdir, err)
+	}
+	if !fi.IsDir() {
+		return fmt.Errorf("output directory %q not a directory; no coverage data written", s.outdir)
+	}
+
+	if (which & metaDataFile) != 0 {
+		if err := s.openMetaFile(metaHash, metaLen); err != nil {
+			return err
+		}
+	}
+	if (which & counterDataFile) != 0 {
+		if err := s.openCounterFile(metaHash); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// emitMetaDataFile emits coverage meta-data to a previously opened
+// temporary file (s.mftmp), then renames the generated file to the
+// final path (s.mfname).
+func (s *emitState) emitMetaDataFile(finalHash [16]byte, tlen uint64) error {
+	if err := writeMetaData(s.mf, s.metalist, cmode, cgran, finalHash); err != nil {
+		return fmt.Errorf("writing %s: %v\n", s.mftmp, err)
+	}
+	if err := s.mf.Close(); err != nil {
+		return fmt.Errorf("closing meta data temp file: %v", err)
+	}
+
+	// Temp file has now been flushed and closed. Rename the temp to the
+	// final desired path.
+	if err := os.Rename(s.mftmp, s.mfname); err != nil {
+		return fmt.Errorf("writing %s: rename from %s failed: %v\n", s.mfname, s.mftmp, err)
+	}
+
+	return nil
+}
+
+// needMetaDataFile returns TRUE if we need to emit a meta-data file
+// for this program run. It should be used only after
+// openOutputFiles() has been invoked.
+func (s *emitState) needMetaDataFile() bool {
+	return s.mf != nil
+}
+
+func writeMetaData(w io.Writer, metalist []rtcov.CovMetaBlob, cmode coverage.CounterMode, gran coverage.CounterGranularity, finalHash [16]byte) error {
+	mfw := encodemeta.NewCoverageMetaFileWriter("<io.Writer>", w)
+
+	// Note: "sd" is re-initialized on each iteration of the loop
+	// below, and would normally be declared inside the loop, but
+	// placed here escape analysis since we capture it in bufHdr.
+	var sd []byte
+	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
+
+	var blobs [][]byte
+	for _, e := range metalist {
+		bufHdr.Data = uintptr(unsafe.Pointer(e.P))
+		bufHdr.Len = int(e.Len)
+		bufHdr.Cap = int(e.Len)
+		blobs = append(blobs, sd)
+	}
+	return mfw.Write(finalHash, blobs, cmode, gran)
+}
+
+func (s *emitState) NumFuncs() (int, error) {
+	var sd []uint32
+	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
+
+	totalFuncs := 0
+	for _, c := range s.counterlist {
+		bufHdr.Data = uintptr(unsafe.Pointer(c.Counters))
+		bufHdr.Len = int(c.Len)
+		bufHdr.Cap = int(c.Len)
+		for i := 0; i < len(sd); i++ {
+			// Skip ahead until the next non-zero value.
+			if sd[i] == 0 {
+				continue
+			}
+
+			// We found a function that was executed.
+			nCtrs := sd[i]
+			totalFuncs++
+
+			// Skip over this function.
+			i += coverage.FirstCtrOffset + int(nCtrs) - 1
+		}
+	}
+	return totalFuncs, nil
+}
+
+func (s *emitState) VisitFuncs(f encodecounter.CounterVisitorFn) error {
+	var sd []uint32
+	bufHdr := (*reflect.SliceHeader)(unsafe.Pointer(&sd))
+
+	dpkg := uint32(0)
+	for _, c := range s.counterlist {
+		bufHdr.Data = uintptr(unsafe.Pointer(c.Counters))
+		bufHdr.Len = int(c.Len)
+		bufHdr.Cap = int(c.Len)
+		for i := 0; i < len(sd); i++ {
+			// Skip ahead until the next non-zero value.
+			if sd[i] == 0 {
+				continue
+			}
+
+			// We found a function that was executed.
+			nCtrs := sd[i+coverage.NumCtrsOffset]
+			pkgId := sd[i+coverage.PkgIdOffset]
+			funcId := sd[i+coverage.FuncIdOffset]
+			cst := i + coverage.FirstCtrOffset
+			counters := sd[cst : cst+int(nCtrs)]
+
+			if s.debug {
+				if pkgId != dpkg {
+					dpkg = pkgId
+					fmt.Fprintf(os.Stderr, "\n=+= %d: pk=%d visit live fcn",
+						i, pkgId)
+				}
+				fmt.Fprintf(os.Stderr, " {i=%d F%d NC%d}", i, funcId, nCtrs)
+			}
+
+			// Vet and/or fix up package ID. A package ID of zero
+			// indicates that there is some new package X that is a
+			// runtime dependency, and this package has code that
+			// executes before its corresponding init package runs.
+			// This is a fatal error that we should only see during
+			// Go development (e.g. tip).
+			ipk := int32(pkgId)
+			if ipk == 0 {
+				fmt.Fprintf(os.Stderr, "\n")
+				reportErrorInHardcodedList(int32(i), ipk, funcId, nCtrs)
+			} else if ipk < 0 {
+				if newId, ok := s.pkgmap[int(ipk)]; ok {
+					pkgId = uint32(newId)
+				} else {
+					fmt.Fprintf(os.Stderr, "\n")
+					reportErrorInHardcodedList(int32(i), ipk, funcId, nCtrs)
+				}
+			} else {
+				// The package ID value stored in the counter array
+				// has 1 added to it (so as to preclude the
+				// possibility of a zero value ; see
+				// runtime.addCovMeta), so subtract off 1 here to form
+				// the real package ID.
+				pkgId--
+			}
+
+			if err := f(pkgId, funcId, counters); err != nil {
+				return err
+			}
+
+			// Skip over this function.
+			i += coverage.FirstCtrOffset + int(nCtrs) - 1
+		}
+		if s.debug {
+			fmt.Fprintf(os.Stderr, "\n")
+		}
+	}
+	return nil
+}
+
+// captureOsArgs converts os.Args() into the format we use to store
+// this info in the counter data file (counter data file "args"
+// section is a generic key-value collection). See the 'args' section
+// in internal/coverage/defs.go for more info. The args map
+// is also used to capture GOOS + GOARCH values as well.
+func captureOsArgs() map[string]string {
+	m := make(map[string]string)
+	m["argc"] = fmt.Sprintf("%d", len(os.Args))
+	for k, a := range os.Args {
+		m[fmt.Sprintf("argv%d", k)] = a
+	}
+	m["GOOS"] = runtime.GOOS
+	m["GOARCH"] = runtime.GOARCH
+	return m
+}
+
+// emitCounterDataFile emits the counter data portion of a
+// coverage output file (to the file 's.cf').
+func (s *emitState) emitCounterDataFile(finalHash [16]byte, w io.Writer) error {
+	cfw := encodecounter.NewCoverageDataWriter(w, coverage.CtrULeb128)
+	if err := cfw.Write(finalHash, capturedOsArgs, s); err != nil {
+		return err
+	}
+	return nil
+}
+
+// markProfileEmitted signals the runtime/coverage machinery that
+// coverate data output files have already been written out, and there
+// is no need to take any additional action at exit time. This
+// function is called (via linknamed reference) from the
+// coverage-related boilerplate code in _testmain.go emitted for go
+// unit tests.
+func markProfileEmitted(val bool) {
+	covProfileAlreadyEmitted = val
+}
+
+func reportErrorInHardcodedList(slot, pkgID int32, fnID, nCtrs uint32) {
+	metaList := getCovMetaList()
+	pkgMap := getCovPkgMap()
+
+	println("internal error in coverage meta-data tracking:")
+	println("encountered bad pkgID:", pkgID, " at slot:", slot,
+		" fnID:", fnID, " numCtrs:", nCtrs)
+	println("list of hard-coded runtime package IDs needs revising.")
+	println("[see the comment on the 'rtPkgs' var in ")
+	println(" <goroot>/src/internal/coverage/pkid.go]")
+	println("registered list:")
+	for k, b := range metaList {
+		print("slot: ", k, " path='", b.PkgPath, "' ")
+		if b.PkgID != -1 {
+			print(" hard-coded id: ", b.PkgID)
+		}
+		println("")
+	}
+	println("remap table:")
+	for from, to := range pkgMap {
+		println("from ", from, " to ", to)
+	}
+}

src/runtime/coverage/hooks.go

@@ -0,0 +1,42 @@
+// Copyright 2022 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.
+
+package coverage
+
+import _ "unsafe"
+
+// initHook is invoked from the main package "init" routine in
+// programs built with "-cover". This function is intended to be
+// called only by the compiler.
+//
+// If 'istest' is false, it indicates we're building a regular program
+// ("go build -cover ..."), in which case we immediately try to write
+// out the meta-data file, and register emitCounterData as an exit
+// hook.
+//
+// If 'istest' is true (indicating that the program in question is a
+// Go test binary), then we tentatively queue up both emitMetaData and
+// emitCounterData as exit hooks. In the normal case (e.g. regular "go
+// test -cover" run) the testmain.go boilerplate will run at the end
+// of the test, write out the coverage percentage, and then invoke
+// markProfileEmitted() to indicate that no more work needs to be
+// done. If however that call is never made, this is a sign that the
+// test binary is being used as a replacement binary for the tool
+// being tested, hence we do want to run exit hooks when the program
+// terminates.
+func initHook(istest bool) {
+	// Note: hooks are run in reverse registration order, so
+	// register the counter data hook before the meta-data hook
+	// (in the case where two hooks are needed).
+	runOnNonZeroExit := true
+	runtime_addExitHook(emitCounterData, runOnNonZeroExit)
+	if istest {
+		runtime_addExitHook(emitMetaData, runOnNonZeroExit)
+	} else {
+		emitMetaData()
+	}
+}
+
+//go:linkname runtime_addExitHook runtime.addExitHook
+func runtime_addExitHook(f func(), runOnNonZeroExit bool)

src/runtime/coverage/testsupport.go

@@ -0,0 +1,207 @@
+// Copyright 2022 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.
+
+package coverage
+
+import (
+	"fmt"
+	"internal/coverage"
+	"internal/coverage/calloc"
+	"internal/coverage/cformat"
+	"internal/coverage/cmerge"
+	"internal/coverage/decodecounter"
+	"internal/coverage/decodemeta"
+	"internal/coverage/pods"
+	"os"
+)
+
+// processCoverTestDir is called (via a linknamed reference) from
+// testmain code when "go test -cover" is in effect. It is not
+// intended to be used other than internally by the Go command's
+// generated code.
+func processCoverTestDir(dir string, cfile string, cm string, cpkg string) error {
+	cmode := coverage.ParseCounterMode(cm)
+	if cmode == coverage.CtrModeInvalid {
+		return fmt.Errorf("invalid counter mode %q", cm)
+	}
+
+	// Emit meta-data and counter data.
+	ml := getCovMetaList()
+	if len(ml) == 0 {
+		// This corresponds to the case where we have a package that
+		// contains test code but no functions (which is fine). In this
+		// case there is no need to emit anything.
+	} else {
+		if err := emitMetaDataToDirectory(dir, ml); err != nil {
+			return err
+		}
+		if err := emitCounterDataToDirectory(dir); err != nil {
+			return err
+		}
+	}
+
+	// Collect pods from test run. For the majority of cases we would
+	// expect to see a single pod here, but allow for multiple pods in
+	// case the test harness is doing extra work to collect data files
+	// from builds that it kicks off as part of the testing.
+	podlist, err := pods.CollectPods([]string{dir}, false)
+	if err != nil {
+		return fmt.Errorf("reading from %s: %v", dir, err)
+	}
+
+	// Open text output file if appropriate.
+	var tf *os.File
+	var tfClosed bool
+	if cfile != "" {
+		var err error
+		tf, err = os.Create(cfile)
+		if err != nil {
+			return fmt.Errorf("internal error: opening coverage data output file %q: %v", cfile, err)
+		}
+		defer func() {
+			if !tfClosed {
+				tfClosed = true
+				tf.Close()
+			}
+		}()
+	}
+
+	// Read/process the pods.
+	ts := &tstate{
+		cm:    &cmerge.Merger{},
+		cf:    cformat.NewFormatter(cmode),
+		cmode: cmode,
+	}
+	for _, p := range podlist {
+		if err := ts.processPod(p); err != nil {
+			return err
+		}
+	}
+
+	// Emit percent.
+	if err := ts.cf.EmitPercent(os.Stdout, cpkg, true); err != nil {
+		return err
+	}
+
+	// Emit text output.
+	if tf != nil {
+		if err := ts.cf.EmitTextual(tf); err != nil {
+			return err
+		}
+		tfClosed = true
+		if err := tf.Close(); err != nil {
+			return fmt.Errorf("closing %s: %v", cfile, err)
+		}
+	}
+
+	return nil
+}
+
+type tstate struct {
+	calloc.BatchCounterAlloc
+	cm    *cmerge.Merger
+	cf    *cformat.Formatter
+	cmode coverage.CounterMode
+}
+
+// processPod reads coverage counter data for a specific pod.
+func (ts *tstate) processPod(p pods.Pod) error {
+	// Open meta-data file
+	f, err := os.Open(p.MetaFile)
+	if err != nil {
+		return fmt.Errorf("unable to open meta-data file %s: %v", p.MetaFile, err)
+	}
+	defer func() {
+		f.Close()
+	}()
+	var mfr *decodemeta.CoverageMetaFileReader
+	mfr, err = decodemeta.NewCoverageMetaFileReader(f, nil)
+	if err != nil {
+		return fmt.Errorf("error reading meta-data file %s: %v", p.MetaFile, err)
+	}
+	newmode := mfr.CounterMode()
+	if newmode != ts.cmode {
+		return fmt.Errorf("internal error: counter mode clash: %q from test harness, %q from data file %s", ts.cmode.String(), newmode.String(), p.MetaFile)
+	}
+	newgran := mfr.CounterGranularity()
+	if err := ts.cm.SetModeAndGranularity(p.MetaFile, cmode, newgran); err != nil {
+		return err
+	}
+
+	// Read counter data files.
+	pmm := make(map[pkfunc][]uint32)
+	for _, cdf := range p.CounterDataFiles {
+		cf, err := os.Open(cdf)
+		if err != nil {
+			return fmt.Errorf("opening counter data file %s: %s", cdf, err)
+		}
+		var cdr *decodecounter.CounterDataReader
+		cdr, err = decodecounter.NewCounterDataReader(cdf, cf)
+		if err != nil {
+			return fmt.Errorf("reading counter data file %s: %s", cdf, err)
+		}
+		var data decodecounter.FuncPayload
+		for {
+			ok, err := cdr.NextFunc(&data)
+			if err != nil {
+				return fmt.Errorf("reading counter data file %s: %v", cdf, err)
+			}
+			if !ok {
+				break
+			}
+
+			// NB: sanity check on pkg and func IDs?
+			key := pkfunc{pk: data.PkgIdx, fcn: data.FuncIdx}
+			if prev, found := pmm[key]; found {
+				// Note: no overflow reporting here.
+				if err, _ := ts.cm.MergeCounters(data.Counters, prev); err != nil {
+					return fmt.Errorf("processing counter data file %s: %v", cdf, err)
+				}
+			}
+			c := ts.AllocateCounters(len(data.Counters))
+			copy(c, data.Counters)
+			pmm[key] = c
+		}
+	}
+
+	// Visit meta-data file.
+	np := uint32(mfr.NumPackages())
+	payload := []byte{}
+	for pkIdx := uint32(0); pkIdx < np; pkIdx++ {
+		var pd *decodemeta.CoverageMetaDataDecoder
+		pd, payload, err = mfr.GetPackageDecoder(pkIdx, payload)
+		if err != nil {
+			return fmt.Errorf("reading pkg %d from meta-file %s: %s", pkIdx, p.MetaFile, err)
+		}
+		ts.cf.SetPackage(pd.PackagePath())
+		var fd coverage.FuncDesc
+		nf := pd.NumFuncs()
+		for fnIdx := uint32(0); fnIdx < nf; fnIdx++ {
+			if err := pd.ReadFunc(fnIdx, &fd); err != nil {
+				return fmt.Errorf("reading meta-data file %s: %v",
+					p.MetaFile, err)
+			}
+			key := pkfunc{pk: pkIdx, fcn: fnIdx}
+			counters, haveCounters := pmm[key]
+			for i := 0; i < len(fd.Units); i++ {
+				u := fd.Units[i]
+				// Skip units with non-zero parent (no way to represent
+				// these in the existing format).
+				if u.Parent != 0 {
+					continue
+				}
+				count := uint32(0)
+				if haveCounters {
+					count = counters[i]
+				}
+				ts.cf.AddUnit(fd.Srcfile, fd.Funcname, fd.Lit, u, count)
+			}
+		}
+	}
+	return nil
+}
+
+type pkfunc struct {
+	pk, fcn uint32
+}

src/runtime/covercounter.go

@@ -0,0 +1,26 @@
+// Copyright 2022 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.
+
+package runtime
+
+import (
+	"internal/coverage/rtcov"
+	"unsafe"
+)
+
+//go:linkname runtime_coverage_getCovCounterList runtime/coverage.getCovCounterList
+func runtime_coverage_getCovCounterList() []rtcov.CovCounterBlob {
+	res := []rtcov.CovCounterBlob{}
+	u32sz := unsafe.Sizeof(uint32(0))
+	for datap := &firstmoduledata; datap != nil; datap = datap.next {
+		if datap.covctrs == datap.ecovctrs {
+			continue
+		}
+		res = append(res, rtcov.CovCounterBlob{
+			Counters: (*uint32)(unsafe.Pointer(datap.covctrs)),
+			Len:      uint64((datap.ecovctrs - datap.covctrs) / u32sz),
+		})
+	}
+	return res
+}

src/runtime/covermeta.go

@@ -24,26 +24,6 @@ var covMeta struct {
 	hardCodedListNeedsUpdating bool
 }
 
-func reportErrorInHardcodedList(slot int32, pkgId int32) {
-	println("internal error in coverage meta-data tracking:")
-	println("encountered bad pkg ID ", pkgId, " at slot ", slot)
-	println("list of hard-coded runtime package IDs needs revising.")
-	println("[see the comment on the 'rtPkgs' var in ")
-	println(" <goroot>/src/internal/coverage/pkid.go]")
-	println("registered list:")
-	for k, b := range covMeta.metaList {
-		print("slot: ", k, " path='", b.PkgPath, "' ")
-		if b.PkgID != -1 {
-			print(" hard-coded id: ", b.PkgID)
-		}
-		println("")
-	}
-	println("remap table:")
-	for from, to := range covMeta.pkgMap {
-		println("from ", from, " to ", to)
-	}
-}
-
 // addCovMeta is invoked during package "init" functions by the
 // compiler when compiling for coverage instrumentation; here 'p' is a
 // meta-data blob of length 'dlen' for the package in question, 'hash'
@@ -80,3 +60,13 @@ func addCovMeta(p unsafe.Pointer, dlen uint32, hash [16]byte, pkpath string, pki
 	// ID zero is reserved as invalid.
 	return uint32(slot + 1)
 }
+
+//go:linkname runtime_coverage_getCovMetaList runtime/coverage.getCovMetaList
+func runtime_coverage_getCovMetaList() []rtcov.CovMetaBlob {
+	return covMeta.metaList
+}
+
+//go:linkname runtime_coverage_getCovPkgMap runtime/coverage.getCovPkgMap
+func runtime_coverage_getCovPkgMap() map[int]int {
+	return covMeta.pkgMap
+}