cmd/go: write coverage to file, add percentage statistic

Move the data dumper to the testing package, where it has access
to file I/O.
Print a percentage value at the end of the run.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/10264045

src/pkg/testing/cover.go

@@ -0,0 +1,91 @@
+// Copyright 2013 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.
+
+// Support for test coverage.
+
+package testing
+
+import (
+	"fmt"
+	"os"
+)
+
+// CoverBlock records the coverage data for a single basic block.
+// NOTE: This struct is internal to the testing infrastructure and may change.
+// It is not covered (yet) by the Go 1 compatibility guidelines.
+type CoverBlock struct {
+	Line0 uint32
+	Col0  uint16
+	Line1 uint32
+	Col1  uint16
+	Stmts uint16
+}
+
+var (
+	coverCounters map[string][]uint32
+	coverBlocks   map[string][]CoverBlock
+)
+
+// RegisterCover records the coverage data accumulators for the tests.
+// NOTE: This struct is internal to the testing infrastructure and may change.
+// It is not covered (yet) by the Go 1 compatibility guidelines.
+func RegisterCover(c map[string][]uint32, b map[string][]CoverBlock) {
+	coverCounters = c
+	coverBlocks = b
+}
+
+// mustBeNil checks the error and, if present, reports it and exits.
+func mustBeNil(err error) {
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "testing: %s\n", err)
+		os.Exit(2)
+	}
+}
+
+// coverReport reports the coverage percentage and writes a coverage profile if requested.
+func coverReport() {
+	var f *os.File
+	var err error
+	if *coverProfile != "" {
+		f, err = os.Create(toOutputDir(*coverProfile))
+		mustBeNil(err)
+		defer func() { mustBeNil(f.Close()) }()
+	}
+
+	var active, total int64
+	packageName := ""
+	for name, counts := range coverCounters {
+		if packageName == "" {
+			// Package name ends at last slash.
+			for i, c := range name {
+				if c == '/' {
+					packageName = name[:i]
+				}
+			}
+		}
+		blocks := coverBlocks[name]
+		for i, count := range counts {
+			stmts := int64(blocks[i].Stmts)
+			total += stmts
+			if count > 0 {
+				active += stmts
+			}
+			if f != nil {
+				_, err := fmt.Fprintf(f, "%s:%d.%d,%d.%d %d %d\n", name,
+					blocks[i].Line0, blocks[i].Col0,
+					blocks[i].Line1, blocks[i].Col1,
+					stmts,
+					count)
+				mustBeNil(err)
+			}
+		}
+	}
+	if total == 0 {
+		total = 1
+	}
+	if packageName == "" {
+		packageName = "package"
+	}
+	fmt.Printf("test coverage for %s: %.1f%% of statements\n", packageName, 100*float64(active)/float64(total))
+}