cmd/{go,cover}: fix for -coverprofile path capture with local pkg

When coverage testing a local package (defined by a relative import
path such as "./foo/bar") the convention when "-coverprofile" is used
has been to capture source files by full pathname, as opposed to
recording the full import path or the invented import path
("command-line-arguments/") created by the go command in the case of
building named Go files. Doing this makes it much easier to use
collected profiles with "go tool -cover -html=<profile>".

The support for this feature/convention wound up being inadvertantly
dropped during the GOEXPERIMENT=coverageredesign implementation; this
patch restores it.

Fixes #56433.

Change-Id: Ib9556fdc86011b00c155caa614ab23e5148f3eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/445917
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/cover/cover.go

@@ -74,7 +74,7 @@ var (
 
 var pkgconfig coverage.CoverPkgConfig
 
-var outputfiles []string // set whe -pkgcfg is in use
+var outputfiles []string // set when -pkgcfg is in use
 
 var profile string // The profile to read; the value of -html or -func
 
@@ -484,6 +484,16 @@ func (f *File) postFunc(fn ast.Node, funcname string, flit bool, body *ast.Block
 	ppath := pkgconfig.PkgPath
 	filename := ppath + "/" + filepath.Base(fnpos.Filename)
 
+	// The convention for cmd/cover is that if the go command that
+	// kicks off coverage specifies a local import path (e.g. "go test
+	// -cover ./thispackage"), the tool will capture full pathnames
+	// for source files instead of relative paths, which tend to work
+	// more smoothly for "go tool cover -html". See also issue #56433
+	// for more details.
+	if pkgconfig.Local {
+		filename = f.name
+	}
+
 	// Hand off function to meta-data builder.
 	fd := coverage.FuncDesc{
 		Funcname: funcname,

src/cmd/go/internal/work/exec.go

@@ -1978,6 +1978,7 @@ func (b *Builder) writeCoverPkgInputs(a *Action, pconfigfile string, covoutputsf
 		// depending on user demand.
 		Granularity: "perblock",
 		OutConfig:   p.Internal.CoverageCfg,
+		Local:       p.Internal.Local,
 	}
 	if a.Package.Module != nil {
 		pcfg.ModulePath = a.Package.Module.Path

src/cmd/go/testdata/script/cover_test_localpkg_filepath.txt

@@ -0,0 +1,40 @@
+
+[short] skip
+
+# collect coverage profile in text format
+go test -coverprofile=blah.prof prog.go prog_test.go
+
+# should not contain cmd-line pseudo-import-path
+grep prog.go blah.prof
+grep $PWD blah.prof
+! grep command-line-arguments blah.prof
+
+-- prog.go --
+package main
+
+func Mumble(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return 42
+}
+
+func Grumble(y int) int {
+	return -y
+}
+
+func main() {
+}
+
+-- prog_test.go --
+package main
+
+import (
+	"testing"
+)
+
+func TestMumble(t *testing.T) {
+	if x := Mumble(10); x != 42 {
+		t.Errorf("Mumble(%d): got %d want %d", 10, x, 42)
+	}
+}

src/internal/coverage/cmddefs.go

@@ -25,6 +25,13 @@ type CoverPkgConfig struct {
 
 	// Module path for this package (empty if no go.mod in use)
 	ModulePath string
+
+	// Local mode indicates we're doing a coverage build or test of a
+	// package selected via local import path, e.g. "./..." or
+	// "./foo/bar" as opposed to a non-relative import path. See the
+	// corresponding field in cmd/go's PackageInternal struct for more
+	// info.
+	Local bool
 }
 
 // CoverFixupConfig contains annotations/notes generated by the