imports: fix unexpected blank line after package comment

The fix in golang/go#23709 introduced a separate bug where extra blank
lines were sometimes inserted. This fixes that newly introduced bug.

Fixes golang/go#26246

Change-Id: I78131cc1d01ae246922ed9e4336ebb31d1c6cfa1
Reviewed-on: https://go-review.googlesource.com/122538
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

imports/fix_test.go

@@ -915,6 +915,110 @@ func main() {
 	x := math.MaxInt64
 	fmt.Println(strings.Join(",", []string{"hi"}), x)
 }
+`,
+	},
+
+	{
+		name: "issue #26246 1",
+		in: `package main
+
+import (
+	_ "io"
+	_ "net/http"
+	_ "net/http/pprof" // install the pprof http handlers
+	_ "strings"
+)
+
+func main() {
+}
+`,
+		out: `package main
+
+import (
+	_ "io"
+	_ "net/http"
+	_ "net/http/pprof" // install the pprof http handlers
+	_ "strings"
+)
+
+func main() {
+}
+`,
+	},
+
+	{
+		name: "issue #26246 2",
+		in: `package main
+
+import (
+	_ "io"
+	_ "net/http/pprof" // install the pprof http handlers
+	_ "net/http"
+	_ "strings"
+)
+
+func main() {
+}
+`,
+		out: `package main
+
+import (
+	_ "io"
+	_ "net/http"
+	_ "net/http/pprof" // install the pprof http handlers
+	_ "strings"
+)
+
+func main() {
+}
+`,
+	},
+
+	{
+		name: "issue #26246 3",
+		in: `package main
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	_ "net/http/pprof" // install the pprof http handlers
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+func main() {
+	_ = strings.ToUpper("hello")
+	_ = io.EOF
+	var (
+		_ json.Number
+		_ *http.Request
+		_ errors.Frame
+	)
+}
+`,
+		out: `package main
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	_ "net/http/pprof" // install the pprof http handlers
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+func main() {
+	_ = strings.ToUpper("hello")
+	_ = io.EOF
+	var (
+		_ json.Number
+		_ *http.Request
+		_ errors.Frame
+	)
+}
 `,
 	},
 }

imports/sortimports.go

@@ -183,6 +183,17 @@ func sortSpecs(fset *token.FileSet, f *ast.File, specs []ast.Spec) []ast.Spec {
 
 	sort.Sort(byCommentPos(comments))
 
+	// Fixup comments can insert blank lines, because import specs are on different lines.
+	// We remove those blank lines here by merging import spec to the first import spec line.
+	firstSpecLine := fset.Position(specs[0].Pos()).Line
+	for _, s := range specs[1:] {
+		p := s.Pos()
+		line := fset.File(p).Line(p)
+		for previousLine := line - 1; previousLine >= firstSpecLine; {
+			fset.File(p).MergeLine(previousLine)
+			previousLine--
+		}
+	}
 	return specs
 }