cmd/pack: treat compiler's -linkobj output as "compiler object"

Treat the compiler's -linkobj output as "compiler object, which
means "pack c" will "see through" the file and add individual
entry to the new archive, instead of the object as a whole.

This is somewhat peculiar. But Go 1.15's cmd/pack does this,
although seemingly accidental. We just do the same. FWIW, it
does make things more consistent with/without -linkobj flag.

Fixes #43271.

Change-Id: I6b2d99256db7ebf0fa430f85afa7464e334f6bcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/279483
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

2 files changed, 84 insertions, 13 deletions

diff --git a/src/cmd/pack/pack.go b/src/cmd/pack/pack.go
index 82546ea7dc..3dffabe5ec 100644
--- a/src/cmd/pack/pack.go
+++ b/src/cmd/pack/pack.go
@@ -315,20 +315,25 @@ func (ar *Archive) extractContents1(e *archive.Entry, out io.Writer) {
 }
 
 // isGoCompilerObjFile reports whether file is an object file created
-// by the Go compiler, which is an archive file with exactly two entries:
-// __.PKGDEF and _go_.o.
+// by the Go compiler, which is an archive file with exactly one entry
+// of __.PKGDEF, or _go_.o, or both entries.
 func isGoCompilerObjFile(a *archive.Archive) bool {
-	if len(a.Entries) != 2 {
-		return false
-	}
-	var foundPkgDef, foundGo bool
-	for _, e := range a.Entries {
-		if e.Type == archive.EntryPkgDef && e.Name == "__.PKGDEF" {
-			foundPkgDef = true
-		}
-		if e.Type == archive.EntryGoObj && e.Name == "_go_.o" {
-			foundGo = true
+	switch len(a.Entries) {
+	case 1:
+		return (a.Entries[0].Type == archive.EntryGoObj && a.Entries[0].Name == "_go_.o") ||
+			(a.Entries[0].Type == archive.EntryPkgDef && a.Entries[0].Name == "__.PKGDEF")
+	case 2:
+		var foundPkgDef, foundGo bool
+		for _, e := range a.Entries {
+			if e.Type == archive.EntryPkgDef && e.Name == "__.PKGDEF" {
+				foundPkgDef = true
+			}
+			if e.Type == archive.EntryGoObj && e.Name == "_go_.o" {
+				foundGo = true
+			}
 		}
+		return foundPkgDef && foundGo
+	default:
+		return false
 	}
-	return foundPkgDef && foundGo
 }
diff --git a/src/cmd/pack/pack_test.go b/src/cmd/pack/pack_test.go
index 218c7acda6..16a5135800 100644
--- a/src/cmd/pack/pack_test.go
+++ b/src/cmd/pack/pack_test.go
@@ -302,6 +302,72 @@ func TestIssue21703(t *testing.T) {
 	run(goBin, "tool", "compile", "-I", ".", "b.go")
 }
 
+// Test the "c" command can "see through" the archive generated by the compiler.
+// This is peculiar. (See issue )
+func TestCreateWithCompilerObj(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	dir := tmpDir(t)
+	defer os.RemoveAll(dir)
+	src := filepath.Join(dir, "p.go")
+	prog := "package p; var X = 42\n"
+	err := os.WriteFile(src, []byte(prog), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	run := func(args ...string) string {
+		return doRun(t, dir, args...)
+	}
+
+	goBin := testenv.GoToolPath(t)
+	run(goBin, "build", "cmd/pack") // writes pack binary to dir
+	run(goBin, "tool", "compile", "-pack", "-o", "p.a", "p.go")
+	run("./pack", "c", "packed.a", "p.a")
+	fi, err := os.Stat(filepath.Join(dir, "p.a"))
+	if err != nil {
+		t.Fatalf("stat p.a failed: %v", err)
+	}
+	fi2, err := os.Stat(filepath.Join(dir, "packed.a"))
+	if err != nil {
+		t.Fatalf("stat packed.a failed: %v", err)
+	}
+	// For compiler-generated object file, the "c" command is
+	// expected to get (essentially) the same file back, instead
+	// of packing it into a new archive with a single entry.
+	if want, got := fi.Size(), fi2.Size(); want != got {
+		t.Errorf("packed file with different size: want %d, got %d", want, got)
+	}
+
+	// Test -linkobj flag as well.
+	run(goBin, "tool", "compile", "-linkobj", "p2.a", "-o", "p.x", "p.go")
+	run("./pack", "c", "packed2.a", "p2.a")
+	fi, err = os.Stat(filepath.Join(dir, "p2.a"))
+	if err != nil {
+		t.Fatalf("stat p2.a failed: %v", err)
+	}
+	fi2, err = os.Stat(filepath.Join(dir, "packed2.a"))
+	if err != nil {
+		t.Fatalf("stat packed2.a failed: %v", err)
+	}
+	if want, got := fi.Size(), fi2.Size(); want != got {
+		t.Errorf("packed file with different size: want %d, got %d", want, got)
+	}
+
+	run("./pack", "c", "packed3.a", "p.x")
+	fi, err = os.Stat(filepath.Join(dir, "p.x"))
+	if err != nil {
+		t.Fatalf("stat p.x failed: %v", err)
+	}
+	fi2, err = os.Stat(filepath.Join(dir, "packed3.a"))
+	if err != nil {
+		t.Fatalf("stat packed3.a failed: %v", err)
+	}
+	if want, got := fi.Size(), fi2.Size(); want != got {
+		t.Errorf("packed file with different size: want %d, got %d", want, got)
+	}
+}
+
 // doRun runs a program in a directory and returns the output.
 func doRun(t *testing.T, dir string, args ...string) string {
 	cmd := exec.Command(args[0], args[1:]...)