cmd/compile: use file content, not suffix, to distinguish .a and .o files

This allows reading from package storage systems that may not
preserve the .a suffix (used with -importcfg).

Fixes #20579 (combined with CLs earlier in stack).

Change-Id: If2fc6a3d01bd0170a757e1f2ba9a22a4d9be7dbf
Reviewed-on: https://go-review.googlesource.com/44853
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -750,21 +750,6 @@ func arsize(b *bufio.Reader, name string) int {
 	return i
 }
 
-func skiptopkgdef(b *bufio.Reader) bool {
-	// archive header
-	p, err := b.ReadString('\n')
-	if err != nil {
-		log.Fatalf("reading input: %v", err)
-	}
-	if p != "!<arch>\n" {
-		return false
-	}
-
-	// package export block should be first
-	sz := arsize(b, "__.PKGDEF")
-	return sz > 0
-}
-
 var idirs []string
 
 func addidir(dir string) {
@@ -975,22 +960,30 @@ func importfile(f *Val) *types.Pkg {
 	defer impf.Close()
 	imp := bufio.NewReader(impf)
 
-	const pkgSuffix = ".a"
-	if strings.HasSuffix(file, pkgSuffix) {
-		if !skiptopkgdef(imp) {
-			yyerror("import %s: not a package file", file)
-			errorexit()
+	// check object header
+	p, err := imp.ReadString('\n')
+	if err != nil {
+		log.Fatalf("reading input: %v", err)
 	}
+	if len(p) > 0 {
+		p = p[:len(p)-1]
 	}
 
-	// check object header
-	p, err := imp.ReadString('\n')
+	if p == "!<arch>" { // package archive
+		// package export block should be first
+		sz := arsize(imp, "__.PKGDEF")
+		if sz <= 0 {
+			yyerror("import %s: not a package file", file)
+			errorexit()
+		}
+		p, err = imp.ReadString('\n')
 		if err != nil {
 			log.Fatalf("reading input: %v", err)
 		}
 		if len(p) > 0 {
 			p = p[:len(p)-1]
 		}
+	}
 
 	if p != "empty archive" {
 		if !strings.HasPrefix(p, "go object ") {
@@ -1030,7 +1023,7 @@ func importfile(f *Val) *types.Pkg {
 		Ctxt.AddImport(path_)
 	} else {
 		// For file "/Users/foo/go/pkg/darwin_amd64/math.a" record "math.a".
-		Ctxt.AddImport(file[len(file)-len(path_)-len(pkgSuffix):])
+		Ctxt.AddImport(file[len(file)-len(path_)-len(".a"):])
 	}
 
 	// In the importfile, if we find: