cmd/gorebuild: report files missing from posted archive better

The "missing from posted archive" case was checking the wrong variable
and could never trigger. Fortunately, it's fairly harmless, as missing
files would still be caught by gorebuild thanks to check hitting a nil
pointer dereference trying to compare the missing file.

Check the right variable to fix the panic, and print the intended text.

For golang/go#57120.
For golang/go#58884.

Change-Id: I4560a9cc6c53bca37283c004826d728e175a1ff1
Reviewed-on: https://go-review.googlesource.com/c/build/+/556075
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

cmd/gorebuild/io.go

@@ -438,7 +438,7 @@ func DiffArchive[File1, File2 any](log *Log,
 		if !okr && !okp { // duplicate name
 			continue
 		}
-		if !okr {
+		if !okp {
 			log.Printf("%s: missing from posted archive", name)
 			match = false
 			continue

cmd/gorebuild/main_test.go

@@ -0,0 +1,59 @@
+// Copyright 2024 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.
+
+package main_test
+
+import (
+	"testing"
+
+	gorebuild "golang.org/x/build/cmd/gorebuild"
+)
+
+func TestDiffArchive(t *testing.T) {
+	type match bool // A named type for readability of test cases below.
+	for _, tc := range [...]struct {
+		name string
+		a, b map[string]string
+		want match
+	}{
+		{
+			name: "empty",
+			a:    map[string]string{},
+			b:    map[string]string{},
+			want: match(true),
+		},
+		{
+			name: "equal",
+			a:    map[string]string{"file1": "content 1", "file2": "content 2"},
+			b:    map[string]string{"file1": "content 1", "file2": "content 2"},
+			want: match(true),
+		},
+		{
+			name: "different content",
+			a:    map[string]string{"file1": "content 1", "file2": "content 2"},
+			b:    map[string]string{"file1": "content 3", "file2": "content 4"},
+			want: match(false),
+		},
+		{
+			name: "missing file", // file2 in a, but missing in b.
+			a:    map[string]string{"file1": "", "file2": ""},
+			b:    map[string]string{"file1": ""},
+			want: match(false),
+		},
+		{
+			name: "unexpected file", // file3 not in a, but unexpectedly there in b.
+			a:    map[string]string{"file1": "", "file2": ""},
+			b:    map[string]string{"file1": "", "file2": "", "file3": ""},
+			want: match(false),
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var log gorebuild.Log
+			got := gorebuild.DiffArchive(&log, tc.a, tc.b, func(_ *gorebuild.Log, a, b string) bool { return a == b })
+			if got != bool(tc.want) {
+				t.Errorf("got match = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}