internal: don't panic on all/all

If CompareBuildContexts sees all/all, it prefers it instead of
panicking.

Change-Id: I50ff46fa2bbad47cfe5539297a08744dd62b261c
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/305969
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

internal/build_context.go

@@ -46,11 +46,17 @@ func CompareBuildContexts(c1, c2 BuildContext) int {
 	if c1 == c2 {
 		return 0
 	}
-	// We should never see a BuildContext with "all" here.
-	if c1.GOOS == All || c1.GOARCH == All || c2.GOOS == All || c2.GOARCH == All {
-		panic("BuildContext with 'all'")
+	// Although we really shouldn't see a BuildContext with "all" here, we may if the
+	// DB erroneously has both an all/all row and some other row. So just prefer the all/all.
+	if c1 == BuildContextAll {
+		if c2 == BuildContextAll {
+			return 0
+		}
+		return -1
+	}
+	if c2 == BuildContextAll {
+		return 1
 	}
-
 	pos := func(c BuildContext) int {
 		for i, d := range BuildContexts {
 			if c == d {