gomtree: add special cases for tar generation checking

Due to several unsolveable problems in tar generation, such as the
size=... keyword, we have to special case quite a few things in the
checking code. We might want to move this to mtree properly (but I'm
hesitant about ignoring errors that only happen for tar DHes).

Signed-off-by: Aleksa Sarai <asarai@suse.de>

cmd/gomtree/main.go

@@ -62,6 +62,90 @@ var formats = map[string]func([]mtree.InodeDelta) string{
 	},
 }
 
+// isDirEntry returns wheter an mtree.Entry describes a directory.
+func isDirEntry(e mtree.Entry) bool {
+	for _, kw := range e.Keywords {
+		kv := mtree.KeyVal(kw)
+		if kv.Keyword() == "type" {
+			return kv.Value() == "dir"
+		}
+	}
+	// Shouldn't be reached.
+	return false
+}
+
+// filterMissingKeywords is a fairly annoying hack to get around the fact that
+// tar archive manifest generation has certain unsolveable problems regarding
+// certain keywords. For example, the size=... keyword cannot be implemented
+// for directories in a tar archive (which causes Missing errors for that
+// keyword).
+//
+// This function just removes all instances of Missing errors for keywords.
+// This makes certain assumptions about the type of issues tar archives have.
+// Only call this on tar archive manifest comparisons.
+func filterMissingKeywords(diffs []mtree.InodeDelta) []mtree.InodeDelta {
+	newDiffs := []mtree.InodeDelta{}
+loop:
+	for _, diff := range diffs {
+		if diff.Type() == mtree.Modified {
+			// We only apply this filtering to directories.
+			// NOTE: This will probably break if someone drops the size keyword.
+			if isDirEntry(*diff.Old()) || isDirEntry(*diff.New()) {
+				// If this applies to '.' then we just filter everything
+				// (meaning we remove this entry). This is because note all tar
+				// archives include a '.' entry. Which makes checking this not
+				// practical.
+				if diff.Path() == "." {
+					continue
+				}
+
+				// Only filter out the size keyword.
+				// NOTE: This currently takes advantage of the fact the
+				//       diff.Diff() returns the actual slice to diff.keys.
+				keys := diff.Diff()
+				for idx, k := range keys {
+					// Delete the key if it's "size". Unfortunately in Go you
+					// can't delete from a slice without reassigning it. So we
+					// just overwrite it with the last value.
+					if k.Name() == "size" {
+						if len(keys) < 2 {
+							continue loop
+						}
+						keys[idx] = keys[len(keys)-1]
+					}
+				}
+			}
+		}
+
+		// If we got here, append to the new set.
+		newDiffs = append(newDiffs, diff)
+	}
+	return newDiffs
+}
+
+// isTarSpec returns whether the spec provided came from the tar generator.
+// This takes advantage of an unsolveable problem in tar generation.
+func isTarSpec(spec *mtree.DirectoryHierarchy) bool {
+	// Find a directory and check whether it's missing size=...
+	// NOTE: This will definitely break if someone drops the size=... keyword.
+	for _, e := range spec.Entries {
+		if !isDirEntry(e) {
+			continue
+		}
+
+		for _, kw := range e.Keywords {
+			kv := mtree.KeyVal(kw)
+			if kv.Keyword() == "size" {
+				return false
+			}
+		}
+		return true
+	}
+
+	// Should never be reached.
+	return false
+}
+
 func main() {
 	flag.Parse()
 
@@ -327,6 +411,9 @@ func main() {
 			return
 		}
 		if res != nil {
+			if isTarSpec(specDh) || *flTar != "" {
+				res = filterMissingKeywords(res)
+			}
 			if len(res) > 0 {
 				defer os.Exit(1)
 			}