Add initial support for an explicit "SharedTags" field

Things we know we'll need to be able to do with the new `SharedTags` (and are thus included in this initial file format support):

- get list of all shared tags from a manifest (`manifest.GetAllSharedTags()`)
- get list of shared tags from an entry (`entry.SharedTags`)
- get list of entries given a shared tag (`manifest.GetSharedTag(tag)`)

manifest/rfc2822.go

@@ -27,7 +27,8 @@ type Manifest2822Entry struct {
 
 	Maintainers []string `delim:"," strip:"\n\r\t "`
 
-	Tags []string `delim:"," strip:"\n\r\t "`
+	Tags       []string `delim:"," strip:"\n\r\t "`
+	SharedTags []string `delim:"," strip:"\n\r\t "`
 
 	GitRepo   string
 	GitFetch  string
@@ -46,6 +47,7 @@ func (entry Manifest2822Entry) Clone() Manifest2822Entry {
 	// SLICES! grr
 	entry.Maintainers = append([]string{}, entry.Maintainers...)
 	entry.Tags = append([]string{}, entry.Tags...)
+	entry.SharedTags = append([]string{}, entry.SharedTags...)
 	entry.Constraints = append([]string{}, entry.Constraints...)
 	return entry
 }
@@ -60,6 +62,10 @@ func (entry Manifest2822Entry) TagsString() string {
 	return strings.Join(entry.Tags, StringSeparator2822)
 }
 
+func (entry Manifest2822Entry) SharedTagsString() string {
+	return strings.Join(entry.SharedTags, StringSeparator2822)
+}
+
 func (entry Manifest2822Entry) ConstraintsString() string {
 	return strings.Join(entry.Constraints, StringSeparator2822)
 }
@@ -77,6 +83,9 @@ func (entry Manifest2822Entry) ClearDefaults(defaults Manifest2822Entry) Manifes
 	if entry.TagsString() == defaults.TagsString() {
 		entry.Tags = nil
 	}
+	if entry.SharedTagsString() == defaults.SharedTagsString() {
+		entry.SharedTags = nil
+	}
 	if entry.GitRepo == defaults.GitRepo {
 		entry.GitRepo = ""
 	}
@@ -103,6 +112,9 @@ func (entry Manifest2822Entry) String() string {
 	if str := entry.TagsString(); str != "" {
 		ret = append(ret, "Tags: "+str)
 	}
+	if str := entry.SharedTagsString(); str != "" {
+		ret = append(ret, "SharedTags: "+str)
+	}
 	if str := entry.GitRepo; str != "" {
 		ret = append(ret, "GitRepo: "+str)
 	}
@@ -145,6 +157,16 @@ func (entry Manifest2822Entry) HasTag(tag string) bool {
 	return false
 }
 
+// HasSharedTag returns true if the given tag exists in entry.SharedTags.
+func (entry Manifest2822Entry) HasSharedTag(tag string) bool {
+	for _, existingTag := range entry.SharedTags {
+		if tag == existingTag {
+			return true
+		}
+	}
+	return false
+}
+
 func (manifest Manifest2822) GetTag(tag string) *Manifest2822Entry {
 	for _, entry := range manifest.Entries {
 		if entry.HasTag(tag) {
@@ -154,6 +176,27 @@ func (manifest Manifest2822) GetTag(tag string) *Manifest2822Entry {
 	return nil
 }
 
+// GetSharedTag returns a list of entries with the given tag in entry.SharedTags (or the empty list if there are no entries with the given tag).
+func (manifest Manifest2822) GetSharedTag(tag string) []Manifest2822Entry {
+	ret := []Manifest2822Entry{}
+	for _, entry := range manifest.Entries {
+		if entry.HasSharedTag(tag) {
+			ret = append(ret, entry)
+		}
+	}
+	return ret
+}
+
+// GetAllSharedTags returns a list of the sum of all SharedTags in all entries of this image manifest (in the order they appear in the file).
+func (manifest Manifest2822) GetAllSharedTags() []string {
+	fakeEntry := Manifest2822Entry{}
+	for _, entry := range manifest.Entries {
+		fakeEntry.SharedTags = append(fakeEntry.SharedTags, entry.SharedTags...)
+	}
+	fakeEntry.DeduplicateSharedTags()
+	return fakeEntry.SharedTags
+}
+
 func (manifest *Manifest2822) AddEntry(entry Manifest2822Entry) error {
 	if len(entry.Tags) < 1 {
 		return fmt.Errorf("missing Tags")
@@ -165,20 +208,36 @@ func (manifest *Manifest2822) AddEntry(entry Manifest2822Entry) error {
 		return fmt.Errorf("Tags %q has invalid Maintainers: %q (expected format %q)", strings.Join(invalidMaintainers, ", "), MaintainersFormat)
 	}
 
+	entry.DeduplicateSharedTags()
+
 	seenTag := map[string]bool{}
 	for _, tag := range entry.Tags {
 		if otherEntry := manifest.GetTag(tag); otherEntry != nil {
 			return fmt.Errorf("Tags %q includes duplicate tag: %q (duplicated in %q)", entry.TagsString(), tag, otherEntry.TagsString())
 		}
+		if otherEntries := manifest.GetSharedTag(tag); len(otherEntries) > 0 {
+			return fmt.Errorf("Tags %q includes tag conflicting with a shared tag: %q (shared tag in %q)", entry.TagsString(), tag, otherEntries[0].TagsString())
+		}
 		if seenTag[tag] {
 			return fmt.Errorf("Tags %q includes duplicate tag: %q", entry.TagsString(), tag)
 		}
 		seenTag[tag] = true
 	}
+	for _, tag := range entry.SharedTags {
+		if otherEntry := manifest.GetTag(tag); otherEntry != nil {
+			return fmt.Errorf("Tags %q includes conflicting shared tag: %q (duplicated in %q)", entry.TagsString(), tag, otherEntry.TagsString())
+		}
+		if seenTag[tag] {
+			return fmt.Errorf("Tags %q includes duplicate tag: %q (in SharedTags)", entry.TagsString(), tag)
+		}
+		seenTag[tag] = true
+	}
 
 	for i, existingEntry := range manifest.Entries {
 		if existingEntry.SameBuildArtifacts(entry) {
 			manifest.Entries[i].Tags = append(existingEntry.Tags, entry.Tags...)
+			manifest.Entries[i].SharedTags = append(existingEntry.SharedTags, entry.SharedTags...)
+			manifest.Entries[i].DeduplicateSharedTags()
 			return nil
 		}
 	}
@@ -210,6 +269,20 @@ func (entry Manifest2822Entry) InvalidMaintainers() []string {
 	return invalid
 }
 
+// DeduplicateSharedTags will remove duplicate values from entry.SharedTags, preserving order.
+func (entry *Manifest2822Entry) DeduplicateSharedTags() {
+	aggregate := []string{}
+	seen := map[string]bool{}
+	for _, tag := range entry.SharedTags {
+		if seen[tag] {
+			continue
+		}
+		seen[tag] = true
+		aggregate = append(aggregate, tag)
+	}
+	entry.SharedTags = aggregate
+}
+
 type decoderWrapper struct {
 	*control.Decoder
 }
@@ -249,7 +322,7 @@ func Parse2822(readerIn io.Reader) (*Manifest2822, error) {
 	if invalidMaintainers := manifest.Global.InvalidMaintainers(); len(invalidMaintainers) > 0 {
 		return nil, fmt.Errorf("invalid Maintainers: %q (expected format %q)", strings.Join(invalidMaintainers, ", "), MaintainersFormat)
 	}
-	if len(manifest.Global.Tags) > 0 {
+	if len(manifest.Global.Tags) > 0 || len(manifest.Global.SharedTags) > 0 {
 		return nil, fmt.Errorf("global Tags not permitted")
 	}