history: Improve timestamps by changing them less often

Currently we change the version timestamp on every single bump of the
package, which causes a lot of unnecessary noise. Further, this impacts
the usefuless of delta packages by skewing their timestamps from simple
bump-rebuilds.

We now set the timestamp to the time the version last changed, as opposed
to when the release number changed, to signify major differences. This is
extracted from the tag data and analysing the spec at that point in time.

Combined with the latest ypkg change to emit `SOURCE_DATE_EPOCH`, this
should remove much of the jitter in between package rebuilds and restore
sane deltas in no time at all.

Signed-off-by: Ikey Doherty <[email protected]>

src/builder/build.go

@@ -383,7 +383,7 @@ func (p *Package) BuildYpkg(notif PidNotifier, usr *UserInfo, pman *EopkgManager
 	}
 	// Pass unix timestamp of last git update
 	if h != nil && len(h.Updates) > 0 {
-		cmd += fmt.Sprintf(" -t %v", h.Updates[0].Time.UTC().Unix())
+		cmd += fmt.Sprintf(" -t %v", h.GetLastVersionTimestamp())
 	}
 
 	log.WithFields(log.Fields{

src/builder/history.go

@@ -335,3 +335,33 @@ func (p *PackageHistory) WriteXML(path string) error {
 	_, err = fi.WriteString(string(bytes))
 	return err
 }
+
+// GetLastVersionTimestamp will return a timestamp appropriate for us within
+// reproducible builds.
+//
+// This is calculated by using the timestamp from the last explicit version
+// change, and not from simple bumps. The idea here is to only increment the
+// timestamp if we've actually upgraded to a major version, and in general
+// attempt to reduce the noise, and thus, produce better delta packages
+// between minor package alterations
+func (p *PackageHistory) GetLastVersionTimestamp() int64 {
+	lastVersion := p.Updates[0].Package.Version
+	lastTime := p.Updates[0].Time
+
+	if len(p.Updates) < 2 {
+		return lastTime.UTC().Unix()
+	}
+
+	// Walk history and find the last version change, assigning timestamp
+	// as appropriate.
+	for i := 1; i < len(p.Updates); i++ {
+		newVersion := p.Updates[i].Package.Version
+		if newVersion != lastVersion {
+			break
+		}
+		lastVersion = p.Updates[i].Package.Version
+		lastTime = p.Updates[i].Time
+	}
+
+	return lastTime.UTC().Unix()
+}