Implement OSL pave offset

Enables paving starting at an offset after epoch,
which may be used to reduce the size of OSL registry
downloads and empty OSL uploads.

psiphon/common/osl/osl.go

@@ -786,6 +786,9 @@ type PaveLogInfo struct {
 // separated. Otherwise the OSL will still be issued, but be empty (unless
 // the scheme is in omitEmptyOSLsSchemes).
 //
+// If startTime is specified and is after epoch, the pave file will contain
+// OSLs for the first period at or after startTime.
+//
 // As OSLs outside the epoch-endTime range will no longer appear in
 // the registry, Pave is intended to be used to create the full set
 // of OSLs for a distribution site; i.e., not incrementally.
@@ -793,6 +796,7 @@ type PaveLogInfo struct {
 // Automation is responsible for consistently distributing server entries
 // to OSLs in the case where OSLs are repaved in subsequent calls.
 func (config *Config) Pave(
+	startTime time.Time,
 	endTime time.Time,
 	propagationChannelID string,
 	signingPublicKey string,
@@ -820,6 +824,12 @@ func (config *Config) Pave(
 
 			oslTime := scheme.epoch
 
+			if !startTime.IsZero() && !startTime.Before(scheme.epoch) {
+				for oslTime.Before(startTime) {
+					oslTime = oslTime.Add(oslDuration)
+				}
+			}
+
 			for !oslTime.After(endTime) {
 
 				firstSLOKTime := oslTime

psiphon/common/osl/osl_test.go

@@ -363,6 +363,21 @@ func TestOSL(t *testing.T) {
 			omitEmptyOSLsSchemes := []int{}
 
 			firstPaveFiles, err := config.Pave(
+				time.Time{},
+				endTime,
+				propagationChannelID,
+				signingPublicKey,
+				signingPrivateKey,
+				paveServerEntries,
+				omitMD5SumsSchemes,
+				omitEmptyOSLsSchemes,
+				nil)
+			if err != nil {
+				t.Fatalf("Pave failed: %s", err)
+			}
+
+			offsetPaveFiles, err := config.Pave(
+				epoch.Add(500*seedPeriod+seedPeriod/2),
 				endTime,
 				propagationChannelID,
 				signingPublicKey,
@@ -376,6 +391,7 @@ func TestOSL(t *testing.T) {
 			}
 
 			paveFiles, err := config.Pave(
+				time.Time{},
 				endTime,
 				propagationChannelID,
 				signingPublicKey,
@@ -399,10 +415,35 @@ func TestOSL(t *testing.T) {
 
 			for index, paveFile := range paveFiles {
 				if paveFile.Name != firstPaveFiles[index].Name {
-					t.Fatalf("Pave name mismatch")
+					t.Fatalf("pave name mismatch")
 				}
 				if !bytes.Equal(paveFile.Contents, firstPaveFiles[index].Contents) {
-					t.Fatalf("Pave content mismatch")
+					t.Fatalf("pave content mismatch")
+				}
+			}
+
+			// Check that the output of a pave using an unaligned offset from epoch
+			// produces a subset of OSLs with the same IDs and content: the OSL and
+			// SLOK time slots must align.
+
+			if len(offsetPaveFiles) >= len(paveFiles) {
+				t.Fatalf("unexpected pave size")
+			}
+
+			for _, offsetPaveFile := range offsetPaveFiles {
+				found := false
+				for _, paveFile := range paveFiles {
+					if offsetPaveFile.Name == paveFile.Name {
+						if offsetPaveFile.Name != GetOSLRegistryURL("") &&
+							!bytes.Equal(offsetPaveFile.Contents, paveFile.Contents) {
+							t.Fatalf("pave content mismatch")
+						}
+						found = true
+						break
+					}
+				}
+				if !found {
+					t.Fatalf("pave name missing")
 				}
 			}
 

psiphon/common/osl/paver/main.go

@@ -213,6 +213,7 @@ func main() {
 	for propagationChannelID := range allPropagationChannelIDs {
 
 		paveFiles, err := config.Pave(
+			startTime,
 			endTime,
 			propagationChannelID,
 			signingPublicKey,