feat(availability): env var to disable pruning (celestiaorg#3949)

This trick allowed me to reuse an old 8mb snapshot, even though it is already far beyond the availability window. I think it is helpful to add it to main.

share/availability/window.go

@@ -1,6 +1,9 @@
 package availability
 
-import "time"
+import (
+	"os"
+	"time"
+)
 
 const (
 	RequestWindow = 30 * 24 * time.Hour
@@ -11,8 +14,33 @@ const (
 // given AvailabilityWindow. If the window is disabled (0), it returns true for
 // every timestamp.
 func IsWithinWindow(t time.Time, window time.Duration) bool {
+	if windowOverride {
+		window = windowOverrideDur
+	}
 	if window == time.Duration(0) {
 		return true
 	}
 	return time.Since(t) <= window
 }
+
+// windowOverride is a flag that overrides the availability window. This flag is intended for
+// testing purposes only.
+var (
+	windowOverride    bool
+	windowOverrideDur time.Duration
+)
+
+func init() {
+	durationRaw, ok := os.LookupEnv("CELESTIA_OVERRIDE_AVAILABILITY_WINDOW")
+	if !ok {
+		return
+	}
+
+	duration, err := time.ParseDuration(durationRaw)
+	if err != nil {
+		panic("failed to parse CELESTIA_OVERRIDE_AVAILABILITY_WINDOW: " + err.Error())
+	}
+
+	windowOverride = true
+	windowOverrideDur = duration
+}