refactor: add constants + comments describing save boundaries PE-5141

We avoid saving reports around the start and end of the epoch for a
combo of chain stability and height consistency (we want to make sure
all the heights are in the same epoch). This commit adds constants and
comments that describe some the reasoning behind that. Note that the
current save boundaries are conservative and may be adjusted in the
future.

src/system.ts

@@ -252,6 +252,15 @@ export const prescribedObserversSource =
       })
     : undefined;
 
+// Wait for chain stability before saving reports
+const START_HEIGHT_START_OFFSET = MAX_FORK_DEPTH;
+
+// Ensure there is enough time to save the report at the end of the epoch. We
+// use 2 * MAX_FORK_DEPTH because it allows MAX_FORK_DEPTH blocks (somewhat
+// arbitrary, but pleasingly symmetric) before we stop attempting to save
+// altogether for consistency reasons at the end of the epoch.
+const START_HEIGHT_END_OFFSET = 2 * MAX_FORK_DEPTH;
+
 export async function updateAndSaveCurrentReport() {
   try {
     log.info('Generating report...');
@@ -286,8 +295,11 @@ export async function updateAndSaveCurrentReport() {
     });
     const saveAfterHeight =
       report.epochStartHeight +
-      50 +
-      (entropy.readUInt32BE(0) % (EPOCH_BLOCK_LENGTH - 150));
+      START_HEIGHT_START_OFFSET +
+      (entropy.readUInt32BE(0) %
+        (EPOCH_BLOCK_LENGTH -
+          START_HEIGHT_START_OFFSET -
+          START_HEIGHT_END_OFFSET));
 
     const currentHeight = await chainSource.getHeight();