test: rpc_getblockfrompeer.py, remove magic numbers usage

Instead of hardcoding the `pruneblockchain(<height>)` heights,
use 'getblockfileinfo' to obtain the highest block number of
each of the block files.

Making the test more robust and readable by stating which file
is being pruned at every point of time (the goal is to mimic
how the automatic pruning process work).

Github-Pull: bitcoin#27770
Rebased-From: 5110139

Author: furszy

test/functional/rpc_getblockfrompeer.py

@@ -122,10 +122,21 @@ def run_test(self):
         pruned_node = self.nodes[2]
         self.generate(self.nodes[0], 400, sync_fun=self.no_op)
         self.sync_blocks([self.nodes[0], pruned_node])
-        pruneheight = pruned_node.pruneblockchain(300)
-        assert_equal(pruneheight, 248)
+
+        # The goal now will be to mimic the automatic pruning process and verify what happens when we fetch an historic
+        # block at any point of time.
+        #
+        # Starting with three blocks files. The pruning process will prune them one by one. And, at the second pruning
+        # event, the test will fetch the past block. Which will be stored at the latest block file. Which can only be
+        # pruned when the latest block file is full (in this case, the third one), and a new one is created.
+
+        # First prune event, prune first block file
+        highest_pruned_block_num = pruned_node.getblockfileinfo(0)["highest_block"]
+        pruneheight = pruned_node.pruneblockchain(highest_pruned_block_num + 1)
+        assert_equal(pruneheight, highest_pruned_block_num)
         # Ensure the block is actually pruned
-        pruned_block = self.nodes[0].getblockhash(2)
+        fetch_block_num = 2
+        pruned_block = self.nodes[0].getblockhash(fetch_block_num)
         assert_raises_rpc_error(-1, "Block not available (pruned data)", pruned_node.getblock, pruned_block)
 
         self.log.info("Fetch pruned block")
@@ -136,18 +147,29 @@ def run_test(self):
         self.wait_until(lambda: self.check_for_block(node=2, hash=pruned_block), timeout=1)
         assert_equal(result, {})
 
+        # Validate that the re-fetched block was stored at the last, current, block file
+        assert_equal(fetch_block_num, pruned_node.getblockfileinfo(2)["lowest_block"])
+
         self.log.info("Fetched block persists after next pruning event")
         self.generate(self.nodes[0], 250, sync_fun=self.no_op)
         self.sync_blocks([self.nodes[0], pruned_node])
-        pruneheight += 251
-        assert_equal(pruned_node.pruneblockchain(700), pruneheight)
+
+        # Second prune event, prune second block file
+        highest_pruned_block_num = pruned_node.getblockfileinfo(1)["highest_block"]
+        pruneheight = pruned_node.pruneblockchain(highest_pruned_block_num + 1)
+        assert_equal(pruneheight, highest_pruned_block_num)
+        # As the re-fetched block is in the third file, and we just pruned the second one, 'getblock' must work.
         assert_equal(pruned_node.getblock(pruned_block)["hash"], "36c56c5b5ebbaf90d76b0d1a074dcb32d42abab75b7ec6fa0ffd9b4fbce8f0f7")
 
-        self.log.info("Fetched block can be pruned again when prune height exceeds the height of the tip at the time when the block was fetched")
+        self.log.info("Re-fetched block can be pruned again when a new block file is created")
         self.generate(self.nodes[0], 250, sync_fun=self.no_op)
         self.sync_blocks([self.nodes[0], pruned_node])
-        pruneheight += 250
-        assert_equal(pruned_node.pruneblockchain(1000), pruneheight)
+
+        # Third prune event, prune third block file
+        highest_pruned_block_num = pruned_node.getblockfileinfo(2)["highest_block"]
+        pruneheight = pruned_node.pruneblockchain(highest_pruned_block_num + 1)
+        assert_equal(pruneheight, highest_pruned_block_num)
+        # and check that the re-fetched block file is now pruned
         assert_raises_rpc_error(-1, "Block not available (pruned data)", pruned_node.getblock, pruned_block)