Add last_checkpoint argument to zebra-checkpoints (#793)

zebra-utils/src/bin/zebra-checkpoints/args.rs

@@ -14,6 +14,11 @@ pub struct Args {
     #[structopt(default_value = "zcash-cli", short, long)]
     pub cli: String,
 
+    /// Start looking for checkpoints after this height.
+    /// If there is no last checkpoint, we start looking at the Genesis block (height 0).
+    #[structopt(short, long)]
+    pub last_checkpoint: Option<u32>,
+
     /// Passthrough args for `zcash-cli`
     #[structopt(last = true)]
     pub zcli_args: Vec<String>,

zebra-utils/src/bin/zebra-checkpoints/main.rs

@@ -89,19 +89,34 @@ fn main() -> Result<()> {
     cmd.arg("getblockcount");
     // calculate the maximum height
     let height_limit: BlockHeight = cmd_output(&mut cmd)?.trim().parse()?;
+    assert!(height_limit <= BlockHeight::MAX);
     let height_limit = height_limit
         .0
         .checked_sub(BLOCK_REORG_LIMIT.0)
         .map(BlockHeight)
         .expect("zcashd has some mature blocks: wait for zcashd to sync more blocks");
 
+    let starting_height = args::Args::from_args().last_checkpoint.map(BlockHeight);
+    if starting_height.is_some() {
+        // Since we're about to add 1, height needs to be strictly less than the maximum
+        assert!(starting_height.unwrap() < BlockHeight::MAX);
+    }
+    // Start at the next block after the last checkpoint.
+    // If there is no last checkpoint, start at genesis (height 0).
+    let starting_height = starting_height.map_or(0, |BlockHeight(h)| h + 1);
+
+    assert!(
+        starting_height < height_limit.0,
+        "No mature blocks after the last checkpoint: wait for zcashd to sync more blocks"
+    );
+
     // set up counters
     let mut cumulative_bytes: u64 = 0;
     let mut height_gap: BlockHeight = BlockHeight(0);
 
     // loop through all blocks
-    for x in 0..height_limit.0 {
+    for x in starting_height..height_limit.0 {
-        // unfortunatly we need to create a process for each block
+        // unfortunately we need to create a process for each block
         let mut cmd = passthrough_cmd();
 
         // get block data