Add txid collision scanner (#928)

examples/tx_collisions.rs

@@ -0,0 +1,31 @@
+use anyhow::{Context, Result};
+use electrs_rocksdb::{ColumnFamilyDescriptor, IteratorMode, Options, DB};
+
+fn main() -> Result<()> {
+    let path = std::env::args().skip(1).next().context("missing DB path")?;
+    let cf_names = DB::list_cf(&Options::default(), &path)?;
+    let cfs: Vec<_> = cf_names
+        .iter()
+        .map(|name| ColumnFamilyDescriptor::new(name, Options::default()))
+        .collect();
+    let db = DB::open_cf_descriptors(&Options::default(), &path, cfs)?;
+    let cf = db.cf_handle("txid").context("missing column family")?;
+
+    let mut state: Option<(u64, u32)> = None;
+    for row in db.iterator_cf(cf, IteratorMode::Start) {
+        let (curr, _value) = row?;
+        let curr_prefix = u64::from_le_bytes(curr[..8].try_into()?);
+        let curr_height = u32::from_le_bytes(curr[8..].try_into()?);
+
+        if let Some((prev_prefix, prev_height)) = state {
+            if prev_prefix == curr_prefix {
+                eprintln!(
+                    "prefix={:x} heights: {} {}",
+                    curr_prefix, prev_height, curr_height
+                );
+            };
+        }
+        state = Some((curr_prefix, curr_height));
+    }
+    Ok(())
+}