Share pre-computed block between file write children

Previously we constructed a separate block cache for each write child in
`file_gen`. This meant as we got more duplicate file writes we increased the
memory by the prebuild cache size. This lead to situations where lading would
OOM if the number of write children were high enough.

REF SMP-558

Signed-off-by: Brian L. Troutwine <brian.troutwine@datadoghq.com>

Author: blt

CHANGELOG.md

@@ -6,6 +6,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.15.3-rc1]
+### Changed
+- `file_gen` now shares its pre-computed block between write children, reducing
+  memory consumption.
+
 ## [0.15.2]
 ### Fixed
 - Disallow the creation of DogStatsD metrics with no values

src/generator/file_gen.rs

@@ -156,18 +156,18 @@ impl FileGen {
 
         let mut handles = Vec::new();
         let file_index = Arc::new(AtomicU32::new(0));
+        let block_cache = construct_block_cache(&mut rng, &config.variant, &block_chunks, &labels);
+        let block_cache = Arc::new(block_cache);
+
         for _ in 0..config.duplicates {
             let throttle = Throttle::new_with_config(config.throttle, bytes_per_second);
 
-            let block_cache =
-                construct_block_cache(&mut rng, &config.variant, &block_chunks, &labels);
-
             let child = Child {
                 path_template: config.path_template.clone(),
                 maximum_bytes_per_file,
                 bytes_per_second,
                 throttle,
-                block_cache,
+                block_cache: Arc::clone(&block_cache),
                 file_index: Arc::clone(&file_index),
                 rotate: config.rotate,
                 shutdown: shutdown.clone(),
@@ -210,7 +210,7 @@ struct Child {
     maximum_bytes_per_file: NonZeroU32,
     bytes_per_second: NonZeroU32,
     throttle: Throttle,
-    block_cache: Vec<Block>,
+    block_cache: Arc<Vec<Block>>,
     rotate: bool,
     file_index: Arc<AtomicU32>,
     shutdown: Shutdown,