1 files changed, 44 insertions, 8 deletions

diff --git a/src/bin/summain.rs b/src/bin/summain.rs
index 4336ebd..5031006 100644
--- a/src/bin/summain.rs
+++ b/src/bin/summain.rs
@@ -1,17 +1,51 @@
 use anyhow::Context;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 use structopt::StructOpt;
 use summain::ManifestEntry;
 
-fn main() -> anyhow::Result<()> {
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
     let mut opt = Opt::from_args();
     opt.pathnames[..].sort();
-    let v: Vec<anyhow::Result<ManifestEntry>> =
-        opt.pathnames.iter().map(|p| manifest(&p)).collect();
-    for m in v {
-        let m = m?;
+
+    // Get metadata about all files, but don't compute checksum yet.
+    //
+    // This all runs in async worker threads, since it's theoretically
+    // I/O heavy and benefits from async task context switching. We
+    // create a task for each named file, since tasks are very cheap.
+    // We keep the JoinHandle for each task in a vector: there's not
+    // going to be so many handles that the vector becomes impossibly
+    // large: Linux only allows 128 KiB of command line arguments.
+
+    let mut handles = vec![];
+    for filename in opt.pathnames.iter().cloned() {
+        handles.push(tokio::spawn(async move { manifest(filename) }));
+    }
+
+    // Compute checksums for regular files.
+    //
+    // This runs in blocking threads, since it's CPU heavy. We create
+    // another vector of JoinHandles. Again, it won't become too large.
+
+    let mut sumhandles = vec![];
+    for h in handles {
+        let mut m: ManifestEntry = h.await?.await?;
+        let h = tokio::task::spawn_blocking(move || match m.compute_checksum() {
+            Err(e) => Err(e),
+            Ok(_) => Ok(m),
+        });
+        sumhandles.push(h)
+    }
+
+    // Wait for checksums to be available and print manifest.
+    //
+    // Note how this iterates over the results in the right order.
+
+    for h in sumhandles {
+        let m: ManifestEntry = h.await??;
         print!("{}", serde_yaml::to_string(&m)?);
     }
+
     Ok(())
 }
 
@@ -21,6 +55,8 @@ struct Opt {
     pathnames: Vec<PathBuf>,
 }
 
-fn manifest(path: &Path) -> anyhow::Result<ManifestEntry> {
-    ManifestEntry::new(path).with_context(|| format!("{}", path.display()))
+async fn manifest(path: PathBuf) -> anyhow::Result<ManifestEntry> {
+    ManifestEntry::new(&path)
+        .await
+        .with_context(|| format!("{}", path.display()))
 }