sausage-vfs: review fixes (fallible growth, symlink flags, stream cancel, sized reads, timestamps, error kinds, linker shadowing)

API: Layer::stat(path, follow), Layer::readlink, Open::follow,
Stat::symlink and Entry::symlink. Process::idle documents the shutdown
order: abort, await the task, await idle, release locks.

vfs/src/generated.rs

@@ -2,7 +2,8 @@
 use crate::*;
 use std::{collections::BTreeMap, io::ErrorKind as E};
 type Generator = Arc<dyn Fn() -> Result<Vec<u8>> + Send + Sync>;
-/// A tree of generated files. Parent folders are implicit.
+/// A static tree of generated files. Parent folders are implicit.
+/// Generators must be cheap and must not block. Stat can invoke them as well as open.
 #[derive(Default)]
 pub struct Generated {
     files: BTreeMap<String, Generator>,
@@ -29,7 +30,7 @@ fn readonly<T>() -> Result<T> {
 }
 #[async_trait]
 impl Layer for Generated {
-    async fn stat(&self, path: &str) -> Result<Stat> {
+    async fn stat(&self, path: &str, _follow: bool) -> Result<Stat> {
         if let Some(f) = self.files.get(path) {
             return Snapshot(f()?).stat().await;
         }
@@ -38,6 +39,7 @@ impl Layer for Generated {
         }
         Ok(Stat {
             directory: true,
+            symlink: false,
             size: 0,
             accessed: None,
             modified: None,
@@ -67,7 +69,11 @@ impl Layer for Generated {
         }
         Ok(entries
             .into_iter()
-            .map(|(name, directory)| Entry { name, directory })
+            .map(|(name, directory)| Entry {
+                name,
+                directory,
+                symlink: false,
+            })
             .collect())
     }
     async fn open(&self, path: &str, o: Open) -> Result<Arc<dyn File>> {
@@ -103,6 +109,7 @@ impl File for Snapshot {
     async fn stat(&self) -> Result<Stat> {
         Ok(Stat {
             directory: false,
+            symlink: false,
             size: self.0.len() as u64,
             accessed: None,
             modified: None,