Merge #1660

1660: Allow mapdir aliases with starting `/` r=MarkMcCaskey a=MarkMcCaskey

Stripping leading `/` is valid because we mount mapped dirs at `/` regardless.

Resolves #1503 

This solution is a bit hacky, there may be a more elegant solution.  I'll look for one tomorrow morning when I'm more mentally fresh.

- [ ] Add a short description of the the change to the CHANGELOG.md file


Co-authored-by: Mark McCaskey <mark@wasmer.io>

lib/wasi/src/state/builder.rs

@@ -438,7 +438,10 @@ impl PreopenDirBuilder {
 
     /// Make this preopened directory appear to the WASI program as `alias`
     pub fn alias(&mut self, alias: &str) -> &mut Self {
-        self.alias = Some(alias.to_string());
+        // We mount at preopened dirs at `/` by default and multiple `/` in a row
+        // are equal to a single `/`.
+        let alias = alias.trim_start_matches(b'/');
+        self.alias = Some(alias);
 
         self
     }

tests/wasi-wast/wasi/snapshot1/mapdir_with_leading_slash.wast

@@ -0,0 +1,7 @@
+;; This file was generated by https://github.com/wasmerio/wasi-tests
+
+(wasi_test "mapdir_with_leading_slash.wasm"
+  (map_dirs "/hamlet:test_fs/hamlet")
+  (assert_return (i64.const 0))
+  (assert_stdout "File exists? true\nSCENE III. A room in the castle.\n\n    Enter KING CLAUDIUS, ROSENCRANTZ, and GUILDENSTERN \n\nKING CLAUDIUS\n\n    I like him not, nor stands it safe with us\n    To let his madness range. Therefore prepare you;\n    I your commission will forthwith dispatch,\n   \n")
+)

tests/wasi-wast/wasi/tests/mapdir_with_leading_slash.rs

@@ -0,0 +1,23 @@
+// WASI:
+// mapdir: /hamlet:test_fs/hamlet
+
+use std::fs;
+use std::io::Read;
+use std::path::PathBuf;
+
+fn main() {
+    #[cfg(not(target_os = "wasi"))]
+    let mut base = PathBuf::from("test_fs/hamlet");
+    #[cfg(target_os = "wasi")]
+    let mut base = PathBuf::from("hamlet");
+
+    base.push("act3/scene3.txt");
+
+    println!("File exists? {}", base.exists());
+
+    let mut f = fs::File::open(&base).unwrap();
+    let mut s = String::new();
+    f.read_to_string(&mut s).unwrap();
+
+    println!("{}", s.chars().take(256).collect::<String>());
+}

tests/wasi-wast/wasi/unstable/mapdir_with_leading_slash.wast

@@ -0,0 +1,7 @@
+;; This file was generated by https://github.com/wasmerio/wasi-tests
+
+(wasi_test "mapdir_with_leading_slash.wasm"
+  (map_dirs "/hamlet:test_fs/hamlet")
+  (assert_return (i64.const 0))
+  (assert_stdout "File exists? true\nSCENE III. A room in the castle.\n\n    Enter KING CLAUDIUS, ROSENCRANTZ, and GUILDENSTERN \n\nKING CLAUDIUS\n\n    I like him not, nor stands it safe with us\n    To let his madness range. Therefore prepare you;\n    I your commission will forthwith dispatch,\n   \n")
+)