wasi: adds preopen test for use in homebrew (#1309)

This adds a test to verify the wazero binary produced by homebrew.
Notably, this was made to be small enough to copy/paste in hex format.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

imports/wasi_snapshot_preview1/testdata/print_args.wat

@@ -1,5 +1,5 @@
-;; $wasi_arg is a WASI command which copies null-terminated args to stdout.
+;; $print_args is a WASI command which copies null-terminated args to stdout.
-(module $wasi_arg
+(module $print_args
 	;; args_get reads command-line argument data.
 	;;
 	;; See https://github.com/WebAssembly/WASI/blob/snapshot-01/phases/snapshot/docs.md#-args_getargv-pointerpointeru8-argv_buf-pointeru8---errno
@@ -24,9 +24,9 @@
     ;; See https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#memories%E2%91%A7
     (memory (export "memory") 1)
 
-    ;; $iovs are offset/length pairs in memory fd_write copies to the file descriptor.
+    ;; $iovs are offset/length pairs in memory fd_write copies to the file
-    ;; $main will only write one offset/length pair, corresponding to null-terminated args.
+    ;; descriptor. $main will only write one offset/length pair.
-    (global $iovs i32 i32.const 1024) ;; 1024 is an arbitrary offset larger than the args.
+    (global $iovs i32 i32.const 1024) ;; 1024 is an arbitrary offset
 
     ;; WASI parameters are usually memory offsets, you can ignore values by writing them to an unread offset.
     (global $ignored i32 i32.const 32768)

imports/wasi_snapshot_preview1/testdata/print_prestat_dirname.wat

@@ -0,0 +1,42 @@
+;; $$print_prestat_dirname is a WASI command which copies the first preopen dirname to stdout.
+(module $print_prestat_dirname
+    (import "wasi_snapshot_preview1" "fd_prestat_get"
+        (func $wasi.fd_prestat_get (param $fd i32) (param $result.prestat i32) (result (;errno;) i32)))
+
+    (import "wasi_snapshot_preview1" "fd_prestat_dir_name"
+        (func $wasi.fd_prestat_dir_name (param $fd i32) (param $result.path i32)  (param $result.path_len i32) (result (;errno;) i32)))
+
+    (import "wasi_snapshot_preview1" "fd_write"
+        (func $wasi.fd_write (param $fd i32) (param $iovs i32) (param $iovs_len i32) (param $result.size i32) (result (;errno;) i32)))
+
+    (memory (export "memory") 1 1)
+
+    (func $main (export "_start")
+        ;; First, we need to know the size of the prestat dir name.
+        (call $wasi.fd_prestat_get
+            (i32.const 3) ;; preopen FD
+            (i32.const 0) ;; where to write prestat
+        )
+        drop ;; ignore the errno returned
+
+        ;; Next, write the dir name to offset 8 (past the prestat).
+        (call $wasi.fd_prestat_dir_name
+            (i32.const 3) ;; preopen FD
+            (i32.const 8) ;; where to write dir_name
+            (i32.load (i32.const 4)) ;; length is the last part of the prestat
+        )
+        drop ;; ignore the errno returned
+
+        ;; Now, convert the prestat to an iovec [offset, len] writing offset=8.
+        (i32.store (i32.const 0) (i32.const 8))
+
+        ;; Finally, write the dirname to stdout via its iovec [offset, len].
+        (call $wasi.fd_write
+            (i32.const 1) ;; stdout
+            (i32.const 0) ;; where's the iovec
+            (i32.const 1) ;; only one iovec
+            (i32.const 0) ;; overwrite the iovec with the ignored result.
+        )
+        drop ;; ignore the errno returned
+    )
+)

imports/wasi_snapshot_preview1/usage_test.go

@@ -8,13 +8,14 @@ import (
 
 	"github.com/tetratelabs/wazero"
 	"github.com/tetratelabs/wazero/imports/wasi_snapshot_preview1"
+	"github.com/tetratelabs/wazero/internal/fstest"
 	"github.com/tetratelabs/wazero/internal/testing/require"
 )
 
-// wasiArg was compiled from testdata/wasi_arg.wat
+// pringArgsWasm was compiled from testdata/wasi_arg.wat
 //
-//go:embed testdata/wasi_arg.wasm
+//go:embed testdata/print_args.wasm
-var wasiArg []byte
+var pringArgsWasm []byte
 
 func TestInstantiateModule(t *testing.T) {
 	ctx := context.Background()
@@ -28,7 +29,7 @@ func TestInstantiateModule(t *testing.T) {
 	sys := wazero.NewModuleConfig().WithStdout(&stdout)
 	wasi_snapshot_preview1.MustInstantiate(ctx, r)
 
-	compiled, err := r.CompileModule(ctx, wasiArg)
+	compiled, err := r.CompileModule(ctx, pringArgsWasm)
 	require.NoError(t, err)
 
 	// Re-use the same module many times.
@@ -46,3 +47,27 @@ func TestInstantiateModule(t *testing.T) {
 		require.NoError(t, mod.Close(ctx))
 	}
 }
+
+// printPrestatDirname was compiled from testdata/print_prestat_dirname.wat
+//
+//go:embed testdata/print_prestat_dirname.wasm
+var printPrestatDirname []byte
+
+// TestInstantiateModule_Prestat
+func TestInstantiateModule_Prestat(t *testing.T) {
+	ctx := context.Background()
+
+	r := wazero.NewRuntime(ctx)
+	defer r.Close(ctx)
+
+	var stdout bytes.Buffer
+
+	wasi_snapshot_preview1.MustInstantiate(ctx, r)
+
+	_, err := r.InstantiateWithConfig(ctx, printPrestatDirname, wazero.NewModuleConfig().
+		WithStdout(&stdout).
+		WithFSConfig(wazero.NewFSConfig().WithFSMount(fstest.FS, "/wazero")))
+	require.NoError(t, err)
+
+	require.Equal(t, "/wazero", stdout.String())
+}

internal/wasip1/wasi.go

@@ -1,5 +1,4 @@
-// Package wasi_snapshot_preview1 is a helper to remove package cycles re-using
+// Package wasip1 is a helper to remove package cycles re-using constants.
-// constants.
 package wasip1
 
 import (