Makes filesystem test simpler and removes use of panic in examples (#395)

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

.github/workflows/commit.yaml

@@ -117,7 +117,8 @@ jobs:
           docker buildx build -t wazero:test --platform linux/${{ matrix.arch }} .
 
       - name: Run built test binaries
-        run: find . -name "*.test" | xargs -Itestbin docker run --platform linux/${{ matrix.arch }} -v $(pwd)/testbin:/test --rm -t wazero:test
+        # This runs all tests compiled above in sequence. Note: This mounts /tmp to allow t.TempDir() in tests.
+        run: find . -name "*.test" | xargs -Itestbin docker run --platform linux/${{ matrix.arch }} -v $(pwd)/testbin:/test --tmpfs /tmp --rm -t wazero:test
 
   bench:
     name: Benchmark

examples/file_system_test.go

@@ -12,9 +12,49 @@ import (
 	"github.com/tetratelabs/wazero/wasi"
 )
 
-// filesystemWasm was compiled from TinyGo testdata/file_system.go
-//go:embed testdata/file_system.wasm
-var filesystemWasm []byte
+// catGo is the TinyGo source
+//go:embed testdata/cat.go
+var catGo []byte
+
+// catWasm was compiled from catGo
+//go:embed testdata/cat.wasm
+var catWasm []byte
+
+// Test_Cat writes the input file to stdout, just like `cat`.
+//
+// This is a basic introduction to the WebAssembly System Interface (WASI).
+// See https://github.com/WebAssembly/WASI
+func Test_Cat(t *testing.T) {
+	r := wazero.NewRuntime()
+
+	// First, configure where the WebAssembly Module (Wasm) console outputs to (stdout).
+	stdoutBuf := bytes.NewBuffer(nil)
+	wasiConfig := wazero.NewWASIConfig().WithStdout(stdoutBuf)
+
+	// Next, configure a sand-boxed filesystem to include one file.
+	file := "cat.go" // arbitrary file
+	memFS := wazero.WASIMemFS()
+	err := writeFile(memFS, file, catGo)
+	require.NoError(t, err)
+	wasiConfig.WithPreopens(map[string]wasi.FS{".": memFS})
+
+	// Since this runs a main function (_start in WASI), configure the arguments.
+	// Remember, arg[0] is the program name!
+	wasiConfig.WithArgs("cat", file)
+
+	// Now, instantiate WASI with the above configuration.
+	wasi, err := r.InstantiateModule(wazero.WASISnapshotPreview1WithConfig(wasiConfig))
+	require.NoError(t, err)
+	defer wasi.Close()
+
+	// Finally, start the `cat` program's main function (in WASI, this is named `_start`).
+	cat, err := wazero.StartWASICommandFromSource(r, catWasm)
+	require.NoError(t, err)
+	defer cat.Close()
+
+	// To ensure it worked, verify stdout from WebAssembly had what we expected.
+	require.Equal(t, string(catGo), stdoutBuf.String())
+}
 
 func writeFile(fs wasi.FS, path string, data []byte) error {
 	f, err := fs.OpenWASI(0, path, wasi.O_CREATE|wasi.O_TRUNC, wasi.R_FD_WRITE, 0, 0)
@@ -28,40 +68,3 @@ func writeFile(fs wasi.FS, path string, data []byte) error {
 
 	return f.Close()
 }
-
-func readFile(fs wasi.FS, path string) ([]byte, error) {
-	f, err := fs.OpenWASI(0, path, 0, 0, 0, 0)
-	if err != nil {
-		return nil, err
-	}
-
-	buf := bytes.NewBuffer(nil)
-
-	if _, err := io.Copy(buf, f); err != nil {
-		return buf.Bytes(), nil
-	}
-
-	return buf.Bytes(), f.Close()
-}
-
-func Test_file_system(t *testing.T) {
-	r := wazero.NewRuntime()
-
-	memFS := wazero.WASIMemFS()
-	err := writeFile(memFS, "input.txt", []byte("Hello, file system!"))
-	require.NoError(t, err)
-
-	wasiConfig := wazero.NewWASIConfig().WithPreopens(map[string]wasi.FS{".": memFS})
-	wasi, err := r.InstantiateModule(wazero.WASISnapshotPreview1WithConfig(wasiConfig))
-	require.NoError(t, err)
-	defer wasi.Close()
-
-	// Note: TinyGo binaries must be treated as WASI Commands to initialize memory.
-	mod, err := wazero.StartWASICommandFromSource(r, filesystemWasm)
-	require.NoError(t, err)
-	defer mod.Close()
-
-	out, err := readFile(memFS, "output.txt")
-	require.NoError(t, err)
-	require.Equal(t, "Hello, file system!", string(out))
-}

examples/testdata/cat.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"os"
+)
+
+// main is the same as cat: "concatenate and print files."
+func main() {
+	// Start at arg[1] because args[0] is the program name.
+	for i := 1; i < len(os.Args); i++ {
+		bytes, err := os.ReadFile(os.Args[i])
+		if err != nil {
+			os.Exit(1)
+		}
+
+		// Use write to avoid needing to worry about Windows newlines.
+		os.Stdout.Write(bytes)
+	}
+}

examples/testdata/file_system.go

@@ -1,25 +0,0 @@
-package main
-
-import (
-	"io"
-	"os"
-)
-
-func main() {
-	fileIn, err := os.Open("input.txt")
-	if err != nil {
-		panic(err)
-	}
-	defer fileIn.Close()
-
-	fileOut, err := os.Create("output.txt")
-	if err != nil {
-		panic(err)
-	}
-	defer fileOut.Close()
-
-	_, err = io.Copy(fileOut, fileIn)
-	if err != nil {
-		panic(err)
-	}
-}

examples/testdata/stdio.go

@@ -12,11 +12,11 @@ func main() {
 	for s.Scan() {
 		line := s.Text()
 		if _, err := fmt.Printf("Hello, %s!\n", strings.TrimSpace(line)); err != nil {
-			panic(err)
+			os.Exit(1)
 		}
 
 		if _, err := fmt.Fprintln(os.Stderr, "Error Message"); err != nil {
-			panic(err)
+			os.Exit(1)
 		}
 	}
 }