fix assumptions about how fd_prestat_get may be called

The current implementation of `fd_prestat_get` does not appears to handle the following cases properly:

- fd_prestat_get is called on the same file descriptor again
- fd_prestat_get is called on stdio file descriptors (fd < 3)
- fd_prestat_get is called in non sequential order of file descriptors

Author: Techatrix

wasm-wasi-core/src/common/process.ts

@@ -242,7 +242,7 @@ export abstract class WasiProcess {
 
 		this.environmentService = EnvironmentWasiService.create(
 			this.fileDescriptors, this.programName,
-			this.preOpenDirectories.entries(), options
+			Array.from(this.preOpenDirectories.entries()), options
 		);
 		this.processService = {
 			proc_exit: async (_memory, exitCode: exitcode) => {

wasm-wasi-core/src/common/service.ts

@@ -191,11 +191,13 @@ export interface EnvironmentOptions extends Omit<ProcessOptions, 'args' | 'trace
 }
 
 export namespace EnvironmentWasiService {
-	export function create(fileDescriptors: FileDescriptors, programName: string, preStats: IterableIterator<[string, DeviceDriver]>, options: EnvironmentOptions): EnvironmentWasiService {
+	export function create(fileDescriptors: FileDescriptors, programName: string, preStats: [string, DeviceDriver][], options: EnvironmentOptions): EnvironmentWasiService {
 
 		const $encoder: RAL.TextEncoder = RAL().TextEncoder.create(options?.encoding);
 		const $preStatDirnames: Map<fd, string> = new Map();
 
+		fileDescriptors.switchToRunning(3 + preStats.length);
+
 		const result: EnvironmentWasiService = {
 			args_sizes_get: (memory: ArrayBuffer, argvCount_ptr: ptr<u32>, argvBufSize_ptr: ptr<u32>): Promise<errno> => {
 				let count = 0;
@@ -262,16 +264,19 @@ export namespace EnvironmentWasiService {
 			},
 			fd_prestat_get: async (memory: ArrayBuffer, fd: fd, bufPtr: ptr<prestat>): Promise<errno> => {
 				try {
-					const next = preStats.next();
-					if (next.done === true) {
-						fileDescriptors.switchToRunning(fd);
+					if (fd < 3) {
+						return Errno.badf;
+					}
+					if (fd - 3 >= preStats.length) {
 						return Errno.badf;
 					}
-					const [ mountPoint, driver ] = next.value;
-					const fileDescriptor = await driver.fd_create_prestat_fd(fd);
-					fileDescriptors.add(fileDescriptor);
-					fileDescriptors.setRoot(driver, fileDescriptor);
-					$preStatDirnames.set(fileDescriptor.fd, mountPoint);
+					const [ mountPoint, driver ] = preStats[fd - 3];
+					if (!fileDescriptors.has(fd)) {
+						const fileDescriptor = await driver.fd_create_prestat_fd(fd);
+						fileDescriptors.add(fileDescriptor);
+						fileDescriptors.setRoot(driver, fileDescriptor);
+						$preStatDirnames.set(fileDescriptor.fd, mountPoint);
+					}
 					const view = new DataView(memory);
 					const prestat = Prestat.create(view, bufPtr);
 					prestat.preopentype = Preopentype.dir;
@@ -1124,7 +1129,7 @@ export namespace FileSystemService {
 		const clock = Clock.create();
 		return Object.assign(
 			{},
-			EnvironmentWasiService.create(fileDescriptors, 'virtualRootFileSystem', preOpens.entries(), {}),
+			EnvironmentWasiService.create(fileDescriptors, 'virtualRootFileSystem', Array.from(preOpens.entries()), {}),
 			DeviceWasiService.create(deviceDrivers, fileDescriptors, clock, virtualRootFileSystem, options)
 		);
 	}

wasm-wasi-core/src/common/test/index.ts

@@ -91,7 +91,7 @@ export function createWasiService(workspaceContent: WorkspaceContent): WasiServi
 
 	const clock = Clock.create();
 	const fileSystemService = DeviceWasiService.create(deviceDrivers, fileDescriptors, clock, undefined, options);
-	const environmentService = EnvironmentWasiService.create(fileDescriptors, 'testApp', preOpenDirectories.entries(), options);
+	const environmentService = EnvironmentWasiService.create(fileDescriptors, 'testApp', Array.from(preOpenDirectories.entries()), options);
 	const clockService = ClockWasiService.create(clock);
 	const result: WasiService = Object.assign({}, NoSysWasiService, environmentService, clockService, fileSystemService);
 	return result;