Test reading from stdin in shell environments (#21926)

Also refactor FS_stdin_getChar so that it only includes the shell
code when needed.

I verified that this test runs under spidermonkey and v8.

Author: sbc100

src/library_fs_shared.js

@@ -136,33 +136,38 @@ addToLibrary({
         try {
           bytesRead = fs.readSync(fd, buf, 0, BUFSIZE);
         } catch(e) {
-          // Cross-platform differences: on Windows, reading EOF throws an exception, but on other OSes,
-          // reading EOF returns 0. Uniformize behavior by treating the EOF exception to return 0.
+          // Cross-platform differences: on Windows, reading EOF throws an
+          // exception, but on other OSes, reading EOF returns 0. Uniformize
+          // behavior by treating the EOF exception to return 0.
           if (e.toString().includes('EOF')) bytesRead = 0;
           else throw e;
         }
 
         if (bytesRead > 0) {
           result = buf.slice(0, bytesRead).toString('utf-8');
-        } else {
-          result = null;
         }
       } else
 #endif
+#if ENVIRONMENT_MAY_BE_WEB
       if (typeof window != 'undefined' &&
         typeof window.prompt == 'function') {
         // Browser.
         result = window.prompt('Input: ');  // returns null on cancel
         if (result !== null) {
           result += '\n';
         }
-      } else if (typeof readline == 'function') {
+      } else
+#endif
+#if ENVIRONMENT_MAY_BE_SHELL
+      if (typeof readline == 'function') {
         // Command line.
         result = readline();
-        if (result !== null) {
+        if (result) {
           result += '\n';
         }
-      }
+      } else
+#endif
+      {}
       if (!result) {
         return null;
       }

test/other/metadce/test_metadce_cxx_ctors1.gzsize

@@ -1 +1 @@
-9840
+9828

test/other/metadce/test_metadce_cxx_ctors1.jssize

@@ -1 +1 @@
-24247
+24180

test/other/metadce/test_metadce_cxx_ctors2.gzsize

@@ -1 +1 @@
-9823
+9811

test/other/metadce/test_metadce_cxx_ctors2.jssize

@@ -1 +1 @@
-24215
+24148

test/other/metadce/test_metadce_cxx_except.gzsize

@@ -1 +1 @@
-10931
+10915

test/other/metadce/test_metadce_cxx_except.jssize

@@ -1 +1 @@
-28164
+28097

test/other/metadce/test_metadce_cxx_except_wasm.gzsize

@@ -1 +1 @@
-9807
+9794

test/other/metadce/test_metadce_cxx_except_wasm.jssize

@@ -1 +1 @@
-24140
+24073

test/other/metadce/test_metadce_cxx_mangle.gzsize

@@ -1 +1 @@
-10932
+10915

test/other/metadce/test_metadce_cxx_mangle.jssize

@@ -1 +1 @@
-28164
+28097

test/other/metadce/test_metadce_cxx_noexcept.gzsize

@@ -1 +1 @@
-9840
+9828

test/other/metadce/test_metadce_cxx_noexcept.jssize

@@ -1 +1 @@
-24247
+24180

test/other/metadce/test_metadce_cxx_wasmfs.gzsize

@@ -1 +1 @@
-5210
+5194

test/other/metadce/test_metadce_cxx_wasmfs.jssize

@@ -1 +1 @@
-12062
+11995

test/other/metadce/test_metadce_files_js_fs.gzsize

@@ -1 +1 @@
-7802
+7790

test/other/metadce/test_metadce_files_js_fs.jssize

@@ -1 +1 @@
-19271
+19204

test/other/metadce/test_metadce_files_wasmfs.gzsize

@@ -1 +1 @@
-3133
+3110

test/other/metadce/test_metadce_files_wasmfs.jssize

@@ -1 +1 @@
-6812
+6745

test/test_other.py

@@ -1649,19 +1649,18 @@ def test_export_all_and_exported_functions(self):
   @also_with_wasmfs
   @crossplatform
   def test_stdin(self, args):
-    create_file('in.txt', 'abcdef\nghijkl')
+    create_file('in.txt', 'abcdef\nghijkl\n')
+    self.set_setting('ENVIRONMENT', 'node,shell')
     self.emcc(test_file('module/test_stdin.c'), args=args, output_filename='out.js')
 
     for engine in config.JS_ENGINES:
-      if engine == config.V8_ENGINE:
-        continue # no stdin support in v8 shell
       engine[0] = os.path.normpath(engine[0])
-      print(engine, file=sys.stderr)
       # work around a bug in python's subprocess module
       # (we'd use self.run_js() normally)
       delete_file('out.txt')
       cmd = jsrun.make_command(os.path.normpath('out.js'), engine)
       cmd = shared.shlex_join(cmd)
+      print(cmd, file=sys.stderr)
       if WINDOWS:
         os.system(f'type "in.txt" | {cmd} >out.txt')
       else: # posix