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