Handle failure of lseek in fflush

Unlike on linux (it seems) the emscripten lseek can fail for various
reasons.  For example, we do not support lseek on TTY or pipe file
descriptors.

Musl, it seems, just assumes that the seek succeeds and adjusts its
internal buffers accordingly.

Fixes: #21791

Author: sbc100

system/lib/libc/musl/src/stdio/fflush.c

@@ -34,11 +34,22 @@ int fflush(FILE *f)
 	}
 
 	/* If reading, sync position, per POSIX */
+#if __EMSCRIPTEN__
+	/* Handle failues of lseek, which can happen in emscripten, e.g. for stdin etc */
+	if (f->rpos != f->rend) {
+		if (f->seek(f, f->rpos-f->rend, SEEK_CUR) == 0) {
+			/* Clear read and write modes */
+			f->wpos = f->wbase = f->wend = 0;
+			f->rpos = f->rend = 0;
+		}
+	}
+#else
 	if (f->rpos != f->rend) f->seek(f, f->rpos-f->rend, SEEK_CUR);
 
 	/* Clear read and write modes */
 	f->wpos = f->wbase = f->wend = 0;
 	f->rpos = f->rend = 0;
+#endif
 
 	FUNLOCK(f);
 	return 0;

test/common.py

@@ -1488,7 +1488,8 @@ def cleanup(line):
 
   def run_js(self, filename, engine=None, args=None,
              assert_returncode=0,
-             interleaved_output=True):
+             interleaved_output=True,
+             input=None):
     # use files, as PIPE can get too full and hang us
     stdout_file = self.in_dir('stdout')
     stderr_file = None
@@ -1510,7 +1511,8 @@ def run_js(self, filename, engine=None, args=None,
       jsrun.run_js(filename, engine, args,
                    stdout=stdout,
                    stderr=stderr,
-                   assert_returncode=assert_returncode)
+                   assert_returncode=assert_returncode,
+                   input=input)
     except subprocess.TimeoutExpired as e:
       timeout_error = e
     except subprocess.CalledProcessError as e:
@@ -1935,7 +1937,8 @@ def _build_and_run(self, filename, expected_output, args=None,
                      check_for_error=True, force_c=False, emcc_args=None,
                      interleaved_output=True,
                      regex=False,
-                     output_basename=None):
+                     output_basename=None,
+                     input=None):
     logger.debug(f'_build_and_run: {filename}')
 
     if no_build:
@@ -1961,7 +1964,8 @@ def _build_and_run(self, filename, expected_output, args=None,
     for engine in engines:
       js_output = self.run_js(js_file, engine, args,
                               assert_returncode=assert_returncode,
-                              interleaved_output=interleaved_output)
+                              interleaved_output=interleaved_output,
+                              input=input)
       js_output = js_output.replace('\r\n', '\n')
       if expected_output:
         if type(expected_output) not in [list, tuple]:

test/jsrun.py

@@ -86,7 +86,7 @@ def require_engine(engine):
 def run_js(filename, engine, args=None,
            stdin=None, stdout=PIPE, stderr=None, cwd=None,
            full_output=False, assert_returncode=0, skip_check=False,
-           timeout=DEFAULT_TIMEOUT):
+           timeout=DEFAULT_TIMEOUT, **kwargs):
   """Execute javascript code generated by tests, with possible timeout."""
 
   # We used to support True here but we no longer do.  Assert here just in case.
@@ -106,7 +106,8 @@ def run_js(filename, engine, args=None,
         stderr=stderr,
         cwd=cwd,
         timeout=timeout,
-        universal_newlines=True)
+        universal_newlines=True,
+        **kwargs)
   except Exception:
     # the failure may be because the engine is not present. show the proper
     # error in that case

test/other/test_stdin_fflush.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include <string.h>
+
+int main(int argc, char** argv) {
+  char buf[10] = {0};
+
+  int cnt = 0;
+  while (!feof(stdin)) {
+    int cnt = fread(buf, 1, 3, stdin);
+    printf("read %d bytes: '%s'\n", cnt, buf);
+    fflush(stdin);
+    memset(buf, 0, sizeof(buf));
+  }
+
+  printf("done\n");
+  return 0;
+}

test/other/test_stdin_fflush.out

@@ -0,0 +1,6 @@
+read 3 bytes: 'foo'
+read 3 bytes: '
+ba'
+read 2 bytes: 'r
+'
+done

test/test_other.py

@@ -1704,22 +1704,26 @@ def test_export_all_and_exported_functions(self):
   def test_stdin(self, args):
     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')
+    self.emcc(test_file('module/test_stdin.c'), args=args)
 
     for engine in config.JS_ENGINES:
       engine[0] = os.path.normpath(engine[0])
       # 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 = jsrun.make_command(os.path.normpath('a.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
-        os.system(f'cat in.txt | {cmd} > out.txt')
+        os.system(f'{cmd} > out.txt < in.txt')
       self.assertContained('abcdef\nghijkl\neof', read_file('out.txt'))
 
+  @crossplatform
+  def test_stdin_fflush(self):
+    self.do_other_test('test_stdin_fflush.c', input='foo\nbar\n')
+
   @crossplatform
   def test_module_stdin(self):
     self.set_setting('FORCE_FILESYSTEM')