Handle failure of lseek in fflush (#23624)

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. This PR update fflush such that it can
handle the failure of lseek.

The alternative to this approach would be to add `seek` support to file
our stdin implementation but that looks like a bigger change. Since it
seems reasonable for stdout and stdin to not be seekable updating musl
to handle this case seems like the right thing to do for now. If we find
a other code in the wild that depends on stdin supporting lseek we can
reconsider.

The following (14) test expectation files were updated by
running the tests with `--rebaseline`:
```                                                                      
other/codesize/test_codesize_cxx_ctors1.size: 129141 => 129147 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_ctors2.size: 128553 => 128559 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_except.size: 170809 => 170815 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_except_wasm.size: 144517 => 144523 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_except_wasm_legacy.size: 142092 => 142098 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_lto.size: 122093 => 122099 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_mangle.size: 232619 => 232625 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_noexcept.size: 131704 => 131710 [+6 bytes / +0.00%]
other/codesize/test_codesize_cxx_wasmfs.size: 169111 => 169117 [+6 bytes / +0.00%]
other/codesize/test_codesize_files_wasmfs.size: 50049 => 50055 [+6 bytes / +0.01%]
other/codesize/test_codesize_hello_O0.size: 15101 => 15105 [+4 bytes / +0.03%]
other/codesize/test_codesize_minimal_O0.size: 1156 => 1160 [+4 bytes / +0.35%]
other/test_unoptimized_code_size.wasm.size: 15101 => 15105 [+4 bytes / +0.03%]
other/test_unoptimized_code_size_strict.wasm.size: 15101 => 15105 [+4 bytes / +0.03%]
                                                                         
Average change: +0.03% (+0.00% - +0.35%)                                 
```      

Fixes: #21791

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

@@ -1492,7 +1492,8 @@ def cleanup(line):
 
   def run_js(self, filename, engine=None, args=None,
              assert_returncode=0,
-             interleaved_output=True):
+             interleaved_output=True,
+             **kwargs):
     # use files, as PIPE can get too full and hang us
     stdout_file = self.in_dir('stdout')
     stderr_file = None
@@ -1514,7 +1515,8 @@ 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,
+                   **kwargs)
     except subprocess.TimeoutExpired as e:
       timeout_error = e
     except subprocess.CalledProcessError as e:
@@ -1937,9 +1939,9 @@ def _build_and_run(self, filename, expected_output, args=None,
                      includes=None,
                      assert_returncode=0, assert_identical=False, assert_all=False,
                      check_for_error=True, force_c=False, emcc_args=None,
-                     interleaved_output=True,
                      regex=False,
-                     output_basename=None):
+                     output_basename=None,
+                     **kwargs):
     logger.debug(f'_build_and_run: {filename}')
 
     if no_build:
@@ -1963,9 +1965,7 @@ def _build_and_run(self, filename, expected_output, args=None,
     if len(engines) == 0:
       self.fail('No JS engine present to run this test with. Check %s and the paths therein.' % config.EM_CONFIG)
     for engine in engines:
-      js_output = self.run_js(js_file, engine, args,
-                              assert_returncode=assert_returncode,
-                              interleaved_output=interleaved_output)
+      js_output = self.run_js(js_file, engine, args, assert_returncode=assert_returncode, **kwargs)
       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.
@@ -96,6 +96,7 @@ def run_js(filename, engine, args=None,
     raise Exception('output file not found: ' + filename)
 
   command = make_command(os.path.abspath(filename), engine, args)
+  kwargs.setdefault('text', True)
   if common.EMTEST_VERBOSE:
     print(f"Running: '{shared.shlex_join(command)}'")
   try:
@@ -106,7 +107,7 @@ def run_js(filename, engine, args=None,
         stderr=stderr,
         cwd=cwd,
         timeout=timeout,
-        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/codesize/test_codesize_cxx_ctors1.size

@@ -1 +1 @@
-129141
+129147

test/other/codesize/test_codesize_cxx_ctors2.size

@@ -1 +1 @@
-128553
+128559

test/other/codesize/test_codesize_cxx_except.size

@@ -1 +1 @@
-170809
+170815

test/other/codesize/test_codesize_cxx_except_wasm.size

@@ -1 +1 @@
-144517
+144523

test/other/codesize/test_codesize_cxx_except_wasm_legacy.size

@@ -1 +1 @@
-142092
+142098

test/other/codesize/test_codesize_cxx_lto.size

@@ -1 +1 @@
-122093
+122099

test/other/codesize/test_codesize_cxx_mangle.size

@@ -1 +1 @@
-232619
+232625

test/other/codesize/test_codesize_cxx_noexcept.size

@@ -1 +1 @@
-131704
+131710

test/other/codesize/test_codesize_cxx_wasmfs.size

@@ -1 +1 @@
-169111
+169117

test/other/codesize/test_codesize_files_wasmfs.size

@@ -1 +1 @@
-50049
+50055

test/other/codesize/test_codesize_hello_O0.size

@@ -1 +1 @@
-15101
+15105

test/other/codesize/test_codesize_minimal_O0.size

@@ -1 +1 @@
-1156
+1160

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/other/test_unoptimized_code_size.wasm.size

@@ -1 +1 @@
-15101
+15105

test/other/test_unoptimized_code_size_strict.wasm.size

@@ -1 +1 @@
-15101
+15105

test/test_other.py

@@ -1730,6 +1730,11 @@ def test_module_stdin(self):
     self.emcc_args += ['--pre-js', 'pre.js']
     self.do_runf('module/test_stdin.c', 'hello, world!')
 
+  @crossplatform
+  def test_stdin_fflush(self):
+    # Force text=False here so that newlines are not treated differently on windows.
+    self.do_other_test('test_stdin_fflush.c', input=b'foo\nbar\n', text=False)
+
   @crossplatform
   def test_module_stdout_stderr(self):
     self.set_setting('FORCE_FILESYSTEM')