Test fgets, signals, and SA_RESTART interactions

The new test only passes thanks to the previous commit.

libc/stdio/fgets_unlocked.c

@@ -55,11 +55,7 @@ char *fgets_unlocked(char *s, int size, FILE *f) {
         if (t) break;
       } else {
         if ((c = fgetc_unlocked(f)) == -1) {
-          if (ferror_unlocked(f) == EINTR) {
-            continue;
-          } else {
-            break;
-          }
+          break;
         }
         *p++ = c & 255;
         if (c == '\n') break;

test/libc/stdio/fgets_interrupt_test.c

@@ -0,0 +1,119 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2024 Cadence Ember                                                 │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/isystem/errno.h"
+#include "libc/isystem/signal.h"
+#include "libc/isystem/stddef.h"
+#include "libc/isystem/unistd.h"
+#include "libc/stdio/stdio.h"
+#include "libc/testlib/testlib.h"
+
+#define MY_TEST_STRING_1 "He"
+#define MY_TEST_STRING_2 "llo world!"
+
+char buf[20] = {0};
+int pipes[2];
+int pid;
+int got_sigusr1 = 0;
+
+void sigusr1_handler(int) {
+  got_sigusr1 = 1;
+}
+
+void write_pipe(int send_signal_before_end) {
+  // Set up pipe for writing
+  close(pipes[0]);
+  FILE *stream = fdopen(pipes[1], "w");
+
+  // Start writing the first part of the stream
+  fputs(MY_TEST_STRING_1, stream);
+
+  // Send SIGUSR1 to parent (if we're currently testing that)
+  if (send_signal_before_end) {
+    kill(getppid(), SIGUSR1);
+  }
+
+  // Send rest of stream
+  fputs(MY_TEST_STRING_2, stream);
+
+  // Close stream - this will cause the parent's fgets to end
+  fclose(stream);
+}
+
+void read_pipe() {
+  // Set up pipe for reading
+  close(pipes[1]);
+  FILE *stream = fdopen(pipes[0], "r");
+
+  // Read with fgets
+  fgets(buf, 20, stream);
+
+  // Tidy up
+  fclose(stream);
+}
+
+void setup_signal_and_pipe(uint64_t sa_flags) {
+  // Set up SIGUSR1 handler
+  struct sigaction sa = {.sa_handler = sigusr1_handler, .sa_flags = sa_flags};
+  if (sigaction(SIGUSR1, &sa, NULL) == -1) {
+    perror("sigaction");
+    _exit(1);
+  }
+
+  // Set up pipe between parent and child
+  if (pipe(pipes) == -1) {
+    perror("pipe");
+    _exit(1);
+  }
+}
+
+TEST(fgets_eintr, testThatFgetsReadsFromPipeNormally) {
+  setup_signal_and_pipe(0);  // 0 = no SA_RESTART
+  ASSERT_NE(-1, (pid = fork()));
+  if (!pid) {
+    write_pipe(0);  // 0 = no signal
+    _exit(0);
+  }
+  read_pipe();
+  EXPECT_STREQ(MY_TEST_STRING_1 MY_TEST_STRING_2, buf);
+}
+
+TEST(fgets_eintr, testThatTheSignalInterruptsFgets) {
+  setup_signal_and_pipe(0);  // 0 = no SA_RESTART
+  ASSERT_NE(-1, (pid = fork()));
+  if (!pid) {
+    write_pipe(1);  // 1 = signal
+    _exit(0);
+  }
+  read_pipe();
+  EXPECT_STRNE(MY_TEST_STRING_1 MY_TEST_STRING_2, buf);
+  EXPECT_EQ(EINTR, errno);
+}
+
+TEST(fgets_eintr, testThatFgetsRestartsWhenSaRestartIsSet) {
+  setup_signal_and_pipe(SA_RESTART);  // SA_RESTART
+  ASSERT_NE(-1, (pid = fork()));
+  if (!pid) {
+    write_pipe(1);  // 1 = signal
+    _exit(0);
+  }
+  read_pipe();
+  EXPECT_STREQ(MY_TEST_STRING_1 MY_TEST_STRING_2, buf);
+  EXPECT_NE(EINTR, errno);
+  EXPECT_EQ(1, got_sigusr1);
+}