Fix vfprintf and derived functions not handling write errors (#798)

The C standard states:
> The fprintf function returns the number of characters transmitted,
> or a negative value if an output or encoding error occurred or if
> the implementation does not support a specified width length
> modifier.
- C Standard, 7.23.6.1.15. The fprintf function

However, cosmopolitan fails to return a negative value in the case of
an output error, meaning that a program such as:

 #include <stdio.h>

int main()
{
    FILE *fp = fopen("/dev/full", "w");

    setbuf(fp, NULL);
    printf("fprintf: %d\n", fprintf(fp, "test\n"));
    printf("fflush: %d\n", fflush(fp));
}

will, under cosmopolitan, print that no error occured in either of the
calls to fprintf and fflush.

This patch fixes this, along with the associated GitHub issue,
https://github.com/jart/cosmopolitan/issues/784

libc/stdio/vfprintf_unlocked.c

@@ -44,7 +44,7 @@ static int vfprintfputchar(const char *s, struct state *t, size_t n) {
   } else {
     rc = 0;
   }
-  return 0;
+  return rc;
 }
 
 /**

test/libc/fmt/fprintf_test.c

@@ -0,0 +1,34 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Gabriel Ravier                                                │
+│                                                                              │
+│ 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/dce.h"
+#include "libc/stdio/stdio.h"
+#include "libc/testlib/testlib.h"
+
+TEST(fprintf, testWriteError) {
+  // Only Linux, NetBSD and FreeBSD are known to have /dev/full
+  if (!IsLinux() && !IsNetbsd() && !IsFreebsd()) return;
+
+  FILE *fp = fopen("/dev/full", "w");
+  ASSERT_NE(fp, NULL);
+
+  setbuf(fp, NULL);
+  ASSERT_LT(fprintf(fp, "test"), 0);
+
+  ASSERT_EQ(fclose(fp), 0);
+}

test/libc/fmt/test.mk

@@ -55,6 +55,9 @@ $(TEST_LIBC_FMT_OBJS): private				\
 	DEFAULT_CCFLAGS +=				\
 		-fno-builtin
 
+o/$(MODE)/test/libc/fmt/fprintf_test.com.runs:		\
+	private .UNVEIL += w:/dev/full
+
 .PHONY: o/$(MODE)/test/libc/fmt
 o/$(MODE)/test/libc/fmt:				\
 		$(TEST_LIBC_FMT_BINS)			\