[lib][libc] rework a bit of the stdio code to handle errors more closely to spec

Some of the recent refactoring may cause some of the inner error codes
to be eaten or misinterpreted. Restructure fread/fwrite to handle these
a bit more gracefully.

There's some careful use with ssize_t vs size_t here to try to deal with
negatves values properly.

lib/libc/stdio.c

@@ -26,6 +26,8 @@
         STDIO_FIELDS            \
     }
 
+// The three main standard io file descriptors that just
+// point to the main console.
 FILE __stdio_FILEs[3] = {
     DEFINE_STDIO_DESC(0), /* stdin */
     DEFINE_STDIO_DESC(1), /* stdout */
@@ -40,6 +42,8 @@ FILE *fopen(const char *filename, const char *mode) {
         return NULL;
     }
 
+    // TODO: handle more open modes
+
     stream->use_fs = true;
     stream->fs_handle.offset = 0;
     stream->fs_handle.readonly = (!strchr(mode, 'w') && !strchr(mode, 'a'));
@@ -77,16 +81,78 @@ int fclose(FILE *stream) {
     return 0;
 }
 
-size_t fread(void *ptr, size_t size, size_t count, FILE *stream) {
+// Inner version of fread that returns a ssize_t to include the internal error code
+// along with the length.
+static ssize_t fread_error(void *ptr, size_t size, size_t count, FILE *stream) {
+    if (count == 0 || size == 0) {
+        return 0;
+    }
 #if defined(WITH_LIB_FS)
     if (stream->use_fs) {
-        size_t rsize = fs_read_file(stream->fs_handle.handle, ptr,
+        ssize_t rsize = fs_read_file(stream->fs_handle.handle, ptr,
                              stream->fs_handle.offset, size * count);
+        if (rsize <= 0) {
+            return rsize;
+        }
         stream->fs_handle.offset += rsize;
         return rsize / size;
     }
 #endif // WITH_LIB_FS
-    return io_read(stream->io, ptr, size * count) / size;
+    ssize_t rsize = io_read(stream->io, ptr, size * count);
+    if (rsize <= 0) {
+        return rsize;
+    }
+    return rsize / size;
+}
+
+// Inner version of fwrite that returns a ssize_t to include the internal error code
+// along with the length.
+static ssize_t fwrite_error(const void *ptr, size_t size, size_t count, FILE *stream) {
+    if (count == 0 || size == 0) {
+        return 0;
+    }
+#if defined(WITH_LIB_FS)
+    if (stream->use_fs) {
+        if (stream->fs_handle.readonly) {
+            // return error here?
+            return 0;
+        }
+
+        // TODO: deal with append
+
+        ssize_t wsize = fs_write_file(stream->fs_handle.handle, ptr,
+                                     stream->fs_handle.offset, size * count);
+        if (wsize <= 0) {
+            return wsize;
+        }
+        stream->fs_handle.offset += wsize;
+
+        return wsize / size;
+    }
+#endif
+    ssize_t wsize = io_write(stream->io, ptr, size * count);
+    if (wsize <= 0) {
+        return wsize;
+    }
+    return wsize / size;
+}
+
+size_t fread(void *ptr, size_t size, size_t count, FILE *stream) {
+    ssize_t read = fread_error(ptr, size, count, stream);
+    if (read < 0) {
+        // TODO: save error for ferror()
+        return 0;
+    }
+    return read;
+}
+
+size_t fwrite(const void *ptr, size_t size, size_t count, FILE *stream) {
+    ssize_t written = fwrite_error(ptr, size, count, stream);
+    if (written < 0) {
+        // TODO: save error for ferror()
+        return 0;
+    }
+    return written;
 }
 
 int fflush(FILE *stream) {
@@ -149,10 +215,14 @@ long ftell(FILE *stream) {
     return 0;
 }
 
-
 int fputc(int _c, FILE *fp) {
     unsigned char c = _c;
-    return fwrite(&c, /*size=*/1, /*count=*/1, fp);
+
+    size_t written = fwrite(&c, /*size=*/1, /*count=*/1, fp);
+    if (written == 0) {
+        return EOF;
+    }
+    return c;
 }
 
 int putchar(int c) {
@@ -161,55 +231,33 @@ int putchar(int c) {
 
 int puts(const char *str) {
     int err = fputs(str, stdout);
-    if (err >= 0)
+    if (err >= 0) {
         err = fputc('\n', stdout);
+    }
     return err;
 }
 
 int fputs(const char *s, FILE *fp) {
     size_t len = strlen(s);
-    return fwrite(s, /*size=*/1, /*count=*/len, fp);
-}
-
-size_t fwrite(const void *ptr, size_t size, size_t count, FILE *stream) {
-    if (stream == stdin) {
+    if (len == 0) {
         return 0;
     }
 
-    if (stream == stdout || stream == stderr) {
-        size_t bytes_written;
-
-        if (size == 0 || count == 0)
-            return 0;
-
-        // fast path for size == 1
-        if (likely(size == 1)) {
-            return io_write(stream->io, ptr, count);
-        }
-
-        bytes_written = io_write(stream->io, ptr, size * count);
-        return bytes_written / size;
+    size_t written = fwrite(s, /*size=*/1, /*count=*/len, fp);
+    if (written == 0) {
+        return EOF;
     }
-
-#if defined(WITH_LIB_FS)
-    if (stream->fs_handle.readonly) {
-        return 0;
-    }
-
-    size_t wsize = fs_write_file(stream->fs_handle.handle, ptr,
-                                 stream->fs_handle.offset, size * count);
-    stream->fs_handle.offset += wsize;
-
-    return wsize / size;
-#endif // WITH_LIB_FS
     return 0;
 }
 
 int fgetc(FILE *fp) {
     char c;
-    ssize_t ret = fread(&c, /*size=*/1, /*count=*/1, fp);
 
-    return (ret > 0) ? c : EOF;
+    size_t err = fread(&c, /*size=*/1, /*count=*/1, fp);
+    if (err == 0) {
+        return EOF;
+    }
+    return c;
 }
 
 int getchar(void) {
@@ -236,7 +284,8 @@ char *fgets(char *s, int size, FILE *stream) {
 
 int _fprintf_output_func(const char *str, size_t len, void *state) {
     FILE *fp = (FILE *)state;
-    return fwrite(str, /*size=*/1, /*count=*/len, fp);
+
+    return fwrite_error(str, /*size=*/1, /*count=*/len, fp);
 }
 
 int vfprintf(FILE *fp, const char *fmt, va_list ap) {