vips_target_finish can fail silently resulting in corrupted or null output

[flga]

Bug report

Describe the bug
Currently vips_target_finish returns void, this can be a problem because it needs to call vips_target_flush which can fail.
This silences any errors that might occur on the last write to the target, and can be particularly devious if all the writes up to that point fit into the target buffer, causing the only write to actually occur to fail silently.

In summary, when using the streaming api (at least _custom) any output that is less than VIPS_TARGET_BUFFER_SIZE can fail unbeknownst to the caller.

To Reproduce
Load and save a small image with a write implementation that fails.

#include <stdio.h>
#include <unistd.h>
#include <vips/vips.h>

gint64 write_cb(VipsTargetCustom *target_custom, gpointer data, gint64 length, gpointer user_data) {
  printf("quick, duck! it's a cosmic ray!\n");
  return -1;
};

int main() {
  VipsSource *source = vips_source_new_from_file("box.jpg");
  VipsTargetCustom *target_custom = vips_target_custom_new();
  g_signal_connect(target_custom, "write", G_CALLBACK(write_cb), NULL);

  VipsImage *img;
  if (vips_jpegload_source(source, &img, "access", VIPS_ACCESS_SEQUENTIAL, NULL)) {
    printf("%s\n", vips_error_buffer());
    return 1;
  }

  if (vips_jpegsave_target(img, VIPS_TARGET(target_custom), NULL)) {
    printf("%s\n", vips_error_buffer());
    return 1;
  }

  printf("phew!\n");
  return 0;
}

box.jpg

Environment

• Vips: 7561d4e

[jcupitt]

You're right, I remember worrying about this.

I've made a PR for a fix.