Save in the #convert format to an extensionless destination

[flavorjones]

Motivation / Background

This is the output side of #147.

HotCell receives the destination as an open file descriptor, but it can't pass /dev/fd/N as destination:, because the savers pick the format from the path's extension. So HotCell saves to a scratch file named output.png and then copies that file into the descriptor (transforming.rb). That is an extra write and read of every output, plus tmpfs space for the scratch file.

ImageProcessing::Vips.source(src).convert("png").call(destination: "/dev/fd/5")
# Vips::Error: VipsForeignSave: "/dev/fd/5" is not a known file format

ImageProcessing::MiniMagick.source(src).loader(inherit_fds: [out]).convert("jpg").call(destination: "/dev/fd/5")
# writes a PNG

Detail

When the destination has no extension, save in the #convert format.

• vips: pick the saver with Vips.vips_foreign_find_save(".#{format}") and filter options as select_valid_saver_options does. Options in the destination's [...] suffix still apply, as with write_to_file.
• MiniMagick: write to "#{format}:#{destination}". A format that isn't a bare word (for example, one containing a colon) raises ImageProcessing::Error, because ImageMagick would read the rest as part of the output path.

Destinations with an extension are unchanged, even when the extension disagrees with #convert. Pipelines without #convert are unchanged. .saver(saver:) still takes precedence.

Pipeline passes format: to Processor.call only in the new case, and Processor.call passes it to save_image as a third positional argument only in the new case, so existing custom processors are unaffected elsewhere.

Additional information

When the destination has no extension, ImageProcessing::Vips::Processor.save_image now calls the libvips save operation directly instead of calling Vips::Image#write_to_file. It therefore skips the garbage collection that write_to_file runs after each save (Vips::Image#write_gc, which is private). The existing .saver(saver:) option already calls the save operation directly and skips it too.

.saver(saver:) doesn't solve this problem, because its value is a libvips save operation name rather than a file format, and it disables the filtering of unsupported saver options.

Closes #149

[janko]

Looks good, thank you!