Implement streamtype=1 option for tables-only JPEG encoding #7491
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bgilbert force-pushed the jpeg-tables-only branch from e8ebc48 to 2a5eac0 Compare 
We already support streamtype=2 to skip producing JPEG tables, but streamtype=1, which skips everything but the tables, was never implemented. The streamtype=1 stub code dates to Git pre-history, so it's not immediately clear why. Implement the missing support. jpeg_write_tables() can't resume after a full output buffer (it fails with JERR_CANT_SUSPEND), so it might seem that Pillow needs to pre-compute the necessary buffer size. However, in the normal case of producing an interchange stream, the tables are written via the same libjpeg codepath during the first jpeg_write_scanlines() call, and table writes aren't resumable there either. Thus, any buffer large enough for the normal case will also be large enough for a tables-only file. The streamtype option isn't documented and this commit doesn't change that. It does add a test though. Co-authored-by: Andrew Murray <radarhere@users.noreply.github.com>
bgilbert force-pushed the jpeg-tables-only branch from 2a5eac0 to 4d7372b Compare 
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| Updated based on suggestions in bgilbert#1. |
radarhere approved these changes Oct 31, 2023
radarhere added a commit to radarhere/Pillow that referenced this pull request Dec 8, 2023
radarhere added a commit to radarhere/Pillow that referenced this pull request Dec 8, 2023
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We already support
streamtype=2to skip producing JPEG tables, butstreamtype=1, which skips everything but the tables, was never implemented. Thestreamtype=1stub code dates to Git pre-history, so it's not immediately clear why. Implement the missing support.jpeg_write_tables()can't resume after a full output buffer (it fails withJERR_CANT_SUSPEND), so it might seem that Pillow needs to pre-compute the necessary buffer size. However, in the normal case of producing an interchange stream, the tables are written via the same libjpeg codepath during the firstjpeg_write_scanlines()call, and table writes aren't resumable there either. Thus, any buffer large enough for the normal case will also be large enough for a tables-only file.I've opted to implement the early exit with a
gotorather than refactoring the state machine. If thegotois not desired, it would be possible to add a dedicated cleanup state, do an early return after writing tables, and require a second call into the state machine to finish cleanup.The
streamtypeoption isn't documented and this PR doesn't change that. It does add a test though.