does borg check without --repair wrongly report "healed" chunks?

[ThomasWaldmann]

borg 1.4.0

Since this is a continuation of some related testing just thought I'd note here for anyone interested some unexpected behavior from check's reporting when used without --repair following previously zeroed out chunks.

I found that after --repair had zeroed an intentionally corrupted chunk, then I ran a create backup successfully, that if I next ran check but without --repair it would report as having healed the missing chunks, when it actually hadn't.

Steps:

• Removed the repo hints, index and integrity files as part of this test (was checking how Borg would respond to both missing repo metadata and a corrupted chunk file simultaneously).
• Intentionally corrupted the largest data file in the repo. Since I have a PNG image in the source data larger than other files in the source data these larger data files have always contained the image data for these tests.
• Ran check with --verify-data and --repair. This zeroed out the corrupted chunks.
• Ran a successful backup with create.
• Ran check with --verify-data but without --repair. And this is where it reported the PNG image as being healed in all backups (removed time values from the log):

...
{"type": "log_message", "message": "borg-backup-test-1: home/user/Documents/test-data/test.png: Healed previously missing file chunk! (Byte 0-475076, Chunk acd0521e02a47fde43e172300e6728268517a5676ebe7079007644aff35f1dac).", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-1: home/user/Documents/test-data/test.png: Completely healed previously damaged file!", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "Analyzing archive borg-backup-test-2 (5/8)", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-2: home/user/Documents/test-data/test.png: Healed previously missing file chunk! (Byte 0-475076, Chunk acd0521e02a47fde43e172300e6728268517a5676ebe7079007644aff35f1dac).", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-2: home/user/Documents/test-data/test.png: Completely healed previously damaged file!", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "Analyzing archive borg-backup-test-3 (6/8)", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-3: home/user/Documents/test-data/test.png: Healed previously missing file chunk! (Byte 0-475076, Chunk acd0521e02a47fde43e172300e6728268517a5676ebe7079007644aff35f1dac).", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-3: home/user/Documents/test-data/test.png: Completely healed previously damaged file!", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "Analyzing archive borg-backup-test-4 (7/8)", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-4: home/user/Documents/test-data/test.png: Healed previously missing file chunk! (Byte 0-475076, Chunk acd0521e02a47fde43e172300e6728268517a5676ebe7079007644aff35f1dac).", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "borg-backup-test-4: home/user/Documents/test-data/test.png: Completely healed previously damaged file!", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "Analyzing archive borg-backup-test-5 (8/8)", "levelname": "INFO", "name": "borg.archive"}
{"type": "log_message", "message": "Archive consistency check complete, no problems found.", "levelname": "INFO", "name": "borg.archive"}

• However after using mount to check if it had actually healed I found only the latest backup contained the openable PNG image. While the prior backups all errored when opening it.
• So I ran check with --repair and it reported the heals like the prior output but this time it actually worked for all the backups it reported (confirmed by re-mounting the backups).

Originally posted by @chocmake in #8527 (comment)