René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"\"Old\" filename in patch files generated for testing does not exist and can cause failures","articleBody":"Most patch files generated for testing start like this:\n\n```diff\n--- a/alef\t2026-05-05 17:02:25.388687288 -0400\n+++ target/unified-diff//alef\t2026-05-05 17:02:25.388687288 -0400\n...\n```\n\nNote `patch` does not use TWO  filenames; it uses only one name because it patches files \"in place\". But `patch` input == `diff` output and it makes sense for `diff` output to have \"old\" _and_ \"new\" filenames.\n\n`a/alef` does not normally not exist and tests normally pass because GNU patch silently falls back on the \"new\" filename. Except when `a/alef` exists, and then the corresponding test(s) to fail.\n\nhttps://www.gnu.org/software/diffutils/manual/html_node/Multiple-Patches.html documents GNU precedence and that the \"old\" file name has precedence over the \"new\" filename.\n\nTrivial reproduction:\n\n```\ncd diffutils\nmkdir a\ntouch a/alef  a/alefn  a/alef_  a/alefx  a/alefr  a/fuzz.file\ncargo test\n\n\n---- context_diff::tests::test_permutations stdout ----\n\nthread 'context_diff::tests::test_permutations' (98977) panicked at src/context_diff.rs:458:33:\nOutput { status: ExitStatus(unix_wait_status(256)), stdout: \"patching file a/alef\\nHunk #1 FAILED at 1.\\n1 out of 1 hunk FAILED -- saving rejects to file a/alef.rej\\n\", stderr: \"\" }\n```\n\nI hit this problem because some other (and unfortunately unknown) test issue left bogus `a/alef*` file(s) behind in my workspace. I didn't bother cleaning them up because I assumed some test would keep recreating them - and that cost me a lot of time.\n\n","author":{"url":"https://github.com/marc-hb","@type":"Person","name":"marc-hb"},"datePublished":"2026-05-05T21:28:57.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/223/diffutils/issues/223"}