fuzz: Use __AFL_SHM_ID for naming test directories #34445

1. 
   marcofleon commented at 2:17 pm on January 29, 2026: contributor

   During long multicore fuzzing campaigns with AFL++, stale datadirs can eventually accumulate from time outs, resulting in disk running out of space (see #28811). The easiest way to reproduce this is by running our utxo_total_supply target using multiple cores with AFL++ and observing the crashes that occur because of all the directories in /tmp/test_common\ bitcoin/utxo_total_supply/.

   Fix this by using the AFL++ shared memory ID to name the test dirs and cleaning it up before each setup. This ID is unique per AFL++ instance, so multiple cores can run in parallel without conflicts.

   Fixes https://github.com/bitcoin/bitcoin/issues/28811

2. fuzz: Use `__AFL_SHM_ID` for naming test directories

   Use the AFL++ shared memory ID environment variable to create
   a deterministic datadir path. This prevents accumulation of stale
   directories after a fuzz iteration crashes or times out. During
   long fuzz campaigns, this accumulation has occasionally resulted
   in running out of disk space.

   d3e681bc06
3. DrahtBot added the label Fuzzing on Jan 29, 2026
4. 
   DrahtBot commented at 2:17 pm on January 29, 2026: contributor

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5. dergoegge approved
6. 
   dergoegge commented at 4:20 pm on January 29, 2026: member

   utACK d3e681bc06758fe0686cd96fcfd4a1c4c5af62b4
7. 
   maflcko commented at 5:27 pm on January 29, 2026: member

   Seems fine, but are there any docs? https://duckduckgo.com/?q=__AFL_SHM_ID+site%3Aaflplus.plus only gives one libafl result?
8. 
   marcofleon commented at 6:04 pm on January 29, 2026: contributor

   Yeah not much documentation (if any), but I figured it’s fine to use as it’s been around forever (since 2.52c):

   https://github.com/AFLplusplus/AFLplusplus/blob/bcc0347eefa05275cea221f8baad8abe7e091872/config.h#L271

   And it’s such a critical part of how AFL++ tells the target where to write coverage.

9. 
   maflcko commented at 6:20 pm on January 29, 2026: member

   lgtm ACK d3e681bc06758fe0686cd96fcfd4a1c4c5af62b4
10. fanquake merged this on Jan 30, 2026
11. fanquake closed this on Jan 30, 2026

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marcofleon DrahtBot dergoegge maflcko

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