build(flags): preserve flag-value ABI by pinning new flags to high bits#31
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Adding DB_MPOOL_AIO and DB_TXN_SNAPSHOT_SAFE to their set_flags/txn_begin flag groups made the api_flags bit allocator repack those groups, which renumbered the public values of several existing flags (DB_NOFLUSH, DB_NOLOCKING, DB_NOPANIC, DB_OVERWRITE, DB_PANIC_ENVIRONMENT, DB_REGION_INIT, DB_TIME_NOTGRANTED, DB_YIELDCPU, DB_TXN_WAIT). That is an ABI change for the public flag constants. Add a __PIN=<value> annotation to dist/api_flags so a flag can be assigned a fixed bit that the greedy allocator reserves up-front and skips. Pin DB_MPOOL_AIO to 0x00100000 and DB_TXN_SNAPSHOT_SAFE to 0x00000800 -- both free in their APIs at the pre-change baseline -- so every pre-existing public flag keeps its original value and the change is purely additive. Verified: src/dbinc_auto/api_flags.in now differs from the pre-AIO baseline by exactly the two added #defines, with zero value changes to existing flags; regenerated Java/C# constants accordingly. TCL test001/012 and ssi001/002 (which exercise DB_TXN_SNAPSHOT_SAFE) pass.
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What
Preserve the public flag-constant ABI by pinning the two recently-added
flags to high, previously-unused bits instead of letting the
api_flagsbit allocator repack their groups.
Why
Adding
DB_MPOOL_AIO(AIO Stage 2, #24) andDB_TXN_SNAPSHOT_SAFE(SSI) totheir flag groups made the greedy bit allocator renumber the public values
of several existing flags:
Changing public flag values is an ABI break for the constants (a
precompiled application that baked in the old value and links a new libdb
would pass the wrong flag).
How
Add a
__PIN=<value>annotation todist/api_flags. A pinned flag's bit isreserved in its APIs' masks before the greedy pass and skipped during
allocation, so it takes a fixed bit without disturbing any other flag.
Pin
DB_MPOOL_AIO = 0x00100000andDB_TXN_SNAPSHOT_SAFE = 0x00000800—both free in their respective APIs at the pre-change baseline.
Verification
src/dbinc_auto/api_flags.innow differs from the pre-AIO baseline(
eae0c147e) by exactly the two added#defines — zero value changes toany existing flag.
DbConstantsaccordingly (they revert to baselinevalues).
test001,test012, andssi001/ssi002(which exercise
DB_TXN_SNAPSHOT_SAFE) pass.