fix(c-api) wasm_limits_t contains Pages, not Bytes + sentinel value
#1690
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When building a `wasm_memorytype_t` with `wasm_memorytype_new`, we pass a `wasm_limits_t`, where `min` and `max` represent `Pages`. This semantics is set by `wasm_memorytype_new` itself where `min` and `max` from `wasm_limits_t` are used to compute `Pages`, which are then passed to `MemoryType`. Then, in `wasm_memorytype_limits`, we expect to get the same `wasm_limits_t` given to `wasm_memorytype_new`. But it's not! The same `MemoryType` is read, good. The `minimum` and `maximum` fields are `Pages`, good. Then, we compute the `min` and `max` values for the resulting `wasm_limits_t`, which receive `Page.bytes().0`, not good! We don't want the number of bytes, but the number of pages. This patch fixes that.
In the `wasm.h` header file, it is defined by `wasm_limits_max_default`.
Hywan
changed the title
wasm_limits_t contains Pages, not Bytes.wasm_limits_t contains Pages, not Bytes + sentinel value
syrusakbary
approved these changes
Oct 8, 2020
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bors r+ |
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Build succeeded: |
MarkMcCaskey
reviewed
Oct 8, 2020
Comment on lines
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| min: md.minimum.0 as _, | ||
| max: md | ||
| .maximum | ||
| .map(|max| max.0 as _) | ||
| .unwrap_or(LIMITS_MAX_SENTINEL), |
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Good catch, I didn't see that
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When building a
wasm_memorytype_twithwasm_memorytype_new, wepass a
wasm_limits_t, whereminandmaxrepresentPages. Thissemantics is set by
wasm_memorytype_newitself whereminandmaxfrom
wasm_limits_tare used to computePages, which are then passedto
MemoryType.Then, in
wasm_memorytype_limits, we expect to get the samewasm_limits_tgiven towasm_memorytype_new. But it's not!The same
MemoryTypeis read, good. Theminimumandmaximumfields are
Pages, good. Then, we compute theminandmaxvaluesfor the resulting
wasm_limits_t, which receivePage.bytes().0, notgood! We don't want the number of bytes, but the number of pages.
This patch fixes that.
Edit: This patch also fixes the max limit sentinel value. In
wasm.h, it is defined bywasm_limits_max_default(0xffffffff). In our code, it is correctly used inwasm_memorytype_new, but not inwasm_memorytype_limits. A new constant has been introduced:LIMITS_MAX_SENTINEL. Not super happy with this name though.