fix(shorebird): let the createForSnapshots tests supply region lengths - #170
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The three CreateForSnapshots tests have crashed since they were added. createForSnapshots asks the VM for each region's length, and the tests hand it 4 byte strings, so the parser reads a header out of arbitrary bytes and dereferences it. Production cannot use the mapping's own size instead, because an AOT snapshot's regions arrive as fml::SymbolMapping, whose GetSize() is 0 for a dlsym'd symbol address that carries no extent. Only the snapshot header knows. So the lookup becomes a parameter, defaulted to the Dart API, and the tests pass their own. Production keeps calling Dart_SnapshotDataSize and Dart_SnapshotInstrSize through the default overload, which is what the single caller uses.
easymac
approved these changes
Sep 21, 2026
| // Production has to ask the Dart VM. An AOT snapshot's regions arrive as | ||
| // fml::SymbolMapping, whose GetSize() is 0 because a dlsym'd symbol address | ||
| // carries no extent. Only the snapshot header knows where the region ends. | ||
| using RegionSizer = std::function<size_t(const uint8_t* region)>; |
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Consider passing the size directly here; I think it may be simpler for callers to use something like createForSnapshots(*snapshot, data.size(), instructions.size())
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Summary
The three CreateForSnapshots tests have crashed since they were added, so the blob ordering they guard has never been checked. They hand
createForSnapshots4 byte strings, and it asks the VM to parse a snapshot header out of them. Production cannot use the mapping's own size instead, because AOT regions arrive asfml::SymbolMapping, whoseGetSize()is 0 for a dlsym'd address that carries no extent. So the length lookup became a parameter, defaulted to the Dart API, rather than production getting weaker to suit a test.Testing
shorebird_unittests41 of 41, the three crashing cases passing by name. This suite has no CI lane, so I built and ran it by hand against arm64 debug unopt.