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Add helper function to check if a value has been initialized #10524

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jlesquembre opened this issue Apr 16, 2024 · 0 comments · Fixed by #10555
Closed

Add helper function to check if a value has been initialized #10524

jlesquembre opened this issue Apr 16, 2024 · 0 comments · Fixed by #10555
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c api Nix as a C library with a stable interface feature Feature request or proposal

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@jlesquembre
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Is your feature request related to a problem? Please describe.

In the C API, the way we set a value is roughly like this:

Value *value = nix_alloc_value(context , state);
nix_init_int(context, value, 42);
nix_get_int(ctx, value);

Calling nix_get_int without prior initialization with nix_init_int will result in a segmentation fault.
This is because attempts to get the type of an uninitialized value triggers a call to abort():

nix/src/libexpr/value.hh

Lines 268 to 287 in 74e4bc9

inline ValueType type(bool invalidIsThunk = false) const
{
switch (internalType) {
case tInt: return nInt;
case tBool: return nBool;
case tString: return nString;
case tPath: return nPath;
case tNull: return nNull;
case tAttrs: return nAttrs;
case tList1: case tList2: case tListN: return nList;
case tLambda: case tPrimOp: case tPrimOpApp: return nFunction;
case tExternal: return nExternal;
case tFloat: return nFloat;
case tThunk: case tApp: return nThunk;
}
if (invalidIsThunk)
return nThunk;
else
abort();
}

Describe the solution you'd like

I suggest to add an isInitialized method to Value for checking initialization in the C API. This would allow us to raise an exception instead of calling abort.

I'm happy to open a PR with these changes, but I'd like to confirm if the approach makes sense.

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@jlesquembre jlesquembre added the feature Feature request or proposal label Apr 16, 2024
jlesquembre added a commit to jlesquembre/nix that referenced this issue Apr 18, 2024
@jlesquembre
Add isInitialized to nix::Value
ec0d8f6 
Add a method to check if a value has been initialized. This helps avoid
segfaults when calling `type()`.
Useful in the context of the new C API.

Closes NixOS#10524
@jlesquembre jlesquembre mentioned this issue Apr 18, 2024
Merged
jlesquembre added a commit to jlesquembre/nix that referenced this issue Apr 18, 2024
@jlesquembre
Add isInitialized to nix::Value
0be906b 
Add a method to check if a value has been initialized. This helps avoid
segfaults when calling `type()`.
Useful in the context of the new C API.

Closes NixOS#10524
@roberth roberth added the c api Nix as a C library with a stable interface label Apr 19, 2024
jlesquembre added a commit to jlesquembre/nix that referenced this issue Apr 21, 2024
@jlesquembre
Add isInitialized to nix::Value
5cc4af5 
Add a method to check if a value has been initialized. This helps avoid
segfaults when calling `type()`.
Useful in the context of the new C API.

Closes NixOS#10524
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Add isValid to nix::Value jlesquembre/nix
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@roberth @jlesquembre