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Nix Bindings

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Nix C API https://github.com/NixOS/nix/blob/master/doc/manual/src/c-api.md, language-specific bindings, and their use cases (currently broken link: https://nixos.org/manual/nix/unstable/c-api)22 Servers

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19 Mar 2025
@o-santi:matrix.orgLeonardo Santiagoshould I be able to copy derivation closures between remote stores without passing through my own local machine?12:08:35
@o-santi:matrix.orgLeonardo SantiagoI tried doing it through the FFI but it grinds to a halt and never returns12:08:52
@xokdvium:matrix.orgxokdvium changed their display name from xokdvium to Sergei Zimmerman (xokdvium).21:12:16
22 Mar 2025
@yorickvp:matrix.orgyorickvp joined the room.14:18:57
@yorickvp:matrix.orgyorickvpin theory yes, in practice I think the copying code does some calls that aren't efficient if both stores are remote14:20:54
28 Mar 2025
@roberthensing:matrix.orgRobert Hensing (roberth)Heads-up regarding libflake initialization breaking in 2.28 https://github.com/roberth/nix/blob/0c75581d8b7cfbfa7a8db9b5dcbf0cbf0709009f/doc/manual/rl-next/c-api-flake-init.md18:06:59
@roberthensing:matrix.orgRobert Hensing (roberth)I've decided to keep it simple and break this, since the changed solution is a recent addition, and C API progress is slow enough as is. Contributions welcome :)18:09:40
2 Apr 2025
@roberthensing:matrix.orgRobert Hensing (roberth) PR adding exposing flake locking functionality in the C API 16:17:09
4 Apr 2025
@roberthensing:matrix.orgRobert Hensing (roberth)rfr ^14:46:50
@roberthensing:matrix.orgRobert Hensing (roberth) we don't have a solution for actually loading settings yet 14:47:30
@o-santi:matrix.orgLeonardo Santiago I'm getting SIGSEGV when calling nix_get_attr_byname with (what it seems to me are) completely valid arguments, that shouldn't even throw an error. 16:15:40
@o-santi:matrix.orgLeonardo Santiago

This is as much information as I can gather from gdb, as I don't know how to enable debug information on the nix C API, for some reason setting dontStrip = true doesn't work for the nix-{x}-c packages.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff6aaee5a in nix::ExprVar::eval(nix::EvalState&, nix::Env&, nix::Value&) ()
   from /nix/store/8dlfn55w3b3szqsph1dp2i2jqg0di1gr-nix-expr-2.27.0/lib/libnixexpr.so
16:17:55
@o-santi:matrix.orgLeonardo SantiagoNot sure how to improve this report, the nix expression is fairly complex and I can't include it, but the program itself should just call eval on an expression from file then access its attributes one by one in a list. I'd like to get some line number information going on but I don't seem to be able to.16:20:23
@o-santi:matrix.orgLeonardo Santiago *

This is as much information as I can gather from gdb, as I don't know how to enable debug information on the nix C API .so libraries, for some reason setting dontStrip = true doesn't work for the nix-{x}-c packages.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff6aaee5a in nix::ExprVar::eval(nix::EvalState&, nix::Env&, nix::Value&) ()
   from /nix/store/8dlfn55w3b3szqsph1dp2i2jqg0di1gr-nix-expr-2.27.0/lib/libnixexpr.so
16:20:44
@o-santi:matrix.orgLeonardo Santiago *

This is as much information as I can gather from gdb (showing only toplevel, the callstack is 22k calls deep), as I don't know how to enable debug information on the nix C API .so libraries, for some reason setting dontStrip = true doesn't work for the nix-{x}-c packages.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff6aaee5a in nix::ExprVar::eval(nix::EvalState&, nix::Env&, nix::Value&) ()
   from /nix/store/8dlfn55w3b3szqsph1dp2i2jqg0di1gr-nix-expr-2.27.0/lib/libnixexpr.so
19:12:38
@roberthensing:matrix.orgRobert Hensing (roberth) Did you use .overrideAllMesonComponents? 22:07:19
6 Apr 2025
@o-santi:matrix.orgLeonardo SantiagoNope, and not a particular weird version of nixpkgs. Stable one, tried both with nix 2.25 and now 2.27.0 and both of them showed the same error.20:34:29
@roberthensing:matrix.orgRobert Hensing (roberth)Hmm ok. For 2.27 you'd need it because of the libraries being separate derivations, but that's not an explanation for the 2.25 behavior fwiw22:11:08
@roberthensing:matrix.orgRobert Hensing (roberth)2.27 is worth another try with that method, because it uses a different build system, so it might not have the 2.25 problem22:11:56
7 Apr 2025
@o-santi:matrix.orgLeonardo Santiago what do you mean? What does .overrideAllMesonComponents do? 14:05:11
@o-santi:matrix.orgLeonardo Santiago * what do you mean? What does .overrideAllMesonComponents do and how am I supposed to use it? 14:05:18
@o-santi:matrix.orgLeonardo Santiago Okay, it finally clicked to me that the reason that I had no line number and source information was because I was compiling nix-expr without source information. When I managed to build it with debug information, it stopped segfaulting :D 16:22:22
@o-santi:matrix.orgLeonardo Santiago

This is what I did, and if I change nix-expr-c for nix.libs.nix-expr-c it goes back to segfaulting

mkDebug = p: p.overrideAttrs (old: old // {                                                                                                           
          dontStrip = true;                                                                                                                                   
          mesonBuildType = "debugoptimized";                                                                                                                  
          NIX_CFLAGS_COMPILE = toString (old.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og";                                                                        
});                                                                                                                                                   
nix-expr-c = (mkDebug nix.libs.nix-expr-c).override { nix-expr = mkDebug nix.libs.nix-expr; };
nix-deps = map mkDebug [ nix.libs.nix-store-c nix-expr-c nix.libs.nix-util-c ];
16:23:56
@o-santi:matrix.orgLeonardo Santiago *

This is what I did, and if I change nix-expr-c for nix.libs.nix-expr-c it goes back to segfaulting

inputs = { 
  nix.url` = "github:nixos/nix/2.27.0;
};
outputs = { ... } : { 
...
nix = inputs.nix.packages.${system}.nix;
mkDebug = p: p.overrideAttrs (old: old // {                                                                                                           
          dontStrip = true;                                                                                                                                   
          mesonBuildType = "debugoptimized";                                                                                                                  
          NIX_CFLAGS_COMPILE = toString (old.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og";                                                                        
});                                                                                                                                                   
nix-expr-c = (mkDebug nix.libs.nix-expr-c).override { nix-expr = mkDebug nix.libs.nix-expr; };
nix-deps = map mkDebug [ nix.libs.nix-store-c nix-expr-c nix.libs.nix-util-c ];
16:24:48
@o-santi:matrix.orgLeonardo Santiago *

This is what I did, and if I change nix-expr-c for nix.libs.nix-expr-c it goes back to segfaulting

inputs = { 
  nix.url` = "github:nixos/nix/2.27.0";
};
outputs = { ... } : { 
...
nix = inputs.nix.packages.${system}.nix;
mkDebug = p: p.overrideAttrs (old: old // {                                                                                                           
          dontStrip = true;                                                                                                                                   
          mesonBuildType = "debugoptimized";                                                                                                                  
          NIX_CFLAGS_COMPILE = toString (old.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og";                                                                        
});                                                                                                                                                   
nix-expr-c = (mkDebug nix.libs.nix-expr-c).override { nix-expr = mkDebug nix.libs.nix-expr; };
nix-deps = map mkDebug [ nix.libs.nix-store-c nix-expr-c nix.libs.nix-util-c ];
16:24:52
@o-santi:matrix.orgLeonardo Santiago *

This is what I did, and if I change nix-expr-c for nix.libs.nix-expr-c it goes back to segfaulting

inputs = { 
  nix.url = "github:nixos/nix/2.27.0";
};
outputs = { ... } : { 
...
nix = inputs.nix.packages.${system}.nix;
mkDebug = p: p.overrideAttrs (old: old // {                                                                                                           
          dontStrip = true;                                                                                                                                   
          mesonBuildType = "debugoptimized";                                                                                                                  
          NIX_CFLAGS_COMPILE = toString (old.NIX_CFLAGS_COMPILE or "") + " -ggdb -Og";                                                                        
});                                                                                                                                                   
nix-expr-c = (mkDebug nix.libs.nix-expr-c).override { nix-expr = mkDebug nix.libs.nix-expr; };
nix-deps = map mkDebug [ nix.libs.nix-store-c nix-expr-c nix.libs.nix-util-c ];
16:24:58
@o-santi:matrix.orgLeonardo Santiago Robert Hensing (roberth): Is this UB? 16:27:29
@o-santi:matrix.orgLeonardo Santiago * Robert Hensing (roberth): Could this be UB? 16:41:45
@o-santi:matrix.orgLeonardo Santiago

Removing mesonBuildType and NIX_CFLAGS_COMPILE and keeping only dontStrip finally made it segfault with debug symbols:

#0  0x00007ffff68ac8d7 in nix::EvalState::callFunction (this=0x5b7b00, fun=..., args=..., vRes=..., pos=...) at <artificial>:1507                           
#1  0x00007ffff68b034c in nix::ExprCall::eval (this=0x648580, state=..., env=..., v=...) at ../stl_iterator.h:90                                            
#2  0x00007ffff68af62b in nix::EvalState::forceValue (pos=..., v=..., this=0x5b7b00) at /build/source/src/libexpr/build/value.hh:96                         
#3  nix::ExprSelect::eval (this=0x648540, state=..., env=..., v=...) at <artificial>:1443                                                                   
#4  0x00007ffff68af158 in nix::EvalState::forceValue (pos=..., v=..., this=0x5b7b00) at /build/source/src/libexpr/build/value.hh:96                         
#5  nix::ExprVar::eval (this=<optimized out>, state=..., env=..., v=...) at <artificial>:1372                                                               
#6  0x00007ffff68af3d9 in nix::ExprSelect::eval (this=0x17f49f0, state=..., env=..., v=...) at <artificial>:1402                                            
#7  0x00007ffff68b0ed1 in nix::ExprOpConcatLists::eval (this=0x17f4a30, state=..., env=..., v=...) at <artificial>:1948                                     
#8  0x00007ffff6905617 in nix::EvalState::forceValue (pos=..., v=..., this=<optimized out>, this=<optimized out>, v=..., pos=...)                           
    at /nix/store/9c9h0zbqa9xfk6pk4387l1h9pj28r692-nix-util-2.27.0-dev/include/nix/aligned_buffer.h:96                                                      
#9  nix::EvalState::forceList (errorCtx=..., pos=..., v=..., this=<optimized out>, this=<optimized out>, v=..., pos=..., errorCtx=...)                      
    at /nix/store/9c9h0zbqa9xfk6pk4387l1h9pj28r692-nix-util-2.27.0-dev/include/nix/aligned_buffer.h:136                                                     
#10 nix::prim_foldlStrict (state=..., pos=..., args=0x7fffff670c00, v=...) at /build/source/src/libexpr/build/new_allocator.h:3485
16:48:55
@roberthensing:matrix.orgRobert Hensing (roberth) I've never seen something like <artificial>:1507 before 20:23:52

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