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12796 :: Data inadvertently wiped when combining `=copy` with iterators

• giuliano (orginal) [2025-04-15T19:55:36+02:00] view original

  Hi there,

  In tracking down a bug in my code, I've observed the following behavior I can't seem to be able to explain (Nim 2.x, refc only):

  
  import std/sequtils
  
  type
    SomeType* = object
  
  proc `=copy`*(dest: var SomeType, src: SomeType) =
    discard
  
  type
    PeerId* = object
      data: seq[int]
    
    SwarmPeerCtx* = object
      id*: PeerId
      blocks*: SomeType
    
    Swarm* = ref object of RootObj
      peers*: seq[SwarmPeerCtx]
  
  iterator peerIt(self: Swarm): SwarmPeerCtx =
    for i in 0 .. high(self.peers):
      yield self.peers[i]
  
  proc getPeers*(self: Swarm): seq[SwarmPeerCtx] =
    var peers = self.peerIt().toSeq
    return peers
  
  let
    swarm = Swarm()
    peerId = PeerId(data: @[1, 2, 3, 4])
  
  swarm.peers.add(SwarmPeerCtx(id: peerId))
  
  doAssert (swarm.peerIt().toSeq)[0].id.data == @[1, 2, 3, 4] # Passes
  doAssert swarm.getPeers()[0].id.data == @[1, 2, 3, 4] # Fails, .data is empty
  Run

  Removing the =copy proc fixes it. More weirdly, however, moving the type declaration for SomeType into the block with the other types; e.g.:

  
  import std/sequtils
  
  type
    PeerId* = object
      data: seq[int]
    
    SomeType* = object
    
    SwarmPeerCtx* = object
      id*: PeerId
      blocks*: SomeType
    
    Swarm* = ref object of RootObj
      peers*: seq[SwarmPeerCtx]
  
  proc `=copy`*(dest: var SomeType, src: SomeType) =
    discard
  
  ...
  Run

  also fixes it. The problem only seems to affect refc, and it doesn't happen in older versions of NIm (tried 1.6.14, works fine). Am I doing anything silly here, or should I file this as a bug?

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• morturo (orginal) [2025-04-15T22:40:22+02:00] view original

  You have an empty copy proc, and the problem is that the value isn't being copied? Also, I always thought copy and others were meant to be used with ARC/ORC.

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• giuliano (orginal) [2025-04-15T23:10:03+02:00] view original

  The =copy is defined for IrrelevantType, but what's getting wiped out is the string value in Inner. And this is just a minimal example: any type which defines a =copy seems to trigger this behavior (including, say, Table in std/tables).

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• Araq (orginal) [2025-04-15T23:21:22+02:00] view original

  Custom hooks are not really supported with refc. We might fix these bugs anyway.

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• giuliano (orginal) [2025-04-15T23:32:10+02:00] view original

  Yeah my original code actually uses a Table and doesn't define any custom hooks on its own.

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• Araq (orginal) [2025-04-16T08:04:40+02:00] view original

  Do you think it'd be helpful if I file a bug for this?

  Definitely.

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