If a readTree node is a derivation, readTree will always call overrideAttrs on it, which will silently discard information.
I think I understand the rationale for why readTree prefers overrideAttrs when available - it avoids inadvertently changing a derivation by merging arbitrary values on top of it, while it is much safer to leverage passthru which avoids any inadvertent cache-busting.
Unfortunately, this ends up causing some very non-intuitive behavior. Anything that is merged on top of a derivation using simple tools like // gets silently discarded, which can be very surprising.
For a contrived example, consider a readTree node that looks like this:
# customCowsay.nix
{ pkgs, ...}:
pkgs.cowsay // { hello = "here is a value that I am very explicitly merging on top"; }Under readTree, if I try to reference customCowsay.hello, I will get an error that the attribute hello doesn't exist. How can that be? I most certainly added it! It takes a person well versed in nix, and willing to go hunting, to understand that this problem is that our value is getting transformed by calling overrideAttrs.
For a more concrete example, consider lib.hiPrio in nixpkgs. It sets meta.priority on an attrset using simple merging. So if we try to do something such as:
# customCowsay.nix
{ pkgs, ...}:
pkgs.lib.hiPrio pkgs.cowsayThen the priority will be lost.
If a readTree node is a derivation, readTree will always call
overrideAttrson it, which will silently discard information.I think I understand the rationale for why readTree prefers
overrideAttrswhen available - it avoids inadvertently changing a derivation by merging arbitrary values on top of it, while it is much safer to leveragepassthruwhich avoids any inadvertent cache-busting.Unfortunately, this ends up causing some very non-intuitive behavior. Anything that is merged on top of a derivation using simple tools like
//gets silently discarded, which can be very surprising.For a contrived example, consider a readTree node that looks like this:
Under readTree, if I try to reference
customCowsay.hello, I will get an error that the attributehellodoesn't exist. How can that be? I most certainly added it! It takes a person well versed in nix, and willing to go hunting, to understand that this problem is that our value is getting transformed by callingoverrideAttrs.For a more concrete example, consider
lib.hiPrioin nixpkgs. It setsmeta.priorityon an attrset using simple merging. So if we try to do something such as:Then the priority will be lost.