buildGoModule: don't inherit build hooks when building go-modules (#225349)

* buildGoModule: don't inherit postBuild hook when building go-modules

This is a slight revert of 5ce647b8bf
(#212800).

Inheriting these hooks in the `.go-modules` derivation can be confusing:
One doesn't expect them to run when generating the fixed output modules
derivation, but only on the main derivation. A `postBuild` hook that
adds some files to $out will cause a very hard to debug issue[1].

This commit adds support for a dedicated `modPostBuild` hook that will
be used only by the derivation building `.go-modules`. Additionally,
`go.section.md` now explains these attributes behavior better.

[1]:
https://discourse.nixos.org/t/cant-update-a-go-package-getting-go-inconsistent-vendoring/27063/6

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

doc/languages-frameworks/go.section.md

@@ -20,6 +20,7 @@ In the following is an example expression using `buildGoModule`, the following a
 
   To obtain the actual hash, set `vendorHash = lib.fakeSha256;` and run the build ([more details here](#sec-source-hashes)).
 - `proxyVendor`: Fetches (go mod download) and proxies the vendor directory. This is useful if your code depends on c code and go mod tidy does not include the needed sources to build or if any dependency has case-insensitive conflicts which will produce platform dependant `vendorHash` checksums.
+- `modPostBuild`: Shell commands to run after the build of the go-modules executes `go mod vendor`, and before calculating fixed output derivation's `vendorHash` (or `vendorSha256`). Note that if you change this attribute, you need to update `vendorHash` (or `vendorSha256`) attribute.
 
 ```nix
 pet = buildGoModule rec {
@@ -114,7 +115,16 @@ done
 
 ## Attributes used by the builders {#ssec-go-common-attributes}
 
-Both `buildGoModule` and `buildGoPackage` can be tweaked to behave slightly differently, if the following attributes are used:
+Many attributes [controlling the build phase](#variables-controlling-the-build-phase) are respected by both `buildGoModule` and `buildGoPackage`. Note that `buildGoModule` reads the following attributes also when building the `vendor/` go-modules fixed output derivation as well:
+
+- [`sourceRoot`](#var-stdenv-sourceRoot)
+- [`prePatch`](#var-stdenv-prePatch)
+- [`patches`](#var-stdenv-patches)
+- [`patchFlags`](#var-stdenv-patchFlags)
+- [`postPatch`](#var-stdenv-postPatch)
+- [`preBuild`](#var-stdenv-preBuild)
+
+In addition to the above attributes, and the many more variables respected also by `stdenv.mkDerivation`, both `buildGoModule` and `buildGoPackage` respect Go-specific attributes that tweak them to behave slightly differently:
 
 ### `ldflags` {#var-go-ldflags}
 

pkgs/build-support/go/module.nix

@@ -83,12 +83,16 @@ let
     inherit (args) src;
     inherit (go) GOOS GOARCH;
 
+    # The following inheritence behavior is not trivial to expect, and some may
+    # argue it's not ideal. Changing it may break vendor hashes in Nixpkgs and
+    # out in the wild. In anycase, it's documented in:
+    # doc/languages-frameworks/go.section.md
     prePatch = args.prePatch or "";
     patches = args.patches or [];
     patchFlags = args.patchFlags or [];
     postPatch = args.postPatch or "";
     preBuild = args.preBuild or "";
-    postBuild = args.postBuild or "";
+    postBuild = args.modPostBuild or "";
     sourceRoot = args.sourceRoot or "";
 
     GO111MODULE = "on";