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Jade Lovelace
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content/posts/debugging-nix-package-building.md
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content/posts/debugging-nix-package-building.md
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date = "2023-10-19"
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draft = false
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path = "/blog/debugging-nix-package-building"
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tags = ["nix"]
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title = "Debugging Nix build inconsistencies: manual vs automatic build"
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+++
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This is a quick post about an issue I debugged in a package build that didn't
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matter, which may however still be instructive.
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I was packaging [ios-webkit-debug-proxy] for NixOS, not having noticed it was
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already there (oops! I noticed after fixing my build), and I ran into an issue
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where my build was not working, so I tried building it manually, and it worked.
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Wat.
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[ios-webkit-debug-proxy]: https://github.com/google/ios-webkit-debug-proxy
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Here is the `package.nix` I was using:
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```nix
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{ stdenv, fetchFromGitHub, autoconf, automake, libtool, pkg-config, libimobiledevice }:
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stdenv.mkDerivation {
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pname = "ios-webkit-debug-proxy";
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version = "unstable-2023-09-23";
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src = fetchFromGitHub {
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owner = "google";
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repo = "ios-webkit-debug-proxy";
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rev = "c5d2e959043293ee7d0587618af2bcc4dde7feef";
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sha256 = "sha256-vBrYbCzH83BqB7uTybEmF/yp2N40qJp+97lu+nKulXM=";
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};
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preConfigure = ''
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NOCONFIGURE=1 ./autogen.sh
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'';
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# These are the fix ;)
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# dontAddDisableDepTrack = true;
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# dontDisableStatic = true;
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nativeBuildInputs = [
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autoconf
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automake
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libtool
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pkg-config
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];
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buildInputs = [
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libimobiledevice
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];
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}
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```
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This is what was happening:
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```
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$ nix build -L --impure --expr 'with import <nixpkgs> {}; (callPackage ../package.nix {})'
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error: builder for '/nix/store/0ci7kyfmcbbxq5g5svzlp316y98vvgng-ios-webkit-debug-proxy-unstable-202
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3-09-23.drv' failed with exit code 2;
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last 10 log lines:
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> gcc -DHAVE_CONFIG_H -I. -I.. -I../include -I../src -I/nix/store/n4wwsam3rk07mghksrp46sn
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lmf7cdi33-libimobiledevice-1.3.0+date=2023-04-30-dev/include -I/nix/store/cykm9dyc58l9jfilgrd0aflfn
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znzrc39-libplist-2.3.0-dev/include -I/nix/store/cykm9dyc58l9jfilgrd0aflfnznzrc39-libplist-2.3.0-dev
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/include -I/nix/store/3g9fg7cdh3234xi5b14v1fp135q8ix49-libusbmuxd-2.0.2+date=2023-04-30/include -I/
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nix/store/lqsfdc4p9vax2h55csza9iw46zjpghl6-openssl-3.0.10-dev/include -g -O2 -Wall -Werror -c -o ..
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/src/socket_manager.o ../src/socket_manager.c
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> ../src/socket_manager.c:34:10: fatal error: socket_manager.h: No such file or directory
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> 34 | #include "socket_manager.h"
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> | ^~~~~~~~~~~~~~~~~~
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> compilation terminated.
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> make[2]: *** [Makefile:464: ../src/socket_manager.o] Error 1
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> make[2]: Leaving directory '/build/source/examples'
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> make[1]: *** [Makefile:410: all-recursive] Error 1
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> make[1]: Leaving directory '/build/source'
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> make: *** [Makefile:342: all] Error 2
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For full logs, run 'nix log /nix/store/0ci7kyfmcbbxq5g5svzlp316y98vvgng-ios-webkit-debug-pro
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$ nix develop --impure --expr 'with import <nixpkgs> {}; (callPackage ../package.nix {})'
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$ cd manually-cloned-src
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$ NOCONFIGURE=1 ./autogen.sh
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$ make -j8
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(succeeds)
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```
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So, we know that the issue is one of two problems, either:
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* The build relies on an impurity of not being in the sandbox. This symptom
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makes no sense for that, but if this is your problem, use [breakpointHook]
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and [cntr] to enter the build environment.
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* There is some inconsistency in the actual build process. If this is the case,
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we *believe*, in this case, that Make must be doing something different,
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since the `../src/socket_manager.c` target does not appear as being compiled
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in the successful build (it is only used in the link stage!).
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One way to verify this is to [manually run the build] and see if the
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behaviour still is occurring. I expect that in this case, it would have
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(extremely hilariously!) caused the bug to reproduce differently due to
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`dontAddDisableDepTrack` being automatically set in nix shells.
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[manually run the build]: https://jade.fyi/blog/building-nix-derivations-manually/
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[breakpointHook]: https://nixos.org/manual/nixpkgs/stable/#breakpointhook
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[cntr]: https://github.com/Mic92/cntr
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What I did was to run the failing `nix build` with `--keep-failed`, then looked
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in `/tmp/nix-build-*` for the failed tree. From this failed tree I found I
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could reproduce the failure again using `make`.
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Let's figure out what differs between these directory trees. To do this, we can
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use [diffoscope], an advanced diff tool that can compare directory structures
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with nice structural diffs.
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[diffoscope]: https://diffoscope.org/
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We believe that it's an inconsistent Makefile, so let's dig through the output and
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test that:
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```
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--- ./src/Makefile
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+++ /tmp/nix-build-ios-webkit-debug-proxy-unstable-2023-09-23.drv-0/source/src/Makefile
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+#include ./$(DEPDIR)/sha1.Plo # am--include-marker
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+#include ./$(DEPDIR)/socket_manager.Plo # am--include-marker
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+#include ./$(DEPDIR)/webinspector.Plo # am--include-marker
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+#include ./$(DEPDIR)/websocket.Plo # am--include-marker
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-include ./$(DEPDIR)/base64.Plo # am--include-marker
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-include ./$(DEPDIR)/char_buffer.Plo # am--include-marker
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-include ./$(DEPDIR)/device_listener.Plo # am--include-marker
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```
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Interesting, so the Makefile would have included some things in the successful
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build that were not in the other build. For context, the way that autotools
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works is:
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* `Makefile.am` defines the rules to generate the `Makefile.in`, using
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`automake`.
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* `Makefile.in` is templated to `Makefile` when `./configure` is run.
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Autotools is a turducken of string templating on more string templating on more
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string templating. String templates generate string templates which generate
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new string templates which finally generate a Makefile.
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In a distribution tarball, `autoconf` and `automake` were already run so
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`configure` and `Makefile.in` will be included in the tarball.
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If we look for where that probably came from in `src/Makefile.in`, we find:
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```
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@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/webinspector.Plo@am__quote@ # am--include-marker
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@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/websocket.Plo@am__quote@ # am--include-marker
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```
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We also observe in `config.status`:
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```
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-AMDEPBACKSLASH='\'
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-AMDEP_FALSE='#'
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-AMDEP_TRUE=''
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+AMDEPBACKSLASH=''
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+AMDEP_FALSE=''
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+AMDEP_TRUE='#'
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...
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-am_cv_CC_dependencies_compiler_type=gcc3
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+am_cv_CC_dependencies_compiler_type=none
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...
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-am__fastdepCC_FALSE='#'
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-am__fastdepCC_TRUE=''
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+am__fastdepCC_FALSE=''
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+am__fastdepCC_TRUE='#'
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```
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and in `config.log`:
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```
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This file contains any messages produced by compilers while
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running configure, to aid debugging if configure makes a mistake.
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It was created by ios_webkit_debug_proxy configure 1.9.0, which was
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generated by GNU Autoconf 2.71. Invocation command line was
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- $ ./configure
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+ $ ./configure --disable-static --disable-dependency-tracking --prefix=/nix/store/19fsba4r1c0jwsqcdjs76y83ndah7vyh-ios-webkit-debug-proxy-unstable-2023-09-23
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...
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configure:4539: checking dependency style of gcc
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-configure:4651: result: gcc3
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+configure:4651: result: none
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...
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configure:13417: checking whether to build static libraries
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-configure:13421: result: yes
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+configure:13421: result: no
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```
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Well. It looks like the synthesis of all of this is that different configure
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arguments were used, causing this. But why?!
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As is often the case,
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[`setup.sh`](https://github.com/nixos/nixpkgs/blob/d5139e30179237517e09174561b82bdacc1c2a03/pkgs/stdenv/generic/setup.sh#L1262-L1276)
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is responsible:
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```
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$ rg -C3 -- --disable-dependency-track ~/dev/nixpkgs
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/home/jade/dev/nixpkgs/pkgs/stdenv/generic/setup.sh
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1260- fi
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1261-
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1262- if [[ -f "$configureScript" ]]; then
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1263: # Add --disable-dependency-tracking to speed up some builds.
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1264- if [ -z "${dontAddDisableDepTrack:-}" ]; then
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1265- if grep -q dependency-tracking "$configureScript"; then
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1266: prependToVar configureFlags --disable-dependency-tracking
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1267- fi
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1268- fi
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1269-
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```
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and likewise for `--disable-static`, if `dontDisableStatic` is unset.
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Indeed, changing the derivation to set both `dontAddDisableDepTrack` and
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`dontDisableStatic` fixes the build.
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This all took altogether far longer than it should have to derive a hypothesis
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for why it was broken.
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