pahole: patch to force single-threaded mode if reproducibility is des…

…ired

pahole's processes DWARF -> BTF in a pseudo-random order if multi
threading is enabled (default in the Linux kernel). This can be fixed in dependent
derivation by making sure users don't ever pass "-j" or "-j N", but it
seems easier and safer to just detect whether reprodubility is desired
(by proxy: if SOURCE_DATE_EPOCH is set) and ignore the multi-threading
flag.

pkgs/development/tools/misc/pahole/default.nix

@@ -26,6 +26,8 @@ stdenv.mkDerivation rec {
     musl-obstack
   ];
 
+  patches = [ ./threading-reproducibility.patch ];
+
   # Put libraries in "lib" subdirectory, not top level of $out
   cmakeFlags = [ "-D__LIB=lib" "-DLIBBPF_EMBEDDED=OFF" ];
 

pkgs/development/tools/misc/pahole/threading-reproducibility.patch

@@ -0,0 +1,18 @@
+diff --git a/pahole.c b/pahole.c
+index 6fc4ed6..a4e306f 100644
+--- a/pahole.c
++++ b/pahole.c
+@@ -1687,8 +1687,11 @@ static error_t pahole__options_parser(int key, char *arg,
+ 		  class_name = arg;			break;
+ 	case 'j':
+ #if _ELFUTILS_PREREQ(0, 178)
+-		  conf_load.nr_jobs = arg ? atoi(arg) :
+-					    sysconf(_SC_NPROCESSORS_ONLN) * 1.1;
++		  // Force single thread if reproducibility is desirable.
++		  if (!getenv("SOURCE_DATE_EPOCH")) {
++			  conf_load.nr_jobs = arg ? atoi(arg) :
++						    sysconf(_SC_NPROCESSORS_ONLN) * 1.1;
++		  }
+ #else
+ 		  fputs("pahole: Multithreading requires elfutils >= 0.178. Continuing with a single thread...\n", stderr);
+ #endif