man-db: remove NixOS-specific configuration

When using --with-config-file, all man-db programs completely ignore the
systemwide configuration in /etc/man_db.conf: it means on NixOS there is
no way to change the configuration without rebuilding man-db, which in
turn causes a mass-rebuild.

To solve this problem this commit removes the NixOS-specific
configuration in man-db, which wasn't the appropriate place to begin
with: the package is expected to work on non-NixOS systems as well. Also
a small patch now ensure /etc/man_db.conf is used, if available, before
the bundled configuration.

pkgs/tools/misc/man-db/default.nix

@@ -15,18 +15,17 @@ stdenv.mkDerivation rec {
   buildInputs = [ libpipeline db groff ]; # (Yes, 'groff' is both native and build input)
   checkInputs = [ libiconv /* for 'iconv' binary */ ];
 
+  patches = [ ./systemwide-man-db-conf.patch ];
+
   postPatch = ''
     # Remove all mandatory manpaths. Nixpkgs makes no requirements on
     # these directories existing.
     sed -i 's/^MANDATORY_MANPATH/# &/' src/man_db.conf.in
 
-    # Add Nixpkgs and NixOS-related manpaths
+    # Add Nix-related manpaths
-    echo "MANPATH_MAP	/run/current-system/sw/bin		/run/current-system/sw/share/man" >> src/man_db.conf.in
-    echo "MANPATH_MAP	/run/wrappers/bin			/run/current-system/sw/share/man" >> src/man_db.conf.in
     echo "MANPATH_MAP	/nix/var/nix/profiles/default/bin	/nix/var/nix/profiles/default/share/man" >> src/man_db.conf.in
 
     # Add mandb locations for the above
-    echo "MANDB_MAP	/run/current-system/sw/share/man	/var/cache/man/nixos" >> src/man_db.conf.in
     echo "MANDB_MAP	/nix/var/nix/profiles/default/share/man	/var/cache/man/nixpkgs" >> src/man_db.conf.in
   '';
 
@@ -34,7 +33,6 @@ stdenv.mkDerivation rec {
     "--disable-setuid"
     "--disable-cache-owner"
     "--localstatedir=/var"
-    # Don't try /etc/man_db.conf by default, so we avoid error messages.
     "--with-config-file=${placeholder "out"}/etc/man_db.conf"
     "--with-systemdtmpfilesdir=${placeholder "out"}/lib/tmpfiles.d"
     "--with-systemdsystemunitdir=${placeholder "out"}/lib/systemd/system"

pkgs/tools/misc/man-db/systemwide-man-db-conf.patch

@@ -0,0 +1,39 @@
+commit 9089291006a4258c39c75a920ad536b61504251a
+Author: rnhmjoj <rnhmjoj@inventati.org>
+Date:   Fri May 1 19:32:15 2020 +0200
+
+    check for systemwide man_db.conf before the bundled one
+
+diff --git a/src/manp.c b/src/manp.c
+index 5441339..0bbf566 100644
+--- a/src/manp.c
++++ b/src/manp.c
+@@ -841,18 +841,24 @@ void read_config_file (bool optional)
+ 	}
+ 
+ 	if (getenv ("MAN_TEST_DISABLE_SYSTEM_CONFIG") == NULL) {
+-		config_file = fopen (CONFIG_FILE, "r");
++		const char *config_filepath;
++		if (access ("/etc/man_db.conf", F_OK) != -1) {
++			config_filepath = "/etc/man_db.conf";
++		} else {
++			config_filepath = CONFIG_FILE;
++		}
++		config_file = fopen (config_filepath, "r");
+ 		if (config_file == NULL) {
+ 			if (optional)
+ 				debug ("can't open %s; continuing anyway\n",
+-				       CONFIG_FILE);
++				       config_filepath);
+ 			else
+ 				error (FAIL, 0,
+ 				       _("can't open the manpath "
+ 					 "configuration file %s"),
+-				       CONFIG_FILE);
++				       config_filepath);
+ 		} else {
+-			debug ("From the config file %s:\n", CONFIG_FILE);
++			debug ("From the config file %s:\n", config_filepath);
+ 
+ 			add_to_dirlist (config_file, 0);
+ 			fclose (config_file);