Make GnuPG automatically find a pinentry installed by Guix. Try using
$HOME or, if that variable is not set, use the system password database,
or fall back to looking in "/".

More information:

https://bugs.gnu.org/24076

diff --git a/common/homedir.c b/common/homedir.c
index 4b6e46e88..f7ae68ba5 100644
--- a/common/homedir.c
+++ b/common/homedir.c
@@ -33,6 +33,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <pwd.h>
 
 #ifdef HAVE_W32_SYSTEM
 #include <winsock2.h>   /* Due to the stupid mingw64 requirement to
@@ -67,6 +68,10 @@
  * gnupg_homedir and gnupg_set_homedir.  Malloced.  */
 static char *the_gnupg_homedir;
 
+/* The user's home directory. Used in Guix to help GnuPG find the
+ * pinentry. */
+static char *the_user_homedir;
+
 /* Flag indicating that home directory is not the default one.  */
 static byte non_default_homedir;
 
@@ -509,6 +514,25 @@ gnupg_homedir (void)
   return the_gnupg_homedir;
 }
 
+/* Return the user's home directory */
+const char *
+user_homedir (void)
+{
+  const char *dir;
+  dir = getenv("HOME");
+  if (dir == NULL)
+    {
+       struct passwd *pw = NULL;
+       pw = getpwuid (getuid ());
+       if (pw != NULL)
+         dir = pw->pw_dir;
+       else
+         dir = "/";
+    }
+  if (!the_user_homedir)
+    the_user_homedir = make_absfilename (dir, NULL);
+  return the_user_homedir;
+}
 
 /* Return whether the home dir is the default one.  */
 int
@@ -971,6 +995,7 @@ get_default_pinentry_name (int reset)
   } names[] = {
     /* The first entry is what we return in case we found no
        other pinentry.  */
+    { user_homedir, "/.guix-profile/bin/pinentry" },
     { gnupg_bindir, DIRSEP_S "pinentry" EXEEXT_S },
 #ifdef HAVE_W32_SYSTEM
     /* Try Gpg4win directory (with bin and without.) */
ion value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=92c3d1c3d059d662f2e5dc1ecdce99f3eb13bfd0'>root</a>/<a href='/guix/log/nix?id=92c3d1c3d059d662f2e5dc1ecdce99f3eb13bfd0'>nix</a>/<a href='/guix/log/nix/libstore?id=92c3d1c3d059d662f2e5dc1ecdce99f3eb13bfd0'>libstore</a>/<a href='/guix/log/nix/libstore/local-store.hh?id=92c3d1c3d059d662f2e5dc1ecdce99f3eb13bfd0'>local-store.hh</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/nix/libstore/local-store.hh?id=92c3d1c3d059d662f2e5dc1ecdce99f3eb13bfd0&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-12-08 22:30:08 +0100'>2020-12-08</span></td><td><a href='/guix/commit/nix/libstore/local-store.hh?id=a618a8c6203d4cf57f12873a86797b8685b11e14'>daemon: Factorize substituter agent spawning.</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/libstore/local-store.hh (class LocalStore)[substituter]: New
method.
[runningSubstituter]: Turn into a shared_ptr.
* nix/libstore/local-store.cc (LocalStore::querySubstitutablePaths):
Call 'substituter' instead of using inline code.
(LocalStore::querySubstitutablePathInfos): Likewise.
(LocalStore::substituter): New method.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-08 22:30:08 +0100'>2020-12-08</span></td><td><a href='/guix/commit/nix/libstore/local-store.hh?id=79c6614f58a57b985daf8940766319e440311db0'>daemon: Use 'Agent' to spawn 'guix substitute --query'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/libstore/local-store.hh (RunningSubstituter): Remove.
(LocalStore)[runningSubstituter]: Change to unique_ptr&lt;Agent&gt;.
[setSubstituterEnv, didSetSubstituterEnv]: Remove.
[getLineFromSubstituter, getIntLineFromSubstituter]: Take an 'Agent'.
* nix/libstore/local-store.cc (LocalStore::~LocalStore): Remove
reference to 'runningSubstituter'.
(LocalStore::setSubstituterEnv, LocalStore::startSubstituter): Remove.
(LocalStore::getLineFromSubstituter): Adjust to 'run' being an 'Agent'.
(LocalStore::querySubstitutablePaths): Spawn substituter agent if
needed.  Adjust to 'Agent' interface.
(LocalStore::querySubstitutablePathInfos): Likewise.
* nix/libstore/build.cc (SubstitutionGoal::tryToRun): Remove call to
'setSubstituterEnv' and add 'setenv' call for "_NIX_OPTIONS" instead.
(SubstitutionGoal::finished): Remove 'readLine' call for 'dummy'.
* guix/scripts/substitute.scm (%allow-unauthenticated-substitutes?):
Remove second argument to 'make-parameter'.
(process-query): Call 'warn-about-missing-authentication'
when (%allow-unauthenticated-substitutes?) is #t.
(guix-substitute): Wrap body in 'parameterize'.  Set 'guix-warning-port'
too.  No longer exit when 'substitute-urls' returns the empty list.  No
longer print newline initially.
* tests/substitute.scm (test-quit): Parameterize 'current-error-port' to
account for the port changes in 'guix-substitute'.
</span></span></td><td>Ludovic Courtès</td></tr>