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.) */
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