New decrypt flags GPGME_DECRYPT_LISTONLY.HEADmaster

* src/gpgme.h.in (GPGME_DECRYPT_LISTONLY): New.
* src/decrypt.c (op_data_t): Add member list_only.
(_gpgme_decrypt_status_handler): Do not return NO_DATA in list_only
mode.
(_gpgme_op_decrypt_init_result): Add arg flags and set the list_only
flag.
(_gpgme_decrypt_start): Pss flags to the init function.
* src/decrypt-verify.c (decrypt_verify_start): Ditto.
* src/engine-gpg.c (gpg_decrypt): Add --list-only if flag is set.

* tests/run-decrypt.c (main): Add option --list-only.

8 files changed, 36 insertions, 7 deletions

diff --git a/NEWS b/NEWS
index 7544d42c..6de14424 100644
--- a/NEWS
+++ b/NEWS
@@ -7,6 +7,9 @@ Noteworthy changes in version 2.0.0 (unreleased)
  * New function gpgme_op_random_value to get a cryptographically
    strong unsigned integer random value.   [T6694]
 
+ * New decrypt flag to skip the actual decryption so that information
+   about the recipients can be retrieved.
+
  * Removed the gpgme_attr_t enums and their functions which were
    deprecated since 2003.  [rMd54d6eaa64]
 
@@ -22,6 +25,7 @@ Noteworthy changes in version 2.0.0 (unreleased)
  gpgme_op_random_value                   NEW.
  GPGME_RANDOM_MODE_NORMAL                NEW.
  GPGME_RANDOM_MODE_ZBASE32               NEW.
+ GPGME_DECRYPT_LISTONLY                  NEW.
  gpgme_attr_t                            REMOVED.
  gpgme_get_sig_ulong_attr                REMOVED.
  gpgme_get_sig_string_attr               REMOVED.
diff --git a/doc/gpgme.texi b/doc/gpgme.texi
index fa93083c..bc4b1ac5 100644
--- a/doc/gpgme.texi
+++ b/doc/gpgme.texi
@@ -5830,6 +5830,15 @@ be an OpenPGP message with only the encryption layer removed.  This
 requires GnuPG 2.1.12 and works only for OpenPGP.  This is the
 counterpart to @code{GPGME_ENCRYPT_WRAP}.
 
+@item GPGME_DECRYPT_LIST
+@since{2.0.0}
+
+The @code{GPGME_DECRYPT_LIST} symbol specifies that the actual
+decryption step of an OpenPGP message shall be skipped.  This can be
+used to information on the keyids of the recipients of some encrypted
+data.  Note that most other result items have no or no useful
+information in this case.
+
 @end table
 
 The function returns the error codes as described for
diff --git a/src/decrypt-verify.c b/src/decrypt-verify.c
index 3ff15feb..94ebeb90 100644
--- a/src/decrypt-verify.c
+++ b/src/decrypt-verify.c
@@ -62,7 +62,7 @@ decrypt_verify_start (gpgme_ctx_t ctx, int synchronous,
   if (err)
     return err;
 
-  err = _gpgme_op_decrypt_init_result (ctx, plain);
+  err = _gpgme_op_decrypt_init_result (ctx, plain, flags);
   if (err)
     return err;
 
diff --git a/src/decrypt.c b/src/decrypt.c
index e5d2f7c7..c6030eef 100644
--- a/src/decrypt.c
+++ b/src/decrypt.c
@@ -44,7 +44,10 @@ typedef struct
 
   int okay;
 
-  /* A flag telling that the a decryption failed and two optional error
+  /* Indicates that list only mode is active.  */
+  int list_only;
+
+  /* A flag telling that the decryption failed and two optional error
    * codes to further specify the failure for public key decryption and
    * symmetric decryption.  */
   int failed;
@@ -419,7 +422,7 @@ _gpgme_decrypt_status_handler (void *priv, gpgme_status_code_t code,
           /* Generic decryption failed error code.  */
           return gpg_error (GPG_ERR_DECRYPT_FAILED);
         }
-      else if (!opd->okay)
+      else if (!opd->okay && !opd->list_only)
         {
           /* No data was found.  */
           return gpg_error (GPG_ERR_NO_DATA);
@@ -540,7 +543,8 @@ decrypt_status_handler (void *priv, gpgme_status_code_t code, char *args)
 
 
 gpgme_error_t
-_gpgme_op_decrypt_init_result (gpgme_ctx_t ctx, gpgme_data_t plaintext)
+_gpgme_op_decrypt_init_result (gpgme_ctx_t ctx, gpgme_data_t plaintext,
+                               gpgme_decrypt_flags_t flags)
 {
   gpgme_error_t err;
   void *hook;
@@ -552,6 +556,7 @@ _gpgme_op_decrypt_init_result (gpgme_ctx_t ctx, gpgme_data_t plaintext)
   if (err)
     return err;
 
+  opd->list_only = !!(flags & GPGME_DECRYPT_LISTONLY);
   opd->last_recipient_p = &opd->result.recipients;
   opd->plaintext_dserial = _gpgme_data_get_dserial (plaintext);
   return 0;
@@ -571,7 +576,7 @@ _gpgme_decrypt_start (gpgme_ctx_t ctx, int synchronous,
   if (err)
     return err;
 
-  err = _gpgme_op_decrypt_init_result (ctx, plain);
+  err = _gpgme_op_decrypt_init_result (ctx, plain, flags);
   if (err)
     return err;
 
diff --git a/src/engine-gpg.c b/src/engine-gpg.c
index 66303263..c0391d11 100644
--- a/src/engine-gpg.c
+++ b/src/engine-gpg.c
@@ -1952,6 +1952,9 @@ gpg_decrypt (void *engine,
         err = add_arg (gpg, "--unwrap");
     }
 
+  if (!err && (flags & GPGME_DECRYPT_LISTONLY))
+    err = add_arg (gpg, "--list-only");
+
   if (!err && export_session_key)
     err = add_gpg_arg (gpg, "--show-session-key");
 
diff --git a/src/gpgme.h.in b/src/gpgme.h.in
index 705d9741..69d9f54c 100644
--- a/src/gpgme.h.in
+++ b/src/gpgme.h.in
@@ -1491,6 +1491,7 @@ typedef enum
   {
     GPGME_DECRYPT_VERIFY = 1,
     GPGME_DECRYPT_ARCHIVE = 2,
+    GPGME_DECRYPT_LISTONLY = 16,
     GPGME_DECRYPT_UNWRAP = 128
   }
 gpgme_decrypt_flags_t;
diff --git a/src/ops.h b/src/ops.h
index f2333717..ff59fcc5 100644
--- a/src/ops.h
+++ b/src/ops.h
@@ -86,7 +86,8 @@ gpgme_error_t _gpgme_verify_status_handler (void *priv,
 
 /* From decrypt.c.  */
 gpgme_error_t _gpgme_op_decrypt_init_result (gpgme_ctx_t ctx,
-                                             gpgme_data_t plaintext);
+                                             gpgme_data_t plaintext,
+                                             gpgme_decrypt_flags_t flags);
 gpgme_error_t _gpgme_decrypt_status_handler (void *priv,
 					     gpgme_status_code_t code,
 					     char *args);
diff --git a/tests/run-decrypt.c b/tests/run-decrypt.c
index c724e407..cbde49dc 100644
--- a/tests/run-decrypt.c
+++ b/tests/run-decrypt.c
@@ -90,6 +90,7 @@ show_usage (int ex)
          "  --no-symkey-cache               disable the use of that cache\n"
          "  --ignore-mdc-error              allow decryption of legacy data\n"
          "  --unwrap         remove only the encryption layer\n"
+         "  --list-only      no actual decryption\n"
          "  --large-buffers  use large I/O buffer\n"
          "  --sensitive      mark data objects as sensitive\n"
          "  --output FILE    write output to FILE instead of stdout\n"
@@ -215,6 +216,11 @@ main (int argc, char **argv)
           raw_output = 1;
           argc--; argv++;
         }
+      else if (!strcmp (*argv, "--list-only"))
+        {
+          flags |= GPGME_DECRYPT_LISTONLY;
+          argc--; argv++;
+        }
       else if (!strcmp (*argv, "--output"))
         {
           argc--; argv++;
@@ -445,7 +451,7 @@ main (int argc, char **argv)
     {
       if (!raw_output)
         print_result (result);
-      if (!output)
+      if (!output && !(flags & GPGME_DECRYPT_LISTONLY))
         {
           if (!raw_output)
             fputs ("Begin Output:\n", stdout);