generator: Introduce error code (errcode) concept.

There was a lot of repeated code to map return types (eg. RErr)
to error cases (eg. -1 or NULL).

This commit introduces an error code type and two functions to
map return types to error codes and error codes to strings.

11 files changed, 315 insertions, 223 deletions

diff --git a/generator/generator_c.ml b/generator/generator_c.ml
index 99cf327f..9b88376c 100644
--- a/generator/generator_c.ml
+++ b/generator/generator_c.ml
@@ -637,14 +637,6 @@ check_state (guestfs_h *g, const char *caller)
 
 ";
 
-  let error_code_of = function
-    | RErr | RInt _ | RInt64 _ | RBool _ -> "-1"
-    | RConstString _ | RConstOptString _
-    | RString _ | RStringList _
-    | RStruct _ | RStructList _
-    | RHashtable _ | RBufferOut _ -> "NULL"
-  in
-
   (* Generate code to check String-like parameters are not passed in
    * as NULL (returning an error if they are).
    *)
@@ -667,7 +659,17 @@ check_state (guestfs_h *g, const char *caller)
           pr "  if (%s == NULL) {\n" n;
           pr "    error (g, \"%%s: %%s: parameter cannot be NULL\",\n";
           pr "           \"%s\", \"%s\");\n" shortname n;
-          pr "    return %s;\n" (error_code_of ret);
+          let errcode =
+            match errcode_of_ret ret with
+            | `CannotReturnError ->
+                if shortname = "test0rconstoptstring" then (* XXX hack *)
+                  `ErrorIsNULL
+                else
+                  failwithf
+                    "%s: RConstOptString function has invalid parameter '%s'"
+                    shortname n
+            | (`ErrorIsMinusOne |`ErrorIsNULL) as e -> e in
+          pr "    return %s;\n" (string_of_errcode errcode);
           pr "  }\n";
           pr_newline := true
 
@@ -689,7 +691,11 @@ check_state (guestfs_h *g, const char *caller)
           pr "      optargs->%s == NULL) {\n" n;
           pr "    error (g, \"%%s: %%s: optional parameter cannot be NULL\",\n";
           pr "           \"%s\", \"%s\");\n" shortname n;
-          pr "    return %s;\n" (error_code_of ret);
+          let errcode =
+            match errcode_of_ret ret with
+            | `CannotReturnError -> assert false
+            | (`ErrorIsMinusOne |`ErrorIsNULL) as e -> e in
+          pr "    return %s;\n" (string_of_errcode errcode);
           pr "  }\n";
           pr_newline := true
 
@@ -711,7 +717,11 @@ check_state (guestfs_h *g, const char *caller)
         pr "  if (optargs->bitmask & UINT64_C(0x%Lx)) {\n" mask;
         pr "    error (g, \"%%s: unknown option in guestfs_%%s_argv->bitmask (this can happen if a program is compiled against a newer version of libguestfs, then dynamically linked to an older version)\",\n";
         pr "           \"%s\", \"%s\");\n" shortname shortname;
-        pr "    return %s;\n" (error_code_of ret);
+        let errcode =
+          match errcode_of_ret ret with
+          | `CannotReturnError -> assert false
+          | (`ErrorIsMinusOne |`ErrorIsNULL) as e -> e in
+        pr "    return %s;\n" (string_of_errcode errcode);
         pr "  }\n";
         pr "\n";
   in
@@ -887,24 +897,24 @@ check_state (guestfs_h *g, const char *caller)
           shortname style;
       pr "{\n";
       pr "  int trace_flag = g->trace;\n";
-      let errcode =
-        match ret with
-        | RErr | RInt _ | RBool _ ->
-            pr "  int r;\n"; Some "-1"
-        | RInt64 _ ->
-            pr "  int64_t r;\n"; Some "-1"
-        | RConstString _ ->
-            pr "  const char *r;\n"; Some "NULL"
-        | RConstOptString _ ->
-            pr "  const char *r;\n"; None
-        | RString _ | RBufferOut _ ->
-            pr "  char *r;\n"; Some "NULL"
-        | RStringList _ | RHashtable _ ->
-            pr "  char **r;\n"; Some "NULL"
-        | RStruct (_, typ) ->
-            pr "  struct guestfs_%s *r;\n" typ; Some "NULL"
-        | RStructList (_, typ) ->
-            pr "  struct guestfs_%s_list *r;\n" typ; Some "NULL" in
+      (match ret with
+       | RErr | RInt _ | RBool _ ->
+           pr "  int r;\n"
+       | RInt64 _ ->
+           pr "  int64_t r;\n"
+       | RConstString _ ->
+           pr "  const char *r;\n"
+       | RConstOptString _ ->
+           pr "  const char *r;\n"
+       | RString _ | RBufferOut _ ->
+           pr "  char *r;\n"
+       | RStringList _ | RHashtable _ ->
+           pr "  char **r;\n"
+       | RStruct (_, typ) ->
+           pr "  struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "  struct guestfs_%s_list *r;\n" typ
+      );
       pr "\n";
       check_null_strings shortname style;
       reject_unknown_optargs shortname style;
@@ -913,14 +923,14 @@ check_state (guestfs_h *g, const char *caller)
       generate_c_call_args ~handle:"g" style;
       pr ";\n";
       pr "\n";
-      (match errcode with
-       | Some errcode ->
-           pr "  if (r != %s) {\n" errcode;
+      (match errcode_of_ret ret with
+       | (`ErrorIsMinusOne | `ErrorIsNULL) as errcode ->
+           pr "  if (r != %s) {\n" (string_of_errcode errcode);
            trace_return ~indent:4 shortname style "r";
            pr "  } else {\n";
            trace_return_error ~indent:4 shortname style;
            pr "  }\n";
-       | None ->
+       | `CannotReturnError ->
            trace_return shortname style "r";
       );
       pr "\n";
@@ -933,7 +943,10 @@ check_state (guestfs_h *g, const char *caller)
   List.iter (
     fun (shortname, (ret, args, optargs as style), _, _, _, _, _) ->
       let name = "guestfs_" ^ shortname in
-      let error_code = error_code_of ret in
+      let errcode =
+        match errcode_of_ret ret with
+        | `CannotReturnError -> assert false
+        | (`ErrorIsMinusOne | `ErrorIsNULL) as e -> e in
 
       (* Generate the action stub. *)
       if optargs = [] then
@@ -1014,7 +1027,7 @@ check_state (guestfs_h *g, const char *caller)
       (* Check we are in the right state for sending a request. *)
       pr "  if (check_state (g, \"%s\") == -1) {\n" shortname;
       trace_return_error ~indent:4 shortname style;
-      pr "    return %s;\n" error_code;
+      pr "    return %s;\n" (string_of_errcode errcode);
       pr "  }\n";
       pr "  guestfs___set_busy (g);\n";
       pr "\n";
@@ -1048,7 +1061,7 @@ check_state (guestfs_h *g, const char *caller)
               pr "    error (g, \"%%s: size of input buffer too large\", \"%s\");\n"
                 shortname;
               pr "    guestfs___end_busy (g);\n";
-              pr "    return %s;\n" error_code;
+              pr "    return %s;\n" (string_of_errcode errcode);
               pr "  }\n";
               pr "  args.%s.%s_val = (char *) %s;\n" n n n;
               pr "  args.%s.%s_len = %s_size;\n" n n n
@@ -1087,7 +1100,7 @@ check_state (guestfs_h *g, const char *caller)
       pr "  if (serial == -1) {\n";
       pr "    guestfs___end_busy (g);\n";
       trace_return_error ~indent:4 shortname style;
-      pr "    return %s;\n" error_code;
+      pr "    return %s;\n" (string_of_errcode errcode);
       pr "  }\n";
       pr "\n";
 
@@ -1100,7 +1113,7 @@ check_state (guestfs_h *g, const char *caller)
             pr "  if (r == -1) {\n";
             pr "    guestfs___end_busy (g);\n";
             trace_return_error ~indent:4 shortname style;
-            pr "    return %s;\n" error_code;
+            pr "    return %s;\n" (string_of_errcode errcode);
             pr "  }\n";
             pr "  if (r == -2) /* daemon cancelled */\n";
             pr "    goto read_reply;\n";
@@ -1125,7 +1138,7 @@ check_state (guestfs_h *g, const char *caller)
       pr "  if (r == -1) {\n";
       pr "    guestfs___end_busy (g);\n";
       trace_return_error ~indent:4 shortname style;
-      pr "    return %s;\n" error_code;
+      pr "    return %s;\n" (string_of_errcode errcode);
       pr "  }\n";
       pr "\n";
 
@@ -1133,7 +1146,7 @@ check_state (guestfs_h *g, const char *caller)
         (String.uppercase shortname);
       pr "    guestfs___end_busy (g);\n";
       trace_return_error ~indent:4 shortname style;
-      pr "    return %s;\n" error_code;
+      pr "    return %s;\n" (string_of_errcode errcode);
       pr "  }\n";
       pr "\n";
 
@@ -1152,7 +1165,7 @@ check_state (guestfs_h *g, const char *caller)
       pr "    free (err.error_message);\n";
       pr "    free (err.errno_string);\n";
       pr "    guestfs___end_busy (g);\n";
-      pr "    return %s;\n" error_code;
+      pr "    return %s;\n" (string_of_errcode errcode);
       pr "  }\n";
       pr "\n";
 
@@ -1163,7 +1176,7 @@ check_state (guestfs_h *g, const char *caller)
             pr "  if (guestfs___recv_file (g, %s) == -1) {\n" n;
             pr "    guestfs___end_busy (g);\n";
             trace_return_error ~indent:4 shortname style;
-            pr "    return %s;\n" error_code;
+            pr "    return %s;\n" (string_of_errcode errcode);
             pr "  }\n";
             pr "\n";
         | _ -> ()
@@ -1254,16 +1267,16 @@ check_state (guestfs_h *g, const char *caller)
           | [] -> "g"
           | args -> name_of_argt (List.hd (List.rev args)) in
 
-        let rerrcode, rtype =
+        let rtype =
           match ret with
-          | RErr | RInt _ | RBool _ -> "-1", "int "
-          | RInt64 _ -> "-1", "int64_t "
-          | RConstString _ | RConstOptString _ -> "NULL", "const char *"
-          | RString _ | RBufferOut _ -> "NULL", "char *"
-          | RStringList _ | RHashtable _ -> "NULL", "char **"
-          | RStruct (_, typ) -> "NULL", sprintf "struct guestfs_%s *" typ
+          | RErr | RInt _ | RBool _ -> "int "
+          | RInt64 _ -> "int64_t "
+          | RConstString _ | RConstOptString _ -> "const char *"
+          | RString _ | RBufferOut _ -> "char *"
+          | RStringList _ | RHashtable _ -> "char **"
+          | RStruct (_, typ) -> sprintf "struct guestfs_%s *" typ
           | RStructList (_, typ) ->
-              "NULL", sprintf "struct guestfs_%s_list *" typ in
+              sprintf "struct guestfs_%s_list *" typ in
 
         (* The regular variable args function, just calls the _va variant. *)
         generate_prototype ~extern:false ~semicolon:false ~newline:true
@@ -1309,17 +1322,22 @@ check_state (guestfs_h *g, const char *caller)
             pr "      break;\n";
         ) optargs;
 
+        let errcode =
+          match errcode_of_ret ret with
+          | `CannotReturnError -> assert false
+          | (`ErrorIsMinusOne | `ErrorIsNULL) as e -> e in
+
         pr "    default:\n";
         pr "      error (g, \"%%s: unknown option %%d (this can happen if a program is compiled against a newer version of libguestfs, then dynamically linked to an older version)\",\n";
         pr "             \"%s\", i);\n" shortname;
-        pr "      return %s;\n" rerrcode;
+        pr "      return %s;\n" (string_of_errcode errcode);
         pr "    }\n";
         pr "\n";
         pr "    uint64_t i_mask = UINT64_C(1) << i;\n";
         pr "    if (optargs_s.bitmask & i_mask) {\n";
         pr "      error (g, \"%%s: same optional argument specified more than once\",\n";
         pr "             \"%s\");\n" shortname;
-        pr "      return %s;\n" rerrcode;
+        pr "      return %s;\n" (string_of_errcode errcode);
         pr "    }\n";
         pr "    optargs_s.bitmask |= i_mask;\n";
         pr "  }\n";
diff --git a/generator/generator_capitests.ml b/generator/generator_capitests.ml
index 5b40cc2e..196b1fb8 100644
--- a/generator/generator_capitests.ml
+++ b/generator/generator_capitests.ml
@@ -792,25 +792,23 @@ and generate_test_command_call ?(expect_error = false) ?test test_name cmd =
         ) optargs;
       );
 
-      let error_code =
-        match style_ret with
-        | RErr | RInt _ | RBool _ -> pr "    int r;\n"; "-1"
-        | RInt64 _ -> pr "    int64_t r;\n"; "-1"
-        | RConstString _ | RConstOptString _ ->
-            pr "    const char *r;\n"; "NULL"
-        | RString _ -> pr "    char *r;\n"; "NULL"
-        | RStringList _ | RHashtable _ ->
-            pr "    char **r;\n";
-            pr "    size_t i;\n";
-            "NULL"
-        | RStruct (_, typ) ->
-            pr "    struct guestfs_%s *r;\n" typ; "NULL"
-        | RStructList (_, typ) ->
-            pr "    struct guestfs_%s_list *r;\n" typ; "NULL"
-        | RBufferOut _ ->
-            pr "    char *r;\n";
-            pr "    size_t size;\n";
-            "NULL" in
+      (match style_ret with
+       | RErr | RInt _ | RBool _ -> pr "    int r;\n"
+       | RInt64 _ -> pr "    int64_t r;\n"
+       | RConstString _ | RConstOptString _ ->
+           pr "    const char *r;\n"
+       | RString _ -> pr "    char *r;\n"
+       | RStringList _ | RHashtable _ ->
+           pr "    char **r;\n";
+           pr "    size_t i;\n"
+       | RStruct (_, typ) ->
+           pr "    struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "    struct guestfs_%s_list *r;\n" typ
+       | RBufferOut _ ->
+           pr "    char *r;\n";
+           pr "    size_t size;\n"
+      );
 
       pr "    suppress_error = %d;\n" (if expect_error then 1 else 0);
       if optargs = [] then
@@ -861,11 +859,21 @@ and generate_test_command_call ?(expect_error = false) ?test test_name cmd =
 
       pr ");\n";
 
-      if not expect_error then
-        pr "    if (r == %s)\n" error_code
-      else
-        pr "    if (r != %s)\n" error_code;
-      pr "      return -1;\n";
+      (match errcode_of_ret style_ret, expect_error with
+       | `CannotReturnError, _ -> ()
+       | `ErrorIsMinusOne, false ->
+           pr "    if (r == -1)\n";
+           pr "      return -1;\n";
+       | `ErrorIsMinusOne, true ->
+           pr "    if (r != -1)\n";
+           pr "      return -1;\n";
+       | `ErrorIsNULL, false ->
+           pr "    if (r == NULL)\n";
+           pr "      return -1;\n";
+       | `ErrorIsNULL, true ->
+           pr "    if (r != NULL)\n";
+           pr "      return -1;\n";
+      );
 
       (* Insert the test code. *)
       (match test with
diff --git a/generator/generator_daemon.ml b/generator/generator_daemon.ml
index f15ae612..21a58882 100644
--- a/generator/generator_daemon.ml
+++ b/generator/generator_daemon.ml
@@ -82,21 +82,20 @@ and generate_daemon_actions () =
       (* Generate server-side stubs. *)
       pr "static void %s_stub (XDR *xdr_in)\n" name;
       pr "{\n";
-      let error_code =
-        match ret with
-        | RErr | RInt _ -> pr "  int r;\n"; "-1"
-        | RInt64 _ -> pr "  int64_t r;\n"; "-1"
-        | RBool _ -> pr "  int r;\n"; "-1"
-        | RConstString _ | RConstOptString _ ->
-            failwithf "RConstString|RConstOptString cannot be used by daemon functions"
-        | RString _ -> pr "  char *r;\n"; "NULL"
-        | RStringList _ | RHashtable _ -> pr "  char **r;\n"; "NULL"
-        | RStruct (_, typ) -> pr "  guestfs_int_%s *r;\n" typ; "NULL"
-        | RStructList (_, typ) -> pr "  guestfs_int_%s_list *r;\n" typ; "NULL"
-        | RBufferOut _ ->
-            pr "  size_t size = 1;\n";
-            pr "  char *r;\n";
-            "NULL" in
+      (match ret with
+       | RErr | RInt _ -> pr "  int r;\n"
+       | RInt64 _ -> pr "  int64_t r;\n"
+       | RBool _ -> pr "  int r;\n"
+       | RConstString _ | RConstOptString _ ->
+           failwithf "RConstString|RConstOptString cannot be used by daemon functions"
+       | RString _ -> pr "  char *r;\n"
+       | RStringList _ | RHashtable _ -> pr "  char **r;\n"
+       | RStruct (_, typ) -> pr "  guestfs_int_%s *r;\n" typ
+       | RStructList (_, typ) -> pr "  guestfs_int_%s_list *r;\n" typ
+       | RBufferOut _ ->
+           pr "  size_t size = 1;\n";
+           pr "  char *r;\n"
+      );
 
       if args <> [] || optargs <> [] then (
         pr "  struct guestfs_%s_args args;\n" name;
@@ -223,11 +222,16 @@ and generate_daemon_actions () =
         pr ";\n" in
 
       (match ret with
+       | RConstOptString _ -> assert false
        | RErr | RInt _ | RInt64 _ | RBool _
-       | RConstString _ | RConstOptString _
+       | RConstString _
        | RString _ | RStringList _ | RHashtable _
        | RStruct (_, _) | RStructList (_, _) ->
-           pr "  if (r == %s)\n" error_code;
+           let errcode =
+             match errcode_of_ret ret with
+             | `CannotReturnError -> assert false
+             | (`ErrorIsMinusOne | `ErrorIsNULL) as e -> e in
+           pr "  if (r == %s)\n" (string_of_errcode errcode);
            pr "    /* do_%s has already called reply_with_error */\n" name;
            pr "    goto done;\n";
            pr "\n"
@@ -235,7 +239,7 @@ and generate_daemon_actions () =
            pr "  /* size == 0 && r == NULL could be a non-error case (just\n";
            pr "   * an ordinary zero-length buffer), so be careful ...\n";
            pr "   */\n";
-           pr "  if (size == 1 && r == %s)\n" error_code;
+           pr "  if (size == 1 && r == NULL)\n";
            pr "    /* do_%s has already called reply_with_error */\n" name;
            pr "    goto done;\n";
            pr "\n"
diff --git a/generator/generator_java.ml b/generator/generator_java.ml
index dedf9625..767c94cf 100644
--- a/generator/generator_java.ml
+++ b/generator/generator_java.ml
@@ -374,40 +374,40 @@ Java_com_redhat_et_libguestfs_GuestFS__1close
       pr ")\n";
       pr "{\n";
       pr "  guestfs_h *g = (guestfs_h *) (long) jg;\n";
-      let error_code, no_ret =
-        match ret with
-        | RErr -> pr "  int r;\n"; "-1", ""
-        | RBool _
-        | RInt _ -> pr "  int r;\n"; "-1", "0"
-        | RInt64 _ -> pr "  int64_t r;\n"; "-1", "0"
-        | RConstString _ -> pr "  const char *r;\n"; "NULL", "NULL"
-        | RConstOptString _ -> pr "  const char *r;\n"; "NULL", "NULL"
-        | RString _ ->
-            pr "  jstring jr;\n";
-            pr "  char *r;\n"; "NULL", "NULL"
-        | RStringList _
-        | RHashtable _ ->
-            pr "  jobjectArray jr;\n";
-            pr "  int r_len;\n";
-            pr "  jclass cl;\n";
-            pr "  jstring jstr;\n";
-            pr "  char **r;\n"; "NULL", "NULL"
-        | RStruct (_, typ) ->
-            pr "  jobject jr;\n";
-            pr "  jclass cl;\n";
-            pr "  jfieldID fl;\n";
-            pr "  struct guestfs_%s *r;\n" typ; "NULL", "NULL"
-        | RStructList (_, typ) ->
-            pr "  jobjectArray jr;\n";
-            pr "  jclass cl;\n";
-            pr "  jfieldID fl;\n";
-            pr "  jobject jfl;\n";
-            pr "  struct guestfs_%s_list *r;\n" typ; "NULL", "NULL"
-        | RBufferOut _ ->
-            pr "  jstring jr;\n";
-            pr "  char *r;\n";
-            pr "  size_t size;\n";
-            "NULL", "NULL" in
+      (match ret with
+       | RErr -> pr "  int r;\n"
+       | RBool _
+       | RInt _ -> pr "  int r;\n"
+       | RInt64 _ -> pr "  int64_t r;\n"
+       | RConstString _ -> pr "  const char *r;\n"
+       | RConstOptString _ -> pr "  const char *r;\n"
+       | RString _ ->
+           pr "  jstring jr;\n";
+           pr "  char *r;\n"
+       | RStringList _
+       | RHashtable _ ->
+           pr "  jobjectArray jr;\n";
+           pr "  int r_len;\n";
+           pr "  jclass cl;\n";
+           pr "  jstring jstr;\n";
+           pr "  char **r;\n"
+       | RStruct (_, typ) ->
+           pr "  jobject jr;\n";
+           pr "  jclass cl;\n";
+           pr "  jfieldID fl;\n";
+           pr "  struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "  jobjectArray jr;\n";
+           pr "  jclass cl;\n";
+           pr "  jfieldID fl;\n";
+           pr "  jobject jfl;\n";
+           pr "  struct guestfs_%s_list *r;\n" typ
+       | RBufferOut _ ->
+           pr "  jstring jr;\n";
+           pr "  char *r;\n";
+           pr "  size_t size;\n"
+      );
+
       List.iter (
         function
         | Pathname n
@@ -527,10 +527,19 @@ Java_com_redhat_et_libguestfs_GuestFS__1close
       ) args;
 
       (* Check for errors. *)
-      pr "  if (r == %s) {\n" error_code;
-      pr "    throw_exception (env, guestfs_last_error (g));\n";
-      pr "    return %s;\n" no_ret;
-      pr "  }\n";
+      (match errcode_of_ret ret with
+       | `CannotReturnError -> ()
+       | `ErrorIsMinusOne ->
+           pr "  if (r == -1) {\n";
+           pr "    throw_exception (env, guestfs_last_error (g));\n";
+           pr "    return -1;\n";
+           pr "  }\n"
+       | `ErrorIsNULL ->
+           pr "  if (r == NULL) {\n";
+           pr "    throw_exception (env, guestfs_last_error (g));\n";
+           pr "    return NULL;\n";
+           pr "  }\n"
+      );
 
       (* Return value. *)
       (match ret with
diff --git a/generator/generator_ocaml.ml b/generator/generator_ocaml.ml
index 393a0629..ced6fb42 100644
--- a/generator/generator_ocaml.ml
+++ b/generator/generator_ocaml.ml
@@ -430,31 +430,28 @@ copy_table (char * const * argv)
         ) optargs
       );
 
-      let error_code =
-        match ret with
-        | RErr -> pr "  int r;\n"; "-1"
-        | RInt _ -> pr "  int r;\n"; "-1"
-        | RInt64 _ -> pr "  int64_t r;\n"; "-1"
-        | RBool _ -> pr "  int r;\n"; "-1"
-        | RConstString _ | RConstOptString _ ->
-            pr "  const char *r;\n"; "NULL"
-        | RString _ -> pr "  char *r;\n"; "NULL"
-        | RStringList _ ->
-            pr "  size_t i;\n";
-            pr "  char **r;\n";
-            "NULL"
-        | RStruct (_, typ) ->
-            pr "  struct guestfs_%s *r;\n" typ; "NULL"
-        | RStructList (_, typ) ->
-            pr "  struct guestfs_%s_list *r;\n" typ; "NULL"
-        | RHashtable _ ->
-            pr "  size_t i;\n";
-            pr "  char **r;\n";
-            "NULL"
-        | RBufferOut _ ->
-            pr "  char *r;\n";
-            pr "  size_t size;\n";
-            "NULL" in
+      (match ret with
+       | RErr -> pr "  int r;\n"
+       | RInt _ -> pr "  int r;\n"
+       | RInt64 _ -> pr "  int64_t r;\n"
+       | RBool _ -> pr "  int r;\n"
+       | RConstString _ | RConstOptString _ ->
+           pr "  const char *r;\n"
+       | RString _ -> pr "  char *r;\n"
+       | RStringList _ ->
+           pr "  size_t i;\n";
+           pr "  char **r;\n"
+       | RStruct (_, typ) ->
+           pr "  struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "  struct guestfs_%s_list *r;\n" typ
+       | RHashtable _ ->
+           pr "  size_t i;\n";
+           pr "  char **r;\n"
+       | RBufferOut _ ->
+           pr "  char *r;\n";
+           pr "  size_t size;\n"
+      );
       pr "\n";
 
       pr "  caml_enter_blocking_section ();\n";
@@ -487,8 +484,15 @@ copy_table (char * const * argv)
         | StringList _ | DeviceList _ | Pointer _ -> ()
       ) optargs;
 
-      pr "  if (r == %s)\n" error_code;
-      pr "    ocaml_guestfs_raise_error (g, \"%s\");\n" name;
+      (match errcode_of_ret ret with
+       | `CannotReturnError -> ()
+       | `ErrorIsMinusOne ->
+           pr "  if (r == -1)\n";
+           pr "    ocaml_guestfs_raise_error (g, \"%s\");\n" name;
+       | `ErrorIsNULL ->
+           pr "  if (r == NULL)\n";
+           pr "    ocaml_guestfs_raise_error (g, \"%s\");\n" name;
+      );
       pr "\n";
 
       (match ret with
diff --git a/generator/generator_php.ml b/generator/generator_php.ml
index d405656d..4431147f 100644
--- a/generator/generator_php.ml
+++ b/generator/generator_php.ml
@@ -357,28 +357,27 @@ PHP_FUNCTION (guestfs_last_error)
       );
 
       (* Return value. *)
-      let error_code =
-        match ret with
-        | RErr -> pr "  int r;\n"; "-1"
-        | RBool _
-        | RInt _ -> pr "  int r;\n"; "-1"
-        | RInt64 _ -> pr "  int64_t r;\n"; "-1"
-        | RConstString _ -> pr "  const char *r;\n"; "NULL"
-        | RConstOptString _ -> pr "  const char *r;\n"; "NULL"
-        | RString _ ->
-            pr "  char *r;\n"; "NULL"
-        | RStringList _ ->
-            pr "  char **r;\n"; "NULL"
-        | RStruct (_, typ) ->
-            pr "  struct guestfs_%s *r;\n" typ; "NULL"
-        | RStructList (_, typ) ->
-            pr "  struct guestfs_%s_list *r;\n" typ; "NULL"
-        | RHashtable _ ->
-            pr "  char **r;\n"; "NULL"
-        | RBufferOut _ ->
-            pr "  char *r;\n";
-            pr "  size_t size;\n";
-            "NULL" in
+      (match ret with
+       | RErr -> pr "  int r;\n"
+       | RBool _
+       | RInt _ -> pr "  int r;\n"
+       | RInt64 _ -> pr "  int64_t r;\n"
+       | RConstString _ -> pr "  const char *r;\n"
+       | RConstOptString _ -> pr "  const char *r;\n"
+       | RString _ ->
+           pr "  char *r;\n"
+       | RStringList _ ->
+           pr "  char **r;\n"
+       | RStruct (_, typ) ->
+           pr "  struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "  struct guestfs_%s_list *r;\n" typ
+       | RHashtable _ ->
+           pr "  char **r;\n"
+       | RBufferOut _ ->
+           pr "  char *r;\n";
+           pr "  size_t size;\n"
+      );
 
       (* Call the function. *)
       if optargs = [] then
@@ -410,9 +409,17 @@ PHP_FUNCTION (guestfs_last_error)
         ) args;
 
       (* Check for errors. *)
-      pr "  if (r == %s) {\n" error_code;
-      pr "    RETURN_FALSE;\n";
-      pr "  }\n";
+      (match errcode_of_ret ret with
+       | `CannotReturnError -> ()
+       | `ErrorIsMinusOne ->
+           pr "  if (r == -1) {\n";
+           pr "    RETURN_FALSE;\n";
+           pr "  }\n"
+       | `ErrorIsNULL ->
+           pr "  if (r == NULL) {\n";
+           pr "    RETURN_FALSE;\n";
+           pr "  }\n"
+      );
       pr "\n";
 
       (* Convert the return value. *)
diff --git a/generator/generator_python.ml b/generator/generator_python.ml
index bc570a88..937c092d 100644
--- a/generator/generator_python.ml
+++ b/generator/generator_python.ml
@@ -293,21 +293,20 @@ py_guestfs_close (PyObject *self, PyObject *args)
         pr "  struct guestfs_%s_argv *optargs = &optargs_s;\n" name;
       );
 
-      let error_code =
-        match ret with
-        | RErr | RInt _ | RBool _ -> pr "  int r;\n"; "-1"
-        | RInt64 _ -> pr "  int64_t r;\n"; "-1"
-        | RConstString _ | RConstOptString _ ->
-            pr "  const char *r;\n"; "NULL"
-        | RString _ -> pr "  char *r;\n"; "NULL"
-        | RStringList _ | RHashtable _ -> pr "  char **r;\n"; "NULL"
-        | RStruct (_, typ) -> pr "  struct guestfs_%s *r;\n" typ; "NULL"
-        | RStructList (_, typ) ->
-            pr "  struct guestfs_%s_list *r;\n" typ; "NULL"
-        | RBufferOut _ ->
-            pr "  char *r;\n";
-            pr "  size_t size;\n";
-            "NULL" in
+      (match ret with
+       | RErr | RInt _ | RBool _ -> pr "  int r;\n"
+       | RInt64 _ -> pr "  int64_t r;\n"
+       | RConstString _ | RConstOptString _ ->
+           pr "  const char *r;\n"
+       | RString _ -> pr "  char *r;\n"
+       | RStringList _ | RHashtable _ -> pr "  char **r;\n"
+       | RStruct (_, typ) -> pr "  struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "  struct guestfs_%s_list *r;\n" typ
+       | RBufferOut _ ->
+           pr "  char *r;\n";
+           pr "  size_t size;\n"
+      );
 
       List.iter (
         function
@@ -457,10 +456,19 @@ py_guestfs_close (PyObject *self, PyObject *args)
             pr "  free (%s);\n" n
       ) args;
 
-      pr "  if (r == %s) {\n" error_code;
-      pr "    PyErr_SetString (PyExc_RuntimeError, guestfs_last_error (g));\n";
-      pr "    return NULL;\n";
-      pr "  }\n";
+      (match errcode_of_ret ret with
+       | `CannotReturnError -> ()
+       | `ErrorIsMinusOne ->
+           pr "  if (r == -1) {\n";
+           pr "    PyErr_SetString (PyExc_RuntimeError, guestfs_last_error (g));\n";
+           pr "    return NULL;\n";
+           pr "  }\n"
+       | `ErrorIsNULL ->
+           pr "  if (r == NULL) {\n";
+           pr "    PyErr_SetString (PyExc_RuntimeError, guestfs_last_error (g));\n";
+           pr "    return NULL;\n";
+           pr "  }\n"
+      );
       pr "\n";
 
       (match ret with
diff --git a/generator/generator_ruby.ml b/generator/generator_ruby.ml
index 6d627b05..49ddf1ff 100644
--- a/generator/generator_ruby.ml
+++ b/generator/generator_ruby.ml
@@ -184,21 +184,20 @@ static VALUE ruby_guestfs_close (VALUE gv)
         pr "\n";
       );
 
-      let error_code =
-        match ret with
-        | RErr | RInt _ | RBool _ -> pr "  int r;\n"; "-1"
-        | RInt64 _ -> pr "  int64_t r;\n"; "-1"
-        | RConstString _ | RConstOptString _ ->
-            pr "  const char *r;\n"; "NULL"
-        | RString _ -> pr "  char *r;\n"; "NULL"
-        | RStringList _ | RHashtable _ -> pr "  char **r;\n"; "NULL"
-        | RStruct (_, typ) -> pr "  struct guestfs_%s *r;\n" typ; "NULL"
-        | RStructList (_, typ) ->
-            pr "  struct guestfs_%s_list *r;\n" typ; "NULL"
-        | RBufferOut _ ->
-            pr "  char *r;\n";
-            pr "  size_t size;\n";
-            "NULL" in
+      (match ret with
+       | RErr | RInt _ | RBool _ -> pr "  int r;\n"
+       | RInt64 _ -> pr "  int64_t r;\n"
+       | RConstString _ | RConstOptString _ ->
+           pr "  const char *r;\n"
+       | RString _ -> pr "  char *r;\n"
+       | RStringList _ | RHashtable _ -> pr "  char **r;\n"
+       | RStruct (_, typ) -> pr "  struct guestfs_%s *r;\n" typ
+       | RStructList (_, typ) ->
+           pr "  struct guestfs_%s_list *r;\n" typ
+       | RBufferOut _ ->
+           pr "  char *r;\n";
+           pr "  size_t size;\n"
+      );
       pr "\n";
 
       if optargs = [] then
@@ -217,8 +216,15 @@ static VALUE ruby_guestfs_close (VALUE gv)
             pr "  free (%s);\n" n
       ) args;
 
-      pr "  if (r == %s)\n" error_code;
-      pr "    rb_raise (e_Error, \"%%s\", guestfs_last_error (g));\n";
+      (match errcode_of_ret ret with
+       | `CannotReturnError -> ()
+       | `ErrorIsMinusOne ->
+           pr "  if (r == -1)\n";
+           pr "    rb_raise (e_Error, \"%%s\", guestfs_last_error (g));\n"
+       | `ErrorIsNULL ->
+           pr "  if (r == NULL)\n";
+           pr "    rb_raise (e_Error, \"%%s\", guestfs_last_error (g));\n"
+      );
       pr "\n";
 
       (match ret with
diff --git a/generator/generator_types.ml b/generator/generator_types.ml
index b6c27309..50e96040 100644
--- a/generator/generator_types.ml
+++ b/generator/generator_types.ml
@@ -1,5 +1,5 @@
 (* libguestfs
- * Copyright (C) 2009-2010 Red Hat Inc.
+ * Copyright (C) 2009-2011 Red Hat Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -203,6 +203,8 @@ and argt =
      *)
   | Pointer of (string * string)
 
+type errcode = [ `CannotReturnError | `ErrorIsMinusOne | `ErrorIsNULL ]
+
 type flags =
   | ProtocolLimitWarning  (* display warning about protocol size limits *)
   | DangerWillRobinson	  (* flags particularly dangerous commands *)
diff --git a/generator/generator_utils.ml b/generator/generator_utils.ml
index 27b543d4..f62b1956 100644
--- a/generator/generator_utils.ml
+++ b/generator/generator_utils.ml
@@ -1,5 +1,5 @@
 (* libguestfs
- * Copyright (C) 2009-2010 Red Hat Inc.
+ * Copyright (C) 2009-2011 Red Hat Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -28,6 +28,21 @@ open Printf
 
 open Generator_types
 
+let errcode_of_ret = function
+  | RConstOptString _ ->
+      `CannotReturnError
+  | RErr | RInt _ | RBool _ | RInt64 _ ->
+      `ErrorIsMinusOne
+  | RConstString _
+  | RString _ | RBufferOut _
+  | RStringList _ | RHashtable _
+  | RStruct _ | RStructList _ ->
+      `ErrorIsNULL
+
+let string_of_errcode = function
+  | `ErrorIsMinusOne -> "-1"
+  | `ErrorIsNULL -> "NULL"
+
 (* Generate a uuidgen-compatible UUID (used in tests).  However to
  * avoid having the UUID change every time we rebuild the tests,
  * generate it as a function of the contents of the
diff --git a/generator/generator_utils.mli b/generator/generator_utils.mli
index 5104a162..5dc4da25 100644
--- a/generator/generator_utils.mli
+++ b/generator/generator_utils.mli
@@ -1,5 +1,5 @@
 (* libguestfs
- * Copyright (C) 2009-2010 Red Hat Inc.
+ * Copyright (C) 2009-2011 Red Hat Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -20,6 +20,17 @@
 
 (** Useful utility functions. *)
 
+val errcode_of_ret : Generator_types.ret -> Generator_types.errcode
+(** Map [ret] type to the error indication that the action returns,
+    eg. [errcode_of_ret RErr] => [`ErrorIsMinusOne] (meaning that
+    these actions return [-1]).
+
+    Note that [RConstOptString] cannot return an error indication, and
+    this returns [`CannotReturnError].  Callers must deal with it. *)
+
+val string_of_errcode : [`ErrorIsMinusOne|`ErrorIsNULL] -> string
+(** Return errcode as a string.  Untyped for [`CannotReturnError]. *)
+
 val uuidgen : unit -> string
 (** Generate a random UUID (used in tests). *)