test-json: Avoid use of /dev/stdin to fix builds in limited chroots.

The chroots in which we often build Open vSwitch don't have /proc and
thus cannot support /dev/stdin, because on Linux that is a symlink to
/proc/self/fd/0.  So avoid using /dev/stdin in the testsuite.

lib/json.c

@@ -1002,18 +1002,34 @@ json_from_string(const char *string)
 struct json *
 json_from_file(const char *file_name)
 {
-    struct json_parser *p;
     struct json *json;
     FILE *stream;
 
-    /* Open file. */
     stream = fopen(file_name, "r");
     if (!stream) {
         return json_string_create_nocopy(
             xasprintf("error opening \"%s\": %s", file_name, strerror(errno)));
     }
+    json = json_from_stream(stream);
+    fclose(stream);
+
+    return json;
+}
+
+/* Parses the contents of 'stream' as a JSON object or array, and returns a
+ * newly allocated 'struct json'.  The caller must free the returned structure
+ * with json_destroy() when it is no longer needed.
+ *
+ * The file must be encoded in UTF-8.
+ *
+ * See json_from_string() for return value semantics.
+ */
+struct json *
+json_from_stream(FILE *stream)
+{
+    struct json_parser *p;
+    struct json *json;
 
-    /* Read and parse file. */
     p = json_parser_create(JSPF_TRAILER);
     for (;;) {
         char buffer[BUFSIZ];
@@ -1026,13 +1042,11 @@ json_from_file(const char *file_name)
     }
     json = json_parser_finish(p);
 
-    /* Close file and check for I/O errors. */
     if (ferror(stream)) {
         json_destroy(json);
         json = json_string_create_nocopy(
-            xasprintf("error reading \"%s\": %s", file_name, strerror(errno)));
+            xasprintf("error reading JSON stream: %s", strerror(errno)));
     }
-    fclose(stream);
 
     return json;
 }

lib/json.h

@@ -111,6 +111,7 @@ void json_parser_abort(struct json_parser *);
 
 struct json *json_from_string(const char *string);
 struct json *json_from_file(const char *file_name);
+struct json *json_from_stream(FILE *stream);
 
 /* Serializing JSON. */
 

tests/test-json.c

@@ -67,23 +67,17 @@ refill(FILE *file, void *buffer, size_t buffer_size, size_t *n, size_t *used)
 }
 
 static bool
-parse_multiple(const char *input_file)
+parse_multiple(FILE *stream)
 {
     struct json_parser *parser;
     char buffer[BUFSIZ];
     size_t n, used;
-    FILE *file;
     bool ok;
 
-    file = fopen(input_file, "r");
-    if (!file) {
-        ovs_fatal(errno, "Cannot open \"%s\"", input_file);
-    }
-
     parser = NULL;
     n = used = 0;
     ok = true;
-    while (used < n || refill(file, buffer, sizeof buffer, &n, &used)) {
+    while (used < n || refill(stream, buffer, sizeof buffer, &n, &used)) {
         if (!parser && isspace((unsigned char) buffer[used])) {
             /* Skip white space. */
             used++;
@@ -113,6 +107,7 @@ int
 main(int argc, char *argv[])
 {
     const char *input_file;
+    FILE *stream;
     bool ok;
 
     set_program_name(argv[0]);
@@ -146,14 +141,15 @@ main(int argc, char *argv[])
     }
 
     input_file = argv[optind];
-    if (!strcmp(input_file, "-")) {
-        input_file = "/dev/stdin";
+    stream = !strcmp(input_file, "-") ? stdin : fopen(input_file, "r");
+    if (!stream) {
+        ovs_fatal(errno, "Cannot open \"%s\"", input_file);
     }
 
     if (multiple) {
-        ok = parse_multiple(input_file);
+        ok = parse_multiple(stream);
     } else {
-        ok = print_and_free_json(json_from_file(input_file));
+        ok = print_and_free_json(json_from_stream(stream));
     }
 
     return !ok;