message-params: Allow parameter strings to contain escaped curly braces

The patch adds the possibility to escape curly braces within parameter strings
and introduces several new functions that can be used for writing parameters.

For writing, the structure pa_message_params, which is a wrapper for pa_strbuf
has been created. Following new write functions are available:

pa_message_params_new() - creates a new pa_message_params structure
pa_message_params_free() - frees a pa_message_params structure
pa_message_param_to_string_free() - converts a pa_message_param to string and
frees the structure
pa_message_params_begin_list() - starts a list
pa_message_params_end_list() - ends a list
pa_message_params_write_string() - writes a string to a pa_message_params structure
pa_message_params_write_raw() - writes a raw string to a pa_message_params structure

For string parameters that contain curly braces or backslashes, those characters
will be escaped when using pa_message_params_write_string(), while write_raw() will
put the string into the buffer without any changes.

For reading, pa_message_params_read_string() reverts the changes that
pa_message_params_write_string() might have introduced.

The patch also adds more restrictions on the object path name. Now only
alphanumeric characters and one of "_", ".", "-" and "/" are allowed.
The path name may not end with a / or contain a double slash. If the user
specifies a trailing / when sending a message, it will be silently removed.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/51>

src/pulse/message-params.c

@@ -28,23 +28,33 @@
 #include <pulse/xmalloc.h>
 
 #include <pulsecore/macro.h>
+#include <pulsecore/strbuf.h>
+#include <pulsecore/core-util.h>
 
 #include "message-params.h"
 
+/* Message parameter structure, a wrapper for pa_strbuf */
+struct pa_message_params {
+    pa_strbuf *buffer;
+};
+
 /* Split the specified string into elements. An element is defined as
  * a sub-string between curly braces. The function is needed to parse
  * the parameters of messages. Each time it is called it returns the
  * position of the current element in result and the state pointer is
- * advanced to the next list element.
+ * advanced to the next list element. On return, the parameter
+ * *is_unpacked indicates if the string is plain text or contains a
+ * sub-list. is_unpacked may be NULL.
  *
  * The variable state points to, should be initialized to NULL before
  * the first call. The function returns 1 on success, 0 if end of string
  * is encountered and -1 on parse error.
  *
  * result is set to NULL on end of string or parse error. */
-static int split_list(char *c, char **result, void **state) {
+static int split_list(char *c, char **result, bool *is_unpacked, void **state) {
     char *current = *state ? *state : c;
     uint32_t open_braces;
+    bool found_backslash = false;
 
     pa_assert(result);
 
@@ -57,29 +67,52 @@ static int split_list(char *c, char **result, void **state) {
     /* Find opening brace */
     while (*current != 0) {
 
-        if (*current == '{')
+        /* Skip escaped curly braces. */
+        if (*current == '\\' && !found_backslash) {
+            found_backslash = true;
+            current++;
+            continue;
+        }
+
+        if (*current == '{' && !found_backslash)
             break;
 
         /* unexpected closing brace, parse error */
-        if (*current == '}')
+        if (*current == '}' && !found_backslash)
             return -1;
 
+        found_backslash = false;
         current++;
     }
 
     /* No opening brace found, end of string */
     if (*current == 0)
-         return 0;
+        return 0;
 
+    if (is_unpacked)
+        *is_unpacked = true;
     *result = current + 1;
+    found_backslash = false;
     open_braces = 1;
 
     while (open_braces != 0 && *current != 0) {
         current++;
-        if (*current == '{')
+
+        /* Skip escaped curly braces. */
+        if (*current == '\\' && !found_backslash) {
+            found_backslash = true;
+            continue;
+        }
+
+        if (*current == '{' && !found_backslash) {
             open_braces++;
-        if (*current == '}')
+            if (is_unpacked)
+                *is_unpacked = false;
+        }
+        if (*current == '}' && !found_backslash)
             open_braces--;
+
+        found_backslash = false;
     }
 
     /* Parse error, closing brace missing */
@@ -96,23 +129,122 @@ static int split_list(char *c, char **result, void **state) {
     return 1;
 }
 
+/* Read functions */
+
 /* Read a string from the parameter list. The state pointer is
  * advanced to the next element of the list. Returns a pointer
- * to a sub-string within c. The result must not be freed. */
+ * to a sub-string within c. Escape characters will be removed
+ * from the string. The result must not be freed. */
 int pa_message_params_read_string(char *c, const char **result, void **state) {
     char *start_pos;
+    char *value = NULL;
     int r;
+    bool is_unpacked = true;
 
     pa_assert(result);
 
-    if ((r = split_list(c, &start_pos, state)) == 1)
-        *result = start_pos;
+    if ((r = split_list(c, &start_pos, &is_unpacked, state)) == 1)
+        value = start_pos;
+
+    /* Check if we got a plain string not containing further lists */
+    if (!is_unpacked) {
+        /* Parse error */
+        r = -1;
+        value = NULL;
+    }
+
+    if (value)
+        *result = pa_unescape(value);
 
     return r;
 }
 
-/* Another wrapper for split_list() to distinguish between reading
- * pure string data and raw data which may contain further lists. */
+/* A wrapper for split_list() to distinguish between reading pure
+ * string data and raw data which may contain further lists. */
 int pa_message_params_read_raw(char *c, char **result, void **state) {
-    return split_list(c, result, state);
+    return split_list(c, result, NULL, state);
+}
+
+/* Write functions. The functions are wrapper functions around pa_strbuf,
+ * so that the client does not need to use pa_strbuf directly. */
+
+/* Creates a new pa_message_param structure */
+pa_message_params *pa_message_params_new(void) {
+    pa_message_params *params;
+
+    params = pa_xnew(pa_message_params, 1);
+    params->buffer = pa_strbuf_new();
+
+    return params;
+}
+
+/* Frees a pa_message_params structure */
+void pa_message_params_free(pa_message_params *params) {
+    pa_assert(params);
+
+    pa_strbuf_free(params->buffer);
+    pa_xfree(params);
+}
+
+/* Converts a pa_message_param structure to string and frees the structure.
+ * The returned string needs to be freed with pa_xree(). */
+char *pa_message_params_to_string_free(pa_message_params *params) {
+    char *result;
+
+    pa_assert(params);
+
+    result = pa_strbuf_to_string_free(params->buffer);
+
+    pa_xfree(params);
+    return result;
+}
+
+/* Writes an opening curly brace */
+void pa_message_params_begin_list(pa_message_params *params) {
+
+    pa_assert(params);
+
+    pa_strbuf_putc(params->buffer, '{');
+}
+
+/* Writes a closing curly brace */
+void pa_message_params_end_list(pa_message_params *params) {
+
+    pa_assert(params);
+
+    pa_strbuf_putc(params->buffer, '}');
+}
+
+/* Writes a string to a message_params structure, adding curly braces
+ * around the string and escaping curly braces within the string. */
+void pa_message_params_write_string(pa_message_params *params, const char *value) {
+    char *output;
+
+    pa_assert(params);
+
+    /* Null value is written as empty element */
+    if (!value)
+        value = "";
+
+    output = pa_escape(value, "{}");
+    pa_strbuf_printf(params->buffer, "{%s}", output);
+
+    pa_xfree(output);
+}
+
+/* Writes a raw string to a message_params structure, adding curly braces
+ * around the string if add_braces is true. This function can be used to
+ * write parts of a string or whole parameter lists that have been prepared
+ * elsewhere (for example an array). */
+void pa_message_params_write_raw(pa_message_params *params, const char *value, bool add_braces) {
+    pa_assert(params);
+
+    /* Null value is written as empty element */
+    if (!value)
+        value = "";
+
+    if (add_braces)
+        pa_strbuf_printf(params->buffer, "{%s}", value);
+    else
+        pa_strbuf_puts(params->buffer, value);
 }