[multiple] reduce redundant NULL buffer checks

This commit is a large set of code changes and results in removal of
hundreds, perhaps thousands, of CPU instructions, a portion of which
are on hot code paths.

Most (buffer *) used by lighttpd are not NULL, especially since buffers
were inlined into numerous larger structs such as request_st and chunk.

In the small number of instances where that is not the case, a NULL
check is often performed earlier in a function where that buffer is
later used with a buffer_* func.  In the handful of cases that remained,
a NULL check was added, e.g. with r->http_host and r->conf.server_tag.

- check for empty strings at config time and set value to NULL if blank
  string will be ignored at runtime; at runtime, simple pointer check
  for NULL can be used to check for a value that has been set and is not
  blank ("")
- use buffer_is_blank() instead of buffer_string_is_empty(),
  and use buffer_is_unset() instead of buffer_is_empty(),
  where buffer is known not to be NULL so that NULL check can be skipped
- use buffer_clen() instead of buffer_string_length() when buffer is
  known not to be NULL (to avoid NULL check at runtime)
- use buffer_truncate() instead of buffer_string_set_length() to
  truncate string, and use buffer_extend() to extend

Examples where buffer known not to be NULL:
  - cpv->v.b from config_plugin_values_init is not NULL if T_CONFIG_BOOL
    (though we might set it to NULL if buffer_is_blank(cpv->v.b))
  - address of buffer is arg (&foo)
    (compiler optimizer detects this in most, but not all, cases)
  - buffer is checked for NULL earlier in func
  - buffer is accessed in same scope without a NULL check (e.g. b->ptr)

internal behavior change:
  callers must not pass a NULL buffer to some funcs.
  - buffer_init_buffer() requires non-null args
  - buffer_copy_buffer() requires non-null args
  - buffer_append_string_buffer() requires non-null args
  - buffer_string_space() requires non-null arg

1 files changed, 5 insertions, 4 deletions

diff --git a/src/mod_indexfile.c b/src/mod_indexfile.c
index 64dbf0cf..5799a23b 100644
--- a/src/mod_indexfile.c
+++ b/src/mod_indexfile.c
@@ -106,12 +106,12 @@ URIHANDLER_FUNC(mod_indexfile_subrequest) {
 			 * index-generator */
 
 		/* temporarily append to base-path buffer to check existence */
-		const uint32_t len = buffer_string_length(b);
-		buffer_append_path_len(b, CONST_BUF_LEN(&ds->value));
+		const uint32_t len = buffer_clen(b);
+		buffer_append_path_len(b, BUF_PTR_LEN(&ds->value));
 
 		const stat_cache_st * const st = stat_cache_path_stat(b);
 
-		buffer_string_set_length(b, len);
+		buffer_truncate(b, len);
 
 		if (NULL == st) {
 			switch (errno) {
@@ -135,7 +135,8 @@ URIHANDLER_FUNC(mod_indexfile_subrequest) {
 		if (ds->value.ptr[0] == '/') {
 			/* replace uri.path */
 			buffer_copy_buffer(&r->uri.path, &ds->value);
-			http_header_env_set(r, CONST_STR_LEN("PATH_TRANSLATED_DIRINDEX"), CONST_BUF_LEN(&r->physical.path));
+			http_header_env_set(r, CONST_STR_LEN("PATH_TRANSLATED_DIRINDEX"),
+			                       BUF_PTR_LEN(&r->physical.path));
 			buffer_copy_buffer(&r->physical.path, &r->physical.doc_root);
 		} else {
 			/* append to uri.path the relative path to index file (/ -> /index.php) */