[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, 3 insertions, 3 deletions

diff --git a/src/plugin.c b/src/plugin.c
index 559eb3a8..814b78ea 100644
--- a/src/plugin.c
+++ b/src/plugin.c
@@ -157,8 +157,8 @@ int plugins_load(server *srv) {
 
 	for (uint32_t i = 0; i < srv->srvconf.modules->used; ++i) {
 		const buffer * const module = &((data_string *)srv->srvconf.modules->data[i])->value;
-		buffer_copy_path_len2(tb, CONST_BUF_LEN(srv->srvconf.modules_dir),
-		                          CONST_BUF_LEN(module));
+		buffer_copy_path_len2(tb, BUF_PTR_LEN(srv->srvconf.modules_dir),
+		                          BUF_PTR_LEN(module));
 #if defined(__WIN32) || defined(__CYGWIN__)
 		buffer_append_string_len(tb, CONST_STR_LEN(".dll"));
 #else
@@ -198,7 +198,7 @@ int plugins_load(server *srv) {
 
 #endif
 		buffer_clear(tb);
-		buffer_append_str2(tb, CONST_BUF_LEN(module),
+		buffer_append_str2(tb, BUF_PTR_LEN(module),
                                        CONST_STR_LEN("_plugin_init"));
 
 #ifdef __WIN32