[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, 4 insertions, 4 deletions

diff --git a/src/mod_scgi.c b/src/mod_scgi.c
index 70cf00b8..bed7b583 100644
--- a/src/mod_scgi.c
+++ b/src/mod_scgi.c
@@ -208,15 +208,15 @@ static handler_t scgi_create_env(handler_ctx *hctx) {
 		size_t len;
 		scgi_env_add(b, CONST_STR_LEN("SCGI"), CONST_STR_LEN("1"));
 		buffer_clear(tb);
-		buffer_append_int(tb, buffer_string_length(b)-10);
+		buffer_append_int(tb, buffer_clen(b)-10);
 		buffer_append_string_len(tb, CONST_STR_LEN(":"));
-		len = buffer_string_length(tb);
+		len = buffer_clen(tb);
 		offset = 10 - len;
 		memcpy(b->ptr+offset, tb->ptr, len);
 		buffer_append_string_len(b, CONST_STR_LEN(","));
 	} else { /* LI_PROTOCOL_UWSGI */
 		/* http://uwsgi-docs.readthedocs.io/en/latest/Protocol.html */
-		size_t len = buffer_string_length(b)-10;
+		size_t len = buffer_clen(b)-10;
 		if (len > USHRT_MAX) {
 			r->http_status = 431; /* Request Header Fields Too Large */
 			r->handler_module = NULL;
@@ -231,7 +231,7 @@ static handler_t scgi_create_env(handler_ctx *hctx) {
 		b->ptr[offset+3] = 0;
 	}
 
-	hctx->wb_reqlen = buffer_string_length(b) - offset;
+	hctx->wb_reqlen = buffer_clen(b) - offset;
 	chunkqueue_prepend_buffer_commit(&hctx->wb);
 	chunkqueue_mark_written(&hctx->wb, offset);
 	hctx->wb.bytes_in  -= (off_t)offset;