util: add osmo_strbuf macros to manipulate the strbuf tail

Upcoming patch adopts osmo_strbuf in logging.c, which sometimes needs to
steal and re-add trailing newline characters, and also needs to let
ctime_r() write to the buffer before updating the osmo_strbuf state.

Related: OS#6284
Related: Ib577a5e0d7450ce93ff21f37ba3262704cbf4752
Change-Id: I997707c328eab3ffa00a78fdb9a0a2cbe18404b4

include/osmocom/core/utils.h

@@ -284,6 +284,12 @@ struct osmo_strbuf {
 /*! Return remaining space for characters and terminating nul in the given struct osmo_strbuf. */
 #define OSMO_STRBUF_REMAIN(STRBUF) ((STRBUF).buf ? (STRBUF).len - ((STRBUF).pos - (STRBUF).buf) : 0)
 
+/*! Return number of actual characters contained in struct osmo_strbuf (without terminating nul). */
+#define OSMO_STRBUF_CHAR_COUNT(STRBUF) ((STRBUF).buf && ((STRBUF).pos > (STRBUF).buf) ? \
+					OSMO_MIN((STRBUF).pos - (STRBUF).buf, \
+						 (STRBUF).len - 1) \
+					: 0)
+
 /*! Like OSMO_STRBUF_APPEND(), but for function signatures that return the char* buffer instead of a length.
  * When using this function, the final STRBUF.chars_needed may not reflect the actual number of characters needed, since
  * that number cannot be obtained from this kind of function signature.
@@ -307,6 +313,16 @@ struct osmo_strbuf {
 		(STRBUF).chars_needed += _sb_l; \
 	} while(0)
 
+void osmo_strbuf_drop_tail(struct osmo_strbuf *sb, size_t n_chars);
+/* Convenience macro. struct osmo_strbuf are typically static to a function scope. Avoid having to type '&', same as
+ * with all the other OSMO_STRBUF_* API. */
+#define OSMO_STRBUF_DROP_TAIL(STRBUF, N_CHARS) osmo_strbuf_drop_tail(&(STRBUF), N_CHARS)
+
+void osmo_strbuf_added_tail(struct osmo_strbuf *sb, size_t n_chars);
+/* Convenience macro. struct osmo_strbuf are typically static to a function scope. Avoid having to type '&', same as
+ * with all the other OSMO_STRBUF_* API. */
+#define OSMO_STRBUF_ADDED_TAIL(STRBUF, N_CHARS) osmo_strbuf_added_tail(&(STRBUF), N_CHARS)
+
 bool osmo_str_startswith(const char *str, const char *startswith_str);
 
 int osmo_float_str_to_int(int64_t *val, const char *str, unsigned int precision);

src/core/libosmocore.map

@@ -510,6 +510,8 @@ osmo_strrb_elements;
 osmo_strrb_get_nth;
 _osmo_strrb_is_bufindex_valid;
 osmo_strrb_is_empty;
+osmo_strbuf_drop_tail;
+osmo_strbuf_added_tail;
 osmo_str_startswith;
 osmo_str_to_int;
 osmo_str_to_int64;

src/core/utils.c

@@ -1211,6 +1211,51 @@ char osmo_luhn(const char* in, int in_len)
 	return (sum * 9) % 10 + '0';
 }
 
+/*! Remove up to N chars from the end of an osmo_strbuf.
+ * |--char-count---| - - chars_needed - - |
+ *               |<---------drop----------|
+ */
+void osmo_strbuf_drop_tail(struct osmo_strbuf *sb, size_t n_chars)
+{
+	size_t drop_n;
+	if (sb->pos <= sb->buf)
+		return;
+	drop_n = OSMO_MIN(sb->chars_needed, n_chars);
+	sb->chars_needed -= drop_n;
+	/* chars_needed was reduced by n_chars, which may have been entirely behind the end of a full buffer, within the
+	 * hypothetical chars_needed. Modify the buffer tail pos only if the buffer is not or longer full now. */
+	if (sb->chars_needed >= OSMO_STRBUF_CHAR_COUNT(*sb))
+		return;
+	sb->pos = sb->buf + sb->chars_needed;
+	*sb->pos = '\0';
+}
+
+/*! Let osmo_strbuf know that n_chars characters (excluding nul) were written to the end of the buffer.
+ * If sb is nonempty, the n_chars are assumed to have been written to sb->pos. If sb is still empty and pos == NULL, the
+ * n_chars are assumed to have been written to the start of the buffer.
+ * Advance sb->pos and sb->chars_needed by at most n_chars, or up to sb->len - 1.
+ * Ensure nul termination. */
+void osmo_strbuf_added_tail(struct osmo_strbuf *sb, size_t n_chars)
+{
+	/* On init of an osmo_strbuf, sb->pos == NULL, which is defined as semantically identical to pointing at the
+	 * start of the buffer. A caller may just write to the buffer and call osmo_strbuf_added_tail(), in which case
+	 * still pos == NULL. pos != NULL happens as soon as the first OSMO_STRBUF_*() API has acted on the strbuf. */
+	if (!sb->pos)
+		sb->pos = sb->buf;
+	sb->chars_needed += n_chars;
+	/* first get remaining space, not counting trailing nul; but safeguard against empty buffer */
+	size_t n_added = OSMO_STRBUF_REMAIN(*sb);
+	if (n_added)
+		n_added--;
+	/* do not add more than fit in sb->len, still ensuring nul termination */
+	n_added = OSMO_MIN(n_added, n_chars);
+	if (n_added)
+		sb->pos += n_added;
+	/* when a strbuf is full, sb->pos may point after the final nul, so nul terminate only when pos is valid. */
+	if (sb->pos < sb->buf + sb->len)
+		*sb->pos = '\0';
+}
+
 /*! Compare start of a string.
  * This is an optimisation of 'strstr(str, startswith_str) == str' because it doesn't search through the entire string.
  * \param str  (Longer) string to compare.

tests/utils/utils_test.c

@@ -31,6 +31,7 @@
 #include <errno.h>
 #include <limits.h>
 #include <inttypes.h>
+#include <string.h>
 
 static void hexdump_test(void)
 {
@@ -1306,6 +1307,50 @@ void strbuf_test_nolen(void)
 	printf("%zu: %s (need=%zu)\n", sb.len, buf, sb.chars_needed);
 }
 
+void strbuf_test_tail_for_buflen(size_t buflen)
+{
+	char buf[buflen];
+	struct osmo_strbuf sb = { .buf = buf, .len = buflen };
+	printf("\n%s(%zu)\n", __func__, buflen);
+
+#define SHOW(N) \
+	printf(#N ": %s sb.chars_needed=%zu sb.pos=&sb.buf[%d]\n", \
+	       osmo_quote_str(buf, -1), sb.chars_needed, (int)(sb.pos - sb.buf))
+
+	/* shorten in steps using OSMO_STRBUF_DROP_TAIL(), removing and re-adding a trailing newline. */
+	OSMO_STRBUF_PRINTF(sb, "banananana\n");
+	SHOW(1);
+	OSMO_STRBUF_DROP_TAIL(sb, 3);
+	SHOW(2);
+	OSMO_STRBUF_PRINTF(sb, "\n");
+	SHOW(3);
+	OSMO_STRBUF_DROP_TAIL(sb, 3);
+	SHOW(4);
+	OSMO_STRBUF_PRINTF(sb, "\n");
+	SHOW(5);
+
+	/* drop trailing newline */
+	OSMO_STRBUF_DROP_TAIL(sb, 1);
+	SHOW(6);
+
+	/* test writing something to the end and letting OSMO_STRBUF_ADDED_TAIL() know later */
+	int n = OSMO_MIN(6, OSMO_STRBUF_REMAIN(sb));
+	if (n)
+		memcpy(sb.pos, "bread\n", n);
+	OSMO_STRBUF_ADDED_TAIL(sb, 6);
+	SHOW(7);
+}
+
+void strbuf_test_tail(void)
+{
+	strbuf_test_tail_for_buflen(64);
+	strbuf_test_tail_for_buflen(32);
+	strbuf_test_tail_for_buflen(16);
+	strbuf_test_tail_for_buflen(8);
+	strbuf_test_tail_for_buflen(4);
+	strbuf_test_tail_for_buflen(1);
+}
+
 static void startswith_test_str(const char *str, const char *startswith_str, bool expect_rc)
 {
 	bool rc = osmo_str_startswith(str, startswith_str);
@@ -2152,6 +2197,7 @@ int main(int argc, char **argv)
 	osmo_str_tolowupper_test();
 	strbuf_test();
 	strbuf_test_nolen();
+	strbuf_test_tail();
 	startswith_test();
 	name_c_impl_test();
 	osmo_print_n_test();

tests/utils/utils_test.ok

@@ -470,6 +470,60 @@ strbuf_test_nolen
 more: 0001011100101010000 (need=19)
 10: 000101110 (need=9)
 
+strbuf_test_tail_for_buflen(64)
+1: "banananana\n" sb.chars_needed=11 sb.pos=&sb.buf[11]
+2: "bananana" sb.chars_needed=8 sb.pos=&sb.buf[8]
+3: "bananana\n" sb.chars_needed=9 sb.pos=&sb.buf[9]
+4: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+5: "banana\n" sb.chars_needed=7 sb.pos=&sb.buf[7]
+6: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+7: "bananabread\n" sb.chars_needed=12 sb.pos=&sb.buf[12]
+
+strbuf_test_tail_for_buflen(32)
+1: "banananana\n" sb.chars_needed=11 sb.pos=&sb.buf[11]
+2: "bananana" sb.chars_needed=8 sb.pos=&sb.buf[8]
+3: "bananana\n" sb.chars_needed=9 sb.pos=&sb.buf[9]
+4: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+5: "banana\n" sb.chars_needed=7 sb.pos=&sb.buf[7]
+6: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+7: "bananabread\n" sb.chars_needed=12 sb.pos=&sb.buf[12]
+
+strbuf_test_tail_for_buflen(16)
+1: "banananana\n" sb.chars_needed=11 sb.pos=&sb.buf[11]
+2: "bananana" sb.chars_needed=8 sb.pos=&sb.buf[8]
+3: "bananana\n" sb.chars_needed=9 sb.pos=&sb.buf[9]
+4: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+5: "banana\n" sb.chars_needed=7 sb.pos=&sb.buf[7]
+6: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+7: "bananabread\n" sb.chars_needed=12 sb.pos=&sb.buf[12]
+
+strbuf_test_tail_for_buflen(8)
+1: "bananan" sb.chars_needed=11 sb.pos=&sb.buf[8]
+2: "bananan" sb.chars_needed=8 sb.pos=&sb.buf[8]
+3: "bananan" sb.chars_needed=9 sb.pos=&sb.buf[8]
+4: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+5: "banana\n" sb.chars_needed=7 sb.pos=&sb.buf[7]
+6: "banana" sb.chars_needed=6 sb.pos=&sb.buf[6]
+7: "bananab" sb.chars_needed=12 sb.pos=&sb.buf[7]
+
+strbuf_test_tail_for_buflen(4)
+1: "ban" sb.chars_needed=11 sb.pos=&sb.buf[4]
+2: "ban" sb.chars_needed=8 sb.pos=&sb.buf[4]
+3: "ban" sb.chars_needed=9 sb.pos=&sb.buf[4]
+4: "ban" sb.chars_needed=6 sb.pos=&sb.buf[4]
+5: "ban" sb.chars_needed=7 sb.pos=&sb.buf[4]
+6: "ban" sb.chars_needed=6 sb.pos=&sb.buf[4]
+7: "ban" sb.chars_needed=12 sb.pos=&sb.buf[4]
+
+strbuf_test_tail_for_buflen(1)
+1: "" sb.chars_needed=11 sb.pos=&sb.buf[1]
+2: "" sb.chars_needed=8 sb.pos=&sb.buf[1]
+3: "" sb.chars_needed=9 sb.pos=&sb.buf[1]
+4: "" sb.chars_needed=6 sb.pos=&sb.buf[1]
+5: "" sb.chars_needed=7 sb.pos=&sb.buf[1]
+6: "" sb.chars_needed=6 sb.pos=&sb.buf[1]
+7: "" sb.chars_needed=12 sb.pos=&sb.buf[1]
+
 startswith_test()
 osmo_str_startswith(NULL, NULL) == true
 osmo_str_startswith("", NULL) == true